The A3 adenosine receptor: an enigmatic player in cell biology

Electromagnetic fields (EMFs) and adenosine receptors modulate prostaglandin E(2) and cytokine release in human osteoarthritic synovial fibroblasts

Synovial fibroblasts (SFs) contribute to the development of osteoarthritis (OA) by the secretion of a wide range of pro-inflammatory mediators, including cytokines and lipid mediators of inflammation. Previous studies suggest that electromagnetic fields (EMFs) may represent a potential therapeutic approach to limit cartilage degradation and control inflammation associated to OA, and that they may act through the adenosine pathway. Therefore, we investigated whether EMFs might modulate inflammatory activities of human SFs from OA patients (OASFs) treated with interleukin-1β (IL-1β), and the possible involvement of adenosine receptors (ARs) in mediating EMF effects. EMF exposure induced a selective increase in A(2A) and A(3) ARs. These increases were associated to changes in cAMP levels, indicating that ARs were functionally active also in EMF-exposed cells. Functional data obtained in the presence of selective A(2A) and A(3) adenosine agonists and antagonists showed that EMFs inhibit the release of prostaglandin E(2) (PGE(2)) and the proinflammatory cytokines interleukin-6 (IL-6) and interleukin-8 (IL-8), while stimulating the release of interleukin-10 (IL-10), an antinflammatory cytokine. These effects seem to be mediated by the EMF-induced upregulation of A(2A) and A(3) ARs. No effects of EMFs or ARs have been observed on matrix degrading enzyme production. In conclusion, this study shows that EMFs display anti-inflammatory effects in human OASFs, and that these EMF-induced effects are in part mediated by the adenosine pathway, specifically by the A(2A) and A(3) AR activation. Taken together, these results open new clinical perspectives to the control of inflammation associated to joint diseases.

A2 adenosine receptors and vascular pathologies

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

Recent Patents on Emerging Therapeutics for the Treatment of Glaucoma, Age Related Macular Degeneration and Uveitis

Advancements in the field and rising interest among pharmaceutical researchers have led to the development of new molecules with enhanced therapeutic activity. Design of new drugs which can target a particular pathway and/or explore novel targets is of immense interest to ocular pharmacologists worldwide. Delivery of suitable pharmacologically active agents at proper dose (within the therapeutic window) to the target tissues without any toxicity to the healthy ocular tissues still remain an elusive task. Moreover, the presence of static and dynamic barriers to drug absorption including the corneal epithelium (lipophilic), corneal and scleral stroma (hydrophilic), conjunctival lymphatics, choroidal vasculature and the blood-ocular barriers also pose a significant challenge for achieving therapeutic drug concentrations at the target site. Although many agents are currently available, new compounds are being introduced for treating various ocular diseases. Deeper understanding of the etiology and complex mechanisms associated with the disease condition would aid in the development of potential therapeutic candidates. Novel small molecules as well as complex biotechnology derived macromolecules with superior efficacy, safety and tolerability are being developed. Therefore, this review article provides an overview of existing drugs, treatment options, advances in emerging therapeutics and related recent patents for the treatment of ocular disorders such as glaucoma, age related macular degeneration (AMD) and uveitis.

Discovery of novel A3 adenosine receptor ligands based on chromone scaffold

A project focused on the discovery of new chemical entities (NCEs) as AR ligands that incorporate a benzo-γ-pyrone [(4H)-1-benzopyran-4-one] substructure has been developed. Accordingly, two series of novel chromone carboxamides placed at positions C2 (compounds 2-13) and C3 (compounds 15-26) of the γ-pyrone ring were synthesized using chromone carboxylic acids (compounds 1 or 14) as starting materials. From this study and on the basis of the obtained structure-activity relationships it was concluded that the chromone carboxamide scaffold represent a novel class of AR ligands. The most remarkable chromones were compounds 21 and 26 that present a better affinity for A3AR (Ki = 3680 nM and Ki = 3750 nM, respectively). Receptor-driven molecular modeling studies provide information on the binding/selectivity data of the chromone. The data so far acquired are instrumental for future optimization of chromone carboxamide as a selective A3AR antagonist.

Highly potent and selective fluorescent antagonists of the human adenosine A₃ receptor based on the 1,2,4-triazolo[4,3-a]quinoxalin-1-one scaffold

The adenosine-A(3) receptor (A(3)AR) is a G protein-coupled receptor that shows promise as a therapeutic target for cancer, glaucoma, and various autoimmune inflammatory disorders, and as such, there is a need for molecular probes to study this receptor. Here, we report a series of fluorescent ligands containing different linkers and fluorophores based around a 1,2,4-triazolo[4,3-a]quinoxalin-1-one antagonist. One of these conjugates (19) displayed high affinity for the A(3)AR (pK(D) = 9.36 ± 0.12) and is >650-fold selective over other adenosine receptor subtypes. Confocal microscopy revealed clear, displaceable membrane labeling of CHO-A(3) cells with 19, with no detectable labeling of CHO-A(1) cells under identical conditions. This fluorescent ligand was also able to specifically label the A(3)AR in HEK293T cells containing a mixed adenosine receptor population. The subtype specificity, along with its excellent imaging properties, make 19 an ideal tool for studying A(3)AR distribution and organization, particularly in the presence of other adenosine receptor subtypes.

P-glycoprotein mediates resistance to A3 adenosine receptor agonist 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-n-methyluronamide in human leukemia cells

We studied effects of 2-chloro-N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA) on apoptosis induction in the K562/Dox cell line, which overexpressed P-glycoprotein (P-gp, ABCB1, MDR1). We found that the K562/Dox cell line was significantly more resistant to Cl-IB-MECA than the maternal cell line K562, which did not express P-gp. Although both cell lines expressed the A3 adenosine receptor (A3AR), cytotoxic effects of Cl-IB-MECA were not prevented by its selective antagonist MRS1523 (3-propyl-6-ethyl-5-[(ethylthio)carbonyl]-2 phenyl-4-propyl-3-pyridine carboxylate). Analysis of cell extracts revealed that the intracellular level of Cl-IB-MECA was significantly lower in the K562/Dox cell line than in the maternal cell line K562. The downregulation of P-gp expression using shRNA targeting ABCB1 gene led to increased intracellular level of Cl-IB-MECA and restored cell sensitivity to this drug. Similarly, valspodar (PSC-833), a specific inhibitor of P-gp, restored sensitivity of the K562/Dox cell line to Cl-IB-MECA with concomitant increase of intracellular level of Cl-IB-MECA in the resistant cell line, while it affected cytotoxicity of Cl-IB-MECA in the sensitive cell line only marginally. An enzyme based assay provided evidence for interaction of P-gp with Cl-IB-MECA. We further observed that cytotoxic effects of Cl-IB-MECA could be augmented by activation of extrinsic cell death pathway by Apo-2L (TRAIL) but not FasL or TNF-α. Our results revealed that Cl-IB-MECA induced an increase in expression of TRAIL receptors in K562 cells, which could sensitize cells to apoptosis induction via an extrinsic cell death pathway. Importantly, these effects were inversely related to P-gp expression. In addition, MRS1523 did not affect Cl-IB-MECA induced expression of TRAIL receptors.

