A2A receptor dependent and A2A receptor independent effects of extracellular adenosine on murine thymocytes in conditions of adenosine deaminase deficiency

Adenosine in the Thymus

Adenosine is an ancient extracellular signaling molecule that regulates various biological functions via activating four G-protein-coupled receptors, A₁, A_2A, A_2B, and A₃ adenosine receptors. As such, several studies have highlighted a role for adenosine signaling in affecting the T cell development in the thymus. Recent studies indicate that adenosine is produced in the context of apoptotic thymocyte clearance. This review critically discusses the involvement of adenosine and its receptors in the complex interplay that exists between the developing thymocytes and the thymic macrophages which engulf the apoptotic cells. This crosstalk contributes to the effective and immunologically silent removal of apoptotic thymocytes, as well as affects the TCR-driven T-cell selection processes.

Prostaglandin E2-mediated adenosinergic effects on CD14+ cells: Self-amplifying immunosuppression in cancer

CD39 and CD73 are surface-expressed ectonucleotidases that hydrolyze ATP in a highly regulated, serial manner into ADP, AMP and adenosine. The end product, adenosine, has both tumor-promoting and immunosuppressive effects. The aim of this study was to determine CD73 expression on immune cells in pleural effusion (PE) in order to have a better understanding of the immune environment in mesothelioma. PE- or blood-derived CD14⁺ cells of mesothelioma patients and healthy donors were analyzed by flow cytometry for the expression of CD39 and CD73. CD73-induction was studied by exposure of CD14⁺ cells to the soluble fraction of PE (sPE), while the signaling mechanism, responsible for CD73 induction, by phosphoflow cytometry and receptor-inhibition studies. We observed CD73 expression on CD14⁺ cells in PE but not peripheral blood of mesothelioma patients or healthy donors. CD73 expression was inducible on CD14⁺ cells with sPE, cyclic-AMP (cAMP)-inducers (forskolin and prostaglandin-E₂ (PGE₂)) and adenosine. Inhibition of PGE₂ receptors or adenosine A₂ receptors blocked CD73-induction by sPE. sPE treatment triggered protein kinase A and p38 activation. However, signal-transducer and activator of transcription 3 (STAT3)-blocking led to enhanced CD73 expression, demonstrating a hitherto unknown negative control of purinergic signaling by STAT3 in CD14⁺ cells. TNFα production by CD73⁺ CD14⁺ cells was significantly impaired in the presence of AMP, confirming immunosuppressive function. Taken together, CD73 expression can be induced by PGE₂, cAMP or adenosine on human CD14⁺ cells. We suggest that targeting this autocrine loop is a valid therapeutic approach in mesothelioma that may also enhance immunotherapy.

Adenosine and adenosine receptors in the pathogenesis and treatment of rheumatic diseases

Adenosine, a nucleoside derived primarily from the extracellular hydrolysis of adenine nucleotides, is a potent regulator of inflammation. Adenosine mediates its effects on inflammatory cells by engaging one or more cell-surface receptors. The expression and function of adenosine receptors on different cell types change during the course of rheumatic diseases, such as rheumatoid arthritis (RA). Targeting adenosine receptors directly for the treatment of rheumatic diseases is currently under study; however, indirect targeting of adenosine receptors by enhancing adenosine levels at inflamed sites accounts for most of the anti-inflammatory effects of methotrexate, the anchor drug for the treatment of RA. In this Review, we discuss the regulation of extracellular adenosine levels and the role of adenosine in regulating the inflammatory and immune responses in rheumatic diseases such as RA, psoriasis and other types of inflammatory arthritis. In addition, adenosine and its receptors are involved in promoting fibrous matrix production in the skin and other organs, and the role of adenosine in fibrosis and fibrosing diseases is also discussed.

Expression of CD73 and A2A receptors in cells from subjects with obesity and type 2 diabetes mellitus

Regulatory T cells have various mechanisms to suppress the inflammatory response, among these, the modulation of the microenvironment through adenosine and with the participation of CD39, CD73 and A2A. The aim of this study was to assess the expression of CD73 and A2A in immune cells and the effect of activation of A2A by an adenosine analogue on apoptosis in patients with obesity and type 2 diabetes mellitus (T2D). CD73 and A2A expression were analyzed by flow cytometry in lymphocyte subpopulations from patients with obesity (n = 22), T2D (n = 22), and healthy subjects (n = 20). Lymphocytes were treated with the selective A2A antagonist (ZM241385) or the selective A2A agonist (CGS21680), and apoptotic cells were detected by Annexin V. We found an increased expression of CD39 coupled to a decrease in CD73 in the patient groups with obesity and T2D compared to the control group in the different studied lymphocyte subpopulations. A2A expression was found to be increased in different subpopulations of lymphocytes from T2D patients. We also detected positive correlations between CD39+ cells and age and BMI. Meanwhile, CD73+ cells showed negative correlations with age, WHR, BMI, FPG, HbAc1, triglycerides and cholesterol. Moreover, an increase in the percentage of apoptotic cells from T2D patients with regard to the groups with obesity and control was observed. In addition, the CD8+ T cells of patients with T2D exhibited decreased apoptosis when treated with the A2A agonist. In conclusion, our data suggest a possible role for CD73 and A2A in inflammation observed in patients with T2D and obesity mediated via apoptosis.

