A3 adenosine receptor activation triggers phosphorylation of protein kinase B and protects rat basophilic leukemia 2H3 mast cells from apoptosis

Adenosine receptors in colon carcinoma tissues and colon tumoral cell lines: focus on the A(3) adenosine subtype

Adenosine may affect several pathophysiological processes, including cellular proliferation, through interaction with A(1), A(2A), A(2B), and A(3) receptors. In this study we characterized adenosine receptors in human colon cancer tissues and in colon cancer cell lines Caco2, DLD1, HT29. mRNA of all adenosine subtypes was detected in cancer tissues and cell lines. At a protein levels low amount of A(1), A(2A), and A(2B) receptors were detected, whilst the A(3) was the most abundant subtype in both cancer tissues and cells, with a pharmacological profile typical of the A(3) subtype. All the receptors were coupled to stimulation/inhibition of adenylyl-cyclase in cancer cells, with the exception of A(1) subtype. Adenosine increased cell proliferation with an EC(50) of 3-12 microM in cancer cells. This effect was not essentially reduced by adenosine receptor antagonists. However dypiridamol, an adenosine transport inhibitor, increased the stimulatory effect induced by adenosine, suggesting an action at the cell surface. Addition of adenosine deaminase makes the A(3) agonist 2-chloro-N6-(3-iodobenzyl)-N-methyl-5'-carbamoyladenosine (Cl-IB-MECA) able to stimulate cell proliferation with an EC(50) of 0.5-0.9 nM in cancer cells, suggesting a tonic proliferative effect induced by endogenous adenosine. This effect was antagonized by 5-N-(4-methoxyphenyl-carbamoyl)amino-8-propyl-2(2furyl)-pyrazolo-[4,3e]-1,2,4-triazolo [1,5-c] pyrimidine (MRE 3008F20) 10 nM. Cl-IB-MECA-stimulated cell proliferation involved extracellular-signal-regulated-kinases (ERK1/2) pathway, as demonstrated by reduction of proliferation with 1,4-diamino-2,3-dicyano-1,4-bis-[2-amino-phenylthio]-butadiene (U0126) and by ERK1/2 phosphorylation. In conclusion this study indicates for the first time that in colon cancer cell lines endogenous adenosine, through the interaction with A(3) receptors, mediates a tonic proliferative effect.

Adenosine protects against suicidal erythrocyte death

Suicidal death of erythrocytes or eryptosis is characterized by cell shrinkage and cell membrane scrambling leading to phosphatidylserine exposure at the erythrocyte surface. The cell membrane scrambling is triggered by an increase in cytosolic Ca(2+) activity and activation of protein kinase C (PKC). Phosphatidylserine exposure fosters adherence of affected erythrocytes to the vascular wall. Thus, microcirculation in ischemic tissues may be impaired by the appearance of eryptotic erythrocytes. Ischemia leads to release of adenosine, which in most tissues leads to vasodilation and protects against cell injury. The present experiments explored whether adenosine influences mechanisms underlying eryptosis. Erythrocyte phosphatidylserine exposure was estimated from annexin V binding, cell volume from forward scatter and cytosolic Ca(2+) activity from Fluo3 fluorescence. Glucose depletion (for 24 or 48 h) significantly increased annexin binding and decreased forward scatter, effects partially reversed by adenosine. The protective effect of adenosine reached statistical significance (s.d.) at > =30 microM. Low Cl(-) solution (Cl(-) exchanged by gluconate for 24 h) similarly increased annexin binding and decreased forward scatter, effects again reversed by adenosine (s.d. at > or =10 and 30 microM, respectively). Similarly, phosphatase inhibitor okadaic acid (OA, 1 microM) and PKC activator phorbol 12-myristate-13-acetate (PMA, 3 microM) significantly enhanced annexin binding and decreased forward scatter. Adenosine significantly blunted the effects of OA and PMA on annexin V binding (s.d. at > or =30 and 10 microM, respectively) and the effect of OA on forward scatter (s.d. at > or =10 microM). In conclusion, adenosine inhibits eryptosis by a mechanism presumably effective downstream of PKC. The effect may participate in the maintenance of microcirculation in ischemic tissue.

Modulation of the Akt/Ras/Raf/MEK/ERK pathway by A₃ adenosine receptor

Downstream A₃ receptor signalling plays an important role in the regulation of cell death and proliferation. Therefore, it is important to determine the molecular pathways involved through A₃ receptor stimulation. The phosphatidylinositide-3-OH kinase (PI3K)/Akt and the Raf/mitogen-activated protein kinase (MAPK/ERK) kinase (MEK)/mitogen-activated protein kinase (MAPK) pathways have central roles in the regulation of cell survival and proliferation. The crosstalk between these two pathways has also been investigated. The focus of this review centres on downstream mediators of A₃ adenosine receptor signalling.

IB-MECA and Cardioprotection

The adenosine A(3) receptor plays an important role in ischemic preconditioning. Activation of the adenosine A(3) receptor with its agonists induces both early and late pharmacological preconditioning through various mechanisms. As the first potent and selective adenosine A(3) receptor agonist, IB-MECA (N(6)-(3-iodobenzyl)-adenosine-5'-N-methylcarboxamide) has been demonstrated to induce cardioprotection against myocardial ischemia/reperfusion injury when given before onset of ischemia by triggering pharmacological preconditioning. More importantly, IB-MECA can also protect the heart even when administered at the onset of reperfusion after ischemia, indicating a strong likelihood that the drug may be useful for the treatment of patients with acute myocardial infarction. However, since IB-MECA has been reported to have lethal effects at higher concentrations, and may cause systemic hypertension in some species, further studies are needed to find the best treatment strategy to increase its therapeutic potential.

N6-(3-iodobenzyl)-adenosine-5'-N-methylcarboxamide confers cardioprotection at reperfusion by inhibiting mitochondrial permeability transition pore opening via glycogen synthase kinase 3 beta

Although the adenosine A(3) receptor agonist N(6)-(3-iodobenzyl)-adenosine-5'-N-methylcarboxamide (IB-MECA) has been reported to be cardioprotective at reperfusion, little is known about the mechanisms underlying the protection. We hypothesized that IB-MECA may protect the heart at reperfusion by preventing the opening of mitochondrial permeability transition pore (mPTP) through inactivation of glycogen synthase kinase (GSK) 3beta. IB-MECA (1 microM) applied during reperfusion reduced infarct size in isolated rat hearts, an effect that was abrogated by the selective A3 receptor antagonist 1,4-dihydro-2-methyl-6-phenyl-4-(phenylethynyl)-3,5-pyridinedicarboxylic acid 3-ethyl-5-[(3-nitrophenyl)-methyl]ester (MRS1334) (100 nM). The effect of IB-MECA was abrogated by the mPTP opener atractyloside (20 microM), implying that the action of IB-MECA may be mediated by inhibition of the mPTP opening. In cardiomyocytes, IB-MECA attenuated oxidant-induced loss of mitochondrial membrane potential (DeltaPsim), which was reversed by MRS1334. IB-MECA also reduced Ca2+-induced mitochondrial swelling. IB-MECA enhanced phosphorylation of GSK-3beta (Ser9) upon reperfusion, and the GSK-3 inhibitor 3-(2,4-dichlorophenyl)-4-(1-methyl-1H-indol-3-yl)-1H-pyrrole-2,5-dione (SB216763) (3 microM) mimicked the protective effect of IB-MECA by attenuating both infarction and the loss of DeltaPsim. In addition, the effect of IB-MECA on GSK-3beta was reversed by wortmannin (100 nM), and IB-MECA was shown to enhance Akt phosphorylation upon reperfusion. In contrast, rapamycin (2 nM) failed to affect GSK-3beta phosphorylation by IB-MECA, and IB-MECA did not alter phosphorylation of either mTOR (Ser2448) or 70s6K (Thr389). Taken together, these data suggest that IB-MECA prevents myocardial reperfusion injury by inhibiting the mPTP opening through the inactivation of GSK-3beta at reperfusion. IB-MECA-induced GSK-3beta inhibition is mediated by the PI3-kinase/Akt signal pathway but not by the mTOR/p70s6K pathway.

Adenosine modulates vascular endothelial growth factor expression via hypoxia-inducible factor-1 in human glioblastoma cells

Hypoxia appears to induce a program which shifts the cellular phenotype toward an increase in extracellular adenosine. Hypoxia-inducible factor-1 (HIF-1) is a key regulator of genes crucial to many aspects of cancer biology. Since in gliomas there is a strong correlation between HIF-1alpha expression, tumor grade and tumor vascularization, the aim of this study was to investigate whether adenosine may regulate HIF-1 in human glioblastoma cell lines. The results indicate that in the human hypoxic A172 and U87MG glioblastoma cell lines adenosine up-regulates HIF-1alpha protein expression via the A(3) receptor subtype. In particular, we investigated the effect of A(3) receptor antagonists on HIF-1 and vascular endothelial growth factor (VEGF) expression. We found that A(3) antagonists inhibit adenosine-induced HIF-1alpha and VEGF protein accumulation in the hypoxic cells. Investigations in the molecular mechanism showed that A(3) receptor stimulation activates p44/p42 and p38 MAPKs that are required for A(3)-induced increase of HIF-1alpha and VEGF. Further studies are required to demonstrate the in vivo relevance of these observations with regard to the proposed role for adenosine as a key element in hypoxia and in tumors.