Cytotoxic purine nucleoside analogues bind to A1, A2A, and A3 adenosine receptors

Fludarabine, clofarabine, and cladribine are anticancer agents which are analogues of the purine nucleoside adenosine. These agents have been associated with cardiac and neurological toxicities. Because these agents are analogues of adenosine, they may act through adenosine receptors to elicit their toxic effects. The objective of this study was to evaluate the ability of cytotoxic nucleoside analogues to bind and activate adenosine receptor subtypes (A(1), A(2A), A(2B), and A(3)). Radioligand binding studies utilizing Chinese hamster ovary cells, stably transfected with adenosine A(1), A(2A), or A(3) receptor subtype, were used to assess the binding affinities of these compounds, whereas adenylyl cyclase activity was used to assess the binding to A(2B) receptors. Clofarabine and cladribine both bound to the A(2A) receptor with a K (i) of 17 and 15 μM, respectively. Clofarabine was the only adenosine analogue to bind to the A(3) receptor with a K (i) of 10 μM, and none of these compounds bound to the A(2B) receptor. Results show that clofarabine, cladribine, and fludarabine bind to the A(1) receptor. In addition, clofarabine, cladribine, and fludarabine were A(1) agonists (IC(50) 3.1, 30, and 30 μM, respectively). Neither pyrimidine nucleoside analogues gemcitabine nor cytarabine associated with any of the adenosine receptor subtypes (K (i) > 100μM). This is the first report of an interaction between all adenosine receptor subtypes and chemotherapeutic nucleoside analogues commonly used in the treatment of cancer. Therefore, activation of these receptors may be at least one mechanism through which fludarabine-associated toxicity occurs.

Emerging adenosine receptor agonists: an update

Adenosine receptors (ARs), the major targets of caffeine and theophylline, comprise four receptor subtypes designated as A(1), A(2A), A(2B) and A(3). Over a dozen AR agonists are currently in clinical trials for various conditions, including cardiac arrhythmias, neuropathic pain, myocardial perfusion imaging, cardiac ischemia, inflammatory diseases and cancer. Adenosine (nonselective), regadenoson (A(2A)) and dipyridamole (act indirectly via ARs) have received regulatory approval for clinical use. The present editorial will give a brief update on the current status of AR agonists in clinical trials.

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. 

Adenosine receptor targeting in health and disease

INTRODUCTION

The adenosine receptors A(1), A(2A), A(2B) and A(3) are important and ubiquitous mediators of cellular signaling that play vital roles in protecting tissues and organs from damage. In particular, adenosine triggers tissue protection and repair by different receptor-mediated mechanisms, including increasing the oxygen supply:demand ratio, pre-conditioning, anti-inflammatory effects and the stimulation of angiogenesis.

AREAS COVERED

The state of the art of the role of adenosine receptors which have been proposed as targets for drug design and discovery, in health and disease, and an overview of the ligands for these receptors in clinical development.

EXPERT OPINION

Selective ligands of A(1), A(2A), A(2B) and A(3) adenosine receptors are likely to find applications in the treatment of pain, ischemic conditions, glaucoma, asthma, arthritis, cancer and other disorders in which inflammation is a feature. The aim of this review is to provide an overview of the present knowledge regarding the role of these adenosine receptors in health and disease.

Ecto-5'-nucleotidase (CD73) decreases mortality and organ injury in sepsis

The extracellular concentrations of adenosine are increased during sepsis, and adenosine receptors regulate the host's response to sepsis. In this study, we investigated the role of the adenosine-generating ectoenzyme, ecto-5'-nucleotidase (CD73), in regulating immune and organ function during sepsis. Polymicrobial sepsis was induced by subjecting CD73 knockout (KO) and wild type (WT) mice to cecal ligation and puncture. CD73 KO mice showed increased mortality in comparison with WT mice, which was associated with increased bacterial counts and elevated inflammatory cytokine and chemokine concentrations in the blood and peritoneum. CD73 deficiency promoted lung injury, as indicated by increased myeloperoxidase activity and neutrophil infiltration, and elevated pulmonary cytokine levels. CD73 KO mice had increased apoptosis in the thymus, as evidenced by increased cleavage of caspase-3 and poly(ADP-ribose) polymerase and increased activation of NF-κB. Septic CD73 KO mice had higher blood urea nitrogen levels and increased cytokine levels in the kidney, indicating increased renal dysfunction. The increased kidney injury of CD73 KO mice was associated with augmented activation of p38 MAPK and decreased phosphorylation of Akt. Pharmacological inactivation of CD73 in WT mice using α, β-methylene ADP augmented cytokine levels in the blood and peritoneal lavage fluid. These findings suggest that CD73-derived adenosine may be beneficial in sepsis.

Infective and inflammatory diarrhoea: mechanisms and opportunities for novel therapies

There have been significant advances in unravelling the cellular mechanisms of diarrhoea in common gut infections and colonic inflammation, as well as in the identification of targets for potential antidiarrhoeal drugs. Infective diarrhoea reflects activation of electrogenic Cl⁻ secretion, inhibition of electroneutral NaCl absorption and in some cases, downregulation of tight junctional proteins and increased apoptosis. In colonic inflammation, diarrhoea mainly reflects impairment of colonic Na⁺ and Cl⁻ absorption by inflammatory cytokines, leading to decreased water absorption. Stimulation of endogenous opiate-dependent pathways, manipulation of epithelial ion (Na⁺, K⁺ and Cl⁻) channels and suppression of proinflammatory cytokine production by a variety of drugs and novel molecules, offer opportunities to move evaluation of these potential antisecretory and anti-inflammatory agents from the laboratory into clinical trials.

