A3 Adenosine Receptors: Protective vs. Damaging Effects Identified Using Novel Agonists and Antagonists

N6-methyladenosine (m6A) is an endogenous A3 adenosine receptor ligand

About 150 post-transcriptional RNA modifications have been identified in all kingdoms of life. During RNA catabolism, most modified nucleosides are resistant to degradation and are released into the extracellular space. In this study, we explored the physiological role of these extracellular modified nucleosides and found that N⁶-methyladenosine (m⁶A), widely recognized as an epigenetic mark in RNA, acts as a ligand for the human adenosine A3 receptor, for which it has greater affinity than unmodified adenosine. We used structural modeling to define the amino acids required for specific binding of m⁶A to the human A3 receptor. We also demonstrated that m⁶A was dynamically released in response to cytotoxic stimuli and facilitated type I allergy in vivo. Our findings implicate m⁶A as a signaling molecule capable of activating G protein-coupled receptors (GPCRs) and triggering pathophysiological responses, a previously unreported property of RNA modifications.

Structure-Affinity Relationships and Structure-Kinetics Relationships of Pyrido[2,1-f]purine-2,4-dione Derivatives as Human Adenosine A3 Receptor Antagonists

We expanded on a series of pyrido[2,1-f]purine-2,4-dione derivatives as human adenosine A₃ receptor (hA₃R) antagonists to determine their kinetic profiles and affinities. Many compounds showed high affinities and a diverse range of kinetic profiles. We found hA₃R antagonists with very short residence time (RT) at the receptor (2.2 min for 5) and much longer RTs (e.g., 376 min for 27 or 391 min for 31). Two representative antagonists (5 and 27) were tested in [³⁵S]GTPγS binding assays, and their RTs appeared correlated to their (in)surmountable antagonism. From a k_on–k_off–K_D kinetic map, we divided the antagonists into three subgroups, providing a possible direction for the further development of hA₃R antagonists. Additionally, we performed a computational modeling study that sheds light on the crucial receptor interactions, dictating the compounds’ binding kinetics. Knowledge of target binding kinetics appears useful for developing and triaging new hA₃R antagonists in the early phase of drug discovery.

Evaluation of SSR161421, a novel orally active adenosine A3 receptor antagonist on pharmacology models

The effects of a novel adenosine A(3) receptor antagonist, SSR161421, were examined on both antigen per se and adenosine receptor agonist-increased airway responses in antigen-sensitized guinea pigs. Adenosine (10(-5)M) and AB-MECA [N6-(4-aminobenzyl)-adenosine-5'-N-methyl-uronamide dihydrochloride] (10(-7)M) increased the antigen response up to 61 ± 3.0% and 88 ± 5.2% of maximal contraction, respectively. The agonists of adenosine A(1) and A(2) adenosine receptors NECA [1-(6-amino-9H-purin-9-yl)-1-deoxy-N-ethyl-b-d-ribofuranuronamide-5'-N-ethylcarboxamidoadenosine], R-PIA [N(6)-R-phenylisopropyladenosine], and CGS21680 (10(-7)M) were ineffective. In vivo intravenous adenosine (600 μg/kg) and AB-MECA (30 μg/kg) increased the threshold antigen dose-induced bronchoconstriction by 214 ± 13.0% and 220 ± 15.2%, respectively. SSR161421 in vitro (IC(50)=5.9 × 10(-7)M) inhibited the AB-MECA-enhanced antigen-induced airway smooth muscle contractions and also in vivo the bronchoconstriction following either intravenous (ED(50)=0.008 mg/kg) or oral (ED(50)=0.03 mg/kg) administration in sensitized guinea pigs. Antigen itself could evoke tracheal contraction in vitro and bronchoconstriction in vivo in antigen-sensitized guinea pigs. SSR161421 (3 × 10(-6)M) decreased the AUC of the antigen-induced contraction-time curve to 20.8 ± 5.4% from the 100% control level. SSR161421 effectively reversed the antigen-induced bronchoconstriction, plasma leak and cell recruitment with EC(50) values of 0.33 mg/kg p.o., 0.02 mg/kg i.p. and 3 mg/kg i.p., respectively.