Modulation of neuroimmunity by adenosine and its receptors: metabolism to mental illness

Adenosine is a pleiotropic bioactive with potent neuromodulatory properties. Due to its ability to easily cross the blood-brain barrier, it can act as a signaling molecule between the periphery and the brain. It functions through four (A1, A2A, A2B, and A3) cell surface G protein-coupled adenosine receptors (ARs) that are expressed in some combination on nearly all cells types within the CNS. By regulating the activity of adenylyl cyclase and changing the intracellular concentration of cAMP, adenosine can alter neuronal function and neurotransmission. A variety of illnesses related to metabolic dysregulation, such as type 1 diabetes and Alzheimer's disease, are associated with an elevated serum concentration of adenosine and a pathogenesis rooted in inflammation. This review describes the accepted physiologic function of adenosine in neurological disease and explores its new potential as a peripheral to central danger signal that can activate the neuroimmune system and contribute to symptoms of sickness and psychopathologies.

NF-κB enhances hypoxia-driven T-cell immunosuppression via upregulation of adenosine A(2A) receptors

Hypoxia affects inflammation by modulating T-cell activation via the adenosinergic system. We supposed that, in turn, inflammation influences cell hypoxic behavior and that stimulation of T-cells in inflammatory conditions involves the concerted action of the nuclear factor κB (NF-κB) and the related hypoxia-inducible factor 1α (HIF-1α) on the adenosinergic system. We addressed this hypothesis by monitoring both transcription factors and four adenosinergic signaling parameters - namely adenosine, adenosine deaminase (ADA), adenosine A2A receptor (A2AR) and cAMP - in T-cells stimulated using phorbol myristate acetate and phytohemagglutinin and submitted to hypoxic conditions which were mimicked using CoCl2 treatment. We found that cell viability was more altered in stimulated than in resting cells under hypoxia. Detailed analysis showed that: i) NF-κB activation remained at basal level in resting hypoxic cells but greatly increased following stimulation, stimulated hypoxic cells exhibiting the higher level; ii) HIF-1α production induced by hypoxia was boosted via NF-κB activation in stimulated cells whereas hypoxia increased HIF-1α production in resting cells without further activating NF-κB; iii) A2AR expression and cAMP production increased in stimulated hypoxic cells whereas adenosine level remained unchanged due to ADA regulation; and iv) the presence of H2S, an endogenous signaling molecule in inflammation, reversed the effect of stimulation on cell viability by down-regulating the activity of transcription factors and adenosinergic immunosuppression. We also found that: i) the specific A2AR agonist CGS-21680 increased the suppressive effect of hypoxia on stimulated T-cells, the antagonist ZM-241385 exhibiting the opposite effect; and ii) Rolipram, a selective inhibitor of cAMP-specific phosphodiesterase 4, and 8-Br-cAMP, a cAMP analog which preferentially activates cAMP-dependent protein kinase A (PKA), increased T-cell immunosuppression whereas H-89, a potent and selective inhibitor of cAMP-dependent PKA, restored cell viability. Together, these data indicate that inflammation enhances T-cell sensitivity to hypoxia via NF-κB activation. This process upregulates A2AR expression and enhances cAMP production and PKA activation, resulting in adenosinergic T-cell immunosuppression that can be modulated via H2S.

Design and evaluation of xanthine based adenosine receptor antagonists: potential hypoxia targeted immunotherapies

Molecular modeling techniques were applied to the design, synthesis and optimization of a new series of xanthine based adenosine A(2A) receptor antagonists. The optimized lead compound was converted to a PEG derivative and a functional in vitro bioassay used to confirm efficacy. Additionally, the PEGylated version showed enhanced aqueous solubility and was inert to photoisomerization, a known limitation of existing antagonists of this class.

Intracellular ATP concentration contributes to the cytotoxic and cytoprotective effects of adenosine

Extracellular adenosine (Ade) interacts with cells by two pathways: by activating cell surface receptors at nanomolar/micromolar concentrations; and by interfering with the homeostasis of the intracellular nucleotide pool at millimolar concentrations. Ade shows both cytotoxic and cytoprotective effects; however, the underlying mechanisms remain unclear. In the present study, the effects of adenosine-mediated ATP on cell viability were investigated. Adenosine treatment was found to be cytoprotective in the low intracellular ATP state, but cytotoxic under the normal ATP state. Adenosine-mediated cytotoxicity and cytoprotection rely on adenosine-derived ATP formation, but not via the adenosine receptor pathway. Ade enhanced proteasome inhibition-induced cell death mediated by ATP generation. These data provide a new pathway by which adenosine exerts dual biological effects on cell viability, suggesting an important role for adenosine as an ATP precursor besides the adenosine receptor pathway.