Localization and function of adenosine receptor subtypes at the longitudinal muscle--myenteric plexus of the rat ileum

Adenosine plays a dual role on acetylcholine (ACh) release from myenteric motoneurons via the activation of high-affinity inhibitory A₁ and facilitatory A(2A) receptors. The therapeutic potential of adenosine-related compounds for controlling intestinal motility and inflammation, prompted us to investigate further the role of low-affinity adenosine receptors, A(2B) and A₃, on electrically-evoked (5 Hz, 200 pulses) [³H]ACh release from myenteric neurons. Immunolocalization studies showed that A(2B) receptors exhibit a pattern of distribution similar to the glial cell marker, GFAP. Regarding A₁ and A₃ receptors, they are mainly distributed to cell bodies of ganglionic myenteric neurons, whereas A(2A) receptors are localized predominantly on cholinergic nerve terminals. Using selective antagonists (DPCPX, ZM241385 and MRS1191), data indicate that modulation of evoked [³H]ACh release is balanced through tonic activation of inhibitory (A₁) and facilitatory (A(2A) and A₃) receptors by endogenous adenosine. The selective A(2B) receptor antagonist, PSB603, alone was devoid of effect and failed to modify the inhibitory effect of NECA. The A₃ receptor agonist, 2-Cl-IB MECA (1-10 nM), concentration-dependently increased the release of [³H]ACh. The effect of 2-Cl-IB MECA was attenuated by MRS1191 and by ZM241385, which selectively block respectively A₃ and A(2A) receptors. In contrast to 2-Cl-IB MECA, activation of A(2A) receptors with CGS21680C attenuated nicotinic facilitation of ACh release induced by focal depolarization of myenteric nerve terminals in the presence of tetrodotoxin. Tandem localization of excitatory A₃ and A(2A) receptors along myenteric neurons explains why stimulation of A₃ receptors (with 2-Cl-IB MECA) on nerve cell bodies acts cooperatively with prejunctional facilitatory A(2A) receptors to up-regulate acetylcholine release. The results presented herein consolidate and expand the current understanding of adenosine receptor distribution and function in the myenteric plexus of the rat ileum, and should be taken into consideration for data interpretation regarding the pathophysiological implications of adenosine on intestinal motility disorders.

Adenosine receptors in health and disease

The adenosine receptors A(1), A(2A), A(2B), and A(3) are important and ubiquitous mediators of cellular signaling, which play vital roles in protecting tissues and organs from damage. In particular, adenosine triggers tissue protection and repair by different receptor-mediated mechanisms, including an increase of oxygen supply/demand ratio, preconditioning, anti-inflammatory effects, and stimulation of angiogenesis. Considerable advances have been recently achieved in the pharmacological and molecular characterization of adenosine receptors, which have been proposed as targets for drug design and discovery. At the present time, it can be speculated that adenosine A(1), A(2A), A(2B), and A(3) receptor-selective ligands may show utility in the treatment of pain, ischemic conditions, glaucoma, asthma, arthritis, cancer, and other disorders in which inflammation is a feature. This chapter documents the present state of knowledge of adenosine receptors' role in health and disease.

CGS 21680, an agonist of the adenosine (A2A) receptor, reduces progression of murine type II collagen-induced arthritis

OBJECTIVE

The aim of our study was to investigate the effect of an adenosine A2A receptor agonist, 2-[p-(2-carboxyethyl)phenylethylamino]-50 ethylcarboxamidoadenosine (CGS 21680), on modulation of the inflammatory response in mice subjected to collagen-induced arthritis (CIA).

METHODS

CIA was induced by intradermal injection of 100 μl of emulsion containing 100 μg of bovine type II collagen (CII) and complete Freund's adjuvant (CFA) at the base of the tail. On Day 21, a second injection of CII in CFA was administered. Immunized mice developed erosive hind paw arthritis. Macroscopic clinical evidence of CIA first appeared as periarticular erythema and edema in the hind paws. The incidence of CIA was 100% by Day 27 in the CII challenged mice and the severity of CIA progressed over a 35-day period, with radiographic evaluation revealing focal resorption of bone. The histopathology of CIA included erosion of cartilage at the joint margins.

RESULTS

Treatment of mice with CGS 21680 starting at the onset of arthritis (Day 25) ameliorated the clinical signs at Days 26-35 and improved histological status in the joint and paw. The degree of oxidative and nitrosative damage was significantly reduced in CGS 21680-treated mice as indicated by elevated levels of malondialdehyde, formation of nitrotyrosine, and activation of poly(ADP-ribose) polymerase. Plasma levels of proinflammatory cytokines such as tumor necrosis factor, interleukin 1ß (IL-1ß) and IL-6 were also reduced by CGS 21680. Treatment with CGS 21680 also decreased the expression of inducible nitric oxide synthase and cyclooxygenase-2.

CONCLUSION

We demonstrate that CGS 21680 exerts an antiinflammatory effect during chronic inflammation and ameliorates the tissue damage associated with CIA.

CGS 21680, an agonist of the adenosine (A2A) receptor, decreases acute lung inflammation

Adenosine A(2A) receptor agonists may be important regulators of inflammation. The aim of this study was to investigate the effects of CGS 21680 (0.1mg/kgi.p.), an agonist of the adenosine (A(2A)) receptor, in a mouse model of carrageenan-induced pleurisy. Injection of carrageenan into the pleural cavity of mice elicited an acute inflammatory response characterised by: infiltration of neutrophils in lung tissues and subsequent lipid peroxidation, increased production of nitric oxide (NO), cytokines such as tumour necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) and increased expression of intercellular adhesion molecule (ICAM-1) and platelet-adhesion molecule (P-selectin). Furthermore, carrageenan induced the expression of nuclear factor-κB (NF-κB), inducible nitric oxide synthase (iNOS), nitrotyrosine, the activation of poly-ADP-ribosyl polymerase (PARP), as well as induced apoptosis (FAS-ligand expression, Bax and Bcl-2 expression) in the lung tissues. Administration of CGS 21680, 30 min prior to challenge with carrageenan, caused a significant reduction of all the parameters of inflammation measured. In addition, to confirm the anti-inflammatory effect of CGS 21680, we have also evaluated the effects of CGS 21680 post-treatment (30 min after the challenge with carrageenan) and we have demonstrated that also it caused a reduction of neutrophil infiltration and the degree of lung injury. Thus, based on these findings we propose that adenosine A(2A) receptor agonists such as CGS 21680 may be useful in the treatment of various inflammatory diseases.

Adenosine activation of A(2B) receptor(s) is essential for stimulated epithelial ciliary motility and clearance

Mucociliary clearance, vital to lung clearance, is dependent on cilia beat frequency (CBF), coordination of cilia, and the maintenance of periciliary fluid. Adenosine, the metabolic breakdown product of ATP, is an important modulator of ciliary motility. However, the contributions of specific adenosine receptors to key airway ciliary motility processes are unclear. We hypothesized that adenosine modulates ciliary motility via activation of its cell surface receptors (A(1), A(2A), A(2B), or A(3)). To test this hypothesis, mouse tracheal rings (MTRs) excised from wild-type and adenosine receptor knockout mice (A(1), A(2A), A(2B), or A(3), respectively), and bovine ciliated bronchial epithelial cells (BBECs) were stimulated with known cilia activators, isoproterenol (ISO; 10 μM) and/or procaterol (10 μM), in the presence or absence of 5'-(N-ethylcarboxamido) adenosine (NECA), a nonselective adenosine receptor agonist [100 nM (A(1), A(2A), A(3)); 10 μM (A(2B))], and CBF was measured. Cells and MTRs were also stimulated with NECA (100 nM or 10 μM) in the presence and absence of adenosine deaminase inhibitor, erythro-9- (2-hydroxy-3-nonyl) adenine hydrochloride (10 μM). Both ISO and procaterol stimulated CBF in untreated cells and/or MTRs from both wild-type and adenosine knockout mice by ~3 Hz. Likewise, CBF significantly increased ~2-3 Hz in BBECs and wild-type MTRs stimulated with NECA. MTRs from A(1), A(2A), and A(3) knockout mice stimulated with NECA also demonstrated an increase in CBF. However, NECA failed to stimulate CBF in MTRs from A(2B) knockout mice. To confirm the mechanism by which adenosine modulates CBF, protein kinase activity assays were conducted. The data revealed that NECA-stimulated CBF is mediated by the activation of cAMP-dependent PKA. Collectively, these data indicate that purinergic stimulation of CBF requires A(2B) adenosine receptor activation, likely via a PKA-dependent pathway.