Adenosine receptors as therapeutic targets

Adenosine receptors are major targets of caffeine, the most commonly consumed drug in the world. There is growing evidence that they could also be promising therapeutic targets in a wide range of conditions, including cerebral and cardiac ischaemic diseases, sleep disorders, immune and inflammatory disorders and cancer. After more than three decades of medicinal chemistry research, a considerable number of selective agonists and antagonists of adenosine receptors have been discovered, and some have been clinically evaluated, although none has yet received regulatory approval. However, recent advances in the understanding of the roles of the various adenosine receptor subtypes, and in the development of selective and potent ligands, as discussed in this review, have brought the goal of therapeutic application of adenosine receptor modulators considerably closer.

A3 adenosine receptors modulate hypoxia-inducible factor-1alpha expression in human A375 melanoma cells

Hypoxia-inducible factor-1 (HIF-1) is a key regulator of genes crucial to many aspects of cancer biology. The purine nucleoside, adenosine, accumulates within many tissues under hypoxic conditions, including that of tumors. Because the levels of both HIF-1 and adenosine are elevated within the hypoxic environment of solid tumors, we investigated whether adenosine may regulate HIF-1. Here we show that, under hypoxic conditions (< 2% O2), adenosine upregulates HIF-1alpha protein expression in a dose-dependent and time-dependent manner, exclusively through the A3 receptor subtype. The response to adenosine was generated at the cell surface because the inhibition of A3 receptor expression, by using small interfering RNA, abolished nucleoside effects. A3 receptor stimulation in hypoxia also increases angiopoietin-2 (Ang-2) protein accumulation through the induction of HIF-1alpha. In particular, we found that A3 receptor stimulation activates p44/p42 and p38 mitogen-activated protein kinases, which are required for A3-induced increase of HIF-1alpha and Ang-2. Collectively, these results suggest a cooperation between hypoxic and adenosine signals that ultimately may lead to the increase in HIF-1-mediated effects in cancer cells.

Generation of adenosine A3 receptor functionally humanized mice for the evaluation of the human antagonists

Although the adenosine A(3) receptor (A3AR), which is a G(i/o) protein-coupled receptor, has attracted considerable interest as a potential target for drugs against asthma or inflammation, the in vivo evaluation of the antagonists using rodents in the first step of drug development has been hampered by the lack of highly potent antagonists for the rodent A3AR. To evaluate the pharmacological effects of human A3AR antagonists in mice, we previously generated A3AR-humanized mice, in which the mouse A3AR gene was replaced by its human counterpart. However, the human A3AR did not lead to the phosphoinositide 3-kinase (PI3K) gamma-signaling pathway such as IgE/antigen-dependent mast cell degranulation, probably due to the uncoupling of the mouse G(i/o) protein(s). To overcome the uncoupling, we here generated A3AR functionally humanized mice by replacing the mouse A3AR gene with a human/mouse chimeric A3AR sequence in which whole intracellular regions of the human A3AR were substituted for the corresponding regions of the mouse A3AR. The chimeric A3AR led to intracellular Ca(2+) elevation and activation of the PI3Kgamma-signaling pathway, which are equivalent to the actions induced by A3AR in wild-type mice. The human A3AR antagonist had the same binding affinities for the chimeric A3AR as the human A3AR and completely antagonized this potentiation. This is the first direct evidence that the uncoupling of mouse G protein(s) to the human A3AR is due to a sequence difference in the intracellular regions of A3AR. The A3AR functionally humanized mice can be widely employed for pharmacological evaluations of the human A3AR antagonists.

Role of central and peripheral adenosine receptors in the cardiovascular responses to intraperitoneal injections of adenosine A1 and A2A subtype receptor agonists

1. The cardiovascular effects of the adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) and the adenosine A2A receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680) were investigated in rats implanted with telemetry transmitters for the measurement of blood pressure and heart rate. 2. Intraperitoneal (i.p.) injections of the adenosine A1 receptor agonist CPA led to dose-dependent decreases in both blood pressure and heart rate. These effects of 0.3 mg kg(-1) CPA were antagonized by i.p. injections of the adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dimethyl-xanthine (CPT), but not by i.p. injections of the adenosine A2A receptor antagonist 3-(3-hydroxypropyl)-8-(m-methoxystyryl)-7-methyl-1-propargylxanthine phosphate disodium salt (MSX-3). Injections (i.p.) of the peripherally acting nonselective adenosine antagonist 8-sulfophenyltheophylline (8-SPT) and the purported nonselective adenosine antagonist caffeine also antagonized the cardiovascular effects of CPA. 3. The adenosine A2A agonist CGS 21680 given i.p. produced a dose-dependent decrease in blood pressure and an increase in heart rate. These effects of 0.5 mg kg(-1) CGS 21680 were antagonized by i.p. injections of the adenosine A2A receptor antagonist MSX-3, but not by i.p. injections of the antagonists CPT, 8-SPT or caffeine. 4. Central administration (intracerebral ventricular) of CGS 21680 produced an increase in heart rate, but no change in blood pressure. MSX-3 given i.p. antagonized the effects of the central injection of CGS 21680. 5. These results suggest that adenosine A1 receptor agonists produce decreases in blood pressure and heart rate that are mediated by A1 receptors in the periphery, with little or no contribution of central adenosine A1 receptors to those effects. 6. The heart rate increasing effect of adenosine A2A agonists appears to be mediated by adenosine A2A receptors in the central nervous system. The blood pressure decreasing effect of adenosine A2A agonists is most probably mediated in the periphery.

Resveratrol-mediated activation of cAMP response element-binding protein through adenosine A3 receptor by Akt-dependent and -independent pathways

A recent study documented a role of adenosine A(3)-Akt-cAMP response element-binding protein (CREB) survival signaling in resveratrol preconditioning of the heart. In this study, we demonstrate that resveratrol-mediated CREB activation can also occur through an Akt-independent pathway. Isolated rat hearts were perfused for 15 min with Krebs-Henseleit bicarbonate (KHB) buffer containing resveratrol in the absence or presence of adenosine A(3) receptor blocker MRS-1191 [3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicar-boxylate], phosphatidylinositol 3 (PI3)-kinase inhibitor LY294002 [2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride], mitogen-activated extracellular signal-regulated protein kinase inhibitor PD098059 [2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one], or a combination of LY294002 and PD098059. All hearts were subsequently subjected to 30-min ischemia followed by 2-h reperfusion. Cardioprotection was examined by determining infarct size, cardiomyocyte apoptosis, and ventricular recovery. Resveratrol phosphorylated both Akt and CREB that was blocked by MRS-1191, which also abolished cardioprotective abilities of resveratrol. LY294002 completely inhibited Akt phosphorylation but partially blocked the phosphorylation of CREB. Inhibition of PI3-kinase also partially blocked resveratrol's ability to precondition the heart. PD098059 partially blocked the phosphorylation of CREB and resveratrol-mediated cardioprotection. Preperfusing the hearts with LY294002 and PD098059 together completely abolished the phosphorylation of CREB, simultaneously inhibiting resveratrol-mediated cardioprotection. The results indicate that resveratrol preconditions the hearts through adenosine A(3) receptor signaling that triggers the phosphorylation of CREB through both Akt-dependent and -independent pathways, leading to cardioprotection.

A3 Adenosine Receptor Activation Inhibits Cell Proliferation via Phosphatidylinositol 3-Kinase/Akt-dependent Inhibition of the Extracellular Signal-regulated Kinase 1/2 Phosphorylation in A375 Human Melanoma Cells

Adenosine exerts its effects through four subtypes of G-protein-coupled receptors: A(1), A(2A), A(2B), and A(3). Stimulation of the human A(3) receptor has been suggested to influence cell death and proliferation. The phosphatidylinositide-3-OH kinase (PI3K)/Akt and the Raf/mitogen-activated protein kinase (MAPK/ERK) kinase (MEK)/mitogen-activated protein kinase (MAPK) pathways have central roles in the regulation of cell survival and proliferation. Due to their importance, the cross-talk between these two pathways has been investigated. Here, we show that the A(3) adenosine receptor agonist Cl-IB-MECA stimulates PI3K-dependent phosphorylation of Akt leading to the reduction of basal levels of ERK1/2 phosphorylation, which in turn inhibits cell proliferation. The response to Cl-IB-MECA was not blocked by A(1), A(2A), or A(2B) receptor antagonists, although it was abolished by A(3) receptor antagonists. Furthermore, the response to Cl-IB-MECA was generated at the cell surface, since the inhibition of A(3) receptor expression, by using small interfering RNA, abolished agonist effects. Using A375 cells, we show that A(3) adenosine receptor stimulation results in PI3K-dependent phosphorylation of Akt, leading to the reduction of basal levels of ERK1/2 phosphorylation, which in turn inhibits cell proliferation.

