Involvement of Cannabinoid Receptors and Adenosine A2B Receptor in Enhanced Migration of Lung Cancer A549 Cells Induced by γ-Ray Irradiation

Residual cancer cells after radiation therapy may acquire malignant phenotypes such as enhanced motility and migration ability, and therefore it is important to identify targets for preventing radiation-induced malignancy in order to increase the effectiveness of radiotherapy. G-Protein-coupled receptors (GPCRs) such as adenosine A2B receptor and cannabinoid receptors (CB1, CB2, and GPR55) may be involved, as they are known to have roles in proliferation, invasion, migration and tumor growth. In this study, we investigated the involvement of A2B and cannabinoid receptors in γ-radiation-induced enhancement of cell migration and actin remodeling, as well as the involvement of cannabinoid receptors in cell migration enhancement via activation of A2B receptor in human lung cancer A549 cells. Antagonists or knockdown of A2B, CB1, CB2, or GPR55 receptor suppressed γ-radiation-induced cell migration and actin remodeling. Furthermore, BAY60-6583 (an A2B receptor-specific agonist) enhanced cell migration and actin remodeling in A549 cells, and this enhancement was suppressed by antagonists or knockdown of CB2 or GPR55, though not CB1 receptor. Our results indicate that A2B receptors and cannabinoid CB1, CB2, and GPR55 receptors all contribute to γ-radiation-induced acquisition of malignant phenotypes, and in particular that interactions of A2B receptor and cannabinoid CB2 and GPR55 receptors play a role in promoting cell migration and actin remodeling. A2B receptor-cannabinoid receptor pathways may be promising targets for blocking the appearance of malignant phenotypes during radiotherapy of lung cancer.

Progress in the pursuit of therapeutic adenosine receptor antagonists

Ever since the discovery of the hypotensive and bradycardiac effects of adenosine, adenosine receptors continue to represent promising drug targets. First, this is due to the fact that the receptors are expressed in a large variety of tissues. In particular, the actions of adenosine (or methylxanthine antagonists) in the central nervous system, in the circulation, on immune cells, and on other tissues can be beneficial in certain disorders. Second, there exists a large number of ligands, which have been generated by introducing several modifications in the structure of the lead compounds (adenosine and methylxanthine), some of them highly specific. Four adenosine receptor subtypes (A1, A2A, A2B, and A3) have been cloned and pharmacologically characterized, all of which are G protein-coupled receptors. Adenosine receptors can be distinguished according to their preferred mechanism of signal transduction: A1 and A3 receptors interact with pertussis toxin-sensitive G proteins of the Gi and Go family; the canonical signaling mechanism of the A2A and of the A2B receptors is stimulation of adenylyl cyclase via Gs proteins. In addition to the coupling to adenylyl cyclase, all four subtypes may positively couple to phospholipase C via different G protein subunits. The development of new ligands, in particular, potent and selective antagonists, for all subtypes of adenosine receptors has so far been directed by traditional medicinal chemistry. The availability of genetic information promises to facilitate understanding of the drug-receptor interaction leading to the rational design of a potentially therapeutically important class of drugs. Moreover, molecular modeling may further rationalize observed interactions between the receptors and their ligands. In this review, we will summarize the most relevant progress in developing new therapeutic adenosine receptor antagonists.

The G(s)-coupled adenosine A(2B) receptor recruits divergent pathways to regulate ERK1/2 and p38

Adenosine A(2B) receptors have been suggested to influence cell differentiation and proliferation. Human adenosine A(2B) receptors expressed in Chinese hamster ovary cells mediate phosphorylation and activation of the extracellular signal-regulated kinase (ERK1/2). Already low concentrations of agonists such as 5'-N-ethylcarboxamidoadenosine (NECA) are effective. Phosphorylation of the stress-activated protein kinase p38 was also potently induced by NECA (EC(50) 18.5 nM). These NECA-induced effects were mimicked by forskolin and 8-Br-cAMP. Inhibition of cAMP-dependent protein kinase (PKA) using H89 (N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide)) blocked phosphorylation of the cAMP response element-binding protein (CREB) and p38, but did not decrease NECA-induced ERK1/2 phosphorylation. NECA activated the small GTPase Rap1, and this was also not blocked by H89. Inhibition of phosphatidylinositol-3'-kinase (PI3K) by wortmannin inhibited adenosine A(2B) receptor-mediated ERK1/2 phosphorylation and activation of Rap1, without affecting CREB and p38 phosphorylation. A(2B) receptor-stimulated protein kinase B phosphorylation was sensitive to wortmannin, but not to H89. Thus, stimulation of adenosine A(2B) receptors activates both ERK1/2 and p38 via cAMP, but the downstream pathways are markedly different. ERK1/2 activation was dependent on PI3K but not on PKA. p38 activation by NECA was instead independent of PI3K but required cAMP and PKA. The potent activation of both MAPKs suggests a physiological role.

Pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine derivatives as adenosine receptor antagonists. Influence of the N5 substituent on the affinity at the human A 3 and A 2B adenosine receptor subtypes: a molecular modeling investigation

A new series of pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidines bearing various substituents at both the N5-pyrimidinyl and N8-pyrazolyl positions have been synthesized, and their binding affinities at the four human adenosine receptor subtypes (hA(1), hA(2A), hA(2B), and hA(3)) have been evaluated. All the described compounds contain arylacetyl moieties at the N5 position and arylalkyl substituents at the N8 position. Surprisingly, all the compounds present their most potent affinities at the hA(2B) adenosine receptor with a range of selectivities against the other subtypes. When bulky groups are present simultaneously at the N5 and N8 positions (e.g., compound 9), the best selectivity for the hA(2B) receptor was observed (K(i)(hA(1)) = 1100 nM; K(i)(hA(2A)) = 800 nM; K(i)(hA(2B)) = 20 nM; K(i)(hA(3)) = 300 nM, K(i)(hA(1)/A(2B)) = 55, K(i)(hA(2A)/A(2B)) = 40, K(i)(hA(3)/hA(2B)) = 15). To understand the molecular significance of these results, we compared the putative TM (transmembrane) binding motif of compound 9 on both hA(2B) and hA(3) receptors. From our docking studies, compound 9 fits neatly inside the TM region of the hA(2B) receptor but not in the corresponding hA(3) region, illustrating significant differences between the two subtypes. The study herein presented permits an understanding of why the bioisosteric replacement of an -NH, present in previously reported hA(3) receptor antagonists, with a -CH(2) group at the N5 position induces such large differences in hA(2B)/hA(3) affinity. In the molecular structure of the hA(3) receptor, two residues, Ser243 (TM6) and Ser271 (TM7), create a hydrophilic region, which seems to permit a better accommodation of the phenylurea series into this putative hA(3) binding site than the phenylacetyl series.

Reduction in preretinal neovascularization by ribozymes that cleave the A2B adenosine receptor mRNA

Adenosine modulates a variety of cellular functions by interacting with specific cell surface G protein-coupled receptors (A1, A2A, A2B, and A3) and is a potential mediator of angiogenesis through the A2B receptor. The lack of a potent, selective A2B receptor inhibitor has hampered its characterization. Our goal was to design a hammerhead ribozyme that would specifically cleave the A2B receptor mRNA and examine its effect on retinal angiogenesis. Ribozymes specific for the mouse and human A2B receptor mRNAs were designed and cloned in expression plasmids. Human embryonic kidney (HEK) 293 cells were transfected with these plasmids and A2B receptor mRNA levels were determined by quantitative real-time RT-PCR. Human retinal endothelial cells (HRECs) were also transfected and cell migration was examined. The effects of these ribozymes on the levels of preretinal neovascularization were determined using a neonatal mouse model of oxygen-induced retinopathy (OIR). We produced a ribozyme with a Vmax of 515+/-125 pmol/min and a Kcat of 36.1+/-8.3 min(-1) (P< or =1x10(-5)). Transfection of HEK293 cells with the plasmid expressing the ribozyme reduced A2B receptor mRNA levels by 45+/-4.8% (P=5.1x10(-5)). Transfection of HRECs reduced NECA-stimulated migration of cells by 47.3+/-1.2% (P=7x10(-4)). Intraocular injection of the constructs into the mouse model reduced preretinal neovascularization by 53.5+/-8.2% (P=4.5x10(-5)). Our results suggest that the A2B receptor ribozyme will provide a tool for the selective inhibition of this receptor and provide further support for the role of A2B receptor in retinal angiogenesis.