Pyrrolo- and pyrazolo-[3,4-e][1,2,4]triazolo[1,5-c]pyrimidines as adenosine receptor antagonists

The discovery and development of adenosine receptor antagonists have represented for years an attractive field of research from the perspective of identifying new drugs for the treatment of widespread disorders such as inflammation, asthma and Parkinson's disease. The present work can be considered as an extension of our structure-activity relationship studies on the pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidine (PTP) nucleus, extensively investigated by us as a useful template, in particular, for the identification of A(2A) and A(3) adenosine receptor antagonists. In order to explore the role of the nitrogen at the 7-position, we performed a new synthetic strategy for the preparation of pyrrolo[3,4-e][1,2,4]triazolo[1,5-c]pyrimidine derivatives which can be considered as 7-deaza analogues of the parent PTPs. We also synthesised a novel series of pyrazolo[3,4-e][1,2,4]triazolo[1,5-c]pyrimidines as junction isomers of the reference compounds. In both cases we obtained some examples of potent antagonists (K(i) in the low nanomolar range) with variable selectivity profiles in relation to the nature of substituents introduced at the C(5)-, N(8)- and/or N(9)-positions. The pyrrolo-triazolo-pyrimidine derivative 9b appeared to be a potent A(3) adenosine receptor antagonist (K(i)=10 nM) with good selectivity over hA(1) (74-fold) and hA(2A) (20-fold) adenosine receptors combined with low activity at the hA(2B) subtype (IC(50)=906 nM). Moreover, some examples of high-affinity A(1)/A(2A) dual antagonists have been identified in both series. This work constitutes a new and important contribution for the comprehension of the interaction between PTPs and adenosine receptors.

Pyrimidine derivatives as potent and selective A3 adenosine receptor antagonists

Two regioisomeric series of diaryl 2- or 4-amidopyrimidines have been synthesized and their adenosine receptor affinities were determined in radioligand binding assays at the four human adenosine receptors (hARs). Some of the ligands prepared herein exhibit remarkable affinities (K(i) < 10 nm) and, most noticeably, the absence of activity at the A(1), A(2A), and A(2B) receptors. The structural determinants that support the affinity and selectivity profiles of the series were highlighted through an integrated computational approach, combining a 3D-QSAR model built on the second generation of GRid INdependent Descriptors (GRIND2) with a novel homology model of the hA(3) receptor. The robustness of the computational model was subsequently evaluated by the design of new derivatives exploring the alkyl substituent of the exocyclic amide group. The synthesis and evaluation of the novel compounds validated the predictive power of the model, exhibiting excellent agreement between predicted and experimental activities.

N6-methoxy-2-alkynyladenosine derivatives as highly potent and selective ligands at the human A3 adenosine receptor

A new series of N6-methoxy-2-(ar)alkynyladenosine derivatives has been synthesized and tested at the human recombinant adenosine receptors. Binding studies demonstrated that the new compounds possess high affinity and selectivity for the A3 subtype. Among them, compounds bearing an N-methylcarboxamido substituent in the 4'-position showed the highest A3 affinity and selectivity. In particular, the N6-methoxy-2-p-acetylphenylethynylMECA (40; Ki A3 = 2.5 nM, A3 selectivity versus A1 = 21 500 and A2A = 4200) results in one of the most potent and selective agonists at the human A3 adenosine receptor reported so far. Furthermore, functional assay, performed with selected new compounds, revealed that the presence of an alkylcarboxamido group in the 4'-position seems to be essential to obtain full agonists at the A3 subtype. Finally, results of molecular docking analysis were in agreement with binding and functional data and could explain the high affinity and potency of the new compounds.

