Occupancy of adenosine receptors raises cyclic AMP alone and in synergy with occupancy of chemoattractant receptors and inhibits membrane depolarization

Programmable stopped-flow injection analysis: A comparative study on the effects of adenosine and its aptamer on respiratory burst of salivary and circulatory neutrophils

Neutrophils play a pivotal role in innate immunity by releasing ROS through respiratory bursts to neutralize various pathogenic factors. However, excessive ROS release can cause tissue damage. Adenosine is an endogenous anti-inflammatory molecule inhibiting respiratory burst to protect the host. Adenosine aptamers with antibody-like properties and good stability are expected to act as adenosine antagonists with functional modulation capability. This study compares the effects of adenosine and its aptamer on the respiratory bursts of salivary polymorphonuclear leukocytes and circulating polymorphonuclear leukocytes using a programmable stopped-flow injection approach, ensuring rapid and efficient analysis while maintaining the neutrophils' viability. The results show that primed salivary polymorphonuclear leukocytes exhibit specificities that differ from circulating polymorphonuclear leukocytes. Adenosine aptamer can function as an inhibitory antagonist that distinguishes between physiologically controlled and excessive priming of neutrophils, showing potential application prospects in immunotherapy.

Adenosine A2A receptor agonist prevents cardiac remodeling and dysfunction in spontaneously hypertensive male rats after myocardial infarction

Background

This work evaluated the hypothesis that 3,4-methylenedioxybenzoyl-2-thienylhydrazone (LASSBio-294), an agonist of adenosine A_2A receptor, could be beneficial for preventing cardiac dysfunction due to hypertension associated with myocardial infarction (MI).

Methods

Male spontaneously hypertensive rats (SHR) were randomly divided into four groups (six animals per group): sham-operation (SHR-Sham), and myocardial infarction rats (SHR-MI) were treated orally either with vehicle or LASSBio-294 (10 and 20 mg.kg⁻¹.d⁻¹) for 4 weeks. Echocardiography and in vivo hemodynamic parameters measured left ventricle (LV) structure and function. Exercise tolerance was evaluated using a treadmill test. Cardiac remodeling was accessed by LV collagen deposition and tumor necrosis factor α expression.

Results

Early mitral inflow velocity was significantly reduced in the SHR-MI group, and there was significant recovery in a dose-dependent manner after treatment with LASSBio-294. Exercise intolerance observed in the SHR-MI group was prevented by 10 mg.kg⁻¹.d⁻¹ of LASS-Bio-294, and exercise tolerance exceeded that of the SHR-Sham group at 20 mg.kg⁻¹.d⁻¹. LV end-diastolic pressure increased after MI, and this was prevented by 10 and 20 mg.kg⁻¹.d⁻¹ of LASSBio-294. Sarcoplasmic reticulum Ca²⁺ ATPase levels were restored in a dose-dependent manner after treatment with LASSBio-294. Fibrosis and inflammatory processes were also counteracted by LASSBio-294, with reductions in LV collagen deposition and tumor necrosis factor α expression.

Conclusion

In summary, oral administration of LASSBio-294 after MI in a dose-dependent manner prevented the development of cardiac dysfunction, demonstrating this compound’s potential as an alternative treatment for heart failure in the setting of ischemic heart disease with superimposed chronic hypertension.

Streptococcus suis adenosine synthase functions as an effector in evasion of PMN-mediated innate immunit

Streptococcus suis serotype 2 (S. suis 2) is a highly invasive pathogen in pigs and humans that can cause severe systemic infection. Sepsis and meningitis are the most common clinical manifestations of S. suis 2 infection. However, the mechanisms of S. suis 2 surviving in human blood remains unclear, so to identify novel virulence factors in evasion of polymorphonuclear leukocyte (PMN)-mediated innate immunity play important roles in developing therapies against S. suis 2 infection. Here, we found that S. suis 2 can escape phagocytic clearance by adenosine synthesis in blood. Through bioinformatics-based analyses we identified a cell wall-anchored protein harbors a 5′-nucleotidase signature sequence and evidence strongly indicated that it can convert adenosine monophosphate (AMP) to adenosine. It was designated as Ssads (the adenosine synthase of S. suis 2). Furthermore, we found that Ssads could impair PMN's defense against S. suis 2 with decreasing of oxidative activity and degranulation of PMNs in human blood via A₂a receptors. Additionally, this enzyme-deficient mutant was found to have diminished virulence in the piglet infection model. Taken together, these results indicate that Ssads play an important role in S. suis 2 escaping human innate immunity in the context of inhibiting PMN's activity by synthesis of adenosine.

Dissociations in the effects of β2-adrenergic receptor agonists on cAMP formation and superoxide production in human neutrophils: support for the concept of functional selectivity

In neutrophils, activation of the β2-adrenergic receptor (β2AR), a Gs-coupled receptor, inhibits inflammatory responses, which could be therapeutically exploited. The aim of this study was to evaluate the effects of various β2AR ligands on adenosine-3',5'-cyclic monophosphate (cAMP) accumulation and N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP)-induced superoxide anion (O2(•-)) production in human neutrophils and to probe the concept of ligand-specific receptor conformations (also referred to as functional selectivity or biased signaling) in a native cell system. This is an important question because so far, evidence for functional selectivity has been predominantly obtained with recombinant systems, due to the inherent difficulties to genetically manipulate human native cells. cAMP concentration was determined by HPLC/tandem mass spectrometry, and O2(•-) formation was assessed by superoxide dismutase-inhibitable reduction of ferricytochrome c. β2AR agonists were generally more potent in inhibiting fMLP-induced O2(•-) production than in stimulating cAMP accumulation. (-)-Ephedrine and dichloroisoproterenol were devoid of any agonistic activity in the cAMP assay, but partially inhibited fMLP-induced O2(•-) production. Moreover, (-)-adrenaline was equi-efficacious in both assays whereas the efficacy of salbutamol was more than two-fold higher in the O2(•-) assay. Functional selectivity was visualized by deviations of ligand potencies and efficacies from linear correlations for various parameters. We obtained no evidence for involvement of protein kinase A in the inhibition of fMLP-induced O2(•-) production after β2AR-stimulation although cAMP-increasing substances inhibited O2(•-) production. Taken together, our data corroborate the concept of ligand-specific receptor conformations with unique signaling capabilities in native human cells and suggest that the β2AR inhibits O2(•-) production in a cAMP-independent manner.

Evidence for ligand-specific conformations of the histamine H(2)-receptor in human eosinophils and neutrophils

The histamine H(2)-receptor (H(2)R) couples to G(S)-proteins and induces adenylyl cyclase-mediated cAMP accumulation. In human neutrophils and eosinophils, the H(2)R reduces chemotactic peptide-stimulated superoxide anion (O(2)(-)) formation. However, pharmacological characterization of the H(2)R in these cells is far from being complete. The aim of this study was to provide a comprehensive profiling of the H(2)R in neutrophils and eosinophils. Histamine inhibited O(2)(-) formation in human neutrophils more effectively than in eosinophils. H(2)R agonists mimicked the effects of histamine and H(2)R antagonists blocked the effects of histamine. We noticed multiple discrepancies in the potencies and efficacies of H(2)R agonists with respect to cAMP accumulation and inhibition of O(2)(-) formation in both cell types. There were also differences in the antagonist profiles between cAMP accumulation and inhibition of O(2)(-) formation in neutrophils. Moreover, the pharmacological profile of the recombinant H(2)R did not match the H(2)R profile in native cells. The H(2)R sequence identified in human neutrophils corresponds to the published H(2)R sequence, excluding the exclusive expression of a new H(2)R isoform as explanation for the differences. Very likely, the differences between ligands are explained by the existence of ligand-specific receptor conformations with unique affinities, potencies and efficacies. Thus, our data provide evidence for the notion that the concept of ligand-specific receptor conformations can be extended from recombinant systems to native cells.

Effects of stimulation of adenosine A2A receptors on lipopolysaccharide-induced production of reactive oxygen species by equine neutrophils

OBJECTIVE

To assess the anti-inflammatory effects of an adenosine analogue on lipopolysaccharide (LPS)-stimulated equine neutrophils.

SAMPLE POPULATION

Neutrophils obtained from 10 healthy horses.