Coordinated adenine nucleotide phosphohydrolysis and nucleoside signaling in posthypoxic endothelium: role of ectonucleotidases and adenosine A2B receptors

Limited oxygen delivery to tissues (hypoxia) is common in a variety of disease states. A number of parallels exist between hypoxia and acute inflammation, including the observation that both influence vascular permeability. As such, we compared the functional influence of activated polymorphonuclear leukocytes (PMN) on normoxic and posthypoxic endothelial cells. Initial studies indicated that activated PMN preferentially promote endothelial barrier function in posthypoxic endothelial cells (>60% increase over normoxia). Extension of these findings identified at least one soluble mediator as extracellular adenosine triphosphate (ATP). Subsequent studies revealed that ATP is coordinately hydrolyzed to adenosine at the endothelial cell surface by hypoxia-induced CD39 and CD73 (>20-and >12-fold increase in mRNA, respectively). Studies in vitro and in cd39-null mice identified these surface ecto-enzymes as critical control points for posthypoxia-associated protection of vascular permeability. Furthermore, insight gained through microarray analysis revealed that the adenosine A2B receptor (AdoRA2B) is selectively up-regulated by hypoxia (>5-fold increase in mRNA), and that AdoRA2B antagonists effectively neutralize ATP-mediated changes in posthypoxic endothelial permeability. Taken together, these results demonstrate transcription coordination of adenine nucleotide and nucleoside signaling at the vascular interface during hypoxia.

cDNA microarray analysis reveals a nuclear factor-kappaB-independent regulation of macrophage function by adenosine

Adenosine is released into the extracellular space from nerve terminals and cells subjected to ischemic stress. This nucleoside modulates a plethora of cellular functions via occupancy of specific receptors. Adenosine is also an important endogenous regulator of macrophage function, because it suppresses the production of a number of proinflammatory cytokines by these cells. However, the mechanisms of this anti-inflammatory effect have not been well characterized. We hypothesized that adenosine may exert some of its anti-inflammatory effects by decreasing activation of the transcription factor nuclear factor-kappaB (NF-kappaB), because gene expression of most of the proinflammatory cytokines inhibited by adenosine is dependent on NF-kappaB activation. Using bacterial lipopolysaccharide (LPS)-stimulated RAW 264.7 macrophages, we found that adenosine as well as adenosine receptor agonists decreased the production of tumor necrosis factor (TNF)-alpha, a typical NF-kappaB-regulated cytokine. This effect of adenosine was not due to an action on the process of TNF-alpha release, because adenosine suppressed also the intracellular levels of TNF-alpha. However, cDNA microarray analysis revealed that mRNA levels of neither TNF-alpha nor other cytokines were altered by adenosine in either LPS-activated or quiescent macrophages. In addition, although LPS induced expression of a number of other, noncytokine genes, including the adenosine A2b receptor, adenosine did not affect the expression of these genes. Furthermore, adenosine as well as adenosine receptor agonists failed to decrease LPS-induced NF-kappaB DNA binding, NF-kappaB promoter activity, p65 nuclear translocation, and inhibitory kappaB degradation. Together, our results suggest that the anti-inflammatory effects of adenosine are independent of NF-kappaB.

Adenosine A 2B receptors modulate cAMP levels and induce CREB but not ERK1/2 and p38 phosphorylation in rat skeletal muscle cells

The present study examined the existence of the adenosine A(1),A(2A), and A(2B) receptors and the effect of receptor activation on cAMP accumulation and protein phosphorylation in primary rat skeletal muscle cells. Presence of mRNA and protein for all three receptors was demonstrated in both cultured and adult rat skeletal muscle. NECA (10(-9)-10(-4)M) increased the cAMP concentration in cultured muscle cells with an EC(50) of (95% confidence interval)=15 (5.9-25.1) micro M, whereas CGS 21680 (10(-9)-10(-4)M) had no effect on cAMP accumulation. Concentrations of [R]-PIA below 10(-6)M had no effect on cAMP accumulation induced by either isoproterenol or forskolin. NECA resulted in phosphorylation of CREB with an EC(50) of (95% confidence interval)=1.7 (0.40-7.02) micro M, whereas ERK1/2 and p38 phosphorylation was unchanged. The results show that, although the A(1),A(2A), and A(2B) receptors are all present in skeletal muscle cells, the effect of adenosine on adenylyl cyclase activation and phosphorylation of CREB is mainly mediated via the adenosine A(2B) receptor.

Medicinal chemistry and pharmacology of A2B adenosine receptors

The low affinity A(2B) adenosine receptor, like any other adenosine receptor subtype, belongs to the super-family of seven transmembrane domain protein-coupled receptors (7TMs GPCR) and is classified by the GPCR database in the family of rhodopsin like receptors (Class A of GPCR). It has been cloned from various species, including rat and human, and its sequences are highly similar across species, ranging from 85% identity between human and mouse and 95% identity between rat and mouse. The A(2B)receptors show a ubiquitous distribution, the highest levels are present in cecum, colon and bladder, followed by blood vessels, lung, eye and mast cells. Through A(2B) receptors adenosine seems to cause mast cells degranulation, vasodilation, cardiac fibroblast proliferation, inhibition of Tumor Necrosis Factor (TNF-alpha), increased synthesis of interleukin-6 (IL-6), stimulation of Cl(-) secretion in intestinal epithelia and hepatic glucose production. Hence, A(2B) adenosine receptor agonists could be useful in the treatment of cardiac diseases like hypertension or myocardial infarction and in the management of septic shock, while antagonists may serve as novel drugs for asthma, Alzheimer's disease, cystic fibrosis and type-II diabetes. No potent and selective A(2B) agonists have been reported so far; 5'-N-ethylcarboxamidoadenosine (NECA) is one of the most active. The monosubstitution on N(6)-position of adenosine is well tolerated and that position appears to be a useful site for increasing A(2B) potency. Among substituents in 2-position of adenosine only 1-alkynyl chains are effective for A(2B) potency. In particular, the (S)-2-hydroxypropynyl substituents brought about the highest activity demonstrating that the A(2B) receptors discriminate between (R) and (S) diastereomers. Hence, (S)-2-phenylhydroxypropynylNECA (PHPNECA), with an EC(50) = 0.22 micro M, proved to be the most potent A(2B) agonist reported so far. Classical antagonists for adenosine receptors are alkylxanthines which show considerable potency at A(2B) receptors. Para substituted 1,3-dialkyl-8-phenylxanthines possess high affinity in binding assays; the 3-unsubstituted 1-alkyl analogues resulted more A(2B) selective with the 8-[4-[(N-(2-hydroxyethyl)carboxamidomethyl)oxy]phenyl]-1-propylxanthine (60) showing the highest affinity (K(i) = 1.2 nM) and selectivity (A(1)/A(2B) = 60, A(2A)/A(2B) = 1,790, A(3)/A(2B) = 360). Among non-xanthine derivatives very promising are substituted purines, in which combination of appropriate substituents in 2-, 8-, and 9-position could lead to very potent and selective A(2B) antagonists.

Activation of murine lung mast cells by the adenosine A3 receptor

Adenosine has been implicated to play a role in asthma in part through its ability to influence mediator release from mast cells. Most physiological roles of adenosine are mediated through adenosine receptors; however, the mechanisms by which adenosine influences mediator release from lung mast cells are not understood. We established primary murine lung mast cell cultures and used real-time RT-PCR and immunofluorescence to demonstrate that the A(2A), A(2B), and A(3) adenosine receptors are expressed on murine lung mast cells. Studies using selective adenosine receptor agonists and antagonists suggested that activation of A(3) receptors could induce mast cell histamine release in association with increases in intracellular Ca(2+) that were mediated through G(i) and phosphoinositide 3-kinase signaling pathways. The function of A(3) receptors in vivo was tested by exposing mice to the A(3) receptor agonist, IB-MECA. Nebulized IB-MECA directly induced lung mast cell degranulation in wild-type mice while having no effect in A(3) receptor knockout mice. Furthermore, studies using adenosine deaminase knockout mice suggested that elevated endogenous adenosine induced lung mast cell degranulation by engaging A(3) receptors. These results demonstrate that the A(3) adenosine receptor plays an important role in adenosine-mediated murine lung mast cell degranulation.

The influence of brain inflammation upon neuronal adenosine A2B receptors

Alzheimer's disease (AD) is associated with glial activation and increased levels of pro-inflammatory cytokines. Epidemiological results suggest that anti-inflammatory therapies can slow the onset of AD. Adenosine, acting at type-2 receptors, is an effective endogenous anti-inflammatory agent that can modulate inflammation both in the periphery and the brain. We investigated changes in the expression of adenosine type-2B (A2B) receptors and a related intracellular second messenger during chronic brain inflammation and following treatment with the non-steroidal anti-inflammatory drug flurbiprofen and its nitric oxide (NO)-donating derivative, HCT1026. Chronic infusion of lipopolysaccharide (LPS) into the 4th ventricle of young rats induced brain inflammation that was associated with microglial activation and reduced neuronal immunoreactivity for adenosine A2B receptors in the cortex. Daily administration of HCT1026, but not flurbiprofen, reduced microglial activation, prevented the down-regulation of A2B receptors and elevated tissue levels of cAMP. The results suggest that a therapy using an NO-releasing NSAID might significantly attenuate the processes that drive the pathology associated with AD and that this process may involve the activation of adenosine A2B receptors.

Analysis of A2a receptor-deficient mice reveals no significant compensatory increases in the expression of A2b, A1, and A3 adenosine receptors in lymphoid organs

Although recent genetic and pharmacologic in vivo studies of acute inflammation models in mice demonstrated that the cyclic AMP-elevating A2a receptor plays a non-redundant role in protection from excessive acute inflammatory tissue damage and in the down-regulation of proinflammatory cytokine production, it remained to be established whether genetic deficiency of the A2a receptor is accompanied by a compensatory up-regulation of the cAMP-elevating A2b receptor and/or other adenosine receptors. Here, we show that most of the cAMP response to adenosine is abolished in lymphoid tissues of A2a receptor-deficient mice, although some response remains in splenocytes. No significant changes were observed in A2b, A1, and A3 mRNA levels in the thymus or lymph nodes of A2a receptor-deficient mice, but small increases in mRNA expression of these receptors were detected in the spleen. These data suggest that regulation of the expression of A2b, A1, and A3 receptors is not affected significantly by the absence of A2a receptors and may provide further explanation of earlier in vivo observations of increased tissue damage and of longer persistence of proinflammatory cytokines in animals with inactivated A2a receptors.

