A2B adenosine receptor activity is reduced in neutrophils from patients with systemic sclerosis

Neutrophils: fast and furious-the nucleotide pathway

Nucleotide signaling is a key element of the neutrophil activation pathway. Neutrophil recruitment and migration to injured tissues is guided by purinergic receptor sensitization, mostly induced by extracellular adenosine triphosphate (ATP) and its hydrolysis product, adenosine (ADO), which is primarily produced by the CD39-CD73 axis located at the neutrophil cell surface. In inflammation unrelated to cancer, neutrophil activation via purinergic signaling aims to eliminate antigens and promote an immune response with minimal damage to healthy tissues; however, an antagonistic response may be expected in tumors. Indeed, alterations in purinergic signaling favor the accumulation of extracellular ATP and ADO in the microenvironment of solid tumors, which promote tumor progression by inducing cell proliferation, angiogenesis, and escape from immune surveillance. Since neutrophils and their N1/N2 polarization spectrum are being considered new components of cancer-related inflammation, the participation of purinergic signaling in pro-tumor activities of neutrophils should also be considered. However, there is a lack of studies investigating purinergic signaling in human neutrophil polarization and in tumor-associated neutrophils. In this review, we discussed the human neutrophil response elicited by nucleotides in inflammation and extrapolated its behavior in the context of cancer. Understanding these mechanisms in cancerous conditions may help to identify new biological targets and therapeutic strategies, particularly regarding tumors that are refractory to traditional chemo- and immunotherapy.

Adenosine Signaling in Autoimmune Disorders

The molecular components of the purinergic system (i.e., receptors, metabolizing enzymes and membrane transporters) are widely expressed in the cells of the immune system. Additionally, high concentrations of adenosine are generated from the hydrolysis of ATP in any "danger" condition, when oxygen and energy availability dramatically drops. Therefore, adenosine acts as a retaliatory metabolite to counteract the nucleotide-mediated boost of the immune reaction. Based on this observation, it can be foreseen that the recruitment with selective agonists of the receptors involved in the immunomodulatory effect of adenosine might represent an innovative anti-inflammatory approach with potential exploitation in autoimmune disorders. Quite surprisingly, pro-inflammatory activity exerted by some adenosine receptors has been also identified, thus paving the way for the hypothesis that at least some autoimmune disorders may be caused by a derailment of adenosine signaling. In this review article, we provide a general overview of the roles played by adenosine on immune cells with a specific focus on the development of adenosine-based therapies for autoimmune disorders, as demonstrated by the exciting data from concluded and ongoing clinical trials.

The Purinergic System as a Pharmacological Target for the Treatment of Immune-Mediated Inflammatory Diseases

Immune-mediated inflammatory diseases (IMIDs) encompass a wide range of seemingly unrelated conditions, such as multiple sclerosis, rheumatoid arthritis, psoriasis, inflammatory bowel diseases, asthma, chronic obstructive pulmonary disease, and systemic lupus erythematosus. Despite differing etiologies, these diseases share common inflammatory pathways, which lead to damage in primary target organs and frequently to a plethora of systemic effects as well. The purinergic signaling complex comprising extracellular nucleotides and nucleosides and their receptors, the P2 and P1 purinergic receptors, respectively, as well as catabolic enzymes and nucleoside transporters is a major regulatory system in the body. The purinergic signaling complex can regulate the development and course of IMIDs. Here we provide a comprehensive review on the role of purinergic signaling in controlling immunity, inflammation, and organ function in IMIDs. In addition, we discuss the possible therapeutic applications of drugs acting on purinergic pathways, which have been entering clinical development, to manage patients suffering from IMIDs.

Purinergic signalling and immune cells

This review article provides a historical perspective on the role of purinergic signalling in the regulation of various subsets of immune cells from early discoveries to current understanding. It is now recognised that adenosine 5'-triphosphate (ATP) and other nucleotides are released from cells following stress or injury. They can act on virtually all subsets of immune cells through a spectrum of P2X ligand-gated ion channels and G protein-coupled P2Y receptors. Furthermore, ATP is rapidly degraded into adenosine by ectonucleotidases such as CD39 and CD73, and adenosine exerts additional regulatory effects through its own receptors. The resulting effect ranges from stimulation to tolerance depending on the amount and time courses of nucleotides released, and the balance between ATP and adenosine. This review identifies the various receptors involved in the different subsets of immune cells and their effects on the function of these cells.