Adenosine A2A receptor stimulation increases angiogenesis by down-regulating production of the antiangiogenic matrix protein thrombospondin 1

Topical adenosine A2A receptor agonists promote wound healing by, among other effects, increasing microvessel formation. Results of representational display analysis of human umbilical vein endothelial cells suggested that A2A receptor occupancy modulates expression of the antiangiogenic matrix protein thrombospondin 1 (TSP1). We therefore determined whether A2A receptor occupation stimulates angiogenesis by modulating TSP1 secretion. Human microvascular endothelial cells (HMVEC) were treated with medium alone, 2-p-[2-carboxyethyl] phenethyl-amino-5'-N-ethylcarboxamido-adenosine (CGS-21680), or 2-[2-(4-chlorophenyl)ethoxy]adenosine (MRE0094), selective A2A receptor agonists. TSP1 protein secretion was down-regulated after treatment with the A2A agonists CGS-21680 or MRE0094 in a dose-dependent manner (EC50 = 6.65 nM and 0.23 microM respectively). The selective A2A receptor antagonist 4-[2-[7-amino-2-(2-furyl)[1,2,4]triazolo-[2,3-a][1,3,5]triazin-5-ylamino]ethyl]phenol (ZM241385) but not the A1 and A2B receptor antagonists diphenylcyclopentylxanthine, enprofylline, and N-(4-acetylphenyl)-2-[4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)phenoxy]acetamide (MRS1706) completely abrogated the A2A receptor agonist-mediated effect on TSP1. Vascular tube formation by HMVEC was increased by adenosine A2A receptor agonists in a dose-dependent fashion (EC50 = 0.1 microM for both), and this effect was reversed by the A2A antagonist. Moreover, in the presence of antibodies to TSP1 and CD36, the receptor for TSP1, the adenosine A2A receptor agonists stimulated no increase in vascular tube formation. These results indicate that the angiogenic effects of adenosine A2A receptor activation are, at least in part, caused by the suppression of TSP1 secretion.

A3 adenosine receptor-mediated protection of the ischemic heart

The A3 adenosine receptor (A3AR) is attributed with multiple beneficial actions in ischemic-reperfused myocardium, including modulation of oncotic and apoptotic cell death and enhancement of contractile function. Additionally, the A3AR may attenuate vascular dysfunction and improve long-term outcome from myocardial insult (modulating hypertrophy and angiogenesis). Available evidence indicates that this receptor sub-type is minimally activated by endogenous adenosine during ischemia (A3AR antagonists exerting no effects on ischemic outcome), and is thus amenable to activation with exogenous agonists. Protected phenotypes arise with both pre- and post-ischemic treatment with A3AR agonists, and transient A3AR agonism also triggers early and delayed preconditioned states. The molecular basis for the varied protective actions of the A3AR remains poorly defined, and may well vary between species (e.g. rodent vs. human) and protective responses (e.g. acute vs. delayed protection). Nonetheless, A3ARs may be more promising as therapeutic "anti-ischemic" targets compared with other adenosine receptor subtypes, since A3AR agonists elicit fewer and less significant side-effects. This review addresses current knowledge and controversy regarding the protective actions (and associated signaling) of A3ARs in ischemic-reperfused heart.

An adenosine analog (IB-MECA) inhibits anchorage-dependent cell growth of various human breast cancer cell lines

A3 adenosine receptor agonists have been reported to influence cell death and survival. Here we report the effects of an A3 adenosine receptor agonist, IB-MECA, on the cell growth of human breast cancer cell lines, MCF-7 (estrogen receptor positive) and MDA-MB468 (estrogen receptor negative). Therefore, this study was aimed to investigate the expression and possible action of A3 receptor in the human breast cancer cell lines. IB-MECA, at 1-100 microM, resulted in a significant cell growth inhibition (P < 0.05) which reached the maximum at 48 h, in the cell lines. In both cell lines, agonist-induced effects were antagonized by pretreatment with a selective A3 adenosine receptor antagonist, MRS1220. Using RT-PCR method, further confirmation was provided by the presence of mRNA of A3 receptor in the cells. In addition, IB-MECA was able to inhibit forskolin-stimulated cAMP levels, which indicate the functional form of A3 receptor on the cell surface of these breast cancer cell lines. These results suggest that the inhibitory effect of IB-MECA on the growth of human breast cancer cell lines is mediated through activation of A3 adenosine receptor.

ERK, PKC and PI3K/Akt pathways mediate extracellular ATP and adenosine-induced proliferation of U138-MG human glioma cell line

OBJECTIVE

Extracellular nucleotides and nucleosides induce proliferation in a set of human glioma cell lines. In this study we investigate the signal transduction pathways involved in ATP and adenosine-mediated proliferation in U138-MG human glioma cells.

METHODS

Cell proliferation was accessed through [(3)H]thymidine incorporation, cell counting and flow cytometry. Protein phosphorylation was detected through Western blotting.

RESULTS

ATP or adenosine (100 microM) induced extracellular signal-regulated protein kinase (ERK), Akt and GSK3beta phosphorylation. The increase in [(3)H]thymidine incorporation induced by ATP or adenosine was decreased when cells were incubated with LY 294002 (by +/-90%), GF 109203X (by +/-76%) or PD 098059 (by +/-63%). The increase in cell numbers with ATP or adenosine was less after a 48-hour treatment of cells with ATP or adenosine plus GF 109203X (by +/-66%) or LY 294002 (by +/-83%). Percentage of cells in S phase was decreased in cells treated with LY 294002 plus ATP when compared to ATP- treated cells.

CONCLUSION

Stimulation of purinergic receptors in U138-MG cells leads to cell proliferation mediated by PI3K/Akt, ERK and PKC signaling. It may be clinically important for pharmacological intervention in gliomas to associate purinergic receptor antagonists and signal transduction pathways blockers.

The receptor mechanism mediating the contractile response to adenosine on lung parenchymal strips from actively sensitised, allergen-challenged Brown Norway rats

Parenchymal strips prepared from lungs removed from actively sensitised Brown Norway rats challenged with allergen show hyperresponsiveness to adenosine. The response is mast cell mediated and a preliminary pharmacological analysis suggested the involvement of a receptor (or receptors) that could not be classified as any of the known adenosine receptor subtypes. We present a further analysis of the response. Male Brown Norway (BN) rats, actively sensitised to ovalbumin (OA), were challenged intratracheally with OA and killed 3 h later to provide parenchymal strip preparations. The augmented contractile responses to adenosine were partially blocked by the 5-HT receptor antagonist, methysergide, or the A(1) receptor antagonist, DPCPX, and abolished in the presence of both antagonists. Responses to high concentrations of the A(1) receptor agonist, CPA were, like those to adenosine, augmented on tissues from allergen-challenged animals and blocked by a combination of methysergide and DPCPX. The A(3) receptor agonist, Cl-IB-MECA, did not contract the tissue, but partially blocked the response to adenosine. A combination of Cl-IB-MECA and methysergide induced a similar degree of blockade to that seen with either drug given alone. Combination of Cl-IB-MECA and/or methysergide with DPCPX abolished the response to adenosine. The effects of the A(3) receptor agonist, inosine, were augmented on tissues from allergen-challenged animals and markedly inhibited by disodium cromoglycate, methysergide or Cl-IB-MECA. Responses to adenosine were abolished when parenchymal strips were taken from rats pretreated 48 h previously with pertussis toxin. 8-SPT, CGS 15943, XAC, MRS 1754, DPCPX and theophylline, at concentrations which inhibit the A(1) A(2A) and/or A(2B) receptors but have negligible affinity for the rat A(3) receptor, inhibited responses to adenosine, but high concentrations were required and blockade was incomplete. MRS 1523 and MRS 1191, which are antagonists at the rat A(3) receptor, had no effect on the response to adenosine. The present results support and clarify our earlier conclusion that an atypical receptor mechanism mediates contraction of the parenchymal strip prepared from the lungs of actively sensitised BN rats challenged with allergen to adenosine. The response arises from a combined effect of adenosine on the A(1) receptor and a receptor with similarities to the A(3) receptor, but where Cl-IB-MECA behaves as an antagonist and MRS 1523 and MRS 1191 are inactive at concentrations that substantially exceed their affinities for the rat A(3) receptor.

In vivo inosine protects alveolar epithelial type 2 cells against hyperoxia-induced DNA damage through MAP kinase signaling

Inosine, a naturally occurring purine with anti-inflammatory properties, was assessed as a possible modulator of hyperoxic damage to the pulmonary alveolar epithelium. Rats were treated with inosine, 200 mg/kg ip, twice daily during 48-h exposure to >90% oxygen. The alveolar epithelial type 2 cells (AEC2) were then isolated and cultured. AEC2 isolated from inosine-treated hyperoxic rats had less DNA damage and had increased antioxidant status compared with AEC2 from hyperoxic rats. Inosine treatment during hyperoxia also reduced the proportion of AEC2 in S and G2/M phases of the cell cycle and increased levels of the DNA repair enzyme 8-oxoguanine DNA glycosylase. Bronchoalveolar lavage (BAL) recovered from hyperoxic, inosine-treated rats contained threefold higher levels of active transforming growth factor-beta than BAL from rats exposed to hyperoxia alone, and Smad2 was activated in AEC2 isolated from these animals. ERK1/2 was activated both in freshly isolated and 24-h-cultured AEC2 by in vivo inosine treatment, whereas blockade of the MAPK pathway in vitro reduced the protective effect of in the vivo inosine treatment. Together, the data suggest that inosine treatment during hyperoxic exposure results in protective signaling mediated through pathways downstream of MEK. Thus inosine may deserve further evaluation for its potential to reduce hyperoxic damage to the pulmonary alveolar epithelium.