2-triazole-substituted adenosines: a new class of selective A3 adenosine receptor agonists, partial agonists, and antagonists

"Click chemistry" was explored to synthesize two series of 2-(1,2,3-triazolyl)adenosine derivatives (1-14). Binding affinity at the human A(1), A(2A), and A(3)ARs (adenosine receptors) and relative efficacy at the A(3)AR were determined. Some triazol-1-yl analogues showed A(3)AR affinity in the low nanomolar range, a high ratio of A(3)/A(2A) selectivity, and a moderate-to-high A(3)/A(1) ratio. The 1,2,3-triazol-4-yl regiomers typically showed decreased A(3)AR affinity. Sterically demanding groups at the adenine C2 position tended to reduce relative A(3)AR efficacy. Thus, several 5'-OH derivatives appeared to be selective A(3)AR antagonists, i.e., 10, with 260-fold binding selectivity in comparison to the A(1)AR and displaying a characteristic docking mode in an A(3)AR model. The corresponding 5'-ethyluronamide analogues generally showed increased A(3)AR affinity and behaved as full agonists, i.e., 17, with 910-fold A(3)/A(1) selectivity. Thus, N(6)-substituted 2-(1,2,3-triazolyl)adenosine analogues constitute a novel class of highly potent and selective nucleoside-based A(3)AR antagonists, partial agonists, and agonists.

Synthesis of hypermodified adenosine derivatives as selective adenosine A3 receptor ligands

We investigated the A(3)AR affinity and selectivity of a series of 2-substituted 3'-azido and 3'-amino adenosine derivatives as well as some 5'-uronamide derivatives thereof. All compounds showed high A(3)AR selectivity. While the 3'-azides appeared to be A(3)AR antagonists with moderate A(3)AR affinity, their 3'-amino congeners exhibit significantly improved A(3)AR affinity and behave as partial agonists. For both the 3'-azides and the 3'-amines, the 5'-methylcarbamoyl modification improved the overall affinity. Introduction of a 2-phenylethynyl substituent provided high affinity for the A(3)AR.

A new orally bioavailable dual adenosine A2B/A3 receptor antagonist with therapeutic potential

The synthesis and SAR of 5-heterocycle-substituted aminothiazole adenosine receptor antagonists is described. Several compounds show high affinity and selectivity for the A2B and A3 receptors. One compound (5f) shows good ADME properties in the rat and as such may be an important new compound in testing the current hypotheses proposing a therapeutic role for a dual A2B/A3 antagonist in allergic diseases.

New pyrrolo[2,1-f]purine-2,4-dione and imidazo[2,1-f]purine-2,4-dione derivatives as potent and selective human A3 adenosine receptor antagonists

Compounds presenting an additional fused ring on the xanthine nucleus have been reported to exhibit antagonistic activity with various levels of affinity and selectivity toward the four adenosine receptors subtypes A(1), A(2A), A(2B), and A(3). This paper reports synthesis and biological evaluation of new 1-benzyl-3-propyl-1H,6H-pyrrolo[2,1-f]purine-2,4-diones and 1-benzyl-3-propyl-1H,8H-imidazo[2,1-f]purine-2,4-diones, among which we identified potent and selective A(3) adenosine receptors antagonists. In particular, 1-benzyl-7-methyl-3-propyl-1H,8H-imidazo[2,1-f]purine-2,4-dione (11e) shows a K(i) (hA(3)) value from binding assay of 0.8 nM.

Role of interstitial ATP and adenosine in the regulation of renal hemodynamics and microvascular function