PROCEDURES

An adenosine analogue (5'-N-ethylcarboxamidoadenosine [NECA]) was tested for its ability to inhibit production of reactive oxygen species (ROS) in LPS-stimulated equine neutrophils. Selective adenosine receptor antagonists were used to identify the receptor subtype responsible for effects. To assess the mechanism of action of NECA, cAMP concentrations were measured, and effects of dibutyryl cAMP (a stable analogue of cAMP) and rolipram (a type 4 phosphodiesterase inhibitor) were investigated.

RESULTS

NECA elicited concentration-dependent inhibition of ROS production that was inhibited by ZM241385, a selective adenosine A(2A) receptor antagonist; this effect of NECA was not affected by the adenosine A(2B) receptor antagonist MRS1706. Also, ZM241385 blocked NECA-induced increases in cAMP concentrations, whereas MRS1706 did not alter this effect of NECA. Rolipram potentiated NECA-induced inhibition of ROS production, and dibutyryl cAMP also inhibited ROS production.

CONCLUSIONS AND CLINICAL RELEVANCE

Activation of adenosine A(2A) receptors inhibited ROS production by LPS-stimulated equine neutrophils in a cAMP-dependent manner. These results suggest that stable adenosine A(2A) receptor agonists may be developed as suitable anti-inflammatory drugs in horses.

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.

[3H]PK11195 binding sites in human neutrophils: effect of fMLP stimulation and modulation in rheumatic diseases

OBJECTIVE

The objectives of this study were to evaluate the [3H]PK11195 binding parameters in a model of acute inflammation, the N-formylmethionine-leucine-phenylalanine (fMLP)-stimulated neutrophil cell membranes, and to analyze if alterations of peripheral-type benzodiazepine receptor (PBR) characteristics occurred in neutrophil cell membranes of patients affected by osteoarthritis (OA), rheumatoid arthritis (RA), and psoriasic arthritis (PA).

DESIGN AND METHODS

Neutrophils were obtained from 15 patients with OA, 15 patients with RA, and 15 patients with PA. fMLP stimulation was performed to aliquots of neutrophils from six healthy individuals. Evaluation of kinetic parameters of PBR was performed using [3H]PK11195, as specific radioligand compared with 15 healthy volunteers.

RESULTS

The results showed a significant decrease of Kd and Bmax in fMLP-stimulated neutrophil membranes. Moreover, an increase of PBR binding sites and affinity value was observed in neutrophils membranes from PA patients.

CONCLUSIONS

Our data suggested a fMLP modulation on [3H]PK11195 binding in human neutrophils. Moreover, our results showed an up-regulation of PBR in neutrophils of PA patients.

Regulation of adenosine receptor subtypes during cultivation of human monocytes: role of receptors in preventing lipopolysaccharide-triggered respiratory burst

Adenosine is a potent anti-inflammatory agent that modulates the function of cells involved in the inflammatory response. Here we show that it inhibits lipopolysaccharide (LPS)-induced formation of reactive oxygen intermediates (ROI) in both freshly isolated and cultured human monocytes. Blocking of adenosine uptake and inactivation of the adenosine-degrading enzyme adenosine deaminase enhanced the inhibitory action of adenosine, indicating that both pathways regulate the extracellular adenosine concentration. Adenosine-mediated inhibition could be reversed by XAC (xanthine amine congener), an antagonist of the adenosine receptor A(2A), and MRS 1220 [N-9-chloro-2-(2-furanyl)[1, 2, 4]-triazolo[1,5-c]quinazolin-5-benzeneacetamide], an A(3) receptor antagonist, in both cell populations, while DPCPX (1,3-dipropyl-8-cyclopentylxanthine), an A(1) receptor antagonist, had no effect. Similar to what was seen with adenosine, CGS 21680, an A(2A) and A(3) receptor agonist, and IB-MECA, a nonselective A(1) and A(3) receptor agonist, dose dependently prevented ROI formation, indicating the involvement of A(3) and probably also A(2A) in the suppressive effect of adenosine. Pretreatment of monocytes with adenosine did not lead to changes in the LPS-induced increase in intracellular calcium levels ([Ca(2+)](i)). Thus, participation of [Ca(2+)](i) in the action of adenosine seems unlikely. The adenosine-mediated suppression of ROI production was found to be more pronounced when monocytes were cultured for 18 h, a time point at which changes in the mRNA expression of adenosine receptors were observed. Most prominent was the increase in the A(2A) receptor mRNA. These data demonstrate that cultivation of monocytes is accompanied by changes in the inhibitory action of adenosine mediated by A(3) and probably also the A(2A) receptor and that regulation of adenosine receptors is an integral part of the monocyte differentiation program.

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.

Role of adenosine in immunomodulation: Review of the literature

OBJECTIVE

Advances in the understanding of sepsis have failed to deliver satisfactory new treatments aimed at attenuating inflammatory-mediated organ dysfunction. Phagocytic cells play a pivotal role in driving the inflammatory response and causing direct tissue injury. Adenoreceptor stimulation may attenuate such inflammatory-mediated damage by down-regulating phagocytic activity and preventing excessive respiratory burst activation.

DATA

A Medline database was used to perform a literature search for all articles relating to the use of adenosine as an immunomodulatory agent.

CONCLUSION

There is convincing evidence to suggest that adenoreceptor modulation can prevent tissue injury through a variety of pathways. The use of adenosine modulation in ischemia/reperfusion injury has been the subject of considerable investigation, although experience with its use in sepsis is limited.

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.

Adenosine up-regulates cyclooxygenase-2 in human granulocytes: impact on the balance of eicosanoid generation

Polymorphonuclear neutrophils (granulocytes; PMNs) are often the first blood cells to migrate toward inflammatory lesions to perform host defense functions. PMNs respond to specific stimuli by releasing several factors and generate lipid mediators of inflammation from the 5-lipoxygenase and the inducible cyclooxygenase (COX)-2 pathways. In view of adenosine's anti-inflammatory properties and suppressive impact on the 5-lipoxygenase pathway, we addressed in this study the impact of this autacoid on the COX-2 pathway. We observed that adenosine up-regulates the expression of the COX-2 enzyme and mRNA. Production of PGE(2) in response to exogenous arachidonic acid was also increased by adenosine and correlated with COX-2 protein levels. The potentiating effect of adenosine on COX-2 could be mimicked by pharmacological increases of intracellular cAMP levels, involving the latter as a putative second messenger for the up-regulation of COX-2 by adenosine. Specific COX-2 inhibitors were used to confirm the predominant role of the COX-2 isoform in the formation of prostanoids by stimulated PMNs. Withdrawal of extracellular adenosine strikingly emphasized the inhibitory potential of PGE(2) on leukotriene B(4) formation and involved the EP(2) receptor subtype in this process. Thus, adenosine may promote a self-limiting regulatory process through the increase of PGE(2) generation, which may result in the inhibition of PMN functions. This study identifies a new aspect of the anti-inflammatory properties of adenosine in leukocytes, introducing the concept that this autacoid may exert its immunomodulatory activities in part by modifying the balance of lipid mediators generated by PMNs.

