Lipopolysaccharide and TNF-alpha modify adenosine A(2A) receptor expression and function in equine monocytes

Anti-Inflammatory Effects of Endogenously Released Adenosine in Synovial Cells of Osteoarthritis and Rheumatoid Arthritis Patients

Exogenous adenosine and its metabolite inosine exert anti-inflammatory effects in synoviocytes of osteoarthritis (OA) and rheumatoid arthritis (RA) patients. We analyzed whether these cells are able to synthesize adenosine/inosine and which adenosine receptors (ARs) contribute to anti-inflammatory effects. The functionality of synthesizing enzymes and ARs was tested using agonists/antagonists. Both OA and RA cells expressed CD39 (converts ATP to AMP), CD73 (converts AMP to adenosine), ADA (converts adenosine to inosine), ENT1/2 (adenosine transporters), all AR subtypes (A₁, A_2A, A_2B and A₃) and synthesized predominantly adenosine. The CD73 inhibitor AMPCP significantly increased IL-6 and decreased IL-10 in both cell types, while TNF only increased in RA cells. The ADA inhibitor DAA significantly reduced IL-6 and induced IL-10 in both OA and RA cells. The A_2AAR agonist CGS 21680 significantly inhibited IL-6 and induced TNF and IL-10 only in RA, while the A_2BAR agonist BAY 60-6583 had the same effect in both OA and RA. Taken together, OA and RA synoviocytes express the complete enzymatic machinery to synthesize adenosine/inosine; however, mainly adenosine is responsible for the anti- (IL-6 and IL-10) or pro-inflammatory (TNF) effects mediated by A_2A- and A_2BAR. Stimulating CD39/CD73 with simultaneous ADA blockage in addition to TNF inhibition might represent a promising therapeutic strategy.

A single peri-sciatic nerve administration of the adenosine 2A receptor agonist ATL313 produces long-lasting anti-allodynia and anti-inflammatory effects in male rats

Neuropathic pain is a widespread problem which remains poorly managed by currently available therapeutics. Peripheral nerve injury and inflammation leads to changes at the nerve injury site, including activation of resident and recruited peripheral immune cells, that lead to neuronal central sensitization and pain amplification. The present series of studies tested the effects of peri-sciatic nerve delivery of single doses of adenosine 2A receptor (A_2aR) agonists on pain and neuroinflammation. The data provide converging lines of evidence supportive that A_2aR agonism at the site of peripheral nerve injury and inflammation is effective in suppressing ongoing neuropathic pain. After A_2aR agonism resolved neuropathic pain, a return of pain enhancement (allodynia) was observed in response to peri-sciatic injection of H-89, which can inhibit protein kinase A, and by peri-sciatic injection of neutralizing antibody against the potent anti-inflammatory cytokine interleukin-10. A_2aR agonist actions at the nerve injury site suppress neuroinflammation, as reflected by decreased release of interleukin-1β and nitric oxide, as well as decreased sciatic expression of markers of monocytes/macrophages and inducible nitric oxide synthase. Taken together, the data are supportive that A_2aR agonists, acting at the level of peripheral nerve injury, may be of therapeutic value in treating chronic pain of neuroinflammatory origin.

Potential Therapeutic Applications of Adenosine A2A Receptor Ligands and Opportunities for A2A Receptor Imaging

Adenosine A_2A receptors (A_2A Rs) are highly expressed in the human striatum, and at lower densities in the cerebral cortex, the hippocampus, and cells of the immune system. Antagonists of these receptors are potentially useful for the treatment of motor fluctuations, epilepsy, postischemic brain damage, or cognitive impairment, and for the control of an immune checkpoint during immunotherapy of cancer. A_2A R agonists may suppress transplant rejection and graft-versus-host disease; be used to treat inflammatory disorders such as asthma, inflammatory bowel disease, and rheumatoid arthritis; be locally applied to promote wound healing and be employed in a strategy for transient opening of the blood-brain barrier (BBB) so that therapeutic drugs and monoclonal antibodies can enter the brain. Increasing A_2A R signaling in adipose tissue is also a potential strategy to combat obesity. Several radioligands for positron emission tomography (PET) imaging of A_2A Rs have been developed in recent years. This review article presents a critical overview of the potential therapeutic applications of A_2A R ligands, the use of A_2A R imaging in drug development, and opportunities and limitations of PET imaging in future research.