Pharmacological preconditioning with resveratrol: role of CREB-dependent Bcl-2 signaling via adenosine A3 receptor activation

Recent studies demonstrated that resveratrol, a grape-derived polyphenolic phytoalexin, provides pharmacological preconditioning (PC) of the heart through a NO-dependent mechanism. Because adenosine receptors play a role in PC, we examined whether they play any role in resveratrol PC. Rats were randomly assigned to groups perfused for 15 min with 1) Krebs-Henseleit bicarbonate buffer (KHB) only; 2) KHB containing 10 microM resveratrol; 3) 10 microM resveratrol + 1 microM 8-cyclopentyl-1,3-dimethylxanthine (CPT; adenosine A(1) receptor blocker); 4) 10 microM resveratrol + 1 microM 8-(3-chlorostyryl)caffeine (CSC; adenosine A(2a) receptor blocker); 5) 10 microM resveratrol + 1 microM 3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS-1191; adenosine A(3) receptor blocker); or 6) 10 microM resveratrol + 3 microM 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride [LY-294002, phosphatidylinositol (PI)3-kinase inhibitor], and groups perfused with adenosine receptor blockers alone. Hearts were then subjected to 30-min ischemia followed by 2-h reperfusion. The results demonstrated significant cardioprotection with resveratrol evidenced by improved ventricular recovery and reduced infarct size and cardiomyocyte apoptosis. CPT and MRS 1191, but not CSC, abrogated the cardioprotective abilities of resveratrol, suggesting a role of adenosine A(1) and A(3) receptors in resveratrol PC. Resveratrol induced expression of Bcl-2 and caused its phosphorylation along with phosphorylation of cAMP response element-binding protein (CREB), Akt, and Bad. CPT blocked phosphorylation of Akt and Bad without affecting CREB, whereas MRS 1191 blocked phosphorylation of all compounds, including CREB. LY-294002 partially blocked the cardioprotective abilities of resveratrol. The results indicate that resveratrol preconditions the heart through activation of adenosine A(1) and A(3) receptors, the former transmitting a survival signal through PI3-kinase-Akt-Bcl-2 signaling pathway and the latter protecting the heart through a CREB-dependent Bcl-2 pathway in addition to an Akt-Bcl-2 pathway.

A3 adenosine receptor signaling contributes to airway inflammation and mucus production in adenosine deaminase-deficient mice

Adenosine signaling has been implicated in chronic lung diseases such as asthma and chronic obstructive pulmonary disease; however, the specific roles of the various adenosine receptors in processes central to these disorders are not well understood. In this study, we have investigated the role(s) of the A(3) adenosine receptor in adenosine-dependent pulmonary inflammation observed in adenosine deaminase (ADA)-deficient mice. The A(3) receptor (A(3)R) was found to be expressed in eosinophils and mucus-producing cells in the airways of ADA-deficient mice. Treatment of ADA-deficient mice with MRS 1523, a selective A(3)R antagonist, prevented airway eosinophilia and mucus production. Similar findings were seen in the lungs of ADA/A(3) double knockout mice. Although eosinophils were decreased in the airways of ADA-deficient mice following antagonism or removal of the A(3)R, elevations in circulating and lung interstitial eosinophils persisted, suggesting signaling through the A(3)R is needed for the migration of eosinophils into the airways. These findings identify an important role for the A(3)R in regulating lung eosinophilia and mucus production in an environment of elevated adenosine.

Identification of Necrosis-Associated Genes in Glioblastoma by cDNA Microarray Analysis

PURPOSE

In the field of cancer research, there has been a paucity of interest in necrosis, whereas studies focusing on apoptosis abound. In neuro-oncology, this is particularly surprising because of the importance of necrosis as a hallmark of glioblastoma (GBM), the most malignant and most common primary brain tumor, and the fact that the degree of necrosis has been shown to be inversely related to patient survival. It is therefore of considerable interest and importance to identify genes and gene products related to necrosis formation.

EXPERIMENTAL DESIGN

We used a nylon cDNA microarray to analyze mRNA expression of 588 universal cellular genes in 15 surgically resected human GBM samples with varying degrees of necrosis. Gene expression was correlated with the degree of necrosis using rank correlation coefficients. The expression of identified genes was compared with their expression in tissue samples from 5 anaplastic astrocytomas (AAs). Immunostaining was used to determine whether genes showing the most positive correlation with necrosis were increasingly expressed in tumor tissues, as grade of necrosis increased.

RESULTS

The hybridization results indicated that 26 genes showed significant correlation with the amount of necrosis. All 26 genes had functions associated with either Ras, Akt, tumor necrosis factor alpha, nuclear factor kappaB, apoptosis, procoagulation, or hypoxia. Nine genes were positively correlated with necrosis grade, and 17 genes were negatively correlated with necrosis grade. There were significant differences in the median expression levels of 3 of the 26 genes between grade III necrosis GBM and anaplastic astrocytoma (AA) samples; all but 1 of the genes had elevated expression when comparing necrosis grade III with AA samples. Two factors, the ephrin type A receptor 1 and the prostaglandin E(2) receptor EP4 subtype, not previously considered in this context, were highlighted because of their particularly high (positive) correlation coefficients; immunostaining showed the products of these two genes to be localized in perinecrotic and necrotic regions and to be overexpressed in grade III GBMs, but not AAs. These two molecules also showed significant correlation with survival of GBM patients (P = 0.0034) in a combined model.

CONCLUSIONS

The application of cDNA expression microarray analysis has identified specific genes and patterns of gene expression that may help elucidate the molecular basis of necrogenesis in GBM. Additional studies will be required to further investigate and confirm these findings.

Animal models for the study of adenosine receptor function

Adenosine receptors represent a family of G-protein coupled receptors that are ubiquitously expressed in a wide variety of tissues. This family contains four receptor subtypes: A1 and A3, which mediate inhibition of adenylyl cyclase; and A2a and A2b, which mediate stimulation of this enzyme. Currently, all receptor subtypes have been genetically deleted in mouse models except for the A2b adenosine receptor, and some have been overexpressed in selective tissues of transgenic mice. Studies involving these transgenic mice indicated that receptor levels are rate limiting, as effects were amplified upon increases in receptor level. The knockout models pointed to clusters of activities related to the physiologies of the cardiovascular and the nervous systems, which are either reduced or enhanced upon specific receptor deletion. Interestingly, the trend of effects on these systems is similar in the A1 and A3 adenosine receptor knockout mice and opposite to the effects observed in the A2a adenosine receptor knockout model. This review summarizes in vitro studies on pathways affected by each adenosine receptor, and primarily focuses on the above in vivo models generated to investigate the physiologic role of adenosine receptors. Furthermore, it illustrates the need for multiple adenosine receptor subtype deficiency studies in mice and the deletion of the A2b subtype.

Adenosine A3 receptor-mediated regulation of p38 and extracellular-regulated kinase ERK1/2 via phosphatidylinositol-3′-kinase

The adenosine A(3) receptor generally couples to the G(i) class of heterotrimeric G proteins, thereby decreasing cAMP levels and also mediating signaling via release of betagamma subunits. Here we describe the central role of phosphatidylinositol-3'-kinase (PI3K) for adenosine A(3) receptor-induced intracellular signaling to the stress-activated protein kinase p38 and the extracellular signal-regulated protein kinases ERK1/2. We used Chinese hamster ovary cells expressing the human adenosine A(3) receptor, phospho-specific antibodies and different pharmacological tools to dissect the signaling pathways involving PI3K. The adenosine receptor agonist 5'N-ethylcarboxamidoadenosine induced a time- and dose-dependent increase in p38 and ERK1/2 phosphorylation, two signaling pathways that appeared also to be activated in the immortalized microglia cell line N13, which expressed endogenous adenosine A(3) receptors. The 5'N-ethylcarboxamidoadenosine-induced effects on p38 and ERK1/2 in CHO cells were blocked by pertussis toxin pretreatment and were sensitive to pharmacological inhibition of PI3K. In addition, inhibition of Rac/Cdc42, small GTPases of the Rho family, by clostridium toxin B, diminished p38 phosphorylation but did not affect ERK1/2. Furthermore, we identified the serine 727 site of signal transducer and activator of transcription STAT3 as a probable downstream target of ERK1/2, and thereby provide evidence that adenosine A(3) receptor mediated ERK1/2 activation has functional consequences.

Inosine stimulates chemotaxis, Ca2+-transients and actin polymerization in immature human dendritic cells via a pertussis toxin-sensitive mechanism independent of adenosine receptors

Inosine is an endogenous purine nucleoside, which is formed by adenosine deaminidase during adenosine breakdown and is released into the extracellular space from the sympathetic nervous system or injured cells. Here, we studied the biological activity of inosine on human dendritic cells (DC), which are specialized antigen presenting cells characterized by their ability to migrate from the blood to peripheral tissues, and then to secondary lymphoid organs where they initiate adaptive immune responses. In immature DC, inosine concentration-dependently stimulated Ca(2+)-transients, actin polymerization, and chemotaxis. Experiments with adenosine receptor antagonists and pertussis toxin (PTX) as well as desensitization studies suggested that the activity of inosine was mediated by a G protein-coupled receptor pathway independent of adenosine receptors. DC, induced to mature by lipopolysaccharide, lost their ability to respond towards inosine with these activities. Moreover, inosine did neither influence membrane expression of CD54, CD80, CD83, CD86, HLA-DR, and MHC class I molecules nor modulated secretion of interleukin (IL)-12, IL-10, and tumor necrosis factor alpha in immature and lipopolysaccharide-matured DC. In aggregate, our study indicates that inosine may be involved in the trafficking control system of immature DC, and mediates its chemotactic activity by a PTX-sensitive mechanism independent of adenosine receptors.