Cyclic AMP-dependent inhibition of human neutrophil oxidative activity by substituted 2-propynylcyclohexyl adenosine A(2A) receptor agonists

Novel 2-propynylcyclohexyl-5'-N:-ehtylcarboxamidoadenosines, trans-substituted in the 4-position of the cyclohexyl ring, were evaluated in binding assays to the four subtypes of adenosine receptors (ARs). Two esters, 4-(3-[6-amino-9-(5-ethylcarbamoyl-3,4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl]-prop-2-ynyl)-cyclohexanecarboxylic acid methyl ester (ATL146e) and acetic acid 4-(3-[6-amino-9-(5-ethylcarbamoyl-3, 4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl] -prop-2-ynyl)-cyclohexylmethyl ester (ATL193) were >50 x more potent than 2-[4-(2-carboxyethyl)phenethylamino]-5'-N:-ethylcarboxamidoadenosine (CGS21680) for human A(2A) AR binding. Human A(2A) AR affinity for substituted cyclohexyl-propynyladenosine analogues was inversely correlated with the polarity of the cyclohexyl side chain. There was a comparable order of potency for A(2A) AR agonist stimulation of human neutrophil [cyclic AMP](i), and inhibition of the neutrophil oxidative burst. ATL146e and CGS21680 were approximately equipotent agonists of human A(3) ARs. We measured the effects of selective AR antagonists on agonist stimulated neutrophil [cyclic AMP](i) and the effect of PKA inhibition on A(2A) AR agonist activity. ATL193-stimulated neutrophil [cyclic AMP](i) was blocked by antagonists with the potency order: ZM241385 (A(2A)-selective)>MRS1220 (A(3)-selective)>>N-(4-Cyano-phenyl)-2-[4-(2,6-dioxo-1,3-dipropyl-2,3,4,5,6,7-hexahydro-1H-purin-8-yl)-phenoxy]-acetamide (MRS1754; A(2B)-selective) approximately 8-(N-methylisopropyl)amino-N(6)-(5'-endohydroxy-endonorbornyl)-9-methyladenine (WRC0571; A(1)-selective). The type IV phosphodiesterase inhibitor, rolipram (100 nM) potentiated ATL193 inhibition of the oxidative burst, and inhibition by ATL193 was counteracted by the PKA inhibitor H-89. The data indicate that activation of A(2A)ARs inhibits neutrophil oxidative activity by activating [cyclic AMP](i)/PKA.

Activation of human neutrophils with chemotactic peptide, opsonized zymosan and the calcium ionophore A23187, but not with a phorbol ester, is accompanied by efflux and store-operated influx of calcium

The objective of this study was to monitor alterations in cellular Ca2+ metabolism following activation of neutrophils with receptor- (chemotactic peptide, FMLP, 1 microM; opsonized zymosan, OZ, 0.5 mg/ml) and non-receptor (calcium ionophore, A23187, 1 microM; phorbol 12-myristate 13-acetate, PMA, 25 ng/ml)-mediated stimuli of the pro-inflammatory functions of these cells. Ca2+ fluxes in activated neutrophils were measured using a fura-2-based spectrofluorimetric method in combination with radiometric (45Ca) procedures which facilitate distinction between net efflux and net influx of the cation. Exposure of neutrophils to receptor-mediated stimuli and to A23187 was associated with an abrupt increase in cytosolic Ca2+ coincident with a rapid efflux of the cation which terminated at around 30 s. In the case of FMLP and OZ, this was followed by a delayed (30-60 s), store-operated influx of Ca2+, which was complete at around 5 min after addition of the stimulus. With A23187, however, influx of Ca2+ occurred immediately following activation of the cells. There were no detectable alterations in cytosolic Ca2+ or measurable net efflux or influx of the cation above control levels in PMA-activated neutrophils. These data demonstrate that FMLP, OZ- and A23187-mediated alterations in neutrophil cytosolic Ca2+ are due to mobilization of both intracellular and extracellular cation, while activation of neutrophils by PMA is independent of alterations in cytosolic Ca2+.

A novel adenosine analog, AMP579, inhibits neutrophil activation, adherence and neutrophil-mediated injury to coronary vascular endothelium

We hypothesized that 1S-[1a,2b,3b, 4a(S*)]-4-[7-[[1-[(3-chloro-2-thienyl)methylpropyl]propyl-amino]-3H-i midazo[4,5-b] pyridyl-3-yl]-N-ethyl-2,3-dihydroxycyclopentane carboxamide (AMP579), a new adenosine analog, inhibits superoxide anion (O(2)(-)) generation and degranulation from canine neutrophils, neutrophil adherence and neutrophil-induced dysfunction to canine coronary artery endothelium by adenosine receptor-mediated mechanisms. AMP579 inhibited O(2)(-) generation (nM/20x10(6) neutrophils) from platelet activating factor (PAF)-activated neutrophil in concentration-dependent manner (4.1+/-0.8 at 10 microM vs. 16.7+/-2.1 in PAF group, P<0.05). This inhibitory effect was blocked by the adenosine A(2A) receptor-selective antagonist, 4-(2-[7-Amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3, 5]triazin-5-ylamino]ethyl)phenol (ZM241385, 17.7+/-2.8, P<0.05), but not by either the adenosine A(1) receptor-selective antagonist, 8-Cyclopentyl-1.3-dipropylxanthine (DPCPX) or the adenosine A(3) receptor-selective antagonist, 9-Chloro-2-(2-furanyl)-5-[(phenylacetyl)amino][1,2,4]-triazolo[1, 5-c]quinazoline (MRS1220). AMP579 inhibited neutrophil degranulation dose-dependently by 38+/-2% at 10 microM (P<0.05). The inhibitory effect of AMP579 was not altered by either DPCPX or MRS1220, but was partially reversed by ZM241385 (69+/-8%, P<0.05 vs. AMP579 10 microM). A total of 10 microM AMP579 reduced neutrophil adherence to thrombin-stimulated endothelium (neutrophils/mm(2)) from 269+/-16 to 44+/-4 (P<0.05); this was reversed by ZM241385, but not by DPCPX or MRS1220. After coincubation of unstimulated neutrophil with thrombin-stimulated endothelium, concentration-relaxation responses to the endothelium receptor-dependent vasodilator, acetylcholine, were reduced (maximum 57+/-5% vs. 120+/-5% in controls, P<0.05). This endothelial dysfunction was attenuated by AMP579 (116+/-7%, P<0. 05). We conclude that AMP579 inhibits neutrophil activation and neutrophil-mediated coronary endothelial dysfunction, primarily by an adenosine A(2A) receptor mechanism.

A peptide of the alpha 3(IV) chain of type IV collagen modulates stimulated neutrophil function via activation of cAMP-dependent protein kinase and Ser/Thr protein phosphatase

Previous reports from our laboratories showed that type IV collagen from anterior lens capsule (ALC) inhibited stimulated neutrophil function. This property was shown to reside in the region comprising residues 185-203 of the non-collagenous domain (NC1) of the alpha 3(IV) chain. We also reported that ALC-type IV collagen or the synthetic alpha 3(IV) 185-203 peptide, induced a rise in intracellular cAMP which persisted for up to 60 minutes. In the present work we extend our previous studies on signal transduction by alpha 3(IV) 185-203 and we provide new data showing the involvement of cAMP-dependent PKA and protein phosphatases. The data also show that the alpha 3(IV) peptide triggered a rise in intracellular calcium that was dependent on phospholipase C activation. Inhibitors of the Ca(2+)/calmodulin system suppressed both the alpha 3(IV) 185-203 peptide-induced cAMP increase and the inhibitory activity of the peptide on f-Met-Leu-Phe triggered O(2)(-) generation. When alpha 3(IV) 185-203 peptide-induced calcium mobilization was blocked by U-73122, an inhibitor of phospholipase C activation, or by BAPTA/AM, a chelator of intracellular calcium, the inhibitory effect of the peptide on PMA-triggered O(2)(-) production was also abolished. These findings provide evidence that signal transduction by the alpha 3(IV) peptide occurs via pathways which involve calcium. Indeed, the cAMP increase was shown to be mediated by adenosine and adenosine A2 receptors and required calcium elevation, since adenosine deaminase, theophilline, dimethylpropargylxanthine, trifluoperazine or autocamtide-2 related inhibitory peptide, suppressed the activity of the alpha 3(IV) peptide. The inhibitory effect of the peptide on f-Met-Leu-Phe-induced O(2)(-) generation was slightly affected by 1 microM KT5720 or H89, two inhibitors of cAMP-dependent PKA, but was completely suppressed by 10 nM calyculin A or 10 microM okadaic acid, two inhibitors of ser/thr phosphatases. These results suggest that Ser/Thr protein phosphatases and/or cAMP-dependent PKA are involved in signal transduction by the alpha 3(IV) 185-203 peptide and is consistent with the concept that adenosine receptor occupancy modulates neutrophil function.