Novel ocular antihypertensive compounds in clinical trials

Introduction:

Glaucoma is a multifactorial disease characterized by progressive optic nerve injury and visual field defects. Elevated intraocular pressure (IOP) is the most widely recognized risk factor for the onset and progression of open-angle glaucoma, and IOP-lowering medications comprise the primary treatment strategy. IOP elevation in glaucoma is associated with diminished or obstructed aqueous humor outflow. Pharmacotherapy reduces IOP by suppressing aqueous inflow and/or increasing aqueous outflow.

Purpose:

This review focuses on novel non-FDA approved ocular antihypertensive compounds being investigated for IOP reduction in ocular hypertensive and glaucoma patients in active clinical trials within approximately the past 2 years.

Methods:

The mode of IOP reduction, pharmacology, efficacy, and safety of these new agents were assessed. Relevant drug efficacy and safety trials were identified from searches of various scientific literature databases and clinical trial registries. Compounds with no specified drug class, insufficient background information, reformulations, and fixed-combinations of marketed drugs were not considered.

Results:

The investigational agents identified comprise those that act on the same targets of established drug classes approved by the FDA (ie, prostaglandin analogs and β-adrenergic blockers) as well as agents belonging to novel drug classes with unique mechanisms of action. Novel targets and compounds evaluated in clinical trials include an actin polymerization inhibitor (ie, latrunculin), Rho-associated protein kinase inhibitors, adenosine receptor analogs, an angiotensin II type 1 receptor antagonist, cannabinoid receptor agonists, and a serotonin receptor antagonist.

Conclusion:

The clinical value of novel compounds for the treatment of glaucoma will depend ultimately on demonstrating favorable efficacy and benefit-to-risk ratios relative to currently approved prostaglandin analogs and β-blockers and/or having complementary modes of action.

Mechanisms involved in the adenosine-induced vasorelaxation to the pig prostatic small arteries

Benign prostatic hypertrophy has been related with glandular ischemia processes and adenosine is a potent vasodilator agent. This study investigates the mechanisms underlying the adenosine-induced vasorelaxation in pig prostatic small arteries. Adenosine receptors expression was determined by Western blot and immunohistochemistry, and rings were mounted in myographs for isometric force recording. A(2A) and A(3) receptor expression was observed in the arterial wall and A(2A)-immunoreactivity was identified in the adventitia-media junction and endothelium. A(1) and A(2B) receptor expression was not obtained. On noradrenaline-precontracted rings, P1 receptor agonists produced concentration-dependent relaxations with the following order of potency: 5'-N-ethylcarboxamidoadenosine (NECA) = CGS21680 > 2-Cl-IB-MECA = 2-Cl-cyclopentyladenosine = adenosine. Adenosine reuptake inhibition potentiated both NECA and adenosine relaxations. Endothelium removal and ZM241385, an A(2A) antagonist, reduced NECA relaxations that were not modified by A(1), A(2B), and A(3) receptor antagonists. Neuronal voltage-gated Ca(2+) channels and nitric oxide (NO) synthase blockade, and adenylyl cyclase activation enhanced these responses, which were reduced by protein kinase A inhibition and by blockade of the intermediate (IK(Ca))- and small (SK(Ca))-conductance Ca(2+)-activated K(+) channels. Inhibition of cyclooxygenase (COX), large-conductance Ca(2+)-activated-, ATP-dependent-, and voltage-gated-K(+) channel failed to modify these responses. These results suggest that adenosine induces endothelium-dependent relaxations in the pig prostatic arteries via A(2A) purinoceptors. The adenosine vasorelaxation, which is prejunctionally modulated, is produced via NO- and COX-independent mechanisms that involve activation of IK(Ca) and SK(Ca) channels and stimulation of adenylyl cyclase. Endothelium-derived NO playing a regulatory role under conditions in which EDHF is non-functional is also suggested. Adenosine-induced vasodilatation could be useful to prevent prostatic ischemia.

Functional selectivity of adenosine receptor ligands

Adenosine receptors are plasma membrane proteins that transduce an extracellular signal into the interior of the cell. Basically every mammalian cell expresses at least one of the four adenosine receptor subtypes. Recent insight in signal transduction cascades teaches us that the current classification of receptor ligands into agonists, antagonists, and inverse agonists relies very much on the experimental setup that was used. Upon activation of the receptors by the ubiquitous endogenous ligand adenosine they engage classical G protein-mediated pathways, resulting in production of second messengers and activation of kinases. Besides this well-described G protein-mediated signaling pathway, adenosine receptors activate scaffold proteins such as β-arrestins. Using innovative and sensitive experimental tools, it has been possible to detect ligands that preferentially stimulate the β-arrestin pathway over the G protein-mediated signal transduction route, or vice versa. This phenomenon is referred to as functional selectivity or biased signaling and implies that an antagonist for one pathway may be a full agonist for the other signaling route. Functional selectivity makes it necessary to redefine the functional properties of currently used adenosine receptor ligands and opens possibilities for new and more selective ligands. This review focuses on the current knowledge of functionally selective adenosine receptor ligands and on G protein-independent signaling of adenosine receptors through scaffold proteins.

Toll-like and adenosine receptor expression in injured skeletal muscle

INTRODUCTION

Many aspects of skeletal muscle regeneration are now considered to be controlled by the innate immune system, specifically macrophages, but the mechanisms for activation and modulation of the innate immune system during injury are not well understood.

METHODS

We analyzed the expression of toll-like receptors (TLRs) and adenosine receptors during traumatic skeletal muscle injury. mRNA expression and immunostaining of these receptors were evaluated in mouse skeletal muscle injured by freezing.

RESULTS

Expression of nearly all mammalian TLRs was induced at 1 and/or 3 days postinjury with a common trend for higher expression at day 3. Injury also elicited a dramatic increase in the expression of adenosine receptors A(2B) and A(3) but not A(1) and A(2A) .