Low efficacy adenosine A1 agonists inhibit striatal acetylcholine release in rats improving central selectivity of action

The objective of this study was to characterize the effects of the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) and its low efficacy derivatives 2'-deoxy-CPA (2DCPA), 3'-deoxy-CPA (3DCPA), 8-ethylamino-CPA (8ECPA) and 8-butylamino-CPA (8BCPA) on the release of acetylcholine (ACh) using intrastriatal microdialysis. These low efficacy agonists exhibited lower effects on the cardiovascular system than CPA. A concentration-dependent inhibition of ACh release was observed with a maximum of 60.5+/-2.4% for CPA, 42.5+/-2.3% for 2DCPA, 45.3+/-5.8% for 3DCPA, 57.1+/-1.4% for 8ECPA and 93.1+/-10.9% for 8BCPA, respectively. This effect was counteracted by the adenosine A(1) receptor antagonist 8-cyclopentyltheophylline. These findings show that low efficacy adenosine A(1) agonists inhibit striatal ACh release equally effective as CPA, suggesting that central nervous system-selective actions can be obtained with these compounds.

Endogenous adenosine increases coronary flow by activation of both A2A and A2B receptors in mice

To clarify which adenosine receptor subtype(s) are responsible for regulation of coronary flow through endogenous adenosine, coronary vascular responses were examined in isolated hearts from wild-type (WT) and A(2A) knockout (A(2A)KO) mice. Adenosine deaminase inhibitor, erythro-9-hydroxy-nonyl-adenine (EHNA), and adenosine kinase inhibitor, iodotubericidine (ITU), were used to examine the effects of endogenous adenosine. Combined infusion of EHNA and ITU in Balb/c hearts produced comparable increases in coronary flow as exerted by exogenous adenosine while they markedly decreased the heart rate, and these effects were reversed by adenosine receptor antagonist, 8-p-sulfophenyl-theophylline (8-SPT). Similarly, EHNA and ITU increased coronary flow in WT hearts to 422% of baseline, whereas this response was reduced to 144% of baseline in A(2A)KO hearts. Heart rate was equally reduced (approximately 50% of baseline) in both groups. Alloxazine (A(2B) receptor antagonist) abolished EHNA- and ITU-induced coronary flow in A(2A)KO hearts without altering the reduced heart rate. Selective A(1) receptor antagonist, 8-cyclopentyl-1-1,3-dipropylxanthine (DPCPX), reversed EHNA- and ITU-induced decreases in heart rate without altering the elevated coronary flow. These findings suggest that coronary vascular responses to endogenous adenosine mimic those produced by exogenous adenosine and are mediated at least by activation of both A(2A) and A(2B) receptors in isolated mouse hearts.

Mast cell-mediated stimulation of angiogenesis: cooperative interaction between A2B and A3 adenosine receptors

Adenosine is released during tissue injury, ischemia and tumor growth, and promotes angiogenesis. Because mast cells accumulate in the proximity of new blood vessel development, we examined if they may contribute to adenosine-induced angiogenesis. We found that HMC-1 human mast cells express A2A, A2B, and A3 adenosine receptors. The adenosine agonist NECA (100 micromol/L) increased interleukin-8 (IL-8), vascular endothelial growth factor (VEGF), and angiopoietin-2 mRNA expression. NECA-induced secretion of IL-8 and VEGF was verified by ELISA. A2B receptors mediate VEGF and IL-8 secretion because neither CGS21680 (selective A2A agonist) nor IB-MECA (selective A3 agonist) produced this effect, and it was inhibited by the selective A2B antagonist IPDX but not by the selective A2A antagonist SCH58261 or the selective A3 antagonist MRS1191. In contrast, the selective A3 agonist IB-MECA (EC50 1 nmol/L) stimulated angiopoietin-2 expression. Conditioned media from NECA-activated HMC-1 stimulated human umbilical vein endothelial cell proliferation and migration, and induced capillary tube formation. Capillary formation induced by mast cell-conditioned media was maximal if both HMC-1 A2B and A3 receptors were activated, whereas activation of A2B receptor alone was less effective. Thus, adenosine A2B and A3 receptors act in a functional cooperative fashion to promote angiogenesis by a paracrine mechanism involving the differential expression and secretion of angiogenic factors from human mast cells.

A2B receptor activation promotes glycogen synthesis in astrocytes through modulation of gene expression

Adenosine has been proposed as a key factor regulating the metabolic balance between energy supply and demand in the central nervous system. Because astrocytes represent an important cellular element in the control of brain energy metabolism, we investigated whether adenosine could induce long-term changes of glycogen levels in primary cultures of mouse cortical astrocytes. We observed that adenosine increased glycogen content, up to 300%, in a time- (maximum at 8 h) and concentration-dependent manner with an EC(50) of 9.69 microM. Pharmacological experiments using the broad-spectrum agonist 5'-(N-ethylcarboxamido)adenosine (NECA) and specific agonists for the A(1), A(2A), and A(3) receptors [N(6)-cyclopentyladenosine (CPA), CGS-21680, and IB-MECA, respectively] suggest that the effect of adenosine is mediated through activation of the low-affinity A(2B) adenosine receptor subtype. Interestingly, adenosine induces in parallel the expression of the protein targeting to glycogen (PTG), one of the protein phosphatase-1 glycogen-targeting subunits that has been implicated in the control of glycogen levels in various tissues. These results indicate that adenosine can exert long-term control over glycogen levels in astrocytes and might therefore play a significant role in physiological and/or pathological processes involving long-term modulation of brain energy metabolism.

Comparison of human recombinant adenosine A2B receptor function assessed by Fluo-3-AM fluorometry and microphysiometry

1. The aim of this study was to establish the utility of a fluorometric imaging plate reader (FLIPR) assay to assess human adenosine A(2B) receptor function by characterizing its receptor pharmacology and comparing this profile to that obtained using a microphysiometer. 2. FLIPR was used, in conjunction with a Ca(2+)-sensitive dye (Fluo-3-AM), to measure rapid rises in intracellular calcium in a Chinese Hamster Ovary (CHO-K1) cell line stably transfected with both the human A(2B) receptor and a promiscuous G(alpha16) protein. Microphysiometry was used to measure rapid changes in the rate of extracellular acidification in a Human Embryonic Kidney (HEK-293) cell line also stably transfected with human A(2B) receptor. 3. Activation of A(2B) receptors by various ligands caused a concentration-dependent increase in both the intracellular calcium concentration and the extracellular acidification rate in the cells tested, with a similar rank order of potency for agonists: NECA > N(6)-Benzyl NECA > adenosine > or = R-PIA > CPA > S-PIA > CHA > CGS 21680. No comparable effects were observed in the non-transfected control cell lines. 4. The rank order of potency of the agonists examined was the same in all studies, whereas absolute potency and efficacy varied. Thus, all compounds exhibited greater potency in FLIPR than the microphysiometer and the efficacies obtained with CHO-K1 + G(alpha16) + A(2B) cell line and FLIPR were greater than those obtained with HEK-293 + A(2B) cell line in the microphysiometer. 5. ZM-241385 was the most potent of a range of adenosine antagonists tested with a pA(2) of 8.0 in both the FLIPR and microphysiometer assays. 6. In conclusion, the profile of the responses to both A(2B) receptor agonists and antagonists in FLIPR were similar to those obtained by the microphysiometer, although both potency and efficacy values were higher in the FLIPR assay. With this caveat in mind, this study shows that FLIPR coupled with a cell line transfected with both the human A(2B) receptor and a promiscuous G(alpha16) protein provides a useful, high throughput method for the assessment of A(2B) receptor function.

Analysis of DNase I-hypersensitive sites in the chromatin of the chicken adenosine receptor 2B gene reveals multiple cell-type-specific cis-regulatory elements

We have cloned and sequenced the gene encoding the chicken A2B adenosine receptor, a member of the family of G-protein-coupled seven-transmembrane-domain receptors which is activated by extracellular adenosine. The gene occupies about 10 kb of genomic sequences and contains two exons. The promoter of the gene is associated with a CpG-rich island and lacks a canonical TATA-box. Previous work has shown that the chicken A2B adenosine receptor is a direct target gene of the transcription factors v-Myb and c-Myb and that its expression in the hematopoietic system is specific for myelomonocytic as well as erythroid cells. To understand the molecular basis of this cell type specificity we have searched for cis-regulatory DNA elements that direct the expression of the gene to specific hematopoietic lineages. As a first step towards this aim we have mapped DNase I-hypersensitive sites in the chromatin of the A2B adenosine receptor gene. Our studies reveal multiple nuclease-sensitive sites which are located in the intron of the gene and in the upstream region. A number of these sites are cell-type-specific suggesting that they correspond to cell-type-specific cis-regulatory DNA elements. To characterize the possible function of these elements we have performed reporter gene studies. Our results show that several of the nuclease-sensitive regions act as myelomonocytic-specific cis-acting stimulatory elements. Taken together, our data suggest that the expression of the A2B adenosine receptor gene in myelomonocytic cells is controlled by multiple cell type-specific cis-acting sequences located upstream and within the intron of the gene.

Quinazolines as adenosine receptor antagonists: SAR and selectivity for A2B receptors

We have recently reported the discovery of numerous new compounds that are selective inhibitors of all of the subtypes of the adenosine receptor family via a pharmacophore database searching and screening strategy. During the course of this work we made the unexpected discovery of a potent A(2B) receptor antagonist, 4-methyl-7-methoxyquinazolyl-2-(2'-amino-4'-imidazolinone) (38, CMB 6446), which showed selectivity for this receptor and functioned as an antagonist, with a binding K(i) value of 112 nM. We explored the effects of both substituent- and ring-structural variations on the receptor affinity in this series of derivatives, which were found to be mostly non-selective adenosine receptor ligands with K(i) values in the micromolar range. Since no enhancement of A(2B) receptor affinity of 38 was achieved, the previously reported pharmacophore-based searching strategy yielded the most potent and selective structurally-related hit in the database originally searched.