Liver grafts from CD39-overexpressing rodents are protected from ischemia reperfusion injury due to reduced numbers of resident CD4+ T cells

Ischemia-reperfusion injury (IRI) is a major limiting event for successful liver transplantation, and CD4+ T cells and invariant natural killer T (iNKT) cells have been implicated in promoting IRI. We hypothesized that hepatic overexpression of CD39, an ectonucleotidase with antiinflammatory functions, will protect liver grafts after prolonged cold ischemia. CD39-transgenic (CD39tg) and wildtype (WT) mouse livers were transplanted into WT recipients after 18 hours cold storage and pathological analysis was performed 6 hours after transplantation. Serum levels of alanine aminotransferase and interleukin (IL)-6 were significantly reduced in recipients of CD39tg livers compared to recipients of WT livers. Furthermore, less severe histopathological injury was demonstrated in the CD39tg grafts. Immune analysis revealed that CD4+ T cells and iNKT cells were significantly decreased in number in the livers of untreated CD39tg mice. This was associated with a peripheral CD4+ T cell lymphopenia due to defective thymocyte maturation. To assess the relative importance of liver-resident CD4+ T cells and iNKT cells in mediating liver injury following extended cold preservation and transplantation, WT mice depleted of CD4+ T cells or mice genetically deficient in iNKT cells were used as donors. The absence of CD4+ T cells, but not iNKT cells, protected liver grafts from early IRI.

CONCLUSION

Hepatic CD4+ T cells, but not iNKT cells, play a critical role in early IRI following extended cold preservation in a liver transplant model.

Search for adenosine A2A spare receptors on peripheral human lymphocytes

Some ligand–receptor couples involve spare receptors, which are apparent when a maximal response is achieved with only a small fraction of the receptor population occupied. This situation favours cross-reactions with low-affinity ligands, which may be detrimental for cell signaling. In the case of the adenosine A_2A receptors (A_2AR), which have an immunosuppressive effect on lymphocytes through cAMP production, the presence of spare A_2AR remains to be established. We examined the situation using patients over-expressing lymphocyte A_2AR and an agonist-like mAb to A_2AR. We found that maximal mAb binding and functional response varied among the patients whereas the dissociation constant and half-maximal effective concentration had similar mean values (0.19 and 0.18 μM, respectively). Lymphocyte A_2AR expression was correlated to plasma adenosine level and A_2AR occupation but not to A_2AR response. These results are consistent with a lack of a reserve of functional A_2AR on human lymphocytes as a general rule and suggest that the amount and functional state of the expressed A_2AR determine the maximal level of the lymphocyte response to adenosine.

CD73-generated extracellular adenosine in chronic lymphocytic leukemia creates local conditions counteracting drug-induced cell death

Extracellular adenosine (ADO), generated from ATP or ADP through the concerted action of the ectoenzymes CD39 and CD73, elicits autocrine and paracrine effects mediated by type 1 purinergic receptors. We have tested whether the expression of CD39 and CD73 by chronic lymphocytic leukemia (CLL) cells activates an adenosinergic axis affecting growth and survival. By immunohistochemistry, CD39 is widely expressed in CLL lymph nodes, whereas CD73 is restricted to proliferation centers. CD73 expression is highest on Ki-67(+) CLL cells, adjacent to T lymphocytes, and is further localized to perivascular areas. CD39(+)/CD73(+) CLL cells generate ADO from ADP in a time- and concentration-dependent manner. In peripheral blood, CD73 expression occurs in 97/299 (32%) CLL patients and pairs with CD38 and ZAP-70 expression. CD73-generated extracellular ADO activates type 1 purinergic A2A receptors that are constitutively expressed by CLL cells and that are further elevated in proliferating neoplastic cells. Activation of the ADO receptors increases cytoplasmic cAMP levels, inhibiting chemotaxis and limiting spontaneous drug-induced apoptosis of CLL cells. These data are consistent with the existence of an autocrine adenosinergic loop, and support engraftment of leukemic cells in growth-favorable niches, while simultaneously protecting from the action of chemotherapeutic agents.

Adenosine A2A receptor activation protects CD4+ T lymphocytes against activation-induced cell death

Activation-induced cell death (AICD) is initiated by T-cell receptor (TCR) restimulation of already activated and expanded peripheral T cells and is mediated through Fas/Fas ligand (FasL) interactions. Adenosine is a purine nucleoside signaling molecule, and its immunomodulatory effects are mediated by 4 G-protein-coupled receptors: A(1), A(2A), A(2B), and A(3). In this study, we investigated the role of A(2A) receptors in regulating CD4(+) T lymphocyte AICD. Our results showed that the selective A(2A) receptor agonist CGS21680 (EC(50)=15.2-32.6 nM) rescued mouse CD4(+) hybridomas and human Jurkat cells from AICD and that this effect was reversed by the selective A(2A) receptor antagonist ZM241385 (EC(50)=2.3 nM). CGS21680 decreased phosphatidylserine exposure on the membrane, as well as the cleavage of caspase-3, caspase-8 and poly(ADP-ribose) polymerase indicating that A(2A) receptor stimulation blocks the extrinsic apoptotic pathway. In addition, CGS21680 attenuated both Fas and FasL mRNA expression. This decrease in FasL expression was associated with decreased activation of the transcription factor systems NF-kappaB, NF-ATp, early growth response (Egr)-1, and Egr-3. The antiapoptotic effect of A(2A) receptor stimulation was mediated by protein kinase A. Together, these results demonstrate that A(2A) receptor activation suppresses the AICD of peripheral T cells.

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.