CONCLUSIONS

Both receptor types may be potential targets for stimulation of skeletal muscle tissue regeneration and functional restoration after injury.

Animal models of airway diseases

Over the past 20 years, the growing awareness that purinergic signaling events literally shape the immune and inflammatory responses to infection and allergic reactions warranted the development of animal models to assess their importance in vivo in acute lung injury and chronic airway diseases. The pioneer work conducted with the adenosine deaminase (ADA)-deficient mouse provided irrefutable evidence that excess adenosine (ADO) accumulating in the lungs of asthmatic patients, constitutes a powerful mediator of disease severity. These original studies launched the development of murine strains for the two major ectonucleotidases responsible for the generation of airway ADO from ATP release: CD39 and CD73. The dramatic acute lung injury and chronic lung complications, manifested by these knockout mice in response to allergens and endotoxin, demonstrated the critical importance of regulating the availability of ATP and ADO for their receptors. Therapeutic targets are currently evaluated using knockout mice and agonists/antagonists for each ADO receptor (A(1)R, A(2A)R, A(2B)R, and A(3)R) and the predominant ATP receptors (P2Y(2)R and P2X(7)R). This chapter provides an in-depth description of each in vivo study, and a critical view of the therapeutic potentials for the treatment of airway diseases.

Investigational A₃ adenosine receptor targeting agents

INTRODUCTION

Adenosine is an endogenous nucleoside that accumulates in the extracellular space in response to metabolic stress and cell damage. Extracellular adenosine is a signaling molecule that signals by activating four GPCRs: the A(1), A(2A), A(2B) and A(3) receptors. Since the discovery of A(3) adenosine receptors, accumulating evidence has identified these receptors as potential targets for therapeutic intervention.

AREAS COVERED

A(3) adenosine receptors are expressed on the surface of most immune cell types, including neutrophils, macrophages, dendritic cells, lymphocytes and mast cells. A(3) adenosine receptor activation on immune cells governs a broad array of immune cell functions, which include cytokine production, degranulation, chemotaxis, cytotoxicity, apoptosis and proliferation. In accordance with their multitudinous immunoregulatory actions, targeting A(3) adenosine receptors has been shown to impact the course of a wide spectrum of immune-related diseases, such as asthma, rheumatoid arthritis, cancer, ischemia and inflammatory disorders.

EXPERT OPINION

Given the existence of both preclinical and early clinical data supporting the utility of A(3) adenosine receptor ligands in treating immune-related diseases, further development of A(3) adenosine receptor ligands is anticipated.

The identification of the 2-phenylphthalazin-1(2H)-one scaffold as a new decorable core skeleton for the design of potent and selective human A3 adenosine receptor antagonists

Following a molecular simplification approach, we have identified the 2-phenylphthalazin-1(2H)-one (PHTZ) ring system as a new decorable core skeleton for the design of novel hA(3) adenosine receptor (AR) antagonists. Interest for this new series was driven by the structural similarity between the PHTZ skeleton and both the 2-aryl-1,2,4-triazolo[4,3-a]quinoxalin-1-one (TQX) and the 4-carboxamido-quinazoline (QZ) scaffolds extensively investigated in our previously reported studies. Our attention was focused at position 4 of the phthalazine nucleus where different amido and ureido moieties were introduced (compounds 2-20). Some of the new PHTZ compounds showed high hA(3) AR affinity and selectivity, the 2,5-dimethoxyphenylphthalazin-1(2H)-one 18 being the most potent and selective hA(3) AR antagonist among this series (K(i) = 0.776 nM; hA(1)/hA(3) and hA(2A)/hA(3) > 12000). Molecular docking studies on the PHTZ derivatives revealed for these compounds a binding mode similar to that of the previously reported TQX and QZ series, as was expected from the simplification approach.

Cl-IB-MECA enhances TNF-α release in peritoneal macrophages stimulated with LPS

Adenosine receptor A3 (A3R) belongs to the Gi/Gq-coupled receptor family, that leads to the intracellular cAMP reduction and intracellular calcium increase, respectively. A3R is widely expressed and it can play a crucial role in many patho-physiological conditions, including inflammation. Here we investigate the effect of Cl-IB-MECA, A3R agonist, on the production of TNF-α. We found that Cl-IB-MECA enhances LPS-induced TNF-α release in peritoneal macrophages. This effect is reduced by MRS1191, A3R antagonist and by forskolin, activator of adenylyl cyclase. pIκBα increased in LPS+Cl-IB-MECA-treated macrophages, while total IκB kinase-β (IKKβ) reduced. Indeed, p65NF-κB nuclear translocation increased in cells treated with LPS+Cl-IB-MECA. Moreover, IMD 0354, IKKβ inhibitor, significantly abrogated the effect of Cl-IB-MECA on TNF-α release. Inhibition of protein kinase C (PKC) significantly reduced Cl-IB-MECA-induced TNF-α release in LPS-stimulated macrophages. Furthermore, LY-294002, PI3K inhibitor, reduced the TNF-α production enhanced by Cl-IB-MECA, although the phosphorylation status of Akt did not change in cells treated with LPS+Cl-IB-MECA than LPS alone. In summary, these data show that Cl-IB-MECA is able to enhance TNF-α production in LPS-treated macrophages in an NF-κB- dependent manner.

Methylxanthines and inflammatory cells

Both caffeine and theophylline have a variety of roles in regulating inflammatory responses. At pharmacologically relevant concentrations most of the effects of these commonly used methylxanthines are attributable to adenosine receptor blockade and histone deacetylase activation. In addition, at higher concentrations methylxanthines can suppress inflammation by inhibiting phosphodiesterases, thereby elevating intracellular cyclic adenosine monophosphate levels. In summary, methylxanthines regulate inflammation by multiple mechanisms.

Regulation of leukocyte function by adenosine receptors

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

Control of enteric neuromuscular functions by purinergic A(3) receptors in normal rat distal colon and experimental bowel inflammation

BACKGROUND AND PURPOSE

Adenosine A(3) receptors mediate beneficial effects in experimental colitis, but their involvement in enteric neuromuscular functions during bowel inflammation is undetermined. This study investigated the regulatory role of A(3) receptors on colonic motility in the presence of experimental colitis.

EXPERIMENTAL APPROACH

Colitis was induced in rats by 2,4-dinitrobenzenesulfonic acid. A(3) receptors and adenosine deaminase (ADA, adenosine catabolic enzyme) mRNA were examined by RT-PCR. Tissue distribution of A(3) receptors was detected by confocal immunofluorescence. The effects of 2,3-ethyl-4,5-dipropyl-6-phenylpyridine-3-thiocarboxylate-5-carboxylate (MRS1523) (MRS, A(3) receptor antagonist), 2-chloro-N(6) -(3-iodobenzyl)-adenosine-5'-N-methyluronamide (2Cl-IB-MECA) (CIB, A(3) receptor agonist), dipyridamole (DIP, adenosine transport inhibitor) and ADA were assayed on contractile responses evoked by electrical stimulation (ES) or carbachol in colonic longitudinal muscle preparations (LMP).