Adenosine receptor occupancy suppresses chemoattractant-induced phospholipase D activity by diminishing membrane recruitment of small GTPases

Adenosine (Ado) is an important autocrine modulator of neutrophil functions. In this study, we determined the effects of endogenous Ado on fMet-Leu-Phe (fMLP)–induced phospholipase D (PLD) activity in neutrophils. The removal of extracellular Ado by Ado deaminase (ADA) or the blockade of its action by the A2a receptor antagonists 8-(3-chlorostyryl) caffeine (CSC) or CGS15943 markedly increased fMLP-induced PLD activation. The concentration-dependent stimulatory effects of CSC and CGS15943 were abolished by a pretreatment of neutrophil suspensionswith ADA. In contrast, the selective A2a receptor agonist CGS21680 suppressed fMLP-induced PLD activation. Furthermore, inhibition by CGS21680 of fMLP-induced PLD activity was reversed by CSC or CGS15943. The removal of Ado by ADA or the blockade of its action by CSC or CGS15943, markedly increased the membrane recruitment of cytosolic protein kinase C (PKC), RhoA, and ADP-ribosylation factor (ARF) in response to fMLP. As shown for PLD activity, the stimulatory effect of Ado receptor antagonists on PLD cofactors translocation was abolished by a pretreatment of the cells with ADA. Moreover, the membrane translocation of both PKC, RhoA, and ARF in response to fMLP was attenuated by CGS21680 and this effect of the A2a receptor agonist was antagonized by CSC or CGS15943. These data demonstrate that Ado released by neutrophils in the extracellular milieu inhibits PLD activation by blocking membrane association of ARF, RhoA, and PKC through Ado A2a receptor occupancy.

Modulation of the chemotactic peptide- and immunoglobulin G-triggered respiratory burst in human neutrophils by exogenous and endogenous adenosine

The effects of exogenous and endogenous adenosine on the production of oxygen metabolites in neutrophils triggered by the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP) or immunoglobulin G (IgG)-opsonized yeast particles, were investigated. By using luminol-enhanced chemiluminescence, we found that adenosine A1 receptor activation did not affect, whereas adenosine A receptor activation, through a mechanism involving the cyclic AMP (cAMP)-protein kinase A signalling pathway, both inhibited the fMLP- and IgG-triggered respiratory burst. The adenosine-induced inhibition was however more pronounced after exposure to fMLP than to IgG-yeast. Stimulation with fMLP caused an extracellular accumulation of endogenous adenosine, which indicates that this event is a negative-feedback mechanism preventing an uncontrolled activation of chemoattractant-stimulated neutrophils. On the contrary, exposure of neutrophils to IgG-yeast did not appear to accumulate extracellular adenosine, probably due to increased adenosine deaminase activity during phagocytosis. In conclusion, this work accentuates the importance of adenosine, both exogenously applied and endogenously formed, as an inflammatory agent modulating the respiratory burst during the different phases in neutrophil activation.

Differential regulation of human neutrophil FcgammaRIIa (CD32) and FcgammaRIIIb (CD16)-induced Ca2+ transients

Human neutrophils express two structurally distinct receptors for the Fc region of IgG, FcgammaRIIa and FcgammaRIIIb. Although earlier studies have suggested that the functional properties of these receptors are similar, mounting evidence suggests that these receptors are capable of inducing distinct functional responses. Accordingly, we have examined the regulation of intracellular Ca2+ transients induced by each of these receptors alone (homotypic receptor cross-linking) and together (heterotypic receptor cross-linking). The glycosylphosphatidylinositol-anchored FcgammaRIIIb induces a rise in [Ca2+] after homotypic cross-linking that is independent of ligand-mediated engagement of the transmembrane FcgammaRIIa. Both receptors were sensitive to the protein-tyrosine kinase inhibitor methyl 2,5-dihydroxycinnamate, but FcgammaRIIa-induced signaling was uniquely sensitive to the protein-tyrosine kinase inhibitor genistein. FcgammaRIIa but not FcgammaRIIIb engages a cAMP-sensitive and inositol 1,4, 5-trisphosphate-dependent pathway(s) that results in the Ca2+-transient. When these receptors are cross-linked into heterotypic clusters, a synergistic rise in [Ca2+] is observed that is accompanied by synergistic increases in the phospholipase Cgamma-breakdown products inositol 1,4,5-trisphosphate and diglyceride. These data provide a mechanism for the functional differences between these two receptors and suggest the possibility that they can be differentially modulated.

Adenosine inhibits actin dynamics in human neutrophils: evidence for the involvement of cAMP

The mechanisms by which adenosine regulates the inflammatory reaction are poorly characterized. In this study, we investigated the effects of adenosine on neutrophil actin polymerization elicited by the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP) or IgG-opsonized yeast particles. We used bodipy-phallacidin staining in combination with flow cytometry and found that adenosine markedly reduced actin polymerization triggered by IgG-yeast, whereas the effect on the fMLP-response was less pronounced. Similar or even more pronounced effects were obtained with the adenosine A2 receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA), suggesting an A2 receptor-mediated mechanism. The following observations indicate that the A2 receptor-induced effects involve the cAMP-protein kinase A (PKA) signaling pathway: (1) a combination of NECA and the cAMP-specific phosphodiesterase (PDE) inhibitor Ro 20-1724 raised the cAMP content in both unstimulated and stimulated neutrophils and also further inhibited the actin dynamics; (2) the PKA inhibitor H89 reversed the inhibitory effects of NECA on the actin dynamics; (3) Ro 20-1724, isoproterenol and dibutyryl cAMP (DBcAMP) reduced actin polymerization in almost the same way as NECA did. NECA together with Ro 20-1724 impaired the fMLP-induced shape changes and cortical accumulation of actin filaments. In contrast, H89 potentiated the fMLP-induced formation of a submembranous ring of actin filaments. Neutrophils phagocytosing yeast particles in the presence of NECA and Ro 20-1724 were predominantly round in shape, and their ability to extend actin-rich pseudopods around the prey was reduced. These effects were partly antagonized by H89. In correlation with the effects on actin polymerization, NECA more effectively diminished IgG-induced upregulation of the beta2 integrin CD11b/CD18 than such upregulation induced by fMLP. The inhibitory effects of A2-receptor activation on actin dynamics and beta2 integrin expression in neutrophils exposed to IgG-yeast were also associated with a cAMP-dependent reduction of the phagocytic capacity. In conclusion, we show that adenosine inhibits actin dynamics and shape changes in neutrophils via a cAMP-dependent pathway. This finding further characterizes the mechanisms by which adenosine functions as an important modulator of the inflammatory response.

Adenosine inhibits superoxide production in rat peritoneal macrophages via elevation of cAMP level

As a part of host-defense immune system, macrophages can response to a variety of stimulants and produce superoxide. We examined the effect of adenosine as a modulator on superoxide production induced by phorbol ester in rat peritoneal macrophages, using an acetyl-cytochrome c reduction method for its detection. 2-Cl-adenosine, a least metabolizable analog of adenosine, inhibited the superoxide production in a dose-dependent manner, and also showed the increasing effect on intracellular cAMP level. Superoxide production was also inhibited by the several reagents which increased intracellular cAMP level, including dibutyryl-cAMP. 8-bromo-cAMP (cell permeable cAMP analogs), forskolin (an adenyl-cyclase activator). Ro 20-1724 (an phosphodiesterase inhibitor), and propentofylline (a xanthine derivative), but not 8-bromo-cGMP (cell permeable cGMP analog). These results suggest that a high level of extracellular adenosine may attenuate immunity through regulating macrophage functions. On the other hand, the tissue damage which is resulted from an over-production of superoxide can be protected by adenosine and its related drugs via an elevation of intracellular cAMP level.

Purines and neutrophil leukocytes

1. Adenosine is a normal constituent of all body fluids and its levels are raised, for example, by hypoxia and ischemia. In addition, both adenosine and ATP can be released by endothelial cells and neutrophils in response to physiologic stimulation. 2. Human neutrophil leukocytes possess multiple adenosine receptors and P2 purinoceptors. 3. ATP can increase intracellular Ca2+ levels in neutrophils, cause degranulation and enzyme release, potentiate the oxidative burst and enhance their adhesion to the endothelium. ATP is broken down to adenosine by ecto-enzymes. Via A1 receptors, adenosine can increase neutrophil chemotaxis and, via A2A receptors, it can decrease the oxidative burst, degranulation and adhesion to endothelium. 4. Adenosine and adenine nucleotides are important endogenous modulators of neutrophil functions, and drugs may exert important actions via purinoceptors on neutrophil leukocytes.