Adenosine A2A receptors play an active role in mouse bone marrow-derived mesenchymal stem cell development

Bone marrow-derived mesenchymal stem cells (BM-MSCs) play a role in wound healing and tissue repair and may also be useful for organ regeneration. As we have demonstrated previously that A(2A) adenosine receptors (A(2A)R) promote tissue repair and wound healing by stimulating local repair mechanisms and enhancing accumulation of endothelial progenitor cells, we investigated whether A(2A)R activation modulates BM-MSC proliferation and differentiation. BM-MSCs were isolated and cultured from A(2A)-deficient and ecto-5'nucleotidase (CD73)-deficient female mice; the MSCs were identified and quantified by a CFU-fibroblast (CFU-F) assay. Procollagen alpha2 type I expression was determined by Western blotting and immunocytochemistry. MSC-specific markers were examined in primary cells and third-passage cells by cytofluorography. PCR and real time-PCR were used to quantitate adenosine receptor and CD73 expression. There were significantly fewer CFU-Fs in cultures of BM-MSCs from A(2A)R knockout (KO) mice or BM-MSCs treated with the A(2A)R antagonist ZM241385, 1 microM. Similarly, there were significantly fewer procollagen alpha2 type I-positive MSCs in cultures from A(2A)R KO and antagonist-treated cultures as well. In late passage cells, there were significantly fewer MSCs from A(2A) KO mice expressing CD90, CD105, and procollagen type I (P<0.05 for all; n=3). These findings indicate that adenosine and adenosine A(2A)R play a critical role in promoting the proliferation and differentiation of mouse BM-MSCs.

Adenosine A2A receptor activation inhibits T helper 1 and T helper 2 cell development and effector function

Adenosine is an immunosuppressive nucleoside, and adenosine A(2A) receptors inhibit T-cell activation. We investigated the role of A(2A) receptors in regulating T helper (Th)1- and Th2-cell development and effector function. A(2A)-receptor stimulation suppressed the development of T-cell receptor (TCR) -stimulated naive T cells into both Th1 and Th2 cells, as indicated by decreased IFN-gamma production by cells developed under Th1-skewing conditions and decreased interleukin (IL) -4, IL-5, and IL-10 production by cells developed under Th2-skewing conditions. Using A(2A) receptor-deficient mice, we demonstrate that A(2A) receptor activation inhibits Th1- and Th2-cell development by decreasing the proliferation and IL-2 production of naive T cells, irrespective of whether the cells are expanded under Th1- or Th2-skewing environment. Using in vivo established Th1 and Th2 cells, we further demonstrate the nonselective nature of A(2A) receptor-mediated immunosuppressive effects, because A(2A) receptor activation decreased IFN-gamma and IL-4 secretion and mRNA level of TCR-stimulated effector Th1 and Th2 cells, respectively. A(2A) receptor mRNA expression in both Th1 and Th2 effector cells increased following TCR stimulation. In summary, these data demonstrate that A(2A) receptor activation has strong inhibitory actions during early developmental, as well as late effector, stages of Th1- and Th2-cell responses.

A2A receptors in inflammation and injury: lessons learned from transgenic animals

Adenosine regulates the function of the innate and adaptive immune systems through targeting virtually every cell type that is involved in orchestrating an immune/inflammatory response. Of the four adenosine receptors (A(1), A(2A), A(2B), A(3)), A(2A) receptors have taken center stage as the primary anti-inflammatory effectors of extracellular adenosine. This broad, anti-inflammatory effect of A(2A) receptor activation is a result of the predominant expression of A(2A) receptors on monocytes/macrophages, dendritic cells, mast cells, neutrophils, endothelial cells, eosinophils, epithelial cells, as well as lymphocytes, NK cells, and NKT cells. A(2A) receptor activation inhibits early and late events occurring during an immune response, which include antigen presentation, costimulation, immune cell trafficking, immune cell proliferation, proinflammatory cytokine production, and cytotoxicity. In addition to limiting inflammation, A(2A) receptors participate in tissue remodeling and reparation. Consistent with their multifaceted, immunoregulatory action on immune cells, A(2A) receptors have been shown to impact the course of a wide spectrum of ischemic, autoimmune, infectious, and allergic diseases. Here, we review the regulatory roles of A(2A) receptors in immune/inflammatory diseases of various organs, including heart, lung, gut, liver, kidney, joints, and brain, as well as the role of A(2A) receptors in regulating multiple organ failure and sepsis.

P2X7 receptor-dependent cell death is modulated during murine T cell maturation and mediated by dual signaling pathways

Extracellular ATP causes apoptosis and/or necrosis of the hemopoietic lineage through the activation of P2X7 receptors. In this study, we investigated P2X7 receptor-mediated cell death during murine T cell maturation. The expression level and activity of P2X7 receptors, as measured by induction of cell death and pore formation, were higher in splenocytes than thymocytes. Flow cytometric analysis revealed that cell shrinkage was induced by activation of the P2X7 receptor in murine lymphocytes and the responding cells were T cells. Splenic T cells were more responsive than their thymic counterpart. These observations indicate that the system of P2X7 receptor-mediated cell death in T cells could be modulated during T cell maturation. Furthermore, decreased extracellular Cl- suppressed ATP-induced cell shrinkage in splenocytes without inhibiting ERK1/2 phosphorylation, which is reported to mediate necrotic cell death. Treatment with U0126 (a MEK inhibitor) suppressed ATP-induced ERK1/2 phosphorylation without inhibiting cell shrinkage. Moreover, decreased extracellular Cl- and treatment with U0126 suppressed ATP-induced cell death. These observations indicate that the activation of P2X7 receptor leads to T cell death by two independent pathways, one of which is cell shrinkage dependent and the other of which involves the phosphorylation of ERK1/2. In conclusion, we demonstrate increasing P2X7 receptor activity during T cell maturation and the existence of two essential pathways in P2X7 receptor-mediated T cell death. Our findings suggest that ATP-induced cell death of peripheral T lymphocytes is important in P2X7 receptor-regulated immune responses.