KEY RESULTS

RT-PCR showed A(3) receptors and ADA mRNA in normal colon and their increased level in inflamed tissues. Immunofluorescence showed a predominant distribution of A(3) receptors in normal myenteric ganglia and an increased density during colitis. MRS enhanced ES-induced cholinergic contractions in normal LMP, but was less effective in inflamed tissues. After pretreatment with dipyridamole plus ADA, to reduce extracellular adenosine, CIB decreased cholinergic motor responses of normal LMP to ES, with enhanced efficacy in inflamed LMP. A(3) receptor ligands did not affect carbachol-induced contractions in LMP from normal or inflamed colon.

CONCLUSIONS AND IMPLICATIONS

Normally, adenosine modulated colonic cholinergic motility via activation of A(3) receptors in the myenteric plexus. A(3) receptor-mediated tonic inhibitory control by adenosine was impaired in inflamed bowel, despite increased density of functioning and pharmacologically recruitable A(3) receptors.

A₃ receptors are overexpressed in pleura from patients with mesothelioma and reduce cell growth via Akt/nuclear factor-κB pathway

RATIONALE

A strong link has been established between exposure to asbestos and increased risk for pleural malignant mesothelioma (MM). Adenosine plays a key role in inflammatory processes and cancer, where it is involved in the regulation of cell death and proliferation.

OBJECTIVES

The primary aim of this study was to investigate the presence of adenosine receptors (ARs) in human MM pleura (MMP) and healthy mesothelial pleura (HMP). To shed some light on the interaction between adenosine and MM, the presence and functionality of ARs were explored in human healthy mesothelial cells (HMC) and in malignant mesothelioma cells (MMC).

METHODS

ARs were analyzed by using reverse transcriptase-polymerase chain reaction, Western blotting, and saturation binding assays. HMC were treated with crocidolite asbestos, which is the principal risk factor for MM. The role of A₃ ARs on these cellular models, evaluating cAMP production, Akt phosphorylation, and nuclear factor (NF)-κB activation, was investigated. The dual effect of A₃AR stimulation on healthy and cancer cell growth was studied by means of proliferation, apoptosis, and cytotoxicity assays.

MEASUREMENTS AND MAIN RESULTS

A₃AR was up-regulated by 2.5-fold (P < 0.01) in MMP when compared with HMP. Stimulation of A₃ARs decreased proliferation and exerted a cytotoxic and proapoptotic effect on MMC and on HMC exposed to asbestos and tumor necrosis factor-α, but not on HMC with an involvement of the deregulation of Akt/NF-κB cell survival pathway.

CONCLUSIONS

These new findings suggest that A₃AR could represent a pharmacological target to prevent tumor development after asbestos exposure and to treat full-blown MM.

Adenosine neuromodulation and traumatic brain injury

Adenosine is a ubiquitous signaling molecule, with widespread activity across all organ systems. There is evidence that adenosine regulation is a significant factor in traumatic brain injury (TBI) onset, recovery, and outcome, and a growing body of experimental work examining the therapeutic potential of adenosine neuromodulation in the treatment of TBI. In the central nervous system (CNS), adenosine (dys)regulation has been demonstrated following TBI, and correlated to several TBI pathologies, including impaired cerebral hemodynamics, anaerobic metabolism, and inflammation. In addition to acute pathologies, adenosine function has been implicated in TBI comorbidities, such as cognitive deficits, psychiatric function, and post-traumatic epilepsy. This review presents studies in TBI as well as adenosine-related mechanisms in co-morbidities of and unfavorable outcomes resulting from TBI. While the exact role of the adenosine system following TBI remains unclear, there is increasing evidence that a thorough understanding of adenosine signaling will be critical to the development of diagnostic and therapeutic tools for the treatment of TBI.

Induction of apoptosis by A3 adenosine receptor agonist N-(3-iodobenzyl)-adenosine-5'-N-methylcarboxamide in human leukaemia cells: a possible involvement of intracellular mechanism

AIM

The sensitivity of cancer cells which exhibit multi-drug resistance phenotype to A3 adenosine receptor (A3AR) agonist N(6)-(3-iodobenzyl)-adenosine-5'-N-methylcarboxamide (IB-MECA) was studied.

METHODS

To establish direct relationship between P-glycoprotein (P-gp, ABCB1 and MDR1) expression and IB-MECA induced cell death, a straightforward method for precise estimation of intracellular level of this A3AR agonist was developed.

RESULTS

We subjected three human leukaemia cell lines HL-60, K562 and K562/HHT to treatment with micromolar concentrations of IB-MECA. Although all cell lines used expressed A3AR, there was a large difference in their sensitivity to IB-MECA. While HL-60 and K562 cells were almost equally sensitive, the K562/HHT cells, which exhibit a multi-drug resistance phenotype because of overexpression of P-gp, were significantly more resistant. We found that the intracellular level of IB-MECA in K562/HHT cells was approx. 10 times lower than those in HL-60 or K562 cells. Inhibitors of P-gp, including cyclosporine A (CsA) and verapamil (Vpa), increased the intracellular level of IB-MECA and reversed the resistance of K562/HHT cells to this drug. Accordingly, shRNA-mediated down-regulation of P-gp significantly increased the intracellular level of IB-MECA in K562/HHT cells which simultaneously exhibited reduced resistance to this A3AR agonist. In addition, an in vitro enzyme-based assay provided evidence that IB-MECA might serve as a substrate for P-gp.

CONCLUSION

Our results suggest that P-gp overexpression prevents cells from IB-MECA induced apoptosis despite the A3AR expression. Pro-apoptotic effect of IB-MECA seemed to strongly depend on its intracellular accumulation rather than on its interaction with A3AR.

Expression and functional role of adenosine receptors in regulating inflammatory responses in human synoviocytes

BACKGROUND AND PURPOSE

Adenosine is an endogenous modulator, interacting with four G-protein coupled receptors (A(1), A(2A), A(2B) and A(3)) and acts as a potent inhibitor of inflammatory processes in several tissues. So far, the functional effects modulated by adenosine receptors on human synoviocytes have not been investigated in detail. We evaluated mRNA, the protein levels, the functional role of adenosine receptors and their pharmacological modulation in human synoviocytes.

EXPERIMENTAL APPROACH

mRNA, Western blotting, saturation and competition binding experiments, cyclic AMP, p38 mitogen-activated protein kinases (MAPKs) and nuclear factor (NF)-kappaB activation, tumour necrosis factor alpha (TNF-alpha) and interleukin-8 (IL-8) release were assessed in human synoviocytes isolated from patients with osteoarthritis.