The adenosine 2b receptor is recruited to the plasma membrane and associates with E3KARP and Ezrin upon agonist stimulation

We have previously shown that adenosine is formed in the intestinal lumen during active inflammation from neutrophil-derived 5'-AMP. Acting through the adenosine A2b receptor (A2bR), the luminally derived adenosine induces vectorial chloride secretion and a polarized secretion of interleukin-6 to the intestinal lumen. Although some G protein-coupled receptors interact with anchoring or signaling molecules, not much is known in this critical area for the A2bR. We used the model intestinal epithelial cell line, T84, and Caco2-BBE cells stably transfected with GFP-A2b receptor to study the intestinal A2bR. The A2bR is present in both the apical and basolateral membranes of intestinal epithelia. Apical or basolateral stimulation of the A2bR induces recruitment of the receptor to the plasma membrane and caveolar fractions. The A2bR co-immunoprecipitates with E3KARP and ezrin upon agonist stimulation. Ezrin interacts with E3KARP and PKA and the interaction between ezrin and E3KARP is enhanced by agonist stimulation. Our data suggest that the A2bR is recruited to the plasma membrane upon apical or basolateral agonist stimulation and interacts with E3KARP and ezrin. We speculate that such an interaction may not only anchor the A2bR to the plasma membrane but may also function to stabilize the receptor in a signaling complex in the plasma membrane.

Imidazo[2,1-i]purin-5-ones and related tricyclic water-soluble purine derivatives: potent A(2A)- and A(3)-adenosine receptor antagonists

A series of tricyclic imidazo[2,1-i]purinones and ring-enlarged analogues derived from xanthine derivatives have been prepared as adenosine receptor (AR) antagonists. In comparison with xanthines, the tricyclic compounds exhibit increased water solubility due to a basic nitrogen atom, which can be protonated under physiological conditions. Substituents were introduced that confer high affinity for A(2A) or A(3) ARs, respectively. A new capillary electrophoresis method was developed for the determination of the enantiomeric purity of selected chiral products using native and modified beta-cyclodextrins as chiral discriminators. The compounds were investigated in radioligand binding assays at rat brain A(1) and A(2A) ARs. Selected compounds were additionally investigated in radioligand binding assays at human recombinant A(3) ARs and in functional studies (adenylate cyclase assays) at A(1) ARs of rat fat cell membranes, A(2A) ARs of rat PC 12 cell membranes, and mouse A(2B) ARs of NIH 3T3 cell membranes. Structure-activity relationships were similar to those of corresponding xanthine derivatives. The 2-styrylimidazopurinones were less potent at A(2A) ARs as compared to 8-styrylxanthine derivatives. The most potent compound at A(2A) ARs was (S)-1,4-dimethyl-8-ethyl-2-styryl-imidazo[2,1-i]purinone (S-25) exhibiting a K(i) value of 424 nM at rat A(2A) ARs. The compound was highly selective for A(2A) receptors vs A(1) and A(3) ARs. Selectivity vs A(2B) ARs, however, was low. Among the 1-unsubstituted 2-phenyl-imidazo[2,1-i]purin-5-one derivatives, very potent and highly selective antagonists for human A(3) ARs were identified. The most potent A(3) antagonist of the present series was (R)-4-methyl-8-ethyl-2-phenyl-imidazo[2,1-i]purin-5-one (R-24) exhibiting a K(i) value of 2.3 nM and high selectivity for A(3) receptors vs all other AR subtypes.

Adenosine kinase inhibition promotes survival of fetal adenosine deaminase-deficient thymocytes by blocking dATP accumulation

Thymocyte development past the CD4(-)CD8(-) stage is markedly inhibited in adenosine deaminase-deficient (ADA-deficient) murine fetal thymic organ cultures (FTOCs) due to the accumulation of ADA substrates derived from thymocytes failing developmental checkpoints. Such cultures can be rescued by overexpression of Bcl-2, suggesting that apoptosis is an important component of the mechanism by which ADA deficiency impairs thymocyte development. Consistent with this conclusion, ADA-deficient FTOCs were partially rescued by a rearranged T cell receptor beta transgene that permits virtually all thymocytes to pass the beta-selection checkpoint. ADA-deficient cultures were also rescued by the adenosine kinase inhibitor 5'-amino-5'-deoxyadenosine (5'A5'dAdo), indicating that the metabolite responsible for the inhibition of thymocyte development is not adenosine or deoxyadenosine, but a phosphorylated derivative of an ADA substrate. Correction of ADA-deficient FTOCs by 5'A5'dAdo correlated with reduced accumulation of dATP, implicating this compound as the toxic metabolite. In ADA-inhibited FTOCs rescued with a Bcl-2 transgene, however, dATP levels were superelevated, suggesting that cells failing positive and negative selection continued to contribute to the accumulation of ADA substrates. Our data are consistent with dATP-induced mitochondrial cytochrome c release followed by apoptosis as the mechanism by which ADA deficiency leads to reduced thymic T cell production.

N(6)-alkyl-2-alkynyl derivatives of adenosine as potent and selective agonists at the human adenosine A(3) receptor and a starting point for searching A(2B) ligands

A series of N(6)-alkyl-2-alkynyl derivatives of adenosine (Ado) have been synthesized and evaluated for their affinity at human A(1), A(2A), and A(3) receptors and for their potency at A(2B) adenosine receptor subtypes. The corresponding 2-(1-alkynyl) derivatives of 5'-N-ethylcarboxamidoadenosine (NECA) and Ado are used as reference compounds. Binding studies demonstrated that the activities of 2-alkynylAdos were slightly increased for the adenosine A(1) receptor and slightly decreased for both A(3) and A(2B) subtypes compared to those of their corresponding NECA derivatives, whereas the A(2A) receptor affinities of the two series of nucleosides were similar. The presence of a methyl group on N(6) of the 2-alkynyladenosines, inducing an increase in affinity at the human A(3) receptor and a decrease at the other subtypes, resulted in an increase in A(3) selectivity. In particular, 2-phenylethynyl-N(6)-methylAdo (8b) showed an A(3) affinity in the low nanomolar range (K(i)(A(3)) = 3.4 nM), with a A(1)/A(3) and A(2A)/A(3) selectivity of about 500 and 2500, respectively. These findings motivated us to search for the preparation of new selective radioligands for the A(3) subtype; hence, a procedure to introduce a tritiated alkylamino group in these molecules was carried out. As far as the potency at the A(2B) receptor, the type of 2-alkynyl chain and the presence of the ethylcarboxamido group on the sugar seem to be very important; in fact, the (S)-2-phenylhydroxypropynylNECA [(S)-PHPNECA, 1e, EC(50)(A(2B)) = 0.22 microM] proved to be one of the most potent A(2B) agonist reported so far. On the other hand, the (S)-2-phenylhydroxypropynyl-N(6)-ethylAdo (9e, EC(50)(A(2B)) = 0.73 microM) showed a significantly increase of potency at the A(2B) subtype in comparison with the N(6)-methyl, N(6)-isopropyl, and the unsubstituted adenosine derivatives, although it resulted in being less potent than (S)-PHPNECA (1e, EC(50)(A(2B)) = 0.22 microM). These observations suggest that the introduction of an ethyl group in the N(6)-position and an ethylcarboxamido substituent in the 4'-position of (S)-2-phenylhydroxypropynyladenosine could lead to a compound endowed with high potency at the A(2B) receptor.

Induction of interleukin 8 release from the HMC-1 mast cell line: synergy between stem cell factor and activators of the adenosine A(2b) receptor

The HMC-1 mast cell line has both adenosine A(3) and A(2b) receptors on its surface, but only agonists of the A(2b) receptor are effective at releasing interleukin 8. Object of this study was to look for co-factors for adenosine A(2b) receptor activation. There was a powerful and statistically significant synergy for release of IL-8, both at the mRNA level (measured after 4 hr) and protein level (measured after 24 hr), between adenosine A(2b) receptor agonists and stem cell factor (SCF). Suitable concentrations for showing synergy were 100 ng/mL SCF and 3 microM 5'-N-ethylcarboxamidoadenosine (NECA). At these concentrations, the IL-8 released into the culture medium after SCF and NECA together was typically 3-5-fold greater in amount than the sum of the amounts of IL-8 released after exposure to the same concentrations of NECA and SCF separately. Since mast cells may be exposed to both adenosine and stem cell factor in the diseased lung, the synergy observed in this model system may have implications for asthma.

Involvement of K+ channels in adenosine A2A and A2B receptor-mediated hyperpolarization of porcine coronary artery endothelial cells

This study investigated the effects of the following adenosine agonists: 5;-ethylcarboxamidoadenosine (NECA), N6-cyclopentyadenosine (CPA) 2-[p-(2-carboxyethyl)]phenylamino-5;N-ethylcarboxamidoadenosine (CGS-21680), and 2-chloroadenosine (CAD) and its antagonist, 4-(2-[7-amino-2-[2-furyl]]1,2,4-triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM-241385), a selective A2A adenosine receptor antagonist, and the involvement of the K+ATP and KCa channels on the resting membrane potential (RMP) of confluent monolayers of cultured porcine coronary artery endothelial cells (PCAECs). Adenosine agonists and K+ATP channel openers (pinacidil, cromakalim) hyperpolarized cultured PCAECs. The average RMP was -32.31 +/- 1.2 mV. Adenosine agonists at 10-5 M caused a significant increase in RMP to -65.0 +/- 1.5 mV for CAD (a nonselective adenosine receptor agonist) to -75.9 +/- 1.6 mV for CGS-21680 (a selective A2A receptor agonist) and to -87.0 +/- 3.5 mV for NECA (a nonselective A1/A2A/A2B receptor agonist). Pinacidil and cromakalim at 10 microM increased the membrane potential to -76.2 +/- 1.2 mV and -75.22 +/- 0.12 mV, respectively. The hyperpolarization induced by adenosine receptor agonists and KATP openers was inhibited by an application of the K+ATP channel blocker glibenclamide (10 microM), indicating the involvement of the K+ATP channel in the adenosine-mediated hyperpolarization of PCAECs. Moreover, 1-EB10, a selective opener of the maxi-KCa channel, hyperpolarized PCAECs, and the effect of 1-EB10 was completely blocked by a selective, irreversible blocker of the high conductance KCa (maxi-K) channels (penitrem A), but it only partially blocked the effect of NECA. ZM-241385 has no effect on hyperpolarization elicited by K+ATP and KCa channel openers. However, ZM-241385 significantly blocked the hyperpolarization effect of CAD and CGS-21680. ZM-241385 partially blocked the hyperpolarizing effect of NECA, and a combination of ZM-241385 and penitrem A further blocked the hyperpolarizing effect of NECA. These results further support the involvement of K+ channels in adenosine A2A and A2B receptor-mediated hyperpolarization of PCAECs.

