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Gao ZG, Auchampach JA, Jacobson KA. Species dependence of A 3 adenosine receptor pharmacology and function. Purinergic Signal 2023; 19:523-550. [PMID: 36538251 PMCID: PMC9763816 DOI: 10.1007/s11302-022-09910-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/22/2022] [Accepted: 11/26/2022] [Indexed: 12/24/2022] Open
Abstract
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.
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Affiliation(s)
- Zhan-Guo Gao
- Molecular Recognition Section, Laboratory of Bioorganic Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD, 20892-0810, USA.
| | - John A Auchampach
- Department of Pharmacology and Toxicology, and the Cardiovascular Center, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, WI, 53226, USA
| | - Kenneth A Jacobson
- Molecular Recognition Section, Laboratory of Bioorganic Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD, 20892-0810, USA.
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2
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Min HS, Cha JJ, Kim K, Kim JE, Ghee JY, Kim H, Lee JE, Han JY, Jeong LS, Cha DR, Kang YS. Renoprotective Effects of a Highly Selective A3 Adenosine Receptor Antagonist in a Mouse Model of Adriamycin-induced Nephropathy. J Korean Med Sci 2016; 31:1403-12. [PMID: 27510383 PMCID: PMC4974181 DOI: 10.3346/jkms.2016.31.9.1403] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/30/2016] [Accepted: 05/13/2016] [Indexed: 12/15/2022] Open
Abstract
The concentration of adenosine in the normal kidney increases markedly during renal hypoxia, ischemia, and inflammation. A recent study reported that an A3 adenosine receptor (A3AR) antagonist attenuated the progression of renal fibrosis. The adriamycin (ADX)-induced nephropathy model induces podocyte injury, which results in severe proteinuria and progressive glomerulosclerosis. In this study, we investigated the preventive effect of a highly selective A3AR antagonist (LJ1888) in ADX-induced nephropathy. Three groups of six-week-old Balb/c mice were treated with ADX (11 mg/kg) for four weeks and LJ1888 (10 mg/kg) for two weeks as following: 1) control; 2) ADX; and 3) ADX + LJ1888. ADX treatment decreased body weight without a change in water and food intake, but this was ameliorated by LJ1888 treatment. Interestingly, LJ1888 lowered plasma creatinine level, proteinuria, and albuminuria, which had increased during ADX treatment. Furthermore, LJ1888 inhibited urinary nephrin excretion as a podocyte injury marker, and urine 8-isoprostane and kidney lipid peroxide concentration, which are markers of oxidative stress, increased after injection of ADX. ADX also induced the activation of proinflammatory and profibrotic molecules such as TGF-β1, MCP-1, PAI-1, type IV collagen, NF-κB, NOX4, TLR4, TNFα, IL-1β, and IFN-γ, but they were remarkably suppressed after LJ1888 treatment. In conclusion, our results suggest that LJ1888 has a renoprotective effect in ADX-induced nephropathy, which might be associated with podocyte injury through oxidative stress. Therefore, LJ1888, a selective A3AR antagonist, could be considered as a potential therapeutic agent in renal glomerular diseases which include podocyte injury and proteinuria.
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Affiliation(s)
- Hye Sook Min
- Department of Nephrology, Korea University Ansan Hospital, Ansan, Korea
| | - Jin Joo Cha
- Department of Nephrology, Korea University Ansan Hospital, Ansan, Korea
| | - Kitae Kim
- Department of Nephrology, Korea University Ansan Hospital, Ansan, Korea
| | - Jung Eun Kim
- Department of Nephrology, Korea University Ansan Hospital, Ansan, Korea
| | - Jung Yeon Ghee
- Department of Nephrology, Korea University Ansan Hospital, Ansan, Korea
| | - Hyunwook Kim
- Department of Nephrology, Wonkwang University Sanbon Hospital, Gunpo, Korea
| | - Ji Eun Lee
- Department of Nephrology, Wonkwang University Sanbon Hospital, Gunpo, Korea
| | - Jee Young Han
- Department of Pathology, Inha University Medical College, Incheon, Korea
| | - Lak Shin Jeong
- Department of Pharmacy, College of Pharmacy, Seoul National University, Seoul, Korea
| | - Dae Ryong Cha
- Department of Nephrology, Korea University Ansan Hospital, Ansan, Korea
| | - Young Sun Kang
- Department of Nephrology, Korea University Ansan Hospital, Ansan, Korea.
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Borea PA, Varani K, Vincenzi F, Baraldi PG, Tabrizi MA, Merighi S, Gessi S. The A3 adenosine receptor: history and perspectives. Pharmacol Rev 2015; 67:74-102. [PMID: 25387804 DOI: 10.1124/pr.113.008540] [Citation(s) in RCA: 177] [Impact Index Per Article: 19.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023] Open
Abstract
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.