Adenosine A2 receptor occupancy regulates stimulated neutrophil function via activation of a serine/threonine protein phosphatase

Adenosine modulates generation of superoxide anion by neutrophils via occupancy of specific adenosine A2A receptors. However, the intracellular signal transduction pathways by which occupancy of neutrophil adenosine A2A receptors inhibits superoxide anion generation (O2.-) are not well understood. We, therefore, tested the hypothesis that signaling at polymorphonuclear leukocyte (PMN) adenosine receptors proceeds via activation of a serine/threonine protein phosphatase (pp). Both the specific pp1 inhibitor calyculin A (10 nM) and the pp2A inhibitor okadaic acid (10 microM) enhanced O2.- generation (185 +/- 24 and 189 +/- 35% of control, respectively, p < 0.0001 for both, n = 8), as reported previously. Calyculin A, but not okadaic acid, completely reversed inhibition of stimulated O2.- generation by the adenosine A2 receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA; IC50 = 30 nM; p < 0.0001, analysis of variance). Calyculin A also reversed the adenosine receptor-mediated desensitization of bound chemoattractant receptors in neutrophils. Treatment of PMNs with NECA increased the pp1 activity of crude membrane preparations in a time- and dose-dependent fashion (EC50 = 40 nM; p < 0.001, analysis of variance, n = 5). NECA inhibited cytosolic protein phosphatase activity by 78 +/- 12% (p < 0.003, n = 6) but did not shift pp1 catalytic subunit from cytosol to plasma membrane. Similar changes were observed in neutrophil cytoplasts depleted of organelles and nucleus. Moreover, the selective protein kinase A inhibitor KT5720 (10 microM) reversed the capacity of dibutyryl cAMP but not NECA to increase pp1 activity (p < 0.01, n = 5) in keeping with its effects on O2.- generation. Western blot analysis of PMN subcellular fractions demonstrated the presence of pp1alpha and pp1gamma1 but not pp1gamma2 isotypes in both cytosol and plasma membrane but not in azurophil or specific granules. We conclude from these studies that signal transduction by adenosine in PMN proceeds via a novel pathway: cAMP-independent activation of a serine/threonine protein phosphatase in the plasma membrane.

Adenosine modulation of tumor necrosis factor-alpha-induced neutrophil activation

We hypothesized that adenosine, known to be release from inflammatory sites, could lessen the potentially damaging activity of neutrophils (PMN) primed by tumor necrosis factor-alpha (TNF alpha) at sites of infection. We investigated the effect of adenosine on PMN primed with cell-free medium from mononuclear leukocytes (MNL) that had been treated with lipopolysaccharide (LPS) yielding a conditioned medium rich in TNF alpha and on PMN primed with recombinant human TNF alpha (rhTNF alpha). LPS (10 ng/mL) minimally primed PMN, but LPS-MNL-conditioned medium increased PMN chemiluminescence in response to f-Met-Leu-Phe (fMLP) 1242% compared with unprimed PMN. LPS-MNL-conditioned medium contained adenosine (approximately 30 nM). Converting the adenosine in the LPS-MNL-conditioned medium to inosine with adenosine deaminase (ADA) or blocking adenosine binding to PMN with the adenosine receptor antagonist 1,3-dipropyl-8-(phenyl-p-acrylate)-xanthine (BW A1433U) resulted in a near doubling of chemiluminescence. The LPS-MNL-conditioned medium contained TNF alpha (836 pg/mL; approximately 1 U/mL). Recombinant human TNF alpha (1 U/mL) primed PMN for a 1033% increase in chemiluminescence. Added adenosine decreased rhTNF alpha-primed PMN chemiluminescence (IC50 approximately 100 nM), and adenosine (100 nM) decreased both superoxide and myeloperoxidase release from rhTNF alpha-primed fMLP-stimulated PMN. The activity of adenosine was counteracted by ADA and BW A1433U, and the modulating effect of adenosine was on the primed response rather than on priming per se. Thus, physiological concentrations of adenosine reduce the effects of recombinant human TNF alpha and native human TNF alpha (released from LPS-treated MNL) on PMN activity. Endogenous adenosine may preclude or minimize damage to infected tissue by damping the TNF alpha-primed PMN oxidative response.

The specific type IV phosphodiesterase inhibitor rolipram combined with adenosine reduces tumor necrosis factor-alpha-primed neutrophil oxidative activity

Monocytes and macrophages produce tumor necrosis factor-alpha (TNF alpha) in response to microbial products including endotoxin. TNF alpha is a potent primer of neutrophil (PMN) oxidative activity. Certain xanthine phosphodiesterase (PDE) inhibitors such as pentoxifylline have been shown to inhibit stimulated oxidative activity in PMN. In the present study, the non-xanthine PDE type IV inhibitor rolipram (4-[3'-cyclopentyloxy-4'-methoxyphenyl]-2-pyrrolidone) alone and in combination with adenosine is examined as a potential modulator of TNF alpha-primed PMN oxidative activity. Attainable in vivo concentrations of rolipram and physiological concentrations of adenosine alone and together synergistically decreased rhTNF alpha-primed suspended PMN oxidative activity stimulated by the chemoattractant f-met-leu-phe. The rolipram effect was reversible by washing, and rolipram had a comparable effect if added before or after priming, indicating that its effect was on the primed response rather than on priming per se. In addition, rolipram especially when combined with adenosine, decreased rhTNF alpha-stimulated PMN adherence to a fibrinogen-coated surface, and the oxidative burst of rhTNF alpha-stimulated adherent PMN. The specific adenosine A2a receptor agonists CGS 21680 and WRC-0474 had comparable activity to adenosine in these experiments. Adenosine (or CGS 21680) combined with rolipram synergistically increased f-met-leu-phe-stimulated PMN cAMP content. The effects of both adenosine and rolipram with adenosine could be only partly counteracted by treatment of the PMN with the protein kinase A inhibitor KT 5720, indicating that protein phosphorylation is only partially involved. Rolipram activity was about 1000 x (by molar concentration) greater than pentoxifylline in comparable assays. Thus, rolipram, especially when combined with adenosine, has potent modulating effects on PMN activation and may be useful in decreasing inflammatory tissue damage in patients with sepsis.

Characterization of the PGE receptor subtype mediating inhibition of superoxide production in human neutrophils

1. The aims of this study were to characterize the EP receptor subtype mediating the inhibition of superoxide anion generation by formyl methionyl leucine phenylalanine (FMLP)-stimulated human neutrophils, and to test the hypothesis that adenosine 3':5'-cyclic monophosphate (cyclic AMP) is the second messenger mediating the inhibition of the neutrophil by prostaglandin (PG)E2. 2. PGE2 (0.001-10 microM) inhibited FMLP (100 nM)-induced O2-generation from human peripheral blood neutrophils in a concentration-dependent manner, with an EC50 of 0.15 +/- 0.03 microM, and a maximum effect ranging from 36-84% (mean inhibition of 68.7 +/- 2.5%, n = 32). 3. The EP2-receptor agonists, misoprostol, 11-deoxy PGE1, AH13205 and butaprost, all at 10 microM, inhibited O2- generation, causing 95.5 +/- 2.9%, 56.8 +/- 5.2%, 37.1 +/- 6.6% and 18.9 +/- 4.4% inhibition respectively, the latter two being much less effective than PGE2. Similarly, the EP1-receptor agonist, 17-phenyl PGE2 (10 microM), and the EP3/EP1-receptor agonist, sulprostone (10 microM), also inhibited O2- generation, causing 32.2 +/- 7.0% and 15.3 +/- 3.4% inhibition respectively. 4. The non-selective phosphodiesterase inhibitor, isobutyl methylxanthine (IBMX, 0.25 mM) inhibited the FMLP response by 54.5 +/- 5.0%. In addition, IBMX shifted concentration-effect curves for PGE2, misoprostol, 11-deoxy PGE1, butaprost, and AH 13205 to the left, to give EC50s of 0.04 +/- 0.03 (n = 13), 0.07 +/- 0.03 (n = 4), 0.08 +/- 0.03 (n = 4), 0.33 +/- 0.13 (n = 4) and 0.41 +/- 0.2 microM (n = 3) respectively, allowing equieffective concentration-ratios (EECs, PGE2 = 1) of 11.5, 5.3, 50.7 and 12.7 to be calculated. This agrees well with the relative potencies of these agonists at EP2 receptors.5. By contrast, even in the presence of IBMX (0.25 mM), sulprostone and 17-phenyl PGE2 were only effective at the highest concentration (10 microM), and gave EECs of > 700 and 486 respectively, suggesting that EP1 or EP3 receptors are not involved.6. The selective type IV phosphodiesterase inhibitor, rolipram at 2 and 10 nM did not inhibit the FMLP response, but at the higher concentration of 50 nM, it decreased the FMLP response by 46.6 +/-7.3%.However, rolipram shifted concentration-effect curves for PGE2 to the left to give EC50s of 0.06 +/-0.022,0.015 +/- 0.0, 0.012 +/- 0.006 microM at 2, 10 and 50 nM respectively, compared to the control EC50 of0.27+/- 0.09 microM for PGE2.7. The EP4/TP receptor blocking drug, AH 23848B (10 microM, 10 min) did not inhibit 02- generation by PGE2, but was found to potentiate significantly the effect of PGE2 at the lower concentrations of PGE2 tested (0.001-0.1 microM).8. The adenylate cyclase inhibitor, SQ 22,536 (0.1 mM, 2 min) reduced PGE2-induced inhibition of 02-production, giving an EC50 in the absence of SQ 22,536 of 0.24 +/- 0.1, and 1.9 +/- 1.1 AM in its presence.9. These results suggest that inhibition of superoxide generation by PGE2 is mediated by stimulation ofEP2 receptors and activation of adenylate cyclase, leading to the elevation of intracellular levels of cyclic AMP.