Expression of adenosine receptors in the preglomerular microcirculation

The purpose of this study was to systematically investigate the abundance of each of the adenosine receptor subtypes in the preglomerular microcirculation vs. other vascular segments and vs. the renal cortex and medulla. Rat preglomerular microvessels (PGMVs) were isolated by iron oxide loading followed by magnetic separation. For comparison, mesenteric microvessels, segments of the aorta (thoracic, middle abdominal, and lower abdominal), renal cortex, and renal medulla were obtained by dissection. Adenosine receptor protein and mRNA expression were examined by Western blotting, Northern blotting, and RT-PCR. Our results indicate that compared with other vascular segments and renal tissues, A1 and A2B receptor protein and mRNA are abundantly expressed in the preglomerular microcirculation, whereas A2A and A3 receptor protein and mRNA are barely detectable or undetectable in PGMVs. We conclude that, relative to other vascular and renal tissues, A1 and A2B receptors are well expressed in PGMVs, whereas A2A and A3 receptors are notably deficient. Thus A1 and A2B receptors, but not A2A or A3 receptors, may importantly regulate the preglomerular microcirculation.

Adenosine-regulated cell proliferation in pituitary folliculostellate and endocrine cells: differential roles for the A(1) and A(2B) adenosine receptors

A(1) and A(2) adenosine receptors have been identified in the pituitary gland, but the cell type(s) on which they are located and their effects on pituitary cell growth are not known. Therefore, we analyzed the expression of A(1) and A(2) receptors in primary rat anterior pituitary cells, two pituitary folliculostellate (TtT/GF and Tpit/F1) and two pituitary endocrine (GH(3) and AtT20) cell lines, and compared their effects on cell proliferation. In anterior pituitary and folliculostellate cells, adenosine and adenosine receptor agonists (5'-N-ethylcarboxamidoadenosine, a universal agonist, and CGS 21680, an A(2A) receptor agonist) stimulated cAMP levels with a rank order of potency that indicates the presence of functional A(2B) receptors. This stimulation, however, was not observed in either GH(3) or AtT20 cells, where adenosine and the A(1) receptor agonist 2-chloro-N(6)-cyclopentyladenosine inhibited VIP/forskolin-stimulated cAMP production. Expression of A(2B) and A(1) receptors in the folliculostellate cells and that of the A(1) receptor in the endocrine cells were confirmed by RT-PCR, immunocytochemistry, and ligand binding. Adenosine and 5'-N-ethylcarboxamidoadenosine dose-dependently (10 nM to 10 microM) stimulated growth in the folliculostellate, but not in the endocrine, cells, whereas in the latter, 100 microM adenosine and 2-chloro-N(6)-cyclopentyladenosine inhibited cell proliferation by slowing cell cycle progression. These data highlight the differential expression of A(1) and A(2B) adenosine receptors in pituitary cells and provide evidence for opposing effects of adenosine on pituitary folliculostellate and endocrine cell growth.

ATP stimulates peripheral axons in human, rat and mouse--differential involvement of A(2B) adenosine and P2X purinergic receptors

Receptors for ATP have been reported on peripheral nerve terminals. It is a widespread assumption that the axonal membrane does not possess this kind of chemosensitivity, although P2X purinoceptors have been found in isolated rat vagus nerve. Therefore, in the present study, effects of ATP and analogues were tested on the excitability of unmyelinated axons in isolated rat sural nerve, mouse dorsal roots, and human sural nerve. Bath application of ATP to all three types of axonal preparations increased axonal excitability, but the underlying receptors appear to differ in the various preparations. In rat sural nerve, alpha,beta-adenosine-5'-methylene triphosphate produced the strongest excitation. This effect was blocked by pyridoxal-phosphate-6-azophenyl-2',5'-disulphonic acid and indicates the presence of P2X receptors. In mouse dorsal roots, differences were found between fast and slow C-fibres. The latter responded to both P2X receptor and adenosine receptor agonists. In contrast, effects of ATP on faster-conducting C-fibres seem to be caused exclusively by effects of ATP on adenosine receptors. Application of ATP also excited C-fibres in fascicles of isolated human nerve. The pharmacological profile indicates activation of A(2B) adenosine receptors. However, we could not detect P2X receptors in this preparation with our techniques. These data show that the ATP sensitivity of sensory neurones is not restricted to their terminals. Activation of axonal purinergic receptors may contribute to the transduction of sensory, including nociceptive, stimuli.

Structure-activity relationships at human and rat A2B adenosine receptors of xanthine derivatives substituted at the 1-, 3-, 7-, and 8-positions

In the search for improved selective antagonist ligands of the A2B adenosine receptor, which have the potential as antiasthmatic or antidiabetic drugs, we have synthesized and screened a variety of alkylxanthine derivatives substituted at the 1-, 3-, 7-, and 8-positions. Competition for 125I-ABOPX (125I-3-(4-amino-3-iodobenzyl)-8-(phenyl-4-oxyacetate)-1-propylxanthine) binding in membranes of stably transfected HEK-293 cells revealed uniformly higher affinity (<10-fold) of these xanthines for human than for rat A2B adenosine receptors. Binding to rat brain membranes expressing A1 and A2A adenosine receptors revealed greater A2B selectivity over A2A than A1 receptors. Substitution at the 1-position with 2-phenylethyl (or alkyl/olefinic groups) and at N-3 with hydrogen or methyl favored A2B selectivity. Relative to enprofylline 2b, pentoxifylline 35 was equipotent and 1-propylxanthine 3 was >13-fold more potent at human A2B receptors. Most N-7 substituents did not enhance affinity over hydrogen, except for 7-(2-chloroethyl), which enhanced the affinity of theophylline by 6.5-fold to 800 nM. The A2B receptor affinity-enhancing effects of 7-(2-chloroethyl) vs 7-methyl were comparable to the known enhancement produced by an 8-aryl substitution. Among 8-phenyl analogues, a larger alkyl group at the 1-position than at the 3-position favored affinity at the human A2B receptor, as indicated by 1-allyl-3-methyl-8-phenylxanthine, with a K(i) value of 37 nM. Substitution on the 8-phenyl ring indicated that an electron-rich ring was preferred for A2B receptor binding. In conclusion, new leads for the design of xanthines substituted in the 1-, 3-, 7-, and 8-positions as A2B receptor-selective antagonists have been identified.

Differential expression of adenosine receptors in human endothelial cells: role of A2B receptors in angiogenic factor regulation

Adenosine has been reported to stimulate or inhibit the release of angiogenic factors depending on the cell type examined. To test the hypothesis that differential expression of adenosine receptor subtypes contributes to endothelial cell heterogeneity, we studied microvascular (HMEC-1) and umbilical vein (HUVEC) human endothelial cells. Based on mRNA level and stimulation of adenylate cyclase, we found that HUVECs preferentially express A2A adenosine receptors and HMEC-1 preferentially express A2B receptors. Neither cells expressed A1 or A3 receptors. The nonselective adenosine agonist 5'-N-ethylcarboxamidoadenosine (NECA) increased expression of interleukin-8 (IL-8), basic fibroblast growth factor (bFGF), and vascular endothelial growth factor (VEGF) in HMEC-1, but had no effect in HUVECs. In contrast, the selective A2A agonist 2-p-(2-carboxyethyl)phenylethylamino-NECA (CGS 21680) had no effect on expression of these angiogenic factors. Cotransfection of each type of adenosine receptors with a luciferase reporter in HMEC-1 showed that A2B receptors, but not A1, A2A, or A3, activated IL-8 and VEGF promoters. These effects were mimicked by constitutively active alphaG(q), alphaG12, and alphaG13, but not alphaG(s) or alphaG(i1-3). Furthermore, stimulation of phospholipase C indicated coupling of A2B receptors to G(q) proteins in HMEC-1. Thus, differential expression of adenosine receptor subtypes contributes to functional heterogeneity of human endothelial cells. A2B receptors, predominantly expressed in human microvascular cells, modulate expression of angiogenic factors via coupling to G(q), and possibly via G12/13.

[Neuroprotective mechanisms of adenosine action on CNS neurons]

Adenosine is known to play a significant role as modulator of neuronal activity. Extracellular levels of adenosine increase during ischaemia. Adenosine has been reported to have beneficial effects against neurodegenerative diseases, although the mechanisms are not fully clarified. Adenosine A1 receptors are found on neurons and presumably involved in the regulation of the release of excitatory amino acids. Using adenosine kinase inhibitors and novel A1 agonists with potent CNS effects and diminished influence on the cardiovascular system offers therapeutic potential to the treatment of neurodegenerative brain disorders. Recently deleterious effects have been reported following the activation of A2A and A3 adenosine receptor subtypes. The selective A2A receptor antagonists have been demonstrated to markedly reduce cell death after brain ischaemia in the rat. This paper discusses the possible role of adenosine receptor subtypes and their agonists or antagonists in neurodegenerative diseases treatment.