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Affiliation(s)
- Pier Andrea Borea
- Department of Medical Sciences, Pharmacology Section (P.A.B., K.V., F.V., S.M., S.G.), and Department of Pharmaceutical Sciences, University of Ferrara, Italy (P.G.B., M.A.T.)
| | - Katia Varani
- Department of Medical Sciences, Pharmacology Section (P.A.B., K.V., F.V., S.M., S.G.), and Department of Pharmaceutical Sciences, University of Ferrara, Italy (P.G.B., M.A.T.)
| | - Fabrizio Vincenzi
- Department of Medical Sciences, Pharmacology Section (P.A.B., K.V., F.V., S.M., S.G.), and Department of Pharmaceutical Sciences, University of Ferrara, Italy (P.G.B., M.A.T.)
| | - Pier Giovanni Baraldi
- Department of Medical Sciences, Pharmacology Section (P.A.B., K.V., F.V., S.M., S.G.), and Department of Pharmaceutical Sciences, University of Ferrara, Italy (P.G.B., M.A.T.)
| | - Mojgan Aghazadeh Tabrizi
- Department of Medical Sciences, Pharmacology Section (P.A.B., K.V., F.V., S.M., S.G.), and Department of Pharmaceutical Sciences, University of Ferrara, Italy (P.G.B., M.A.T.)
| | - Stefania Merighi
- Department of Medical Sciences, Pharmacology Section (P.A.B., K.V., F.V., S.M., S.G.), and Department of Pharmaceutical Sciences, University of Ferrara, Italy (P.G.B., M.A.T.)
| | - Stefania Gessi
- Department of Medical Sciences, Pharmacology Section (P.A.B., K.V., F.V., S.M., S.G.), and Department of Pharmaceutical Sciences, University of Ferrara, Italy (P.G.B., M.A.T.)
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ZHANG L, FRANCHINI M, WEHRLI ESER M, DIP R. Enhanced IL-6 transcriptional response to adenosine receptor ligands in horses with lower airway inflammation. Equine Vet J 2011; 44:81-7. [DOI: 10.1111/j.2042-3306.2010.00350.x] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
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Sun WC, Berghaus LJ, Moore JN, Hurley DJ, Vandenplas ML, Thompson R, Linden J. Lipopolysaccharide and TNF-alpha modify adenosine A(2A) receptor expression and function in equine monocytes. Vet Immunol Immunopathol 2010; 135:289-95. [PMID: 20056284 DOI: 10.1016/j.vetimm.2009.12.001] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/02/2009] [Revised: 09/09/2009] [Accepted: 12/02/2009] [Indexed: 11/22/2022]
Abstract
Stimulation of adenosine A(2A) receptors results in anti-inflammatory effects in a variety of cell types. Lipopolysaccharide (LPS) and pro-inflammatory cytokines, such as TNF-alpha and IL-1, have been reported to up-regulate the expression of adenosine A(2A) receptors and thereby enhance the functional activity of adenosine A(2A) receptors in human and murine monocyte/macrophage cell lines and in monocytes/macrophages isolated from those species. In this study, we investigated the effects of LPS and TNF-alpha on the expression and functional activity of adenosine A(2A) receptors in isolated equine peripheral blood monocytes. The results of this study indicate that LPS and TNF-alpha up-regulate the transcription of adenosine A(2A) receptors for up to 24h; the response to LPS was of greater magnitude than the response to TNF-alpha. In this study, incubation with LPS, but not with TNF-alpha, resulted in down-regulation of adenosine A(3) receptor mRNA expression. Furthermore, incubation of these cells with LPS significantly increases the surface density of adenosine A(2A) receptors, and incubation with low concentrations of either LPS or TNF-alpha significantly increases the potency of the adenosine A(2A) receptor agonist, ATL313, to inhibit LPS-induced production of TNF-alpha. These findings suggest that the increased expression of adenosine A(2A) receptors and the enhanced functional potency of adenosine A(2A) receptor agonists after exposure to pro-inflammatory substances such as LPS or TNF-alpha may render adenosine A(2A) receptor agonists particularly important in the treatment of the systemic inflammatory response syndrome that occurs secondary to endotoxemia and bacterial infections in adult horses and neonatal foals.
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Abstract
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.