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.

Adenosine A2A receptor inactivation increases survival in polymicrobial sepsis

The mechanisms governing the impairment of bacterial clearance and immune function in sepsis are not known. Adenosine levels are elevated during tissue hypoxia and damage associated with sepsis. Adenosine has strong immunosuppressive effects, many of which are mediated by A(2A) receptors (A(2A)R) expressed on immune cells. We examined whether A(2A)R are involved in the regulation of immune function in cecal ligation and puncture-induced murine polymicrobial sepsis by genetically or pharmacologically inactivating A(2A)R. A(2A)R knockout (KO) mice were protected from the lethal effect of sepsis and had improved bacterial clearance compared with wild-type animals. cDNA microarray analysis and flow cytometry revealed increased MHC II expression in A(2A)-inactivated mice, suggesting improved Ag presentation as a mechanism of protection. Apoptosis was attenuated in the spleen of A(2A) KO mice indicating preserved lymphocyte function. Levels of the immunosuppressive cytokines IL-10 and IL-6 were markedly lower following A(2A)R blockade. Similar to observations with A(2A)R KO mice, an A(2A)R antagonist increased survival even when administered in a delayed fashion. These studies demonstrate that A(2A)R blockade may be useful in the treatment of infection and sepsis.

The role of adenosine in chondrocyte death in murine osteoarthritis and in a murine chondrocyte cell line

OBJECTIVE

To investigate the role of adenosine in chondrocyte death in murine osteoarthritis (OA).

METHODS

5'-Nucleotidase (5'NT) generates adenosine. Enzyme activity was measured histochemically in normal murine and osteoarthritic STR/ort strain tibial cartilage. Adenosine-mediated cell death was investigated in MC615 chondrocyte cultures. Adenosine receptors (ARs) were assessed by reverse transcriptase polymerase chain reaction (RT-PCR). Cellular uptake of [(3)H] adenosine was measured with or without the inhibitor, nitrobenzylthioinosine (NBTI). Cell death was assessed by cell counting and DNA laddering following selective receptor stimulation, or after modulating adenosine metabolism with adenosine deaminase (ADA) or adenosine kinase (AK) inhibitors [erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) and Iodotubericidin (Itub)], or with homocysteine (HC). Markers of apoptosis were assessed by Western blotting. Cell studies were validated by incubating normal murine knee joints in a medium containing adenosine and metabolic inhibitors. Apoptotic chondrocytes were identified with the TUNEL reaction.

RESULTS

5'NT activity in STR/ort tibial cartilage increased with development of OA, especially close to OA lesions. Adenosine induced MC615 cell death in the presence of ADA inhibition (100 microM EHNA), or 1mM HC, or both. Adenosine uptake, mediated by NBTI-sensitive adenosine transporters, was required for cell death. ARs were expressed (A2b>A2a>A1) but were not involved in mediating cell death. Cell death involved the activation of caspase-3 and DNA fragmentation and was prevented by inhibiting caspase activity. However, neither caspase-8 nor caspase-9 was detected. Adenosine+EHNA induced chondrocyte apoptosis in normal murine knee joints.

CONCLUSION

Increased adenosine production may induce chondrocyte apoptosis and play a role in OA in STR/ort mice.

Adenosine A2A agonists in development for the treatment of inflammation

Extracellular adenosine binds specifically to a family of four G protein-coupled cell-surface adenosine receptors (ARs). As the activation of the A2AAR modulates the activity of multiple inflammatory cells including neutrophils, macrophages and T lymphocytes, the receptor is considered to be a promising pharmacological target for the treatment of inflammatory disorders. Although adenosine binds nonselectively to all four AR subtypes, A2AAR selective agonists have been developed and shown to inhibit multiple manifestations of inflammatory cell activation including superoxide anion generation, cytokine production and adhesion molecule expression. A2AAR agonists are also vasodilators, but the inhibition of inflammation occurs at low doses that produce few or no cardiovascular side effects. Therefore, the selective activation of the A2AAR by these compounds holds significant potential in the treatment of inflammation.

Proton-sensing and lysolipid-sensitive G-protein-coupled receptors: A novel type of multi-functional receptors

OGR1, GPR4, G2A, and TDAG8 share 40% to 50% homology with each other and seem to form a family of GPCRs. They have been described as receptors for lipid molecules such as sphingosylphosphorylcholine, lysophosphatidylcholine, and psychosine. Recent studies, however, have revealed that these receptors also sense extracellular protons or pH through histidine residues of receptors and stimulate a variety of intracellular signaling pathways through several species of hetero-trimeric G-proteins, including G(s), G(i), G(q), and G(12/13). Thus, this family of GPCR seems to recognize both lipid molecules and protons as ligands. Although our knowledge of proton-sensing and lysolipid-sensitive GPCRs is preliminary, the receptor levels and ligand levels especially protons are both sensitively modulated in response to a variety of microenvironmental changes. These results suggest a multiple role of proton-sensing GPCRs in a variety of physiological and pathophysiological states.