Ultraviolet irradiation induces apoptosis in human immature, but not in skin mast cells

As diverse pruritic cutaneous diseases respond to ultraviolet treatment, we have examined whether ultraviolet light is capable of inducing apoptosis in mast cells. Human mast cell line 1 (HMC1) derived from a patient with malignant mastocytosis and purified skin mast cells were irradiated with single doses of ultraviolet B or ultraviolet A1, or pretreated with 8-methoxypsoralen prior to ultraviolet A1 exposure. After 0 to 48 h of incubation, the percentage of apoptotic and dead cells was assessed. In HMC1 cells, morphologic features of apoptosis were further evaluated by electron microscopy. All ultraviolet treatment induced apoptosis of HMC1 cells in a time- and dose-dependent manner. Apoptosis was associated with activation of caspase-3, release of cytochrome C, cleavage of poly(ADP-ribose)-polymerase, and nuclear accumulation of p53. In contrast, resting skin mast cells were resistant to ultraviolet light induced apoptosis. After incubation with stem cell factor and interleukin-4 for 2 wk, however, slowly proliferating skin mast cells also underwent apoptosis in response to ultraviolet light. In conclusion, these data demonstrate that ultraviolet light directly affects mast cells, but mainly aims at the proliferating mast cells as found in mastocytosis and mast cell dependent pruritic diseases, where increased numbers are observed due to the recruitment mast cell precursors from the blood.

Acute adenosinergic cardioprotection in ischemic-reperfused hearts

Cells of the cardiovascular system generate and release purine nucleoside adenosine in increasing quantities when constituent cells are "stressed" or subjected to injurious stimuli. This increased adenosine can interact with surface receptors in myocardial, vascular, fibroblast, and inflammatory cells to modulate cellular function and phenotype. Additionally, adenosine is rapidly reincorporated back into 5'-AMP to maintain the adenine nucleotide pool. Via these receptor-dependent and independent (metabolic) paths, adenosine can substantially modify the acute response to ischemic insult, in addition to generating a more sustained ischemia-tolerant phenotype (preconditioning). However, the molecular basis for acute adenosinergic cardioprotection remains incompletely understood and may well differ from more widely studied preconditioning. Here we review current knowledge and some controversies regarding acute cardioprotection via adenosine and adenosine receptor activation.

Extracellular nucleotides and nucleosides induce proliferation and increase nucleoside transport in human glioma cell lines

Extracellular purines (adenosine triphosphate (ATP), adenosine 5'-diphosphate (ADP) and adenosine) and pyrimidines (uridine 5'-triphosphate (UTP) and UDP) are important signaling molecules that mediate diverse biological effects via P1 and P2 purinergic receptors. The human glioma cell lines U87 MG, U251 MG and U138 MG were treated with purines and pyrimidines for 24 or 48 h and proliferation was measured by [3H]-thymidine incorporation, flow cytometry and cell counting. The studies showed that extracellular nucleotides and nucleosides induce proliferation of the studied glioma cells. Incorporation of [3H]-thymidine followed the order of ATP approximately equal to guanosine approximately equal to inosine approximately equal to adenosine > UTP > ADP while ATPgammaS and 2MeSATP had no effect. The effect of ATP was partially inhibited by suramin and by reactive blue 2 (RB2). Co-treatment with the following antagonists of P1 purinoreceptors DPCPX, CPT or 8PT did not block the effect of adenosine while a specific antagonist of the A3 receptor, MRS1220, totally blocked the effect of adenosine. ATP and adenosine also increased the overall uptake of [3H]-thymidine into the cell, producing a positive effect on the [3H]-thymidine incorporation measurements. These data indicate that the uptake of thymidine and proliferation of gliomas can be induced by purines and pyrimidines via both P1 and P2 purinoceptors.

A glance at adenosine receptors: novel target for antitumor therapy

Adenosine can be released from a variety of cells throughout the body, as the result of increased metabolic rates, in concentrations that can have a profound impact on the vasculature, immunoescaping, and growth of tumor masses. It is recognized that the concentrations of this nucleoside are increased in cancer tissues. Therefore, it is not surprising that adenosine has been shown to be a crucial factor in determining the cell progression pathway, either during apoptosis or during cytostatic state. From the perspective of cancer, the most important question then may be "Can activation and/or blockade of the pathways downstream of the adenosine receptor contribute to tumor development?" Rigorous examinations of the role of adenosine in in vivo and in vitro systems need to be investigated. The present review therefore proposes multiple adenosine-sustained ways that could prime tumor development together with the critical combinatorial role played by adenosine receptors in taking a choice between proliferation and death. This review proposes that adenosine acts as a potent regulator of normal and tumor cell growth. It is hypothesized that this effect is dependent on extracellular adenosine concentrations, cell surface expression of different adenosine receptor subtypes, and signal transduction mechanisms activated following the binding of specific agonists. We venture to suggest that the clarification of the role of adenosine and its receptors in cancer development may hold great promise for the treatment of chemotherapy in patients affected by malignancies.

Pyrazolotriazolopyrimidine derivatives sensitize melanoma cells to the chemotherapic drugs: taxol and vindesine

In this study, we have evaluated the "in vitro" modulatory activity of a series of pyrazolotriazolopyrimidine derivatives (PTP-d) in sensitizing malignant melanoma cells to the chemotherapic drugs: taxol and vindesine. To that end, we have described the impact of chemotherapeutic agents on the cell cycle and on the induction of apoptosis when used alone or in combination with PTP-d. We have demonstrated that four PTP-d reduced chemotherapic drugs EC(50) doses of the G(2)/M accumulation with an average of 1.7-fold for taxol and 9.5-fold for vindesine when challenged on A375 human melanoma cell line. This sensitization activity was also confirmed by analyzing the apoptosis degree induced by the chemotherapic drugs. Interestingly, PTP-d had no effects on the response to cytotoxic agents by skin-derived human keratinocyte cells, NCTC 2544. Therefore, we have investigated the signaling pathway sustaining the sensitizing effect of PTP-d, providing functional evidence that active compounds are able to inhibit multidrug resistance-associated ATP-binding cassette drug transporter. These results suggested that PTP-d hold great promise for the treatment of multidrug resistance in cancers, leading to potential new therapies for melanoma.

Evidence for functional adenosine A3 receptors in microglia cells

Adenosine exerts its effects through four subtypes of G-protein-coupled receptors (GPCRs): adenosine A1 and A3 receptors (A3R), which generally couple to Gi proteins and adenosine A2A and A2B receptors that activate Gs proteins. Though there is evidence for the expression of mRNA for the A3R in the central nervous system, evidence for functional receptors has depended on drugs with uncertain specificity. Here, we show that A3Rs mediating functional responses are present in microglia cells. By selectively stimulating the A3R in both primary mouse microglia cells and the N13 microglia cell line with the agonist Cl-IB-MECA, we have found a biphasic, partly Gi protein-dependent influence on the phosphorylation of the extracellular signal-regulated protein kinase 1/2 (ERK1/2). ERK1/2 activation was assessed by immunoblotting with phospho-specific antibodies. The involvement of the A3R in Cl-IB-MECA-induced ERK1/2 phosphorylation was confirmed by demonstrating that those effects are absent in primary mouse microglia cells isolated from mice lacking the gene for the A3R.

A3 adenosine receptor agonist IB-MECA reduces myocardial ischemia-reperfusion injury in dogs

We examined the effect of the A3 adenosine receptor (AR) agonist IB-MECA on infarct size in an open-chest anesthetized dog model of myocardial ischemia-reperfusion injury. Dogs were subjected to 60 min of left anterior descending (LAD) coronary artery occlusion and 3 h of reperfusion. Infarct size and regional myocardial blood flow were assessed by macrohistochemical staining with triphenyltetrazolium chloride and radioactive microspheres, respectively. Four experimental groups were studied: vehicle control (50% DMSO in normal saline), IB-MECA (100 microg/kg iv bolus) given 10 min before the coronary occlusion, IB-MECA (100 microg/kg iv bolus) given 5 min before initiation of reperfusion, and IB-MECA (100 microg/kg iv bolus) given 10 min before coronary occlusion in dogs pretreated 15 min earlier with the ATP-dependent potassium channel antagonist glibenclamide (0.3 mg/kg iv bolus). Administration of IB-MECA had no effect on any hemodynamic parameter measured including heart rate, first derivative of left ventricular pressure, aortic pressure, LAD coronary blood flow, or coronary collateral blood flow. Nevertheless, pretreatment with IB-MECA before coronary occlusion produced a marked reduction in infarct size ( approximately 40% reduction) compared with the control group (13.0 +/- 3.2% vs. 25.2 +/- 3.7% of the area at risk, respectively). This effect of IB-MECA was blocked completely in dogs pretreated with glibenclamide. An equivalent reduction in infarct size was observed when IB-MECA was administered immediately before reperfusion (13.1 +/- 3.9%). These results are the first to demonstrate efficacy of an A3AR agonist in a large animal model of myocardial infarction by mechanisms that are unrelated to changes in hemodynamic parameters and coronary blood flow. These data also demonstrate in an in vivo model that IB-MECA is effective as a cardioprotective agent when administered at the time of reperfusion.

Activation of mast cells by immunoglobulin E-receptor cross-linkage, but not through adenosine receptors, induces A1 expression and promotes survival

BACKGROUND

Mast cells are a potent source of mediators that regulate the inflammatory response in allergy and asthma. Mast cells can be activated through different receptors, for example, via cross-linkage of the high-affinity IgE receptor (Fc epsilon RI) and by adenosine acting on specific receptors. We have recently described mast cell survival of an IgE receptor activation by up-regulation of the anti-apoptotic gene A1.

OBJECTIVE

To compare mast cell survival and expression of A1 after activation through the Fc epsilon RI and by an adenosine agonist.