The role of cyclic AMP, calcium and filamentous actin in adenosine modulation of Fc receptor-mediated phagocytosis in human neutrophils

The role of cyclic AMP, calcium and filamentous actin (F-actin) content during adenosine modulation of Fc receptor (FcR)-mediated phagocytosis in adherent human neutrophils was investigated. Phagocytosis of IgG-opsonized yeast particles was found to be enhanced by pico- to nanomolar concentrations of adenosine or the A1-agonist N6-cyclopentyl-adenosine (CPA) but reduced by micromolar concentrations of adenosine or the A2-agonist 5'-N-ethylcarboxamidoadenosine (NECA). NECA, in the presence of the cAMP-specific phosphodiesterase inhibitor Ro 20-1724, increased the intracellular content of cAMP during phagocytosis. Ro 20-1724 potentiated the NECA-induced reduction of the phagocytic capacity. These observations indicate that cAMP elevations are involved in A2-receptor-mediated inhibition of phagocytosis. NECA, in the presence of Ro 20-1724, markedly enhanced the action polymerization associated with adhesion to the substrate and contact with the phagocytic prey. During advanced phagocytosis, however, the F-actin content reached levels clearly below those observed in control cells. This prolonged depolymerization phase correlated with the A2-receptor-induced cAMP elevation. Depletion of intracellular free calcium abolished the cAMP-elevating effects of NECA, and also completely abrogated the A1- and A2-receptor-mediated effects on phagocytosis. However, since NECA reduced the F-actin content even in Ca(2+)-depleted cells, A2-receptor-mediated inhibition of phagocytosis could not be directly coupled to changes in the overall content of F-actin. Our results indicate that adenosine modulates FcR-mediated phagocytosis in a calcium-dependent way, and does so through 'stimulatory' A1 and 'inhibitory' A2 receptors, and also that cAMP elevation is linked to the A2-receptor-induced inhibition of phagocytosis.

Acting via A2 receptors, adenosine inhibits the upregulation of Mac-1 (Cd11b/CD18) expression on FMLP-stimulated neutrophils

Modulation of neutrophil responses by adenosine may have an important role in limiting tissue injury during inflammation or ischemia-reperfusion. Mac-1 (CD11/CD18), a member of the leukocyte integrin family, participates in neutrophil adhesion to endothelium, in transendothelial migration, and in phagocytosis. Using monoclonal antibodies and flow cytometry, we investigated the effect of adenosine on the increase in plasma membrane expression of Mac-1 which occurs following stimulation of neutrophils with the chemotactic peptide N-formylmethionylleucylphenylalanine (FMLP). Adenosine and 5'N-ethylcarboxamido-adenosine, a potent A2 agonist, each produced a dose-dependent inhibition of as much as 50% of the increase in Mac-1 expression on neutrophils stimulated with FMLP, with an IC50 of approximately 1 nM. The effect of adenosine was blocked by 8-p-sulfophenyltheophylline, an adenosine-receptor antagonist, N6-cyclopentyl-adenosine, an A1-selective agonist, had no effect on FMLP-stimulated Mac-1 expression in the concentration range expected for its action on neutrophil adenosine receptors of the A1 type. We also found that dibutyryl cyclic adenosine monophosphate inhibited the upregulation of Mac-1, and that the effect of adenosine on Mac-1 expression was not reversed by colchicine or vinblastine. We conclude that adenosine acts via A2 receptors to inhibit the upregulation of Mac-1 expression of FMLP-stimulated neutrophils, and that A1 receptors are not involved. This effect of adenosine may help to limit Mac-1-dependent neutrophil exudation at sites of inflammation or ischemia-reperfusion.

Functional characterization of three adenosine receptor types

1. The purpose of the present study was to classify adenosine receptors into A1 and A2 subtypes in a wide range of isolated tissues and cell types (rat adipocytes and atria, guinea-pig ileum and atria (A1); guinea-pig aorta, dog coronary artery and human platelets and neutrophils (A2)) using the R- and S-diastereoisomers of N-phenylisopropyladenosine (PIA), N-cyclopentyladenosine (CPA), the novel compound, N-[(1S,trans)-2-hydroxycyclopentyl]adenosine (GR79236), N-[(2-methylphenyl)methyl]adenosine (metrifudil), 2-(phenylamino)adenosine (CV1808), and 2[[2-[4-(2-carboxyethyl)phenyl]ethyl]amino]-N- ethylcarboxamidoadenosine (CGS21680); N-ethylcarboxamidoadenosine (NECA) was used as a standard. 2. Results obtained in all tissue preparations previously reported to contain A1-receptors could be described by a single rank order of agonist potency: CPA > or = GR79236, R-PIA > or = NECA >> S-PAI > or = metrifudil > or = CV1808, CGS21680. 3. In contrast, two distinct rank orders of agonist potency were observed in preparations previously reported to contain A2-receptors. In dog coronary artery, human neutrophils and platelets the rank order of potency was: CV1808, CGS21680 > or = NECA > R-PIA > or = metrifudil > or = CPA > GR79236 S-PIA. However, in guinea-pig aorta the rank order was: NECA > metrifudil > R-PIA, CPA > CV1808, GR79236 > or = S-PIA, CGS21680. 4. The results of this study are consistent with the existence of three types of adenosine receptor: A1-and two subtypes of A2-receptor. The receptor present in dog coronary artery, human platelets and neutrophils, probably corresponds to the A2a subtype, whilst that present in the guinea-pig aorta may be of the A2b subtype.

Adenosine inhibits fMLP-stimulated adherence and superoxide anion generation by human neutrophils at an early step in signal transduction

The ability of physiological concentrations of adenosine to inhibit formylmethionylleucylphenylalanine (fMLP)-stimulated superoxide anion (O2-) generation, adherence and degranulation is well established in human neutrophils. However, the mechanism of inhibition remains to be determined. To better understand where adenosine blocks the fMLP signal transduction pathway, we examined the ability of adenosine to inhibit neutrophil adherence stimulated by phorbol myristate acetate (PMA), NaF, and A23187; these agents activate intermediate steps in fMLP signal transduction. Adenosine (0.1-100 microM) did not inhibit adherence mediated by these receptor-independent agonists or NaF- and A23187-mediated O2- production. Additionally, NaF and A23187 completely abrogated adenosine inhibition of fMLP-stimulated neutrophil adherence. We also found that pertussis toxin (5 and 10 microM) completely inhibited fMLP-induced neutrophil adherence and O2- generation, indicating that both processes are G protein mediated. Furthermore, fMLP-stimulated GTPase activity in neutrophil membrane preparations was significantly inhibited by adenosine (1 and 10 microM) or 5'-N-ethylcarboxamidoadenosine (1 microM) (NECA). These data indicate that adenosine inhibits a G-protein-dependent pathway of fMLP stimulation by uncoupling G proteins from the fMLP receptor. This may be a general mechanism of adenosine inhibition of cell-surface receptor-mediated signals as both fMLP- and C5a-stimulated neutrophil adherence were inhibited at similar concentrations.