Adenosine-induced apoptosis in EL-4 thymoma cells is caspase-independent and mediated through a non-classical adenosine receptor

Cell death caused by the accumulation of extracellular adenosine is believed to contribute to the profound loss of T lymphocytes in patients with severe combined immunodeficiency disease due to adenosine deaminase deficiency. Although adenosine is known to trigger apoptosis in thymocytes and peripheral T cells, the molecular basis of this effect is not understood. In this study, we show that adenosine-induced apoptosis in mouse EL-4 thymoma cells was associated with the generation of reactive oxygen species and a reduction in mitochondrial transmembrane potential. In addition, cell death was by a caspase-independent mechanism because caspase inhibitors did not protect EL-4 cells from adenosine-induced cytotoxicity. Although reverse transcriptase polymerase chain reaction revealed that EL-4 cells expressed A2b and A3 adenosine receptor subtypes, blockade of A2b and A3 adenosine receptors with receptor-selective antagonists did not attenuate adenosine-induced cell death. Nevertheless, the failure of nucleoside transport inhibitors to prevent adenosine cytotoxicity suggested that adenosine was acting through a cell-surface receptor. In addition, adenosine-induced apoptosis was not due to an accumulation of intracellular cyclic adenosine monophosphate (cAMP) since neither forskolin nor 8-Br-cAMP was cytotoxic for EL-4 cells. Adenosine therefore acts through a non-classical receptor at the cell surface to trigger caspase-independent apoptosis in mouse thymoma cells.

A role for adenosine deaminase in Drosophila larval development

Adenosine deaminase (ADA) is an enzyme present in all organisms that catalyzes the irreversible deamination of adenosine and deoxyadenosine to inosine and deoxyinosine. Both adenosine and deoxyadenosine are biologically active purines that can have a deep impact on cellular physiology; notably, ADA deficiency in humans causes severe combined immunodeficiency. We have established a Drosophila model to study the effects of altered adenosine levels in vivo by genetic elimination of adenosine deaminase-related growth factor-A (ADGF-A), which has ADA activity and is expressed in the gut and hematopoietic organ. Here we show that the hemocytes (blood cells) are the main regulator of adenosine in the Drosophila larva, as was speculated previously for mammals. The elevated level of adenosine in the hemolymph due to lack of ADGF-A leads to apparently inconsistent phenotypic effects: precocious metamorphic changes including differentiation of macrophage-like cells and fat body disintegration on one hand, and delay of development with block of pupariation on the other. The block of pupariation appears to involve signaling through the adenosine receptor (AdoR), but fat body disintegration, which is promoted by action of the hemocytes, seems to be independent of the AdoR. The existence of such an independent mechanism has also been suggested in mammals.

Adenosine deaminase is critically important to survival; congenital deficiency in humans leads to severe immunodeficiency. Here, the authors demonstrate that adenosine deaminase deficiency in flies results in severe developmental defects.

A2A adenosine receptor induction inhibits IFN-gamma production in murine CD4+ T cells

Incubation of purified C57BL/6 murine CD4(+) T lymphocytes with anti-CD3 mAb serves as a model of TCR-mediated activation and results in increased IFN-gamma production and cell surface expression of CD25 and CD69. We demonstrate here that signaling through the TCR causes a rapid (4-h) 5-fold increase in A(2A) adenosine receptor (AR) mRNA, which is correlated with a significant increase in the efficacy of A(2A)AR-mediated cAMP accumulation in these cells. A(2A)AR activation reduces TCR-mediated production of IFN-gamma by 98% with a potency order of 4-{3-[6-amino-9-(5-ethylcarbamoyl-3,4-dihydroxytetrahydrofuran-2-yl)-9H-purin-2-yl]prop-2-ynyl}cyclohexanecarboxylic acid methyl ester (ATL146e; EC(50) = 0.19 +/- 0.03 nM) > 4-{3-[6-amino-9-(5-cyclopropyl-carbamoyl-3,4-dihydroxytetrahydrofuran-2-yl)-9H-purin-2-yl]prop-2-ynyl}piperidine-1-carboxylic acid methyl ester (ATL313; 0.43 +/- 0.06 nM) > 5'-N-ethylcarboxamidoadenosine (3.5 +/- 0.77 nM) > 2-[4-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680; 7.2 +/- 1.4 nM) >> N(6)-cyclohexyladenosine (110 +/- 33 nM) > 2-chloro-N(6)-(3-iodobenzyl)-5'-N-methylcarboxamide (390 +/- 160 nM), similar to the potency order to compete for radioligand binding to the recombinant murine A(2A)AR but not the A(3)AR. The selective A(2A)AR antagonist, 4-(2-[7-amino-2-[2-furyl][1,2,4]triazolo[2,3-a][1,3,5]triazin-5-yl-amino]ethyl)phenol (ZM241385), inhibits the effect of ATL146e with a pA(2) of 0.34 nM and also inhibits the effects of N(6)-cyclohexyl-adenosine and 2-chloro-N(6)-(3-iodobenzyl)-5'-N-methylcarboxamide. In CD4(+) T cells derived from A(2A)AR(-/-) and A(2A)AR(+/-) mice, the IFN-gamma release response to ATL146e is reduced by 100 and 50%, respectively, indicative of a gene dose effect. The response of T cells to the phosphodiesterase inhibitor, 4-(3'-cyclopentyloxy-4'-methoxyphenyl)-2-pyrrolidone (rolipram), is not affected by A(2A)AR deletion. We conclude that the rapid induction of the A(2A)AR mRNA in T cells provides a mechanism for limiting T cell activation and secondary macrophage activation in inflamed tissues.