METHODS

Bone marrow-derived, cultured mouse mast cells (BMCMC) were activated either with IgE+antigen or with the adenosine receptor agonist 5'-N-ethylcarboxamido adenosine (NECA). Release of beta-hexosaminidase, cell viability, phosphorylation of Akt and IkB-alpha, and expression of pro-survival and pro-apoptotic genes were measured after activation.

RESULTS

Activation of BMCMC with NECA caused the release of beta-hexosaminidase, although to a lesser extent than after Fc epsilon RI activation (33% and 98%, respectively). Activation by both NECA and Fc epsilon RI stimulated phosphorylation of Akt (Ser473 and Thr308) and IkB-alpha (Ser32), both of which are implicated in the regulation of cell survival. However, only cells that were activated through Fc epsilon RI, but not by NECA, expressed A1 and exhibited an increased survival rate compared to the control.

CONCLUSION

These results show that adenosine receptor activation of BMCMC does not induce the same survival programme in mast cells as does activation through Fc epsilon RI. These findings may be important for understanding the role that mast cells play in asthma provoked by different stimuli.

A3 adenosine and CB1 receptors activate a PKC-sensitive Cl- current in human nonpigmented ciliary epithelial cells via a G beta gamma-coupled MAPK signaling pathway

(1) We examined A3 adenosine and CB1 cannabinoid receptor-coupled signaling pathways regulating Cl(-) current in a human nonpigmented ciliary epithelial (NPCE) cell line. (2) Whole-cell patch-clamp recordings demonstrated that the A3 receptor agonist, IB-MECA, activates an outwardly rectifying Cl(-)current (I(Cl,Aden)) in NPCE cells, which was inhibited by the adenosine receptor antagonist, CGS-15943 or by the protein kinase C (PKC) activator, phorbol 12,13 dibutyrate (PDBu). (3) Treatment of NPCE cells with pertussis-toxin (PTX), or transfection with the COOH-terminus of beta-adrenergic receptor kinase (ct-betaARK), inhibited I(Cl,Aden). The phosphatidyl inositol 3-kinase (PI3K) inhibitor, wortmannin, had no effect on I(Cl,Aden); however, the mitogen-activated protein kinase kinase (MEK) inhibitor, PD98059, inhibited I(Cl,Aden). (4) Reverse transcription-polymerase chain reaction experiments and immunocytochemistry confirmed mRNA and protein expression for the CB1 receptor in NPCE cells, and the CB1 receptor agonist, Win 55,212-2, activated a PDBu-sensitive Cl(-) current (I(Cl,Win)). (5) Transfection of NPCE cells with the human CB1 (hCB1) receptor, increased I(Cl,Win), consistent with increased receptor expression, and I(Cl,Win) in hCB1 receptor-transfected cells was decreased after application of a CB1 receptor inverse agonist, SR 141716. (6) Constitutive activity for CB1 receptors was not significant in NPCE cells as transfection with hCB1 receptors did not increase basal Cl(-) current, nor was basal current inhibited by SR 141716. (7) I(Cl,Win) was inhibited by PTX preincubation, by transfection with ct-betaARK and by the MEK inhibitor, PD98059, but unaffected by the PI3K inhibitor, wortmannin. (8) We conclude that both A3 and CB1 receptors activate a PKC-sensitive Cl(-) current in human NPCE cells via a G(i/o)/Gbetagamma signaling pathway, in a manner independent of PI3K but involving MAPK.

P2Y6 nucleotide receptor activates PKC to protect 1321N1 astrocytoma cells against tumor necrosis factor-induced apoptosis

1. We recently reported that the activation by UDP of rat P2Y6 nucleotide receptors expressed in 1321N1 astrocytoma cells protected them from TNFalpha-induced apoptosis by suppressing activation of caspase 3 and 8. This study aims to characterize the involvement of intracellular signaling pathways, including kinases involved in the antiapoptotic effect of UDP. 2. Cell death was induced in 1321N1 astrocytoma cells permanently expressing the rat P2Y6 receptor by exposure to TNFalpha in the presence of cycloheximide. The apoptotic fraction was analyzed using flow cytometry. 3. The activation of P2Y6 receptors by UDP both protected the astrocytes from TNF-alpha induced apoptosis and activated protein kinase C (PKC) isotypes. The phorbol ester PMA also activated PKC and protected the cells from TNFalpha-induced cell death. The alpha- and epsilon-isotypes of PKC were both activated in a persistent fashion upon 5-min exposure to either UDP (10 microM) or the phorbol ester PMA (100 nM). The PKCzeta isotype was markedly activated upon UDP treatment. 4. The addition of PKC inhibitors, GF109203X or Gö6976, partially antagonized the protective effect of UDP and reduced the UDP-induced phosphorylation of extracellular signal-regulated protein kinases (Erk). The inhibitors of Erk, PD98,059 or U0126, antagonized UDP-induced protection. 5. The antiapoptotic protein, Akt, was not affected by P2Y6 receptor activation. Incubation of the astrocytes with calcium modifiers BAPTA-AM or dantrolene, did not affect the UDP-induced protection from apoptosis. 6. The addition of phospholipase C (PLC) inhibitors, D609 or U73122, partially antagonized both UDP-induced protection and PKC activation.

Extracellular purines from cells of seminiferous tubules

It has been long postulated that extracellular purines can modulate the function of the male reproductive system by interacting with different purinergic receptors of Sertoli and germinative cells. Many authors have described the biological changes induced by extracellular ATP and/or adenosine in these cells, and some hypothetical models for paracrine communication mediated by purines were proposed; however, the cellular source(s) of these molecules in seminiferous tubules remains unknown. In this study, we demonstrated for the first time that Sertoli cells are able to release ATP (0.3 nmol/mg protein) and adenosine (0.1 nmol/mg protein) in the extracellular medium, while germinative and myoid peritubular cells are able to secrete adenosine (0.02 and 0.37 nmol/mg protein, respectively). Indeed, all the three types of cells were able to release inosine at significant concentrations (about 0.4 nmol/mg protein). This differential secretion depending on the cellular type suggests that these molecules may be involved in the paracrine regulation and/or control of the maturation processes of these cells.

Agonists to the A3 adenosine receptor induce G-CSF production via NF-kappaB activation: a new class of myeloprotective agents

OBJECTIVE

The aim of this study was to evaluate the effect of CF101, a synthetic agonist to the A3 adenosine receptor (A3AR), on the production of granulocyte colony-stimulating factor (G-CSF). The ability of CF101 to act as a myeloprotective agent in chemotherapy-treated mice was tested.

METHODS

CF101 was administered orally to naïve mice and its effect was studied on blood cell counts (coulter counter), serum G-CSF level (ELISA), bone marrow colony-forming cells (soft agar culture), and splenocytes' ability to produce ex vivo G-CSF. Protein extract was prepared from splenocytes and Western blot analysis was carried out to evaluate expression level of key proteins. In an additional set of experiments, CF101 was administered to mice 48 hours after cyclophosphamide treatment and blood cell counts as well as serum G-CSF levels were monitored.

RESULTS

Oral administration of CF101 to naïve mice led to the elevation of serum G-CSF levels, an increase in absolute neutrophil counts (ANC), and bone marrow colony-forming cells. Splenocytes derived from these mice produced higher G-CSF level than controls. The molecular mechanisms underlying the events prior to G-CSF production included the upregulation of NF-kappaB and the upstream kinases phosphoinositide 3-kinase (PI3K), protein kinase B/Akt (PKB/Akt), and IKK. Accelerated recovery of white blood cells and neutrophil counts were observed in cyclophosphamide-treated mice following CF101 administration.

CONCLUSION

CF101 induced upregulation of the PI3K/NF-kappaB pathway leading to G-CSF production, resulting in myeloprotective effect in cyclophosphamide-treated mice.

Signaling pathway from the human adenosine A(3) receptor expressed in Chinese hamster ovary cells to the extracellular signal-regulated kinase 1/2

Adenosine activates four different receptors, the A(1), A(2A), A(2B), and the A(3) receptors, all of which are G protein-coupled. We have previously shown that stimulation of the human adenosine A(3) receptor can induce phosphorylation of extracellular signal-regulated kinase (ERK1/2). Here we show that the adenosine receptor agonist 5' N-ethylcarboxamidoadenosine (NECA) induces phosphorylation and activation of ERK1/2 in Chinese hamster ovary (CHO) cells expressing the human adenosine A(3) receptor (CHO A(3) cells) with the same potency. Pretreatment with pertussis toxin abolished the effect, which also could be blunted by overexpressing the betagamma-sequestering peptide beta-adrenergic receptor kinase-ct, implicating the involvement of betagamma subunits released from G(i/o) proteins. Activation of phosphatidylinositol-3-kinase (PI3K) by adenosine A(3) receptors is inferred from a dose-dependent Ser-phosphorylation of the protein kinase B (Akt). Furthermore the ERK1/2 phosphorylation was sensitive to the PI3K inhibitors wortmannin and LY294002 (2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride) and the MEK inhibitor PD98059 (2'-amino-3'-methoxyflavone), whereas chelation of Ca(2+) with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxymethyl ester) and long-term treatment with phorboldibutyrate did not decrease the adenosine A(3) receptor-mediated ERK1/2 phosphorylation. Thus, Ca(2+) mobilization and conventional and novel protein kinase C (PKC) isoforms are not involved in this pathway. The atypical PKCzeta was not activated by NECA and thus not involved in the A(3) receptor-mediated ERK1/2 phosphorylation. NECA stimulation of CHO A(3) cells activated the small G protein Ras and the dominant negative mutant RasS17N prevented the phosphorylation of ERK1/2. In conclusion, the adenosine A(3) receptor recruits a pathway that involves betagamma release from G(i/o), PI3K, Ras, and MEK to induce ERK1/2 phosphorylation and activation, whereas signaling is independent of Ca(2+), PKC, and c-Src.