Adverse and protective influences of adenosine on the newborn and embryo: implications for preterm white matter injury and embryo protection

Few signaling molecules have the potential to influence the developing mammal as the nucleoside adenosine. Adenosine levels increase rapidly with tissue hypoxia and inflammation. Adenosine antagonists include the methylxanthines caffeine and theophylline. The receptors that transduce adenosine action are the A1, A2a, A2b, and A3 adenosine receptors (ARs). In the postnatal period, A1AR activation may contribute to white matter injury in the preterm infant by altering oligodendrocyte (OL) development. In models of perinatal brain injury, caffeine is neuroprotective against periventricular white matter injury (PWMI) and hypoxic-ischemic encephalopathy (HIE). Supporting the notion that blockade of adenosine action is of benefit in the premature infant, caffeine reduces the incidence of bronchopulmonary dysplasia and CP in clinical studies. In comparison with the adverse effects on the postnatal brain, adenosine acts via A1ARs to play an essential role in protecting the embryo from hypoxia. Embryo protective effects are blocked by caffeine, and caffeine intake during early pregnancy increases the risk of miscarriage and fetal growth retardation. Adenosine and adenosine antagonists play important modulatory roles during mammalian development. The protective and deleterious effects of adenosine depend on the time of exposure and target sites of action.

Adenosine receptor expression in Escherichia coli-infected and cytokine-stimulated human urinary tract epithelial cells

OBJECTIVE

To assess the expression and regulation of adenosine receptors in unstimulated, uropathogenic Escherichia coli (UPEC)-infected and cytokine-stimulated human urinary tract epithelial cells, and to examine the regulation of interleukin (IL)-6 secretion in response to A(2A) receptor activation.

MATERIALS AND METHODS

Human urinary tract epithelial cells (A498, T24 and RT4) were grown in cell culture and stimulated with a mixture of pro-inflammatory cytokines (CM) or UPEC. The expression of adenosine receptors was evaluated using semiquantitative reverse transcription-polymerase chain reaction (RT-PCR), Western blot analysis and immunocytochemistry. IL-6 secretion was measured with an enzyme-linked immunosorbent assay.

RESULTS

RT-PCR analysis showed the presence of transcripts for the A(1), A(2A) and A(2B) receptor subtypes but not for the A(3) receptor in A498 kidney epithelial cells. The expression of A(2A) receptor mRNA increased in A498 epithelial cells exposed to CM and UPEC, while A(1) and A(2B) receptor transcripts decreased or remained unchanged. Up-regulation of A(2A) receptors was confirmed at the protein level using Western blot analysis and immunocytochemistry. There was also an increase in A(2A) receptor mRNA in human bladder epithelial cells (T24 and RT4) and in mouse bladder uroepithelium in response to cytokines and UPEC. IL-6 secretion in UPEC-infected A498 cells was decreased by 38% when exposed to the A(2A) receptor agonist CGS 21680.

CONCLUSION

Our data showed a subtype-selective plasticity among adenosine receptors in urinary tract epithelial cells in response to UPEC-infection and cytokines. There was a consistent up-regulation of A(2A) receptors in kidney and bladder epithelial cells. Functionally, A(2A) receptor activation reduced UPEC-induced IL-6 secretion. These findings suggest that adenosine might be a previously unrecognized regulator of the mucosal response in urinary tract infection.

Allergen inhalation decreases adenosine receptor expression in sputum and blood of asthma patients

BACKGROUND

Adenosine is a signalling nucleoside that has been proposed to contribute to the pathogenesis of asthma. Adenosine is produced in inflammatory environments and acts via adenosine receptors (A(1)R, A(2A)R, A(2B)R, and A(3)R) expressed by a wide variety of cells, resulting in pro- and anti-inflammatory effects.

OBJECTIVE

To compare AR expression in asthma patients and healthy subjects, and to assess the effect of allergen challenge on AR expression of inflammatory cells and on cytokines in peripheral blood and sputum in asthma.