Effects of adenosine and its analogues on actin polymerization in human polymorphonuclear leucocytes

1. The effects of adenosine and its analogues on actin polymerization in human polymorphonuclear leucocytes (PMN) induced by three different chemotactic stimulants, platelet-activating factor (PAF), N-formyl-methionyl-leucyl-phenylalanine (FMLP) and an activated fragment of C5 (C5a) were investigated. 2. Adenosine and its analogues inhibited the actin polymerization induced by these three agents in a concentration-dependent manner and theophylline, a competitive antagonist at adenosine receptors, abolished these inhibitory effects. 3. The adenosine analogue 5'-N-ethylcarboxamideadenosine (NECA) was a more potent inhibitor of actin polymerization than either L-N6-phenylisopropyladenosine (PIA) or adenosine itself; the rank order of potency of these agonists was characteristic of adenosine A2 receptors. 4. Adenosine deaminase (ADA) abolished the inhibitory effect of adenosine and augmented PAF-induced actin polymerization. 5. It was concluded that, at physiological concentrations, adenosine inhibits actin polymerization in PMN via activation of PMN surface membrane adenosine A2 receptors and thus modulates chemotactic stimulus-induced PMN motility.

Adenosine inhibits platelet-activating factor, but not tumour necrosis factor-alpha-induced priming of human neutrophils

Regulation of the respiratory burst and its priming by recombinant human tumour necrosis factor-alpha (rhTNF-alpha) and platelet-activating factor (PAF) were investigated in human polymorphonuclear leucocytes (PMN). Adenosine (0.1-10 microM) pretreatment of PMN concentration-dependently inhibited the superoxide anion generation (O2-) in response to formyl-methionyl-leucyl-phenylalanine (FMLP). The priming by PAF (1 microM) for an increased O2- generation by FMLP-stimulated PMN was completely blocked by adenosine pretreatment. In contrast, rhTNF-alpha-induced priming was unaffected by adenosine. In addition, the direct stimulation of PMN O2- by rhTNF-alpha was also unaffected by adenosine as was rhTNF-alpha-induced PAF synthesis. FMLP-induced PAF synthesis was reduced by adenosine to a similar extent as the inhibition of the respiratory burst. Adenosine also inhibited PAF-, but not FMLP-induced increases in intracellular calcium in PMN. These findings indicate that short-term, direct stimulants (FMLP) or priming agents (PAF) are subject to modulation by the endothelial product adenosine, whereas the priming and direct stimulation of the respiratory burst by the longer-acting agent, rhTNF-alpha is unaffected. Moreover, differential inhibition of PMN activation by adenosine reveals important functional differences in the signalling mechanisms initiated by PAF, FMLP and rhTNF-alpha.

Modulation of barrier function of bovine aortic and pulmonary artery endothelial cells: dissociation from cytosolic calcium content

1. Barrier function and cytosolic free calcium content [Ca2+]i was measured in monolayers of bovine pulmonary artery endothelial cells (BPAEC) and bovine aortic endothelial cells (BAEC). 2. Thrombin (1 u ml-1) increased albumin transfer across monolayers of BPAEC but not BAEC, yet induced biphasic increases in [Ca2+]i in both endothelial cell types, consisting of a rapid, initial phasic component which decayed to a lower, more sustained plateau phase. 3. 4 beta-Phorbol 12-myristate 13-acetate (PMA; 0.3-3000 nM) increased albumin transfer across monolayers of BPAEC and BAEC, but had no effect on basal levels of [Ca2+]i in either endothelial cell type. 4. Treatment of BPAEC and BAEC with forskolin (30 microM), an activator of adenylate cyclase, had no effect on resting transfer of albumin, but inhibited that stimulated by PMA (600 nM). It also inhibited the thrombin (1 u ml-1)-induced increase in albumin transfer across monolayers of BPAEC, but enhanced the plateau phase of the associated increase in [Ca2+]i. 5. Treatment of BPAEC and BAEC with either atriopeptin II (100 nM), an activator of particulate guanylate cyclase, or 8 bromo cyclic GMP (30 microM) had no effect on resting or PMA (600 nM)-stimulated transfer of albumin. Both agents did, however, inhibit the thrombin (1 u ml-1)-induced increase in albumin transfer across monolayers of BPAEC, but had no effect on the associated increase in [Ca2+]i. 6. These data suggest a dissociation between the ability of agents that increase or decrease albumin transfer and their effects on [Ca2+]i. Consequently, activation of protein kinase C may be the major stimulus for trans-endothelial transfer of macromolecular solutes. Endothelial barrier function is enhanced by elevation of either cyclic AMP or cyclic GMP content. Cyclic AMP appears to act by inhibiting the actions of protein kinase C, while cyclic GMP may act to inhibit a key step proximal to activation of this enzyme.

Adenosine inhibits divalent cation influx across human neutrophil plasma membrane via surface adenosine A2 receptors

Adenosine and its analogues inhibited increases in divalent cation influx stimulated by platelet-activating factor (PAF) and formyl-methionyl-leucyl-phenylalanine (FMLP) in a dose-dependent fashion. This effect was antagonized by theophylline, an adenosine receptor antagonist. When extracellular adenosine was removed by adenosine deaminase, the effect of adenosine was completely abolished. Two adenosine analogues with different affinities for adenosine receptor subtypes, 5'-N-ethylcarboxamideadenosine (NECA) and L-N6-phenylisopropyladenosine (PIA), also inhibited divalent cation influx, NECA being more potent than PIA. These results suggest that adenosine and its analogues inhibit divalent cation influx across neutrophil plasma membranes via surface adenosine A2 receptors. Adenosine had little effect on the initial peaks of intracellular free calcium rises induced by chemoattractants, but it inhibited the subsequent rise in free calcium. Since calcium influx through the divalent cation channels or neutrophil plasma membranes is responsible for maintaining free calcium concentration following the initial peaks, we suggest that adenosine modulates neutrophil function by interfering with this calcium influx.

Cross-talk between cAMP and formylmet-leu-phe in human neutrophils: phosphorylation of a 52,000 molecular weight protein

The mechanism of inhibition of neutrophil phagocytic functions by cAMP-elevating agents has not yet been clarified. In the present work, the effects of adenylate cyclase agonists on protein phosphorylation in the formylmethionyl-leucyl-phenylalanine (fMLP)-stimulated human neutrophils were studied. Before stimulation, 32Pi-labelled cells were incubated with adenosine deaminase to remove the endogenously produced adenosine, an adenylate cyclase agonist itself. A protein of about 52,000 molecular weight was rapidly and transiently phosphorylated when neutrophils were stimulated with fMLP in the presence of isoproterenol, prostaglandin E1, histamine or 2-chloroadenosine. This phosphorylation was blocked by the antagonists of the receptors for the above-listed agents. No phosphorylation of the 52,000 molecular weight protein could be observed if either fMLP or the cAMP-elevating agent were applied alone. A calcium ionophore A23187 and dibutyryl-cAMP could replace fMLP and a cAMP-elevating agent, respectively. Phosphorylation of the 52,000 molecular weight protein was also demonstrated in cell lysates in the presence of cAMP, and in membrane preparations in the presence of the catalytic subunit of cAMP-dependent protein kinase. These data suggest that phosphorylation of the 52,000 molecular weight protein in intact cells is dependent on the cross-talk between the fMLP- and the cAMP-signalling pathways, and may thus be involved in the cAMP-regulatory mechanism.