New insights into adenosine-receptor-mediated immunosuppression and the role of adenosine in causing the immunodeficiency associated with adenosine deaminase deficiency

There is growing interest in manipulating adenosine (Ado) signal transduction to control inflammation and autoimmunity. This concept probably originated with the discovery of severe combined immunodeficiency disease (SCID) in infants with inherited deficiency of adenosine deaminase (ADA). However, the basis for immunosuppression by Ado has not been well defined, and effects of 2'-deoxyadenosine (dAdo), which does not activate Ado receptors, have also been implicated in causing SCID. Here I discuss recent evidence that Ado, acting through its A2A receptor, interferes with NF-kappa B activation in antigen-receptor-stimulated B and T lymphocytes. I also assess the relative contributions of Ado and dAdo to the pathogenesis of ADA-deficient SCID.

Adenosine and cAMP are potent inhibitors of the NF-kappa B pathway downstream of immunoreceptors

Anergic B lymphocytes exert compromised signal transduction towards the activation of NF-kappa B in response to B cell antigen receptor (BCR) triggering, whereas activation of the ERK pathway appears normal. How this differential down-regulation of the NF-kappa B pathway is regulated remains still elusive. Here, we demonstrate that stimuli known to enhance 3',5'-cyclic adenosine monophosphate (cAMP) are capable of selectively suppressing the activation both of NF-kappa B downstream of the BCR and Toll-like receptor 4 in splenic B lymphocytes and of the high-affinity receptor for IgE in BM-derived mast cells. This suppression is accomplished by blocking phosphorylation and subsequent degradation of the inhibitor of NF-kappa B. A cAMP-dependent protein kinase (PKA) inhibitor reverses this suppressive effect, indicating that PKA is a downstream effector of cAMP in this process. Importantly, not only drugs that artificially elevate intracellular cAMP levels, but also the nucleoside adenosine, which is known to be a mediator of cellular distress, inhibit the NF-kappa B pathway. This suggests that adenosine-mediated signals represent an important step in the molecular decision process controlling inflammation versus anergic immune responses.

PhysiologicalControl ofImmuneResponse andInflammatoryTissueDamage byHypoxia-InducibleFactors andAdenosineA2AReceptors

Immune cell-mediated destruction of pathogens may result in excessive collateral damage to normal tissues, and the failure to control activated immune cells may cause immunopathologies. The search for physiological mechanisms that downregulate activated immune cells has revealed a critical role for extracellular adenosine and for immunosuppressive A2A adenosine receptors in protecting tissue from inflammatory damage. Tissue damage-associated deep hypoxia, hypoxia-inducible factors, and hypoxia-induced accumulation of adenosine may represent one of the most fundamental and immediate tissue-protecting mechanisms, with adenosine A2A receptors triggering "OFF" signals in activated immune cells. In these regulatory mechanisms, oxygen deprivation and extracellular adenosine accumulation serve as "reporters," while A2A adenosine receptors serve as "sensors" of excessive tissue damage. The A2A receptor-triggered generation of intracellular cAMP then inhibits activated immune cells in a delayed negative feedback manner to prevent additional tissue damage. Targeting A2A adenosine receptors may have important clinical applications.

A(2A) adenosine receptor ligands and proinflammatory cytokines induce PC 12 cell death through apoptosis

A(2A) adenosine receptor-mediated signaling affects a variety of important processes in the central nervous system both in physiological and pathological conditions, and has been indicated as possible novel therapeutic target in several nervous system diseases. In the present work, cell death induction was investigated after neuronal PC 12 cell treatment with proinflammatory cytokines and adenosine receptor ligands. Interleukin-1-beta (IL-1-beta, 500 U/mL), tumor necrosis factor-alpha (TNF-alpha, 1000 U/mL) and the non selective adenosine receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA), caused a significant reduction of cell viability with a maximal effect within 3-48 hr. Moreover, an addictive effect was detected when the cells were simultaneously treated with Interleukin-1-beta and NECA for 3 hr. To investigate the adenosine receptor subtypes involved in PC 12 cell death, the effects of several adenosine receptor agonists/antagonists were evaluated. The endogenous nucleoside, adenosine, and the selective A(2A) adenosine receptor agonist, 2-(carboxyethylphenylethylamino)adenosine-5'-carboxamide (CGS21680) reduced PC 12 cell viability. This effect was counteracted by the selective A(2A) adenosine receptor antagonist, 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3e]-1,2,4-triazolo[1,5c]pyrimidine (SCH58261), but not by selective A(2B) adenosine receptor antagonist N-(4-acethylphenyl)-2-[4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)phenoxy]acetamide (MRS1706), suggesting the specific involvement of A(2A) adenosine receptor subtype in adenosine-mediated cytotoxicity. Moreover, the selective A(1) adenosine receptor agonist, N(6)-cyclohexyladenosine (CHA), did not induce any significant effect on cell viability. By ELISA immunoassay cell death detection and transmission electron microscopy (TEM) we demonstrated that A(2A) adenosine receptor ligands and cytokines induced cell death through an apoptotic pathway. In conclusion, our results showed that A(2A) adenosine receptors are involved in the control of PC 12 cell survival/death and may contribute to modulate cellular activity in response to tissue damage associated with inflammatory mediator production.