METHODS

Asthma patients underwent an allergen challenge, and blood and induced sputum samples were taken before and 24 h after allergen challenge to study inflammatory cells numbers, AR expression and cytokine production. Blood and sputum were investigated at one time point in healthy subjects. AR expression was measured by flow cytometry (blood) or on cytospins using immunocytochemistry (sputum). Cytokines (luminex, ELISA) and adenosine (HPLC) were measured in sputum supernatant.

RESULTS

The percentage of A(2B)R expressing neutrophils in sputum was lower in asthma patients than in healthy subjects (P = 0.016). Allergen challenge decreased A(1)R and A(2A)R expression on neutrophils and A(1)R expression on T cells in peripheral blood (all P < 0.05). Allergen challenge increased IL-8 levels and eosinophil numbers (P < 0.05), whereas it decreased thymic stromal lymphopoietin levels and the percentage of A(1)R expressing macrophages in induced sputum (P < 0.05).

CONCLUSIONS

Allergen challenge has a down-regulatory effect on AR expression in asthma, suggesting a contribution of adenosine-related effector mechanisms in the pathophysiology.

Mechanisms of adenosine-induced cytotoxicity and their clinical and physiological implications

Extracellular ATP (ATPo) and adenosine are cytotoxic to several cancer cell lines, suggesting their potential use for anticancer therapy. Adenosine causes cytotoxicity, either when added exogenously or when generated from ATPo hydrolysis, via mechanisms which are not mutually exclusive and which involve, adenosine receptor activation, pyrimidine starvation and/or increases in intracellular S-adenosylhomocysteine: S-adenosylmethionine ratio. Given that adenosine also appears to protect against cytotoxicity via mechanisms including immunity against damage by oxygen free radicals, an understanding of the contribution of adenosine to ATPo-induced cytotoxicity is thus crucial, when considering any potential therapeutic use for these compounds. However, such an understanding has been largely hindered by the fact that many studies have not focused enough on the possibility that both ATPo and adenosine may mediate cytotoxicity in the same system. Such studies can benefit from use a range of ATPo concentrations when assessing the contribution of adenosine to ATPo-induced cytotoxicity. Whilst future molecular and pharmacological studies are needed to establish the nature of the cytotoxic adenosine receptor, it is possible that more than just one adenosine receptor type is involved and that the cytotoxic receptor(s) type is more likely to have a low affinity for adenosine. Activation of the adenosine receptor(s) would thus lead to cytotoxicity only at relatively high adenosine concentrations, while lower adenosine concentrations mediate non-cytotoxic physiological effects.

Differential expression of adenosine receptors in human neutrophils: up-regulation by specific Th1 cytokines and lipopolysaccharide

Four types of adenosine receptors have been identified in different tissues and cell types, namely, A1, A2A, A2B, and A3 receptors. We report that A2AR but not A2BR mRNA in freshly isolated polymorphonuclear neutrophils (PMN) is maximally up-regulated after 4 h stimulation with lipopolysaccharide (LPS) or tumor necrosis factor alpha (TNF-alpha) and to a lesser extent, with interleukin (IL)-1beta. These effects were maintained up to 21 h. Consistent with changes in A2AR mRNA expression, up-regulation of A2AR protein was also detected after 4 h of LPS or TNF-alpha exposure. Up-regulation of A2AR protein expression was transient and returned to near basal levels after 12 h or 16 h stimulation with TNF-alpha or LPS, respectively. Conversely, IL-1beta failed to promote A2AR protein expression. Suppression of thapsigargin-induced leukotriene synthesis by the selective A2AR agonist CGS-21680 was found to be more pronounced when PMN were cultured for 4 h with LPS or TNF-alpha. In contrast, the up-regulation of A2AR has no impact on CGS-21680-induced inhibition of phospholipase D activation and superoxide production in response to formyl-Met-Leu-Phe. These results demonstrate that the A2AR is up-regulated by specific T helper cell type 1 cytokines and LPS. Although this could represent a potential feedback mechanism to control inflammation, the effect of A2AR up-regulation varied depending on the stimulus used to stimulate PMN functional responses after their incubation with proinflammatory mediators.