Adenosine A(3) receptor activation protects the myocardium from reperfusion/reoxygenation injury

Ischemia-reperfusion induces both necrotic and apoptotic cell death. The ability of adenosine to attenuate reperfusion-induced injury (RI) and the role played by adenosine receptors are unclear. We therefore studied the role of the A(3) receptor (A(3)R) in ameliorating RI using the specific A(3)R agonist 1-[2-chloro-6-[[(3-iodophenyl)methyl]amino]-9H-purin-9-yl]-1-deoxi-N-methyl-b-D-ribofuranuronamide (2-Cl-IB-MECA). Isolated rat hearts and cardiomyocytes were subjected to ischemia or simulated ischemia, followed by reperfusion/reoxygenation. The end points were percent infarction/risk zone and annexin-V (apoptosis) and/or propidium iodide positivity (necrosis), respectively. In isolated hearts, 2-Cl-IB-MECA significantly limited infarct size (44.2 +/- 2.7% in control vs. 21.9 +/- 2.4% at 1 nM and 35.8 +/- 3.3% at 0.1 nM, P < 0.05). In isolated myocytes, apoptosis and necrosis were significantly reduced compared with controls (5.7 +/- 2.6% vs. 17.1 +/- 1.3% and 13.7 +/- 2.0% vs. 23.1 +/- 1.5%, respectively, P < 0.0001). In both models, the beneficial effects were abrogated using the A(3)R antagonist MRS-1191. The involvement of A(2a) receptor activation was also examined. This is the first study to demonstrate that A(3)R activation at reperfusion limits myocardial injury in the isolated rat heart and improves survival in isolated myocytes, possibly by antiapoptotic and antinecrotic mechanisms.

Adenosine receptors as mediators of both cell proliferation and cell death of cultured human melanoma cells

Adenosine displays contradictory effects on cell growth: it improves cell proliferation, but it may also induce apoptosis and impair cell survival. Following the pharmacologic characterization of adenosine receptor expression on the human melanoma cell line A375, we chose A375 as our cellular model to define how the extracellular adenosine signals are conveyed from each receptor. By using selective adenosine receptor agonists or antagonists, we found that A2A stimulation reduced cell viability and cell clone formation, whereas, at the same time, it improved cell proliferation. In support of this finding we demonstrated that the stimulation of A2A adenosine receptors stably expressed in Chinese hamster ovary cell clone reproduced deleterious effects observed in human melanoma cells. A3 stimulation counteracted A2A-induced cell death but also reduced cell proliferation. Furthermore, we found that A3 stimulation ensures cell survival. We demonstrated that adenosine triggers a survival signal via A3 receptor activation and it kills the cell through A2A receptor inducing a signaling pathway that involves protein kinase C and mitogen-activated protein kinases.

Extracellular ATP and UTP activate the protein kinase B/Akt cascade via the P2Y(2) purinoceptor in renal mesangial cells

Extracellular nucleotides can activate a common purinoceptor mediating various cell responses. In this study we report that stimulation of rat mesangial cells with ATP and UTP leads to a rapid activation of the protein kinase B/Akt (PKB) pathway. Time-course studies reveal a rapid and transient phosphorylation of both Ser(473) and Thr(308) of PKB with a maximal effect after 5 min of stimulation. The response is concentration-dependent with a maximal effect at 30 microM of ATP and UTP. Western blot analysis of mesangial cells reveals the expression of the isoenzymes PKB-alpha and PKB-gamma, but not the PKB-beta. ATP and UTP also activate the upstream located PI 3-kinase-dependent kinase. Furthermore, the ATP- and UTP-induced PKB phosphorylation is abolished by two inhibitors of the PI 3-kinase. In addition, suramin, a putative P2Y(2) receptor antagonist, and pertussis toxin, an inhibitor of G(i)/G(o) activation, markedly block ATP- and UTP-induced PKB phosphorylation. A series of ATP and UTP analogues were tested for their ability to stimulate PKB phosphorylation. UTP, ATP and gamma-thio-ATP are the only compounds capable of activating PKB. Stress-induced apoptosis of mesangial cells is reduced by the stable ATP analogue, gamma-thio-ATP, and this inhibitory effect is reversed in the presence of LY 294002. In summary, these results demonstrate that extracellular nucleotides are able to activate the PI 3-kinase/PDK/PKB cascade via the P2Y(2)-receptor and a pertussis toxin-sensitive G(i) protein. Moreover, in mesangial cells this cascade may have an important role in the antiapoptotic response but not in the mitogenic or inflammatory response produced by extracellular nucleotides.

KF26777 (2-(4-bromophenyl)-7,8-dihydro-4-propyl-1H-imidazo[2,1-i]purin-5(4H)-one dihydrochloride), a new potent and selective adenosine A3 receptor antagonist

We investigated the biochemical and pharmacological properties of a new adenosine A(3) receptor antagonist, KF26777 (2-(4-bromophenyl)-7,8-dihydro-4-propyl-1H-imidazo[2,1-i]purin-5(4H)-one dihydrochloride). This compound was characterized using N(6)-(4-amino-3-iodobenzyl)adenosine-5'-N-methyluronamide ([125I]AB-MECA) or [35S]guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding to membranes from human embryonic kidney 293 (HEK293) cells expressing human adenosine A(3) receptors. KF26777 showed a K(i) value of 0.20+/-0.038 nM for human adenosine A(3) receptors labeled with [125I]AB-MECA and possessed 9000-, 2350- and 3100-fold selectivity vs. human adenosine A(1), A(2A) and A(2B) receptors, respectively. The inhibitory mode of binding was competitive. KF26777 inhibited the binding of [35S]GTPgammaS stimulated by 1 microM 2-chloro-N(6)-(3-iodobenzyl)adenosine-5'-N-methyluronamide (Cl-IB-MECA). The IC(50) value was 270+/-85 nM; the compound had no effect on basal activity. Dexamethasone treatment for HL-60 cells, human promyelocytic leukemia, up-regulated functional adenosine A(3) receptors expression, and resulted in the enhanced elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)) via the adenosine A(3) receptor. KF26777 antagonized this [Ca(2+)](i) mobilization induced by Cl-IB-MECA, with a K(B) value of 0.42+/-0.14 nM. These results indicate that KF26777 is a highly potent and selective antagonist of the human adenosine A(3) receptor.

Cardiomyocyte resistance to doxorubicin mediated by A(3) adenosine receptor

Recently, we reported that the activation of A(3) adenosine receptor (A(3)R) in newborn cultured cardiomyocytes by highly selective agonist Cl-IB-MECA (2-chloro-N(6)-(3-iodobenzyl)adenosine-5'-N-methyluronamide) induces protection against the anthracycline antibiotic doxorubicin (DOX) cardiotoxicity. The present study was undertaken to further characterize the cardioprotective action of A(3)R activation by revealing the structural changes in cardiomyocytes elicited upon exposure to DOX. Morphological observations (ultrastructural and immunocytochemical) indicate that after DOX treatment, the cardiomyocytes undergo destructive alterations, and protective action of A(3)R is not connected with its anti-apoptotic activity. A(3)R activation appeared to prevent destructive alterations of cardiomyocyte mitochondria and dissipation of mitochondrial membrane potential. In DOX-treated cardiomyocytes, appearance of disorganized desmin and contractile filaments was related to detrimental alterations in the mitochondrial structure, in particular their position and transmembrane potential. In intact cardiomyocytes, diazoxide, a selective mitochondrial K(ATP) channel opener, induced an increase in ATP synthesis within 15 min of application. Similar effect was obtained by activation of adenosine A(1)R. However, A(3)R agonist Cl-IB-MECA did not affect ATP synthesis. Neither A(1)R agonist CCPA (2-chloro-N(6)-cyclopentyladenosine) nor diazoxide protected cardiomyocytes from the detrimental effects of DOX. Thus, the opening of mitochondrial K(ATP) channels does not seem to be effective during the slow development of anthracycline cytotoxicity. Our results indicate that DOX increases the activity of lysosomes, which may contribute to cell injury in an "oncotic" manner and also demonstrate the proinflammatory potency of the drug. Furthermore, the decreased acidification of cytoplasm upon activation of A(3)R may attenuate the ongoing inflammatory response. The present study identifies a novel role for A(3)R selective agonist Cl-IB-MECA and suggests its importance in regulating cardiac cellular function.

Inosine exerts a broad range of antiinflammatory effects in a murine model of acute lung injury

OBJECTIVE

To investigate the effects of inosine on the acute lung inflammation induced by lipopolysaccharide (LPS) in vivo and on the activation and cytotoxicity elicited by proinflammatory cytokines on human lung epithelial (A549) cells in vitro.

SUMMARY BACKGROUND DATA

Inosine is an endogenous purine recently shown to exert immunomodulatory and antiinflammatory effects.

