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Adenosine plasma level and A2A adenosine receptor expression: correlation with laboratory tests in patients with neurally mediated syncope

Objectives The purpose of this study was to investigate the hypothesis that responses to the ATP test and head-up tilt test (HUT) may be correlated with different purinergic profiles. Design and setting The ATP and HUT identify distinct subsets of patients with neurally mediated syncope (NMS). Adenosine and its A2A receptors (A2AR) may be implicated in the pathophysiology of NMS in patients with positive HUT. Nothing is known about the purinergic profile of patients with positive ATP. Patients and measures This prospective study includes a consecutive series of patients with suspected NMS. All patients underwent both HUT and ATP. Before testing, samples were collected for measurement of baseline adenosine plasma level (APL) and expression. Results A total of 46 patients (25 men and 21 women) with a mean age of 57±18 years were enrolled. The HUT test was positive in 27 patients and the ATP test in 20. Both tests were positive in 9 and negative in 8. High APL was associated with high probability of positive HUT while low APL was associated with high probability of positive ATP. Expression of A2AR was lower in patients with positive ATP than in those with positive HUT. Conclusion These findings indicate that patients with NMS present different purinergic profiles and that responses to HUT and ATP are correlated with these profiles.

Monoclonal antibody–assisted stimulation of adenosine A2A receptors induces simultaneous downregulation of CXCR4 and CCR5 on CD4+ T-cells

Immunocompetent cells express various G-protein-coupled receptors that transduce extracellular signals across the plasma membrane. Among them, CXCR4 and CCR5 chemokines receptors and adenosine A(2A) receptors (A(2A)R) are involved in inflammatory processes. Considering that A(2A)R activation may have incidence on CXCR4 and CCR5 protein expression through heterologous desensitization process, we tested Adonis, an agonist-like monoclonal antibody to A(2A)R on CD4+ CEM T-cells. We found that Adonis inhibited the CEM cell growth, upregulated A(2A)R and downregulated CXCR4 and CCR5 without modifying the CD4 expression. By reducing the expression of CXCR4 and CCR5 chemokines receptors utilized as entry co-receptors by HIV-1 during viral infection of CD4 expressing cells, Adonis stimulation of A(2A)R appears as a valuable means to treat infected cells.

Head-up tilt induced syncope and adenosine A2A receptor gene polymorphism

AIMS High adenosine plasma levels and high expression of adenosine A(2A) receptors are observed in patients with unexplained syncope and a positive head-up tilt test (HUT). This study aimed to evaluate the single nucleotide polymorphism (SNP) (c.1364 T>C) which is the most commonly found polymorphism in the A(2A) receptor gene, in patients with unexplained syncope undergoing HUT. METHODS AND RESULTS One hundred and five patients with unexplained syncope who underwent HUT were included. Fifty-two had a positive test. Receptor genotype determinations were performed in patients and in 121 healthy subjects. Genotype (TT, CC, TC) was determined from DNA leucocytes. The distribution of the polymorphism showed significant (P < 0.0001) difference when the results of HUT were analysed. Fifty-two per cent of patients with a positive HUT had a CC genotype and 34.6% a TC genotype, whereas 13.2% of the patients with a negative HUT had a CC genotype and 71.7% a TC genotype. Patients with a CC genotype had a higher incidence of spontaneous syncopal episodes. CONCLUSION In patients with unexplained syncope, a significant association between high incidence of syncopal episodes, positive HUT, and the presence of the CC variant in the adenosine A(2A) receptor gene was elicited.

NF-κB enhances hypoxia-driven T-cell immunosuppression via upregulation of adenosine A2A 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.

Production of an agonist-like monoclonal antibody to the human A2A receptor of adenosine for clinical use.

The second extracellular loop of the A(2A) receptor (A(2A)R) of adenosine was used to immunize mice for production of Adonis, an IgM monoclonal antibody. Adonis bound to the immunogen peptide and the native receptor in ELISA with K(D) values in 6.51-12.35 nM range. It recognized a linear epitope of 7 amino acids (LFEDVVP) at the C-terminal part of the external loop. Adonis revealed a 45-kDa band in lysate of human peripheral blood mononuclear cells in Western blotting in denaturing conditions. This served to monitor the up-regulation of the A(2A)R expression by caffeine. Adonis stimulated the cAMP production and inhibited the cell proliferation of an A(2A)R transfected stable cell line. These results confirm the immunogenicity and the functional relevance of the second extracellular loop of the A(2A)R. They suggest that Adonis may be of clinical use in various pathological situations to measure the regulation of the A(2A)R expression and to act as A(2A)R agonist drug.