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Sun WC, Moore JN, Hurley DJ, Vandenplas ML, Fortes B, Thompson R, Linden J. Differential modulation of lipopolysaccharide-induced expression of inflammatory genes in equine monocytes through activation of adenosine A2A receptors. Vet Immunol Immunopathol 2010; 134:169-77. [PMID: 19766323 DOI: 10.1016/j.vetimm.2009.08.018] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/04/2009] [Revised: 08/26/2009] [Accepted: 08/26/2009] [Indexed: 01/06/2023]
Abstract
Adenosine is an endogenous nucleoside that has potent receptor-mediated immunomodulatory effects on macrophage/monocyte function. In this study, we determined the effects of an adenosine A(2A) receptor agonist, ATL313, on the expression of mRNAs for four pro-inflammatory mediators, IL-1beta, IL-8, COX-2, and TNF-alpha, and the mRNA and protein for the anti-inflammatory cytokine, IL-10 in equine monocytes incubated with lipopolysaccharide (LPS). The results indicate that ATL313 significantly reduces LPS-induced expression of COX-2 and TNF-alpha, enhances the expression of IL-10 and IL-8, but does not alter the expression of IL-1beta. These effects of ATL313 were reversed by co-incubation with the selective adenosine A(2A) antagonist ZM241385, and were mimicked by the cAMP analogue dibutyryl cAMP. These differential effects of adenosine A(2A) receptor activation were in contrast to those obtained using the P38 MAPK inhibitor, SB203580, which nearly abolished all LPS-induced changes in mRNA expression as well as the production of TNF-alpha protein. These findings, which indicate that adenosine A(2A) receptor activation modulates the transcription of several, but not all, pro-inflammatory mediators and exerts a synergistic effect on the induction of at least one anti-inflammatory cytokine, suggest that selective adenosine A(2A) agonists may reduce the early pro-inflammatory effects of endotoxemia in horses.
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Abstract
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.
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Affiliation(s)
- Michael R Blackburn
- Department of Biochemistry and Molecular Biology, The University of Texas-Houston Medical School, Houston, TX 77030, USA.
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9
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Sun WC, Moore JN, Hurley DJ, Vandenplas ML, Linden JM, Murray TF. Pharmacologic characterization of novel adenosine A2A receptor agonists in equine neutrophils. Am J Vet Res 2007; 68:981-7. [PMID: 17764413 DOI: 10.2460/ajvr.68.9.981] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
OBJECTIVE To evaluate anti-inflammatory effects of several novel adenosine receptor agonists and to determine their specificity for various adenosine receptor subtypes on neutrophils, cells heterologously expressing equine adenosine receptors, or equine brain membranes. SAMPLE POPULATION Neutrophils isolated from 8 healthy horses. PROCEDURES Radioligand binding experiments were performed to compare binding affinities of adenosine receptor agonists to equine adenosine A(1), A(2A), and A(3) receptor subtypes. Effects of these agonists on endotoxin-induced production of reactive oxygen species (ROS) by equine neutrophils and roles of specific adenosine receptor subtypes and cAMP production in mediating these effects were determined. RESULTS Radioligand binding experiments yielded a ranked order of affinity for the brain equine A(2A) receptor on the basis of 50% inhibitory concentrations (IC(50)) of the agonists as follows: ATL307 (IC(50) = 1.9nM) and ATL313 > ATL309 and ATL310 > ATL202 > 2-([p-2- carboxyethyl] phenylethylamino)-5'-N-ethylcarboxyamidoadenosine > 5'-N-ethylcarboxamidoadenosine. Furthermore, ATL313 had approximately 100-fold greater selectivity for A(2A) over A(1) and A(3) receptors. In functional assays with equine neutrophils, the compounds inhibited endotoxin-induced ROS production and stimulated production of cAMP with the same ranked order of potency. Results of experiments performed with selective adenosine receptor antagonists indicated that functional effects of ATL313 were via stimulation of A(2A) receptors. CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that activation of A(2A) receptors exerted anti-inflammatory effects on equine neutrophils and that stable, highly selective adenosine A(2A) receptor agonists may be developed for use in management of horses and other domestic animals with septic and nonseptic inflammatory diseases.
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Affiliation(s)
- Wan-chun Sun
- Department of Large Animal Medicine, College of Veterinary Medicine, University of Georgia, Athens, GA 30602, USA
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10
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Dip RG. Adenosine receptor modulation: potential implications in veterinary medicine. Vet J 2007; 179:38-49. [PMID: 17904398 DOI: 10.1016/j.tvjl.2007.08.005] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/20/2007] [Revised: 07/27/2007] [Accepted: 08/02/2007] [Indexed: 12/01/2022]
Abstract
Adenosine is a purine nucleoside whose concentration increases during inflammation and hypoxia and the many roles of this molecule are becoming better understood. Increased reactivity to adenosine of the airways of asthmatic but not of normal subjects underlines the role of adenosine in airway inflammation. The identification and pharmacological characterisation of different adenosine receptors have stimulated the search for subtype-specific ligands able to modulate the effects of this molecule in a directed way. Several compounds of different chemical classes have been identified as having potential drawbacks, including side effects resulting from the broad distribution of the receptors across the organism, have prevented clinical application. In this article, the effects of adenosine's different receptors and the intracellular signalling pathways are reviewed. The potential of adenosine receptor modulation as a therapeutic target for chronic airway inflammation is considered, taking equine recurrent airway disease and feline asthma as examples of naturally occurring airway obstructive diseases. Other potential applications for adenosine receptor modulation are also discussed. As the intrinsic molecular events of adenosine's mechanism of action become uncovered, new concrete therapeutic approaches will become available for the treatment of various conditions in veterinary medicine.