KEY RESULTS

mRNA and protein for A(1), A(2A), A(2B) and A(3) adenosine receptors are expressed in human synoviocytes. Standard adenosine agonists and antagonists showed affinity values in the nanomolar range and were coupled to stimulation or inhibition of adenylyl cyclase. Activation of A(2A) and A(3) adenosine receptors inhibited p38 MAPK and NF-kappaB pathways, an effect abolished by selective adenosine antagonists. A(2A) and A(3) receptor agonists decreased TNF-alpha and IL-8 production. The phosphoinositide 3-kinase or G(s) pathways were involved in the functional responses of A(3) or A(2A) adenosine receptors. Synoviocyte A(1) and A(2B) adenosine receptors were not implicated in the inflammatory process whereas stimulation of A(2A) and A(3) adenosine receptors was closely associated with a down-regulation of the inflammatory status.

CONCLUSIONS AND IMPLICATIONS

These results indicate that A(2A) and A(3) adenosine receptors may represent a potential target in therapeutic modulation of joint inflammation.

Activation of mast cells by trimeric G protein Gi3; coupling to the A3 adenosine receptor directly and upon T cell contact

Mast cells are key players in mediating and amplifying allergic and inflammatory reactions. Previously, we identified the G-protein, Gi3, as the cellular target of receptor mimetic basic secretagogues that activate mast cell independently of IgE. In this study, we demonstrate that Gi3 is the cellular target of the adenosine A3 receptor (A3R), a G-protein coupled receptor involved in inflammation and the pathophysiology of asthma. By using a cell permeable peptide comprising the C-terminal end of Galphai3 fused to an importation sequence (ALL1) as a selective inhibitor of Gi3 signaling, we show that by coupling to Gi3, the A3R stimulates multiple signaling pathways in human mast cells, leading to upregulation of cytokines, chemokines, and growth factors. We further show that after contact with activated T cell membranes, endogenous adenosine binds to and activates the A3R, resulting in Gi3-mediated signaling. Specifically, the majority of ERK1/2 signaling initiated by contact with activated T cell membranes, is mediated by Gi3, giving rise to ALL1-inhibitable cellular responses. These results unveil the physiological G-protein coupled receptor that couples to Gi3 and establish the important role played by this G-protein in inflammatory conditions that involve adenosine-activated mast cells.

Modulation of metalloproteinase-9 in U87MG glioblastoma cells by A3 adenosine receptors

In this work, we investigated the biological functions of adenosine (ado) in metalloproteinase-9 (MMP-9) regulation in U87MG human glioblastoma cells. The nucleoside was able to increase both MMP-9 mRNA and protein levels through A3 receptors activation. We revealed that A3 receptor stimulation induced an increase of MMP-9 protein levels in cellular extracts of U87MG cells by phosphorylation of extracellular signal-regulated protein kinases (ERK1/2), c-Jun N-terminal kinase/stress-activated protein kinase (pJNK/SAPK), protein kinase B (Akt/PKB) and finally activator protein 1 (AP-1). A3 receptor activation stimulated also an increase of extracellular MMP-9 in the supernatants from U87MG glioblastoma cells. Finally, the Matrigel invasion assay demonstrated that A3 receptors, by inducing an increase in MMP-9 levels, was responsible for an increase of glioblastoma cells invasion. Collectively, these results suggest that ado, through A3 receptors activation, modulates MMP-9 protein levels and plays a role in increasing invasion of U87MG cells.

A(2B) and A(3) adenosine receptors modulate vascular endothelial growth factor and interleukin-8 expression in human melanoma cells treated with etoposide and doxorubicin

Cancer patients undergoing treatment with systemic cancer chemotherapy drugs often have abnormal growth factor and cytokine profiles. Thus, serum levels of interleukin-8 (IL-8) are elevated in patients with malignant melanoma. In addition to IL-8, aggressive melanoma cells secrete, through its transcriptional regulator hypoxia-inducible factor 1 (HIF-1), vascular endothelial growth factor (VEGF), which promotes angiogenesis and metastasis of human cancerous cells. Whether these responses are related to adenosine, a ubiquitous mediator expressed at high concentrations in cancer and implicated in numerous inflammatory processes, is not known and is the focus of this study. We have examined whether the DNA-damaging agents etoposide (VP-16) and doxorubicin can affect IL-8, VEGF, and HIF-1 expressions in human melanoma cancer cells. In particular, we have investigated whether these responses are related to the modulation of the adenosine receptor subtypes, namely, A(1), A(2A), A(2B), and A(3). We have demonstrated that A(2B) receptor blockade can impair IL-8 production, whereas blocking A(3) receptors, it is possible to further decrease VEGF secretion in melanoma cells treated with VP-16 and doxorubicin. This understanding may present the possibility of using adenosine antagonists to reduce chemotherapy-induced inflammatory cytokine production and to improve the ability of chemotherapeutic drugs to block angiogenesis. Consequently, we conclude that adenosine receptor modulation may be useful for refining the use of chemotherapeutic drugs to treat human cancer more effectively.

2-Phenylpyrazolo[4,3-d]pyrimidin-7-one as a new scaffold to obtain potent and selective human A3 adenosine receptor antagonists: new insights into the receptor-antagonist recognition

A molecular simplification approach of previously reported 2-arylpyrazolo[3,4-c]quinolin-4-ones was applied to design 2-arylpyrazolo[4,3-d]pyrimidin-7-one derivatives as new human A(3) adenosine receptor antagonists. Substituents with different lipophilicity and steric hindrance were introduced at the 5-position of the bicyclic scaffold (R(5) = H, Me, Et, Ph, CH(2)Ph) and on the 2-phenyl ring (OMe, Me). Most of the synthesized derivatives were highly potent hA(3) adenosine receptor antagonists, the best being the 2-(4-methoxyphenyl)pyrazolo[4,3-d]pyrimidin-7-one (K(i) = 1.2 nM). The new compounds were also highly selective, being completely devoid of affinity toward hA(1), hA(2A), and hA(2B) adenosine receptors. On the basis of the recently published human A(2A) receptor crystallographic information, we propose a novel receptor-driven hypothesis to explain both A(3) AR affinity and A(3) versus A(2A) selectivity profiles of these new antagonists.

Radiosynthesis of a novel potential adenosine A3 receptor ligand, 5-ethyl 2,4-diethyl-3-((2-[18F]fluoroethyl)sulfanylcarbonyl)-6-phenylpyridine-5-carboxylate ([18F]FE@SUPPY:2)

Since, to date very limited information on the distribution and function of the adenosine A3 receptor is available, the development of suitable radioligands is needed. Recently, we introduced [ 18F]FE@SUPPY (5-(2-[ 18F]fluoroethyl) 2,4-diethyl-3-(ethylsulfanylcarbonyl)-6-phenylpyridine-5-carboxylate) as the first PET-ligand for the A3R. Regarding the metabolic profile – this class of dialkylpyridines comprises two ester functions within one molecule, one carboxylic and one thiocarboxylic – one could expect carboxylesterases significantly contributing to cleavage and degradation. Therefore, our aim was the development of [ 18F]FE@SUPPY:2 (5-ethyl 2,4-diethyl-3-((2-[ 18F]fluoroethyl)sulfanylcarbonyl)-6-phenylpyridine-5-carboxylate), the functional isomer containing the label at the thiocarboxylic moiety.