Adenine nucleotide-induced activation of adenosine A(2B) receptors expressed in Xenopus laevis oocytes: involvement of a rapid and localized adenosine formation by ectonucleotidases

We recently demonstrated that extracellular ATP effectively activates adenosine (Ade) A(2B) receptors indirectly through a localized rapid conversion to Ade by ectonucleotidases on the membrane surface of C6Bu-1 rat glioma cells. These responses were observed even in the presence of adenosine deaminase (ADA). Here, we demonstrate that such responses indeed occur in A(2B) receptor-expressing Xenopus laevis oocytes, which possess endogenous ectonucleotidase activity. In oocytes coexpressing the A(2B) receptor and cystic fibrosis transmembrane conductance regulator (CFTR), Ade induced a concentration-dependent increase in a cyclic AMP-activated CFTR current, a response that was inhibited by the P1 antagonist xanthine-amine congener (XAC). A brief application of ATP and beta,gamma-methylene ATP (beta,gamma-MeATP) also induced the CFTR current in a manner similar to that seen with Ade. Among several nucleotide agonists, ADP, AMP, and adenosine-5'-O-(3-thio)triphosphate induced the CFTR current. Although adenine nucleotide-induced CFTR currents were inhibited by XAC, they were highly resistant to ADA treatment; 5 U/ml ADA was required for inhibition of adenine nucleotide-induced CFTR current, whereas 1 U/ml ADA was sufficient to abolish the Ade-induced response. In addition, the ecto-5'-nucleotidase inhibitor alpha,beta-methylene ADP markedly inhibited the beta,gamma-MeATP-induced response but not the Ade-induced one. These results support our hypothesis that adenine nucleotides are rapidly and locally converted into Ade on the membrane surface, resulting in the activation of A(2B) receptors.

Rhythmic gene expression in pituitary depends on heterologous sensitization by the neurohormone melatonin

In mammals, many daily cycles are driven by a central circadian clock, which is based on the cell-autonomous rhythmic expression of clock genes. It is not clear, however, how peripheral cells are able to interpret the rhythmic signals disseminated from this central oscillator. Here we show that cycling expression of the clock gene Period1 in rodent pituitary cells depends on the heterologous sensitization of the adenosine A2b receptor, which occurs through the nocturnal activation of melatonin mt1 receptors. Eliminating the impact of the neurohormone melatonin simultaneously suppresses the expression of Period1 and evokes an increase in the release of pituitary prolactin. Our findings expose a mechanism by which two convergent signals interact within a temporal dimension to establish high-amplitude, precise and robust cycles of gene expression.

Adenosine receptors: G protein-mediated signalling and the role of accessory proteins

Ever since the discovery of the effects of adenosine in the circulation, adenosine receptors continue to represent a promising drug target. Firstly, this is due to the fact that the receptors are expressed in a large variety of cells; in particular, the actions of adenosine (or, respectively, of the antagonistic methylxanthines) in the central nervous system, in the circulation, on immune cells and on other tissues can be beneficial in certain disorders. Secondly, there exists a large number of ligands, which have been generated by introducing several modifications in the structure of the lead compounds (adenosine and methylxanthine), some of them highly specific. Four adenosine receptor subtypes have been identified by molecular cloning; they belong to the family of G protein-coupled receptors, which transfer signals by activating heterotrimeric G proteins. It has been appreciated recently that accessory proteins impinge on the receptor/G protein interaction and thus modulate the signalling reaction. These accessory components may be thought as adaptors that redirect the signalling pathway to elicit a cell-specific response. Here, we review the recent literature on adenosine receptors and place a focus on the role of accessory proteins in the organisation of adenosine receptor signalling. These components have been involved in receptor sorting, in the control of signal amplification and in the temporal regulation of receptor activity, while the existence of others is postulated on the basis of atypical cellular reactions elicited by receptor activation.

A(2B) adenosine receptors stimulate growth of porcine and rat arterial endothelial cells

The goal of this study was to determine which adenosine receptor subtype mediates growth stimulation by adenosine in arterial endothelial cells. In porcine coronary artery and rat aortic endothelial cells, 2-chloroadenosine (Cl-Ad), a metabolically stable analog of adenosine, stimulated DNA synthesis ((3)H-thymidine incorporation), cellular proliferation (cell number), collagen synthesis ((3)H-proline incorporation), and cell migration. The growth effects of adenosine and Cl-Ad were mimicked by the adenosine receptor agonist 5'-N-methylcarboxamidoadenosine but not by the adenosine receptor agonists N(6)-cyclopentyladenosine, 4-aminobenzyl-5'-N-methylcarboxamidoadenosine or CGS21680, an agonist profile consistent with an A(2B) receptor-mediated effect. The adenosine receptor antagonists KF17837 and 1,3-dipropyl-8-p-sulfophenylxanthine but not 8-cyclopentyl-1,3-dipropylxanthine blocked the growth-stimulatory effects of Cl-Ad and 5'-N-methylcarboxamidoadenosine, an antagonist profile consistent with an A(2) receptor-mediated action. Treatment of endothelial cells with erythro-9-(2-hydroxy-3-nonyl) adenine plus iodotubericidin (inhibitors of adenosine deaminase and adenosine kinase, respectively) induced endothelial cell growth, and these effects were blocked by 1,3-dipropyl-8-p-sulfophenylxanthine and KF17837 but not 8-cyclopentyl-1,3-dipropylxanthine, suggesting that endothelial cell-derived adenosine induces growth via A(2) receptors. The growth-stimulatory effects of Cl-Ad, 5'-N-methylcarboxamidoadenosine, and erythro-9-(2-hydroxy-3-nonyl) adenine plus iodotubericidin were abolished by antisense but not scrambled or sense oligonucleotides to the A(2B) receptor. Our findings strongly support the hypothesis that adenosine induces endothelial cell growth by activating A(2B) receptors. Thus, A(2B) receptors may play a critical role in regulating vascular remodeling associated with endothelial cell proliferation in angiogenesis, collateral vessel development, and recovery after vascular injury. Pharmacological or molecular biological activation of A(2B) receptors may be useful in modulating vascular remodeling.

Cardiac effects of adenosine in A(2A) receptor knockout hearts: uncovering A(2B) receptors

To clarify the relative roles of A(2) adenosine receptor subtypes in the regulation of coronary flow and myocardial contractility, coronary vascular and functional responses to adenosine and its analogs were examined in isolated wild-type (WT) and A(2A) receptor knockout (A(2A)KO) mouse hearts. Nonselective agonists adenosine and 5'-N-ethyl-carboxamido-adenosine (NECA) increased coronary flow in A(2A)KO hearts, albeit with a rightward shift of concentration-response curves and decreased maximal vasodilation compared with WT hearts. 2-p-(2-Carboxy-ethyl)phenethylamino-5'-N-ethyl-carboxamidoadenosine (CGS-21680, a selective A(2A) receptor agonist) increased coronary flow in WT hearts but did not affect A(2A)KO hearts. Adenosine and NECA each elicited equal maximal increases in developed pressure in WT and A(2A)KO hearts, whereas CGS-21680 did not affect developed pressure in A(2A)KO hearts. Alloxazine, a selective A(2B) receptor antagonist, attenuated NECA-induced coronary vasodilation (from 202 +/- 14% to 128 +/- 9% of baseline, P < 0.05) and NECA-induced increases in developed pressure (from 133 +/- 8% to 112 +/- 7% of baseline, P < 0.05) in A(2A)KO hearts. Together, these findings support the conclusion that A(2B) adenosine receptor activation increases coronary flow and developed pressure in isolated murine hearts.

Compartmentalized autocrine signaling to cystic fibrosis transmembrane conductance regulator at the apical membrane of airway epithelial cells

Physical stimulation of airway surfaces evokes liquid secretion, but the events that mediate this vital protective function are not understood. When cystic fibrosis transmembrane conductance regulator (CFTR) channel activity was used as a functional readout, we found signaling elements compartmentalized at both extracellular and intracellular surfaces of the apical cell membrane that activate apical Cl(-) conductance in Calu-3 cells. At the outer surface, ATP was released by physical stimuli, locally converted to adenosine, and sensed by A(2B) adenosine receptors. These receptors couple to G proteins, adenylyl cyclase, and protein kinase A, at the intracellular face of the apical membrane to activate colocalized CFTR. Thus, airways have evolved highly efficient mechanisms to "flush" noxious stimuli from airway surfaces by selective activation of apical membrane signal transduction and effector systems.

Inhibition of human mast cell activation with the novel selective adenosine A(2B) receptor antagonist 3-isobutyl-8-pyrrolidinoxanthine (IPDX)(2)

The antiasthmatic drug enprofylline was the first known selective, though not potent, A(2B) antagonist. On the basis of structure-activity relationships (SARs) of xanthine derivatives, we designed a novel selective adenosine A(2B) receptor antagonist, 3-isobutyl-8-pyrrolidinoxanthine (IPDX), with potency greater than that of enprofylline. IPDX displaced [3H]ZM241385 ([3H]4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a]-[1,3,5]triazin-5-ylamino]ethyl)phenol) from human A(2B) adenosine receptors with a K(i) value of 470 +/- 2 nM and inhibited A(2B)-dependent cyclic AMP (cAMP) accumulation in human erythroleukemia (HEL) cells with a K(B) value of 625 +/- 71 nM. We found that IPDX was more selective than enprofylline toward human A(2B) receptors. It was 38-, 55-, and 82-fold more selective for human A(2B) than for human A(1) (K(i) value of 24 +/- 8 microM), human A(2A) (K(B) value of 36 +/- 8 microM), and human A(3) (K(i) value of 53 +/- 10 microM) adenosine receptors, respectively. IPDX inhibited NECA (5'-N-ethylcarboxamidoadenosine)-induced interleukin-8 secretion in human mast cells (HMC-1) with a potency close to that determined for A(2B)-mediated cAMP accumulation in HEL cells, thus confirming the role of A(2B) adenosine receptors in mediating human mast cell activation. Since adenosine triggers bronchoconstriction in asthmatic patients through human mast cell activation, IPDX may become a basis for the development of new antiasthmatic drugs with improved properties compared with those of enprofylline. Our data demonstrate that IPDX can be used as a tool to differentiate between A(2B) and other adenosine receptor-mediated responses.