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Affiliation(s)
- Ramiro G Dip
- Institute of Pharmacology and Toxicology, University of Zurich-Vetsuisse, Winterthurerstrasse 260, CH-8057 Zurich, Switzerland.
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Gessi S, Merighi S, Varani K, Leung E, Mac Lennan S, Borea PA. The A3 adenosine receptor: an enigmatic player in cell biology. Pharmacol Ther 2007; 117:123-40. [PMID: 18029023 DOI: 10.1016/j.pharmthera.2007.09.002] [Citation(s) in RCA: 180] [Impact Index Per Article: 10.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/05/2007] [Accepted: 09/05/2007] [Indexed: 02/01/2023]
Abstract
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.
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Affiliation(s)
- Stefania Gessi
- Department of Clinical and Experimental Medicine, Pharmacology Unit and Interdisciplinary Center for the Study of Inflammation, Ferrara, Italy
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Sun WC, Moore JN, Hurley DJ, Vandenplas ML, Linden J, Cao Z, Murray TF. Adenosine A2A receptor agonists inhibit lipopolysaccharide-induced production of tumor necrosis factor-alpha by equine monocytes. Vet Immunol Immunopathol 2007; 121:91-100. [PMID: 17913243 DOI: 10.1016/j.vetimm.2007.08.011] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/2007] [Revised: 08/15/2007] [Accepted: 08/20/2007] [Indexed: 11/16/2022]
Abstract
Adenosine is an endogenous nucleoside that regulates many physiological processes by activating one or more adenosine receptor subtypes, namely A1, A2A, A2B and A3. The results of previous studies indicate that adenosine analogues inhibit lipopolysaccharide (LPS)-induced production of reactive oxygen species (ROS) by equine neutrophils primarily through activation of A2A receptors. Because peripheral blood monocytes produce cytokines that are responsible for many of the deleterious effects of LPS, the current study was performed to evaluate the effects of an array of novel adenosine receptor agonists on LPS-induced production of tumor necrosis factor-alpha (TNF-alpha), and to assess the selectively of these agonists for equine adenosine A2A over the A1 receptor. Radioligand binding studies performed with equine tissues expressing adenosine A1 and A2A receptor subtypes yielded a rank order of affinity for the equine A2A receptor of ATL307>ATL309 approximately ATL310 approximately ATL313>ATL202 approximately ATL361 approximately ATL376>ATL372>CGS21680>NECA. Co-incubation of equine peripheral blood monocytes with LPS and these agonists resulted in inhibition of TNF-alpha production with a rank order of potency that strongly correlated with their binding affinities for equine adenosine A2A receptors. Results of experiments performed with one of the adenosine receptor agonists (ATL313) and selective adenosine receptor antagonists confirmed that inhibition of LPS-induced production of TNF-alpha occurred via stimulation of A2A receptors. Although incubation of monocytes with IB-MECA, a compound purported to act as an adenosine A3 receptor agonist, reduced LPS-induced TNF-alpha production, this effect of IB-MECA was inhibited by the A2A selective antagonist ZM241385 but not by the A3 receptor antagonist MRS1220. These results indicate that the adenosine receptor subtype responsible for regulation of LPS-induced cytokine production by equine monocytes is the A2A receptor. To address the signal transduction mechanism responsible for the anti-inflammatory effects of ATL313 in equine monocytes, production of cAMP was compared in the presence and absence of either the adenosine A2A receptor antagonist ZM241385 or the adenosine A2B receptor antagonist MRS1706. In the absence of the antagonists, ATL313 increased production of cAMP; ZM241385 inhibited this effect of ATL313, whereas MRS1706 did not. Furthermore, incubation of monocytes with either the stable analogue of cAMP, dibutyryl cAMP, or forskolin, an activator of adenylyl cyclase, also inhibited LPS-induced production of TNF-alpha production by equine monocytes. Collectively, the results of the current study indicate that adenosine analogues inhibit LPS-induced production of TNF-alpha by equine monocytes primarily via activation of adenosine A2A receptors and do so in a cAMP-dependent manner. The results of this study indicate that stable adenosine analogues that are selective for adenosine A2A receptors may be suitable for development as anti-inflammatory drugs in horses.
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Affiliation(s)
- Wan-Chun Sun
- Department of Large Animal Medicine, College of Veterinary Medicine, University of Georgia, Athens, GA 30602, United States
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