For satisfactory yields in high scale radiosyntheses, a reaction temperature of 75 °C has to be applied for at least 20 min using 20 mg/mL of precursor. So far, 6 complete high-scale radiosyntheses were performed. Starting from an average of 51.2±21.8 GBq (mean±SD) [ 18F]fluoride, 5.8±4.1 GBq of formulated [ 18F]FE@SUPPY:2 (12.0±5.4%, based on [ 18F]fluoride, not corrected for decay) were prepared in 75±8 min.

Normalization of A2A and A3 adenosine receptor up-regulation in rheumatoid arthritis patients by treatment with anti-tumor necrosis factor alpha but not methotrexate

OBJECTIVE

To investigate A(1), A(2A), A(2B), and A(3) adenosine receptors in lymphocytes and neutrophils from patients with early rheumatoid arthritis (ERA) as well as from RA patients treated with methotrexate (MTX) or anti-tumor necrosis factor alpha (anti-TNFalpha), as compared with those in age-matched healthy controls, and to examine correlations between the status and functionality of adenosine receptors and TNFalpha release and NF-kappaB activation.

METHODS

Adenosine receptors were analyzed by saturation binding assays and Western blot analyses. We investigated the potency of typical A(2A) and A(3) agonists in the production of cAMP in control subjects, ERA patients, and RA patients treated with MTX or anti-TNFalpha. In a separate cohort of RA patients, TNFalpha release and NF-kappaB activation were evaluated in plasma and nuclear extracts, respectively.

RESULTS

In ERA patients, we found a high density and altered functionality of A(2A) and A(3) receptors. The binding and functional parameters of A(2A) and A(3) receptors normalized after anti-TNFalpha, but not MTX, treatment. TNFalpha release was increased in ERA patients and in MTX-treated RA patients, whereas in anti-TNFalpha-treated RA patients, release was comparable to that in the controls. NF-kappaB activation was elevated in ERA patients and in MTX-treated RA patients. Anti-TNFalpha treatment mediated decreased levels of NF-kappaB activation.

CONCLUSION

A(2A) and A(3) receptor up-regulation in ERA patients and in MTX-treated RA patients was associated with high levels of TNFalpha and NF-kappaB activation. Treatment with anti-TNFalpha normalized A(2A) and A(3) receptor expression and functionality. This new evidence of A(2A) and A(3) receptor involvement opens the possibility of exploiting their potential role in human diseases characterized by a marked inflammatory component.

Adenosine receptors interacting proteins (ARIPs): Behind the biology of adenosine signaling

Adenosine is a well known neuromodulator in the central nervous system. As a consequence, adenosine can be beneficial in certain disorders and adenosine receptors will be potential targets for therapy in a variety of diseases. Adenosine receptors are G protein-coupled receptors, and are also expressed in a large variety of cells and tissues. Using these receptors as a paradigm of G protein-coupled receptors, the present review focus on how protein-protein interactions might contribute to neurotransmitter/neuromodulator regulation, based on the fact that accessory proteins impinge on the receptor/G protein interaction and therefore modulate receptor functioning. Besides affecting receptor signaling, these accessory components also play a key role in receptor trafficking, internalization and desensitization, as it will be reviewed here. In conclusion, the finding of an increasing number of adenosine receptors interacting proteins, and specially the molecular and functional integration of these accessory proteins into receptorsomes, will open new perspectives in the understanding of particular disorders where these receptors have been proved to be involved.

Adenosine blocks IFN-gamma-induced phosphorylation of STAT1 on serine 727 to reduce macrophage activation

Macrophages are activated by IFN-gamma, a proinflammatory and proatherogenic cytokine that mediates its downstream effects primarily through STAT1. IFN-gamma signaling induces phosphorylation of two STAT1 residues: Tyr(701) (Y701), which facilitates dimerization, nuclear translocation, and DNA binding; and Ser(727) (S727), which enables maximal STAT1 transcription activity. Immunosuppressive molecules such as adenosine in the cellular microenvironment can reduce macrophage inflammatory and atherogenic functions through receptor-mediated signaling pathways. We hypothesized that adenosine achieves these protective effects by interrupting IFN-gamma signaling in activated macrophages. This investigation demonstrates that adding adenosine to IFN-gamma-stimulated murine RAW 264.7 and human THP-1 macrophages results in unique modulation of STAT1 serine and tyrosine phosphorylation events. We show that adenosine inhibits IFN-gamma-induced STAT1 S727 phosphorylation by >30% and phosphoserine-mediated transcriptional activity by 58% but has no effect on phosphorylation of Y701 or receptor-associated JAK tyrosine kinases. Inhibition of the adenosine A(3) receptor with a subtype-specific antagonist (MRS 1191 in RAW 264.7 cells and MRS 1220 in THP-1 cells) reverses this adenosine suppressive effect on STAT1 phosphoserine status by 25-50%. Further, RAW 264.7 A(3) receptor stimulation with Cl-IB-MECA reduces IFN-gamma-induced STAT1 transcriptional activity by 45% and STAT1-dependent gene expression by up to 80%. These data suggest that A(3) receptor signaling is key to adenosine-mediated STAT1 modulation and anti-inflammatory action in IFN-gamma-activated mouse and human macrophages. Because STAT1 plays a key role in IFN-gamma-induced inflammation and foam cell transformation, a better understanding of the mechanisms underlying STAT1 deactivation by adenosine may improve preventative and therapeutic approaches to vascular disease.

Adenosine receptors as targets for therapeutic intervention in asthma and chronic obstructive pulmonary disease

Asthma and chronic obstructive pulmonary disease (COPD) are pulmonary disorders characterized by various degrees of inflammation and tissue remodeling. Adenosine is a signaling molecule that is elevated in the lungs of patients with asthma and COPD. Adenosine elicits its actions by engaging cell surface adenosine receptors, and substantial preclinical evidence suggests that targeting these receptors will provide novel approaches for the treatment of asthma and COPD. Studies in animal models of airway disease suggest that there may be clinical benefit to the use of A(1), A(3) and A(2B) adenosine receptor antagonists in the treatment of features of asthma and/or COPD, while A(2A) agonists may also prove effective. Several adenosine receptor based pharmacologic agents have entered clinical development for the treatment of asthma and COPD; however, the studies have been limited and the efficacy of such approaches is not yet clear.