Adenosine A(2A) and A(2B) receptor activation of erythropoietin production

We have examined the effects of adenosine receptors and protein kinases A and C in the regulation of erythropoietin (Epo) production using hepatocellular carcinoma (Hep3B) cells in culture and in vivo in normal mice under normoxic and hypoxic conditions. CGS-21680, a selective adenosine A(2A) agonist, significantly increased levels of Epo in normoxic Hep3B cell cultures and in serum of normal mice under both normoxic and hypoxic conditions. CGS-21680 also produced a significant increase in Epo mRNA levels in Hep3B cell cultures. SCH-58261, a selective adenosine A(2A) receptor antagonist, significantly inhibited the increase in medium levels of Epo in Hep3B cell cultures exposed to hypoxia (1% O(2)). Enprofylline, a selective adenosine A(2B) receptor antagonist, significantly inhibited the increase in plasma levels of Epo in normal mice exposed to hypoxia. Chelerythrine chloride, an antagonist of protein kinase C activation, significantly inhibited hypoxia-induced increases in serum levels of Epo in normal mice. A model is presented for adenosine in hypoxic regulation of Epo production that involves kinases A and C and phospholipase A(2) pathways.

Differential gene expression of adenosine A1, A2a, A2b, and A3 receptors in the human enteric nervous system

Adenosine receptors (ADORs) in the enteric nervous system may be of importance in the control of motor and secretomotor functions. Gene expression and distribution of neural adenosine A1, A2a, A2b, or A3 receptors (Rs) in the human intestine was investigated using immunochemical, Western blotting, RT-PCR, and short-circuit current (I(sc)) studies. Adenosine A1R, A2aR, A2bR, or A3R mRNAs were differentially expressed in neural and nonneural layers of the jejunum, ileum, colon, and cecum and in HT-29, T-84, T98G, and Bon cell lines. A1R, A2aR, A2bR, and A3R immunoreactivities (IRs) were differentially expressed in PGP 9.5-immunoreactive neurons. A2bR IR occurs exclusively in 50% of submucosal vasoactive intestinal peptide (VIP) neurons (interneurons, secretomotor or motor neurons) in jejunum, but not colon; A2aR is also found in other neurons. A3R IR occurs in 57% of substance P-positive jejunal submucosal neurons (putative intrinsic primary afferent neurons) and less than 10% of VIP neurons. Western blots revealed bands for A3R at 44 kDa, 52 kDa, and 66 kDa. A2aR and A2bR are coexpressed in enteric neurons and epithelial cells. 5'-N-methylcarboxamidoadenosine or carbachol evoked an increase in I(sc). A2bR IR is more prominent than A2aR IR in myenteric neurons, nerve fibers, or glia. A1R is expressed in jejunal myenteric neurons and colonic submucosal neurons. Regional differences also exist in smooth muscle expression of ADOR IR(s). It is concluded that neural and nonneural A1, A2a, A2b, and A3Rs may participate in the regulation of neural reflexes in the human gut. Clear cell and regional differences exist in ADOR gene expression, distribution, localization, and coexpression.

Pharmacological characterization of adenosine receptors in PGT-beta mouse pineal gland tumour cells

1. The adenosine receptor in mouse pinealocytes was identified and characterized using pharmacological and physiological approaches. 2. Expression of the two adenosine receptor subtypes A2B and A3 was detected in mouse pineal glands and PGT-beta cells by polymerase chain reaction and nucleotide sequencing. 3. Adenosine and 5'-N-ethylcarboxamidoadenosine (NECA) evoked cyclic AMP generation but the A2)-selective agonist 2-(4-(2-carboxyethyl)phenylethylamino)adenosine-5'-N-ethylcarboxamideadenosine (CGS 21680) and the A1-specific agonists R-N(6)-(2-phenylisopropyl)adenosine (R-PIA) and N(6)-cyclopentyladenosine (CPA) had little effect on intracellular cyclic AMP levels. The A2B receptor selective antagonists alloxazine and enprofylline completely blocked NECA-mediated cyclic AMP accumulation. 4. Treatment of cells with the A3-selective agonist N(6)-(3-iodobenzyl)-5'-(N-methylcarbamoyl)adenosine (IB-MECA) inhibited the elevation of the cyclic AMP level induced by NECA or isoproterenol in a concentration-dependent manner with maximal inhibition of 40 - 50%. These responses were blocked by the specific A3 adenosine receptor antagonist MRS 1191. Pretreatment of the cells with pertussis toxin attenuated the IB-MECA-induced responses, suggesting that this effect occurred via the pertussis toxin-sensitive inhibitory G proteins. 5. IB-MECA also caused a concentration-dependent elevation in [Ca(2+)]i and IP3 content. Both the responses induced by IB-MECA were attenuated by treatment with U73122 or phorbol 12-myristate 13-acetate. 6. These data suggest the presence of both A2B and A3 adenosine receptors in mouse pineal tumour cells and that the A2B receptor is positively coupled to adenylyl cyclase whereas the A3 receptor is negatively coupled to adenylyl cyclase and also coupled to phospholipase C.

Expression and function of adenosine receptors in human dendritic cells

Dendritic cells (DCs) are specialized antigen-presenting cells characterized by their ability to migrate into target sites, process antigens, and activate naive T cells. In this study, we analyzed the biological activity and intracellular signaling of adenosine by using reverse transcriptase-polymerase chain reaction assays to investigate mRNA expression of A(1), A(2a) and A(3) adenosine receptors in immature and mature human DCs. Functional experiments on adenosine stimulation showed chemotaxis, intracellular calcium transients, and actin polymerization, but no activation of adenylate cyclase in immature DCs. Experiments with receptor isotype-selective agonists and antagonists as well as pertussis toxin revealed that chemotaxis, calcium transients, and actin polymerization were mediated via G(i-) or G(0-)protein-coupled A(1) and A(3) receptors. Maturation of DCs induced by lipopolysaccharide (LPS) resulted in down-regulation of A(1) and A(3) receptor mRNAs, although A(2a) receptor mRNA was still expressed. However, in LPS-differentiated DCs, adenosine and an A(2a) receptor agonist stimulated adenylate cyclase activity, enhanced intracellular cAMP levels, and inhibited interleukin 12 (IL-12) production. These effects could be completely prevented by pretreatment with A(2) receptor antagonist. These findings strongly suggest that adenosine has important but distinct biological effects in DCs activity as a chemotaxin for immature DCs and as a modulator of IL-12 production in mature DCs. These effects can be explained by differential expression of adenosine receptor subtypes.

Methotrexate suppresses NF-kappaB activation through inhibition of IkappaBalpha phosphorylation and degradation

Methotrexate (MTX), a folate antagonist, is a commonly used anti-inflammatory, antiproliferative, and immunosuppressive drug whose mode of action is not fully established. Due to the central role of NF-kappaB in these responses, we postulated that MTX must mediate its effects through suppression of NF-kappaB activation. We investigated the effects of MTX on NF-kappaB activation induced by TNF in Jurkat cells. The treatment of these cells with MTX suppressed TNF-induced NF-kappaB activation with optimum effects occurring at 10 microM MTX for 60 min. These effects were not restricted to Jurkat cells because other cell types were also inhibited. Besides TNF, MTX also suppressed the NF-kappaB activation induced by various other inflammatory stimuli. The suppression of TNF-induced NF-kappaB activation by MTX correlated with inhibition of IkappaBalpha degradation, suppression of IkappaBalpha phosphorylation, abrogation of IkappaBalpha kinase activation, and inhibition of NF-kappaB-dependent reporter gene expression. Because ecto 5' nucleotidase inhibitor (alpha,beta-methylene adenosine-5'-diphosphate) blocked the effect of MTX, adenosine mimicked the effect of MTX, and adenosine A2b receptor antagonist (3,7-dimethyl-1-propargylxanthine) reversed the inhibitory effect of MTX, we suggest that MTX suppresses NF-kappaB activation by releasing adenosine. A partial reversal of MTX-induced NF-kappaB suppression by thymidine and folinic acid indicates the role of the thymidylate synthase pathway also. Overall, our results clearly demonstrate that MTX suppresses NF-kappaB activation through the release of adenosine, which may contribute to the role of MTX in anti-inflammatory, immunomodulatory, and antiproliferative effects.