A glance at adenosine receptors: novel target for antitumor therapy

Adenosine can be released from a variety of cells throughout the body, as the result of increased metabolic rates, in concentrations that can have a profound impact on the vasculature, immunoescaping, and growth of tumor masses. It is recognized that the concentrations of this nucleoside are increased in cancer tissues. Therefore, it is not surprising that adenosine has been shown to be a crucial factor in determining the cell progression pathway, either during apoptosis or during cytostatic state. From the perspective of cancer, the most important question then may be "Can activation and/or blockade of the pathways downstream of the adenosine receptor contribute to tumor development?" Rigorous examinations of the role of adenosine in in vivo and in vitro systems need to be investigated. The present review therefore proposes multiple adenosine-sustained ways that could prime tumor development together with the critical combinatorial role played by adenosine receptors in taking a choice between proliferation and death. This review proposes that adenosine acts as a potent regulator of normal and tumor cell growth. It is hypothesized that this effect is dependent on extracellular adenosine concentrations, cell surface expression of different adenosine receptor subtypes, and signal transduction mechanisms activated following the binding of specific agonists. We venture to suggest that the clarification of the role of adenosine and its receptors in cancer development may hold great promise for the treatment of chemotherapy in patients affected by malignancies.

Genotype is an important determinant of phenotype in adenosine deaminase deficiency

Adenosine deaminase (ADA) deficiency is associated with a broad clinical and mutational spectrum. Defining the relationship of genotype to phenotype among patients with different degrees of immunodeficiency has been complicated because the disease is rare, most mutations are 'private' and patients are often heteroallelic. In recent years, knowledge of ADA structure and systematic expression of mutant alleles have revealed that phenotype is strongly associated with the sum of ADA activity provided by both alleles. A scale for ranking novel ADA alleles based on expression may have utility if newborn screening for primary immunodeficiency disorders is initiated.

The critical role of adenosine A2A receptors in downregulation of inflammation and immunity in the pathogenesis of infectious diseases

Adenosine can be described as a retaliatory metabolite, the production and release of which is usually enhanced under adverse environmental conditions. Binding via specific receptors, adenosine activates endogenous protective mechanisms aiming at the restoration of tissue homeostasis. While adenosinergic downregulation of tissue damage is beneficial in acute inflammation, chronic suppression of the immune system by adenosine may account for immunoparalysis in long-term septic patients.

Use of the A(2A) adenosine receptor as a physiological immunosuppressor and to engineer inflammation in vivo

Inflammation must be inhibited in order to treat, e.g., sepsis or autoimmune diseases or must be selectively enhanced to improve, for example, immunotherapies of tumors or the development of vaccines. Predictable enhancement of inflammation depends upon the knowledge of the "natural" pathways by which it is down-regulated in vivo. Extracellular adenosine and A(2A) adenosine (purinergic) receptors were identified recently as anti-inflammatory signals and as sensors of excessive inflammatory tissue damage, respectively (Ohta A and Sitkovsky M, Nature 2001;414:916-20). These molecules may function as an important part of a physiological "metabolic switch" mechanism, whereby the inflammatory stimuli-produced local tissue damage and hypoxia cause adenosine accumulation and signaling through cyclic AMP-elevating A(2A) adenosine receptors in a delayed negative feedback manner. Patterns of A(2A) receptor expression are activation- and differentiation-dependent, thereby allowing for the "acquisition" of an immunosuppressive "OFF button" and creation of a time-window for immunomodulation. Identification of A(2A) adenosine receptors as "natural" brakes of inflammation provided a useful framework for understanding how tissues regulate inflammation and how to enhance or decrease (engineer) inflammation by targeting this endogenous anti-inflammatory pathway. These findings point to the need of more detailed testing of anti-inflammatory agonists of A(2A) receptors and create a previously unrecognized strategy to enhance inflammation and targeted tissue damage by using antagonists of A(2A) receptors. It is important to further identify the contributions of different types of immune cells at different stages of the inflammatory processes in different tissues to enable the "tailored" treatments with drugs that modulate the signaling through A(2A) purinergic receptors.

Adenosine receptors as mediators of both cell proliferation and cell death of cultured human melanoma cells

Adenosine displays contradictory effects on cell growth: it improves cell proliferation, but it may also induce apoptosis and impair cell survival. Following the pharmacologic characterization of adenosine receptor expression on the human melanoma cell line A375, we chose A375 as our cellular model to define how the extracellular adenosine signals are conveyed from each receptor. By using selective adenosine receptor agonists or antagonists, we found that A2A stimulation reduced cell viability and cell clone formation, whereas, at the same time, it improved cell proliferation. In support of this finding we demonstrated that the stimulation of A2A adenosine receptors stably expressed in Chinese hamster ovary cell clone reproduced deleterious effects observed in human melanoma cells. A3 stimulation counteracted A2A-induced cell death but also reduced cell proliferation. Furthermore, we found that A3 stimulation ensures cell survival. We demonstrated that adenosine triggers a survival signal via A3 receptor activation and it kills the cell through A2A receptor inducing a signaling pathway that involves protein kinase C and mitogen-activated protein kinases.