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.

Danger signal adenosine via adenosine 2a receptor stimulates growth of Porphyromonas gingivalis in primary gingival epithelial cells

Extracellular signaling during inflammation and chronic diseases involves molecules referred to as 'Danger Signals' (DS), including the small molecule adenosine. We demonstrate that primary gingival epithelial cells (GEC) express a family of G-protein coupled receptors known as adenosine receptors, including the high-affinity receptors A1 and A2a and low-affinity receptors A2b and A3. Treatment of Porphyromonas gingivalis-infected GEC with the A2a receptor-specific agonist CGS-21680 resulted in elevated intracellular bacterial replication as determined by fluorescence microscopy and antibiotic protection assay. Additionally, A2a receptor antagonism and knockdown via RNA interference significantly reduced metabolically active intracellular P. gingivalis. Furthermore, analysis of anti-inflammatory mediator cyclic AMP (cAMP) following A2a receptor selective agonist CGS-21680 stimulation induced significantly higher levels of cAMP during P. gingivalis infection, indicating that adenosine signaling may attenuate inflammatory processes associated with bacterial infection. This study reveals that the GEC express functional A2a receptor and P. gingivalis may use the A2a receptor coupled DS adenosine signaling as a means to establish successful persistence in the oral mucosa, possibly via downregulation of the pro-inflammatory response.

The A2A adenosine receptor is a dual coding gene: a novel mechanism of gene usage and signal transduction

The A2A adenosine receptor (A2AR) is a G protein-coupled receptor and a major target of caffeine. The A2AR gene encodes alternative transcripts that are initiated from at least two independent promoters. The different transcripts of the A2AR gene contain the same coding region and 3'-untranslated region and different 5'-untranslated regions that are highly conserved among species. We report here that in addition to the production of the A2AR protein, translation from an upstream, out-of-frame AUG of the rat A2AR gene produces a 134-amino acid protein (designated uORF5). An anti-uORF5 antibody recognized a protein of the predicted size of uORF5 in PC12 cells and rat brains. Up-regulation of A2AR transcripts by hypoxia led to increased levels of both the A2AR and uORF5 proteins. Moreover, stimulation of A2AR increased the level of the uORF5 protein via post-transcriptional regulation. Expression of the uORF5 protein suppressed the AP1-mediated transcription promoted by nerve growth factor and modulated the expression of several proteins that were implicated in the MAPK pathway. Taken together, our results show that the rat A2AR gene encodes two distinct proteins (A2AR and uORF5) in an A2AR-dependent manner. Our study reveals a new example of the complexity of the mammalian genome and provides novel insights into the function of A2AR.

Neuroprotective roles of the P2Y(2) receptor

Purinergic signaling plays a unique role in the brain by integrating neuronal and glial cellular circuits. The metabotropic P1 adenosine receptors and P2Y nucleotide receptors and ionotropic P2X receptors control numerous physiological functions of neuronal and glial cells and have been implicated in a wide variety of neuropathologies. Emerging research suggests that purinergic receptor interactions between cells of the central nervous system (CNS) have relevance in the prevention and attenuation of neurodegenerative diseases resulting from chronic inflammation. CNS responses to chronic inflammation are largely dependent on interactions between different cell types (i.e., neurons and glia) and activation of signaling molecules including P2X and P2Y receptors. Whereas numerous P2 receptors contribute to functions of the CNS, the P2Y(2) receptor is believed to play an important role in neuroprotection under inflammatory conditions. While acute inflammation is necessary for tissue repair due to injury, chronic inflammation contributes to neurodegeneration in Alzheimer's disease and occurs when glial cells undergo prolonged activation resulting in extended release of proinflammatory cytokines and nucleotides. This review describes cell-specific and tissue-integrated functions of P2 receptors in the CNS with an emphasis on P2Y(2) receptor signaling pathways in neurons, glia, and endothelium and their role in neuroprotection.