Rapid agonist-induced desensitization and internalization of the A(2B) adenosine receptor is mediated by a serine residue close to the COOH terminus

The G(s)-coupled rat A(2B) adenosine receptor (A(2B)-AR) was epitope-tagged at the NH(2) terminus with hemagglutinin (HA) and subjected to progressive deletions or point mutations of the COOH terminus in order to determine regions of the receptor that contribute to agonist-induced desensitization and internalization. When expressed stably in Chinese hamster ovary cells, a mutant receptor in which the final 2 amino acids were deleted, the Leu(330)-stop mutant, underwent rapid agonist-induced desensitization and internalization as did the wild type (WT) receptor. However, the Phe(328) and the Gln(325)-stop mutants were resistant to rapid agonist-induced desensitization and internalization. Co-expression of arrestin-2-green fluorescent protein (arrestin-2-GFP) with WT receptor or Leu(330)-stop mutant resulted in rapid translocation of arrestin-2-GFP from cytosol to membrane upon agonist addition. On the other hand, agonist activation of the Phe(328)-stop or Gln(325)-stop mutant did not result in translocation of arrestin-2-GFP from cytosol. A COOH terminus point mutant, S329G, was also unable to undergo rapid agonist-induced desensitization and internalization, indicating that Ser(329) is a critical residue for these processes. A further deletion mutant (Ser(326)-stop) unexpectedly underwent rapid agonist-induced desensitization and internalization. However, activation of this mutant did not promote translocation of arrestin-2-GFP from cytosol to membrane. In addition, whereas WT receptor internalization was markedly inhibited by co-expression of dominant negative mutants of arrestin-2 (arrestin-2-(319-418)), dynamin (dynamin K44A), or Eps-15 (EDelta95-295), Ser(326)-stop receptor internalization was only inhibited by dominant negative mutant dynamin. Taken together these results indicate that Ser(329), close to the COOH terminus of the rat A(2B)-AR, is critical for the rapid agonist-induced desensitization and internalization of the receptor. However, deletion of the COOH terminus also uncovers a motif that is able to redirect internalization of the receptor to an arrestin- and clathrin-independent pathway.

Proliferation, migration, and ERK activation in human retinal endothelial cells through A(2B) adenosine receptor stimulation

PURPOSE

The nucleoside adenosine has been implicated in angiogenesis. A previous study demonstrated that activation of the A(2B) adenosine receptor (AdoR) increases cAMP accumulation, cell proliferation, and VEGF expression in human retinal endothelial cells (HRECs). In the present study, the role of this receptor was further characterized by examination of the effects of the selective A(2B) AdoR antagonists 3-N-propylxanthine (enprofylline) and 3-isobutyl-8-pyrrolidinoxanthine (IPDX) on AdoR-mediated HREC proliferation, capillary tube formation, and signal-transduction pathways.

METHODS

HRECs were exposed to the adenosine analogue 5'-N-ethylcarboxamido-adenosine (NECA) in the absence or presence of AdoR antagonists. Migration was measured using Boyden chambers. Proliferation was assessed by counting cells. Western analysis was used to assess extracellular signal-related kinase (ERK) and cAMP response element-binding protein (CREB) in cell lysates. The effect of AdoR activation on tube formation was studied using cells grown on a synthetic basement membrane matrix.

RESULTS

NECA induced proliferation in a concentration-dependent manner that was inhibited by enprofylline and IPDX. NECA stimulated chemotaxis in a concentration-dependent manner that was also blocked by both A(2B) AdoR antagonists. NECA activated ERK and CREB in HRECs. Both A(2B) AdoR antagonists diminished activation of ERK by NECA exposure. ERK activation was also blocked by the ERK-mitogen-activated protein kinase (MAPK) inhibitor PD98059, but not by the protein kinase A (PKA) inhibitor H-89. CREB activation was blocked by H-89, but not by PD98059, suggesting that ERK activation is independent of PKA. NECA enhanced tube formation on the matrix, whereas both A(2B) AdoR antagonists attenuated this effect.

CONCLUSIONS

The selective A(2B) AdoR antagonists, enprofylline and IPDX, inhibited NECA-stimulated proliferation, ERK activation, cell migration, and capillary tube formation. A(2B) AdoR inhibition may offer a way to inhibit retinal angiogenesis and provide a novel therapeutic approach to treatment of diseases associated with aberrant neovascularization, such as diabetic retinopathy and retinopathy of prematurity.

Inhibition of NF-kappaB-mediated gene transcription by the human A2B adenosine receptor in Chinese hamster ovary cells

NF-kappaB is a transcription factor that plays a vital role in regulating inducible gene expression in immune and inflammatory responses. In view of the well documented effects of adenosine on immune and inflammatory responses, we have explored whether adenosine A1, A2B and A3 receptors regulate NF-kappaB activity in transfected Chinese hamster ovary (CHO) cells using a luciferase reporter gene construct. No increases in NF-kappaB activity were observed in CHO-A1, -A2B and -A3 cells stimulated with the non-selective adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine. Elevating intracellular cyclic AMP (cAMP) levels using forskolin (direct activator of adenylyl cyclase) and rolipram (type IV, cAMP-specific phosphodiesterase inhibitor), inhibited NF-kappaB activity in CHO cells. Adenosine A2B receptor stimulation also inhibited NF-kappaB activity, whereas adenosine A1 and A3 receptor activation had no effect. These data reflect the known coupling of adenosine A2B receptors to increases in cAMP. In conclusion, adenosine A1, A2B and A3 receptors do not directly activate NF-kappaB in CHO cells. However, adenosine A2B receptor activation significantly inhibited NF-kappaB activity. Inhibition of NF-kappaB activity by the adenosine A2B receptor may contribute to the anti-inflammatory effects of adenosine.

Neutrophil-epithelial crosstalk at the intestinal lumenal surface mediated by reciprocal secretion of adenosine and IL-6

Adenosine is formed in the intestinal lumen during active inflammation from neutrophil-derived 5' AMP. Using intestinal epithelial cell line T84, we studied the effect of adenosine on the secretion of IL-6, a proinflammatory cytokine involved in neutrophil degranulation and lymphocyte differentiation. Stimulation of T84 monolayers with either apical or basolateral adenosine induces A2b receptor-mediated increase in IL-6 secretion, which is polarized to the apical (luminal) compartment. In addition, Salmonella typhimurium, TNF-alpha, and forskolin, known inducers of IL-6 secretion in intestinal epithelial cells, also stimulate IL-6 secretion into the apical compartment. We show that IL6 promoter induction by adenosine occurs through cAMP-mediated activation of nuclear cAMP-responsive element-binding protein (CREB). We also show that IL-6 released in the luminal (apical) compartment achieves a sufficient concentration to activate neutrophils (from which the adenosine signal originates), since such IL-6 is found to induce an intracellular [Ca(++)] flux in neutrophils. We conclude that adenosine released in the intestinal lumen during active inflammation may induce IL-6 secretion, which is mediated by cAMP/CREB activation and occurs in an apically polarized fashion. This would allow sequential activation of neutrophil degranulation in the lumen -- a flow of events that would, in an epithelium-dependent fashion, enhance microbicidal activity of neutrophils as they arrive in the intestinal lumen.

Adenosine inhibits neutrophil vascular endothelial growth factor release and transendothelial migration via A2B receptor activation

The effects of adenosine on neutrophil (polymorphonuclear neutrophils; PMN)-directed changes in vascular permeability are poorly characterized. This study investigated whether adenosine modulates activated PMN vascular endothelial growth factor (vascular permeability factor; VEGF) release and transendothelial migration. PMN activated with tumour necrosis factor-alpha (TNF-alpha, 10 ng/mL) were incubated with adenosine and its receptor-specific analogues. Culture supernatants were assayed for VEGF. PMN transendothelial migration across human umbilical vein endothelial cell (HUVEC) monolayers was assessed in vitro. Adhesion molecule receptor expression was assessed flow cytometrically. Adenosine and some of its receptor-specific analogues dose-dependently inhibited activated PMN VEGF release. The rank order of potency was consistent with the affinity profile of human A2B receptors. The inhibitory effect of adenosine was reversed by 3,7-dimethyl-1-propargylxanthine, an A2 receptor antagonist. Adenosine (100 microM) or the A2B receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA, 100 microM) significantly reduced PMN transendothelial migration. However, expression of activated PMN beta2 integrins and HUVEC ICAM-1 were not significantly altered by adenosine or NECA. Adenosine attenuates human PMN VEGF release and transendothelial migration via the A2B receptor. This provides a novel target for the modulation of PMN-directed vascular hyperpermeability in conditions such as the capillary leak syndrome.

[3H]MRS 1754, a selective antagonist radioligand for A(2B) adenosine receptors

MRS 1754 [N-(4-cyanophenyl)-2-[4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)-phenoxy]acetamide] is a selective antagonist ligand of A(2B) adenosine receptors. This is the least well-defined adenosine receptor subtype, and A(2B) antagonists have potential as antiasthmatic drugs. For use as a radioligand, MRS 1754, a p-cyanoanilide xanthine derivative, was tritiated on the propyl groups in a two-step reaction using a p-carboxamido precursor, which was dehydrated to the cyano species using trifluoroacetic anhydride. [3H]MRS 1754 (150 Ci/mmol) bound to recombinant human A(2B) adenosine receptors in membranes of stably transfected HEK-293 cells. Specific binding was saturable, competitive, and followed a one-site model, with a K(D) value of 1.13 +/- 0.12 nM and a B(max) value of 10.9 +/- 0.6 pmol/mg protein. Specific binding utilizing 0.7 nM [3H]MRS 1754 was > 70% of total binding. The affinity calculated from association and dissociation binding constants was 1.22 nM (N = 4). Binding to membranes expressing rat and human A(1) and A(3) adenosine receptors was not significant, and binding in membranes of HEK-293 cells expressing human A(2A) receptors was of low affinity (K(D) > 50 nM). The effects of cations and chelators were explored. Specific binding was constant over a pH range of 4.5 to 6.5, with reduced binding at higher pH. The pharmacological profile in competition experiments with [3H]MRS 1754 was consistent with the structure-activity relationship for agonists and antagonists at A(2B) receptors. The K(i) values of XAC (xanthine amine congener) and CPX (8-cyclopentyl-1,3-dipropylxanthine) were 16 and 55 nM, respectively. NECA (5'-N-ethylcarboxamidoadenosine) competed for [3H]MRS 1754 binding with a K(i) of 570 nM, similar to its potency in functional assays. Thus, [3H]MRS 1754 is suitable as a selective, high-affinity radioligand for A(2B) receptors.