Peripheral plasma adenosine release in patients with chronic heart failure

Objective: Chronic heart failure (CHF) is accompanied by increased adenosine plasma levels (APLs). It is unknown whether adenosine release occurs at the peripheral level or whether the myocardium itself is the source of adenosine release. To answer this question, we evaluated APLs in the coronary sinus of CHF patients during a resynchronisation procedure and compared the values with those at the peripheral level. We also investigated a possible correlation between APLs and ischaemia-modified albumin (IMA) levels, a useful marker of tissue ischaemia. Methods: 19 men and seven women were prospectively included. Blood samples for APLs were collected simultaneously from a brachial vein (peripheral) and from the coronary sinus. Blood samples for brain natriutretic peptide (BNP) and IMA were collected from a brachial vein. Results: APLs from the brachial vein were higher than those from the coronary sinus (1.69 vs 0.75 μM p<0.01). IMA levels were correlated with APLs from the brachial vein (r = 0.59, p<0.01). BNP concentrations were correlated with APLs from the brachial vein (r = 0.73, p<0.001) but not with APLs from the coronary sinus (r = 0.38, p>0.05). BNP concentrations and IMA levels were correlated (r = 0.71, p<0.001). Conclusions: In CHF patients, adenosine release occurs at a peripheral level and not at the myocardium level.

Design, synthesis and biological evaluation of a bivalent μ opiate and adenosine A1 receptor antagonist

The cross talk between different membrane receptors is the source of increasing research. We designed and synthesized a new hetero-bivalent ligand that has antagonist properties on both A(1) adenosine and mu opiate receptors with a K(i) of 0.8+/-0.05 and 0.7+/-0.03 microM, respectively. This hybrid molecule increases cAMP production in cells that over express the mu receptor as well as those over expressing the A(1) adenosine receptor and reverses the antalgic effects of mu and A(1) adenosine receptor agonists in animals.

Expeditious synthesis and biological evaluation of new C-6 1,2,3-triazole adenosine derivatives A1 receptor antagonists or agonists

The synthesis of new C-6 1,2,3-triazole adenosine derivatives via microwave assisted 1,3-dipolar cycloaddition as key step is described. The binding on membranes of cells that over express A(1) adenosine receptors (A(1)AR) was also evaluated. Among them, four compounds increased cAMP production, in a dose-dependent manner acting as antagonists of the A(1)AR, while two compounds act as agonists.

Intracerebroventricular injection of an agonist-like monoclonal antibody to adenosine A2A receptor has antinociceptive effects in mice

Adenosine is a modulator of nociceptive pathways, both at the spinal and supraspinal levels. Adenosine A(1) and A(2A) receptors (A(1)R, A(2A)R) are expressed in the basal ganglia where they are the target of caffeine, the most widely use psychoactive drug which acts as an antagonist to both types of receptors. Given the controversial role of A(2A)R versus A(1)R in modulating pain in brain areas, mice received intracerebroventricular injection of Adonis, an agonist-like monoclonal antibody with high specificity for the A(2A)R and were subjected to behavioral tests investigating nociceptive thresholds. We report that Adonis led to a significant dose-dependent increase in hot-plate and tail-flick latencies in mice and that such increase was prevented by caffeine and ZM 241385, a specific A(2A)R antagonist. The Adonis antinociceptive effects were also inhibited by naloxone, a non selective antagonist for opioid receptors, suggesting that Adonis acts, at least in part, through the stimulation of the endogenous opioid system. These results confirm the A(2A)R as a target for pain control and Adonis as a potential drug with therapeutic interest.

Mononuclear cell adenosine deaminase and CD26/dipeptidylpeptidase-IV activities are sensitive markers of reperfusion during percutaneous transluminal angioplasty

UNLABELLED During ischaemia, the extracellular level of adenosine increases, which has cytotoxic effects. In endothelium, cell surface adenosine deaminase (ADA) complexing CD26 is coordinately induced during ischaemia as part of an adaptative response by eliminating adenosine. We examined whether a similar mechanism exists for mononuclear cells. We studied mononuclear cell surface ADA (MCADA) and dipeptidyl-peptidase IV activity (DPPIV) of membrane CD26 during percutaneous transluminal coronary angioplasty (PTCA) as a model of ischaemia-reperfusion. Enzymatic activities were compared with levels of ischaemia-modified albumin (IMA), a marker of ischaemia-reperfusion. METHODS AND RESULTS Patients (15 men and 5 women) with non-ST segment elevation acute coronary syndrome related to a stenosis of proximal left anterior descending artery were prospectively included before revascularization. MCADA, DPPIV and IMA were measured before PTCA (T0) then 15 (T15) and 120 (T120) minutes after reperfusion. Fifteen healthy control subjects were enrolled. At T0, MCADA and IMA levels were higher in patients than in controls. MCADA decreased at T15 (median, IQR: 8.2 [7.6-9.8] IU) relative to T0 (11.25 [10-13.5] IU, p<0.01) and remained low at T120. DPPIV decreased at T15 (0.9 [0.7-1.1] AU) relative to T0 (1.05 [0.99-1.48] AU; p<0.01) and remained low at T120. IMA level increased only at T120. MCADA and DPPIV were correlated. Our findings are that MCADA and DPPIV decreased rapidly after angioplasty, suggesting that both catalysts are early markers of reperfusion. CONCLUSION MCADA and DPPIV are sensitive and early markers of ischaemia-reperfusion process during PTCA.