Neutrophils from asthmatics exhibit diminished responsiveness to 2-chloroadenosine which is reversed by theophylline. Evidence for a cyclic-AMP-independent pathway on human neutrophils

We have shown previously that neutrophils (PMNs) from patients with asthma have a more potent stimulated respiratory burst than normals and that their respiratory burst is significantly less suppressed with exposure to 2-chloroadenosine (2-CADO). The present studies investigated the basis of this defect in responsiveness to 2-CADO. PMNs obtained from asthmatics either not on theophylline (minus theophylline) or taking theophylline (plus theophylline) generated significantly more superoxide in response to 2 x 10(-8) M FMLP (2.08 +/- 0.36 nmol/5 x 10(5) PMNs (minus theophylline) (P less than 0.01 compared to controls) vs. 2.16 +/- 0.44 (plus theophylline) (P less than 0.01) as compared to controls (1.05 +/- 0.17 nmol). In the presence of FMLP (2 x 10(-8) M), PMNs from the minus theophylline cohort had less 2-CADO (10(-6) M)-mediated suppression of superoxide generation as compared to controls (38.3 +/- 3.8% vs. 67.1 +/- 3.8%; (P less than 0.001). The plus theophylline group exhibited suppression values similar to controls (64.5 +/- 7.2%). Theophylline, in the presence of a physiological concentration of 2-CADO (0.1 microM) accentuated the suppression of the respiratory burst in normals (74.1 +/- 5.9%, 80.1 +/- 4.9% (P less than 0.02) and 84.7 +/- 3.8% (P less than 0.02) at 0, 10, and 100 microM, respectively). PMNs from asthmatics not taking theophylline demonstrated suppression values of 46.2 +/- 6%, 53.8 +/- 6.6% (P = NS), and 63.2 +/- 7.1% (P less than 0.01), respectively. Resting PMNs from normal controls generated 0.97 +/- 0.20 pmol cAMP/10(7) cells compared to 2.83 +/- 0.75 pmol in the presence of 0.1 microM 2-CADO. The combination of 2-CADO and theophylline (10-100 microM) produced cAMP concentrations not significantly different from that observed with 2-CADO alone. These findings support the existence of a novel cAMP-independent adenosine receptor in PMNs. The specific binding of 10(-8)M 3H-labeled 2-CADO (in delta cpm) was 10,358 +/- 1502 (P less than 0.001 compared to controls), 5468 +/- 843 (NS compared to controls), and 3751 +/- 477 in the plus theophylline group, minus theophylline group, and controls, respectively. Such up-regulation of specific binding may represent the effects of theophylline as shown by the specific binding of [3H]2-CADO in PMNs from normal controls exposed to 10 microM theophylline for 30 min (6013 +/- 969) compared to unexposed PMNs (3768 +/- 656; P less than 0.05).(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of oxygen radical production of human polymorphonuclear leukocytes by adenosine: the role of calcium

In polymorphonuclear leukocytes (PMNL) adenosine is a potent inhibitor of stimulus/response coupling, as demonstrated by its adverse action on phagocytosis, degranulation and oxygen radical production. Because this nucleoside can reduce several cell functions by counteracting intracellular calcium ions (Ca2+), the present study investigates the effect of adenosine on oxygen radical production in human PMNL stimulated by N-formylmethionyl-leucyl-phenylalanine (fMet-Leu-Phe), zymosan-activated serum (ZAS) and ionophore A 23187 in a Ca(2+)-dependent manner, and stimulated by phorbol myristate acetate (PMA) and latex in a Ca(2+)-independent manner. The results demonstrate that all the Ca(2+)-dependent stimuli were concentration-dependently inhibited by adenosine. In contrast, leukocyte stimulation by the Ca(2+)-independent activator PMA was not affected by adenosine. Surprisingly, radical formation stimulated by latex beads was concentration-dependently reduced by adenosine. When intracellular Ca2+ was modified by the ionophore clamping technique or the Ca(2+)-buffering capacity of quin-2, latex-induced radical formation could be separated into two parts, one showing Ca(2+)-dependent and the other Ca(2+)-independent activation. In the presence of intracellular Ca2+, adenosine exerted a strong inhibition on the latex-induced cell activation but failed to inhibit in the Ca(2+)-depleted state. In order to elucidate a direct reduction of Ca2+ as the underlying mechanism of adenosine-mediated inhibition, intracellular Ca2+ was measured in PMNL by quin-2 fluorescence. When PMNL were activated by latex, fMet-Leu-Phe and ionophore A 23187, adenosine significantly reduced the stimulated rise in intracellular Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulus-response uncoupling in the neutrophil. Adenosine A2-receptor occupancy inhibits the sustained, but not the early, events of stimulus transduction in human neutrophils by a mechanism independent of actin-filament formation

Generation of superoxide anion (O2-) in response to occupancy of neutrophil chemoattractant receptors requires both early events ('triggering') and sustained signals ('activation'). We have previously demonstrated that occupancy of adenosine A2 receptors inhibits O2- generation by neutrophils. In parallel, adenosine-receptor occupancy promotes association of bound N-formylmethionyl-leucyl-phenylalanine (fMLP) receptors with the cytoskeleton, a process associated with termination of neutrophil activation (stimulus-response uncoupling). We undertook this study to determine whether inhibition of neutrophil function by adenosine-receptor occupancy requires intact actin filaments and to examine the effect of adenosine-receptor occupancy on the stimulated generation of intracellular signals involved in neutrophil triggering and activation. Occupancy of adenosine A2 receptors by 5'-N-ethylcarboxamidoadenosine (NECA, 1 microM) significantly increased (130 +/- 1% of control, P less than 0.001, n = 3) association of [3H]fMLP with cytoskeletal preparations. Cytochalasin B (5 micrograms/ml), an agent which disrupts actin filaments, completely blocked association of [3H]fMLP with cytoskeletal preparations, as previously reported. However, NECA markedly increased association of [3H]fMLP with the cytoskeleton even in the presence of cytochalasin B (P less than 0.0002). Moreover, NECA did not significantly affect either the early (30s) or the late (5 min) formation of actin filaments after stimulation by chemoattractant (fMLP, 0.1-100 nM). Cytochalasin B markedly inhibited actin-filament formation by stimulated neutrophils, and NECA did not reverse the effect of cytochalasin B on actin-filament formation. Adenosine-receptor occupancy did not affect the rapid peak in diacylglycerol generation (less than or equal to 15 s) from either [3H]arachidonate- or [14C]glycerol-labelled phospholipid pools. However, as would be predicted if occupancy of the adenosine receptor was a signal for early termination of cell activation, NECA (1 microM) markedly diminished the slow sustained generation of diacylglycerol. These results suggest that adenosine-A2-receptor occupancy does not affect triggering of the neutrophil, but that occupancy of adenosine receptors is an early signal for the termination of neutrophil activation, i.e. the 'premature' finish of signal transduction. Moreover, these data indicate that at least two pathways are available for increasing the association of ligated chemoattractant receptors with the cytoskeleton of neutrophils: F-actin-dependent and -independent.

Inhibition of neutrophil superoxide production by adenosine released from vascular endothelial cells

To investigate the inhibitory effect of adenosine released by endothelium on neutrophil superoxide (O2-) production, we treated confluent monolayers of cultured human umbilical vein endothelial cells with the enzyme adenosine deaminase, and then added human neutrophils. Superoxide (O2-) production by human neutrophils stimulated with 10(-6) M formyl-methionyl-leucyl-phenylalanine was inhibited by 49% in the presence of a confluent monolayer of human umbilical vein endothelial cells (5.1 +/- 0.1 versus 2.6 +/- 0.3 nmols O2-/10(6) neutrophils). Addition of 0.25 U/ml adenosine deaminase to neutrophils plus endothelial cells restored formyl-methionyl-leucyl-phenylalanine-stimulated neutrophil superoxide production to the level seen with neutrophils alone. Deoxycoformycin (10(-4) M), an inhibitor of adenosine deaminase activity, prevented the increase in superoxide production associated with adenosine deaminase addition. The adenosine analogue 5'-(N-ethylcarboxamido)- adenosine (3 x 10(-4) M) caused increased inhibition of formyl-methionyl-leucylphenylalanine-stimulated superoxide release by neutrophils in the presence of endothelial cells and prevented neutrophil-mediated endothelial cell damage, as measured by release of 3H-2-deoxy-D-glucose. Pairing 2-chloroadenosine (10(-5) M) or 5'-(N-ethylcarboxamido)-adenosine (3 x 10(-4) M) with a cyclic adenosine monophosphate phosphodiesterase inhibitor, 3-isobutyl-l-methyl-xanthine (10-4 M), produced greater inhibition of neutrophil superoxide production than occurred with either compound alone. The results support the hypothesis that vascular endothelial cells protect themselves from neutrophil attack by releasing adenosine to inhibit superoxide production.