METHODS

Mice challenged with intratracheal LPS (50 microg) were treated after 1, 6, and 12 hours with inosine (200 mg/kg intraperitoneal) or vehicle. After 24 hours, bronchoalveolar lavage fluid was obtained to measure proinflammatory (tumor necrosis factor-alpha [TNF-alpha], interleukin [IL]-1beta, IL-6), and antiinflammatory (IL-10, IL-4) cytokines, chemokines (MIP-1alpha and MIP-2), myeloperoxidase activity and total cell counts, nitric oxide production, and proteins. Lung histology and immunohistochemical detection of 3-nitrotyrosine, a marker of nitrosative stress, were performed in inflated-fixed lungs. In vitro, cell viability and production of the chemokine IL-8 were evaluated in A549 cells stimulated with a mixture of cytokines in the presence or absence of inosine.

RESULTS

Inosine downregulated the LPS-induced expression of TNF-alpha, IL-1beta, IL-6 and MIP-2 and tended to reduce MIP-1alpha, whereas it enhanced the production of IL-4. Total leukocyte counts, myeloperoxidase, nitric oxide production, and proteins were all significantly decreased by inosine. The purine also improved lung morphology and suppressed 3-nitrotyrosine staining in the lungs after LPS. Inosine attenuated the cytotoxicity and the expression of IL-8 induced by proinflammatory cytokines in A549 cells.

CONCLUSIONS

Inosine largely suppressed LPS-induced lung inflammation in vivo and reduced the toxicity of cytokines in lung cells in vitro. These data support the proposal that inosine might represent a useful adjunct in the therapy of acute respiratory distress syndrome.

Stimulation of protein kinase B and p70 S6 kinase by the histamine H1 receptor in DDT1MF-2 smooth muscle cells

1. Previous studies have shown that the histamine H(1) receptor activates p42/p44 mitogen-activated protein kinases (MAPK) in DDT(1)MF-2 smooth muscle cells via a phosphatidylinositol 3-kinase (PI-3K)-dependent pathway. In this study the effect of histamine H(1) receptor stimulation on protein kinase B (PKB) and p70 S6 kinase, both of which are downstream targets of PI-3K, has been investigated. Increases in PKB and p70 S6 kinase activation were monitored by Western blotting using phospho-specific PKB (Ser(473)) and p70 S6 kinase (Thr(421)/Ser(424)) antibodies. 2. Histamine stimulated time and concentration-dependent increases in the phosphorylation of PKB and p70 S6 kinase in DDT(1)MF-2 cells. Both responses were completely inhibited by the histamine H(1) receptor antagonist mepyramine and following pre-treatment with pertussis toxin, to block G(i)/G(o) protein dependent pathways. 3. The PI-3K inhibitors wortmannin (IC(50) 5.9+/-0.5 nM) and LY 294002 (IC(50) 6.9+/-0.8 microM) attenuated the increase in PKB phosphorylation induced by histamine (100 microM) in a concentration-dependent manner. 4. Histamine-induced increases in p70 S6 kinase phosphorylation were partially sensitive to rapamycin (20 nM; 68% inhibition) but completely blocked by wortmannin (100 nM), LY 294002 (30 microM) and the MAPK kinase inhibitor PD 98059 (50 microM). 5. In summary, these data demonstrate that the histamine H(1) receptor stimulates PKB and p70 S6 kinase phosphorylation in DDT(1)MF-2 smooth muscle cells. However, functional studies revealed that histamine does not stimulate DDT(1)MF-2 cell proliferation or attenuate staurosporine-induced caspase-3 activity. The challenge for future research will be to link the stimulation of these kinase pathways with the physiological and pathophysiological roles of the histamine H(1) receptor.

Phosphoinositide 3-kinase gamma is an essential amplifier of mast cell function

Mast cells are key regulators in allergy and inflammation, and release histamine upon clustering of their IgE receptors. Here we demonstrate that murine mast cell responses are exacerbated in vitro and in vivo by autocrine signals through G protein-coupled receptors (GPCRs) and require functional phosphoinositide 3-kinase gamma (PI3Kgamma). Adenosine, acting through the A(3) adenosine receptor (A(3)AR) as well as other agonists of G(alphai)-coupled GPCRs, transiently increased PtdIns(3,4,5)P(3) exclusively via PI3Kgamma. PI3Kgamma-derived PtdIns(3,4,5)P(3) was instrumental for initiating a sustained influx of external Ca(2+) and degranulation. Mice lacking PI3Kgamma did not form edema after intradermal injection of adenosine and when challenged by passive systemic anaphylaxis. PI3Kgamma thus relays inflammatory signals through various G(i)-coupled receptors and is central to mast cell function.

Evidence for an atypical receptor mediating the augmented bronchoconstrictor response to adenosine induced by allergen challenge in actively sensitized Brown Norway rats

The bronchoconstrictor response to adenosine is markedly and selectively increased following ovalbumin (OA) challenge in actively sensitized, Brown Norway rats. We present a pharmacological analysis of the receptor mediating this response. Like adenosine, the broad-spectrum adenosine receptor agonist, NECA, induced dose-related bronchoconstriction in actively sensitized, OA-challenged animals. In contrast, CPA, CGS 21680 and 2-Cl-IB-MECA, agonists selective for A(1) A(2A) and A(3) receptors, respectively, induced no, or minimal, bronchoconstriction. Neither the selective A(1) receptor antagonist, DPCPX, nor the selective A(2A) receptor antagonist, ZM 241385, blocked the bronchoconstrictor response to adenosine. MRS 1754, which has similar affinity for rat A(2B) and A(1) receptors, failed to block the bronchoconstrictor response to adenosine despite blockade of the A(1) receptor-mediated bradycardia induced by NECA. 8-SPT and CGS 15943, antagonists at A(1), A(2A), and A(2B) but not A(3) receptors, inhibited the bronchoconstrictor response to adenosine. However, the degree of blockade (approximately 3 fold) did not reflect the plasma concentrations, which were 139 and 21 times greater than the K(B) value at the rat A(2B) receptor, respectively. Adenosine and NECA, but not CPA, CGS 21680 or 2-Cl-IB-MECA, induced contraction of parenchymal strip preparations from actively sensitized OA-challenged animals. Responses to adenosine could not be antagonized by 8-SPT or MRS 1754 at concentrations >50 times their affinities at the rat A(2B) receptor. The receptor mediating the bronchoconstrictor response to adenosine augmented following allergen challenge in actively sensitized BN rats cannot be categorized as one of the four recognized adenosine receptor subtypes.

Neoceptor concept based on molecular complementarity in GPCRs: a mutant adenosine A(3) receptor with selectively enhanced affinity for amine-modified nucleosides

Adenosine A(3) receptors are of interest in the treatment of cardiac ischemia, inflammation, and neurodegenerative diseases. In an effort to create a unique receptor mutant that would be activated by tailor-made synthetic ligands, we mutated the human A(3) receptor at the site of a critical His residue in TM7, previously proposed to be involved in ligand recognition through interaction with the ribose moiety. The H272E mutant receptor displayed reduced affinity for most of the uncharged A(3) receptor agonists and antagonists examined. For example, the nonselective agonist 1a was 19-fold less potent at the mutant receptor than at the wild-type receptor. The introduction of an amino group on the ribose moiety of adenosine resulted in either equipotency or enhanced binding affinity at the H272E mutant relative to wild-type A(3) receptors, depending on the position of the amino group. 3'-Amino-3'-deoxyadenosine proved to be 7-fold more potent at the H272E mutant receptor than at the wild-type receptor, while the corresponding 2'- and 5'-amino analogues did not display significantly enhanced affinities. An 3'-amino-N(6)-iodobenzyl analogue showed only a small enhancement at the mutant (K(i) = 320 nM) vs wild-type receptors. The 3'-amino group was intended for a direct electrostatic interaction with the negatively charged ribose-binding region of the mutant receptor, yet molecular modeling did not support this notion. This design approach is an example of engineering the structure of mutant receptors to recognize synthetic ligands for which they are selectively matched on the basis of molecular complementarity between the mutant receptor and the ligand. We have termed such engineered receptors "neoceptors", since the ligand recognition profile of such mutant receptors need not correspond to the profile of the parent, native receptor.

Pharmacological and biochemical characterization of adenosine receptors in the human malignant melanoma A375 cell line

1. The present work characterizes, from a pharmacological and biochemical point of view, adenosine receptors in the human malignant melanoma A375 cell line. 2. Adenosine receptors were detected by RT - PCR experiments. A1 receptors were characterized using [3H]-DPCPX binding with a KD of 1.9+/-0.2 nM and Bmax of 23+/-7 fmol x mg(-1) of protein. A2A receptors were studied with [3H]-SCH 58261 binding and revealed a KD of 5.1+/-0.2 nM and a Bmax of 220+/-7 fmol x mg(-1) of protein. A3 receptors were studied with the new A3 adenosine receptor antagonist [3H]-MRE 3008F20, the only A3 selective radioligand currently available. Saturation experiments revealed a single high affinity binding site with KD of 3.3+/-0.7 nM and Bmax of 291+/-50 fmol x mg(-1) of protein. 3. The pharmacological profile of radioligand binding on A375 cells was established using typical adenosine ligands which displayed a rank order of potency typical of the different adenosine receptor subtype. 4. Thermodynamic data indicated that radioligand binding to adenosine receptor subtypes in A375 cells was entropy- and enthalpy-driven. 5. In functional assays the high affinity A2A agonists HE-NECA, CGS 21680 and A2A - A2B agonist NECA were able to increase cyclic AMP accumulation in A375 cells whereas A3 agonists Cl-IB-MECA, IB-MECA and NECA were able to stimulate Ca2+ mobilization. In conclusion, all these data indicate, for the first time, that adenosine receptors with a pharmacological and biochemical profile typical of the A1, A2A, A2B and A3 receptor subtype are present on A375 melanoma cell line.