The role of adenosine in the regulation of renal hemodynamics and function has been studied extensively; however, another purine agent, ATP, is also gaining recognition for its paracrine role in the kidney. Adenosine and ATP bind to specific membrane-bound P1 and P2 purinoceptors, respectively, and initiate a variety of biological effects on renal microvascular tone, mesangial cell function, and renal epithelial transport. The purpose of this review is to summarize the potential roles of interstitial ATP and adenosine as regulators of renal hemodynamics and microcirculation. In vitro blood-perfused juxtamedullary nephron preparation was used to assess the roles of ATP and adenosine in the regulation of renal microvascular tone. This approach mimics the adventitial exposure of renal microvascular smooth muscle to ATP and adenosine synthesized locally and released into the interstitial fluid. ATP selectively vasoconstricts afferent but not efferent arterioles via P2X and P2Y receptors, whereas, adenosine vasoconstricts both vascular segments via activation of adenosine A(1) receptors. Furthermore, selective P2X and P2Y receptor stimulation increases intracellular calcium concentration in vascular smooth muscle cells that are freshly isolated from the preglomerular microvasculature. These data support the hypothesis that interstitial ATP plays a critical role in the control of renal microvascular function through mechanisms that are independent of adenosine receptors. We have recently developed a renal microdialysis method to determine the dynamics of ATP and adenosine levels in the renal cortical interstitium. In this review, we also summarize current knowledge pertaining to the alterations in renal interstitial ATP and adenosine in some pathophysiological conditions.

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.

Adenosine receptor agonists: from basic medicinal chemistry to clinical development

Adenosine is a physiological nucleoside which acts as an autocoid and activates G protein-coupled membrane receptors, designated A(1), A(2A), A(2B) and A(3). Adenosine plays an important role in many (patho)physiological conditions in the CNS as well as in peripheral organs and tissues. Adenosine receptors are present on virtually every cell. However, receptor subtype distribution and densities vary greatly. Adenosine itself is used as a therapeutic agent for the treatment of supraventricular paroxysmal tachycardia and arrhythmias and as a vasodilatatory agent in cardiac imaging. During the past 20 years, a number of selective agonists for A(1), A(2A) and A(3) adenosine receptors have been developed, all of them structurally derived from adenosine. Several such compounds are currently undergoing clinical trials for the treatment of cardiovascular diseases (A(1)and A(2A)), pain (A(1)), wound healing (A(2A)), diabetic foot ulcers (A(2A)), colorectal cancer (A(3)) and rheumatoid arthritis (A(3)). Clinical evaluation of some A(1) and A(2A) adenosine receptor agonists has been discontinued. Major problems include side effects due to the wide distribution of adenosine receptors; low brain penetration, which is important for the targeting of CNS diseases; short half-lifes of compounds; or a lack of effects, in some cases perhaps due to receptor desensitisation or to low receptor density in the targeted tissue. Partial agonists, inhibitors of adenosine metabolism (adenosine kinase and deaminase inhibitors) or allosteric activators of adenosine receptors may be advantageous for certain indications, as they may exhibit fewer side effects.

Exploring human adenosine A3 receptor complementarity and activity for adenosine analogues modified in the ribose and purine moiety

In this paper we investigated the influence on affinity, selectivity and intrinsic activity upon modification of the adenosine agonist scaffold at the 3'- and 5'-positions of the ribofuranosyl moiety and the 2- and N6-positions of the purine base. This resulted in the synthesis of various analogues, that is, 3-12 and 24-33, with good hA3AR selectivity and moderate-to-high affinities (as in 32, Ki=27 nM). Interesting was the ability to tune the intrinsic activity depending on the substituent introduced at the 3'-position.

Adenosine A3 receptor-induced CCL2 synthesis in cultured mouse astrocytes

During neuropathological conditions, high concentrations of adenosine are released, stimulating adenosine receptors in neurons and glial cells. It has recently been shown that stimulation of adenosine receptors in glial cells induces the release of neuroprotective substances such as NGF, S-100beta, and interleukin-6 (IL-6). It has therefore been suggested that glial adenosine receptors are involved in neuroprotection. Since recently neuroprotective effects of the chemokine CCL2 (formerly known as MCP-1) have been reported, we investigated the possible effect of adenosine receptor stimulation on glial CCL2 synthesis. Here we show that stimulation of cultured murine astrocytes with the selective adenosine A3 receptor agonist 2-chloro-N6-(3-iodobenzyl)-N-methyl-5'-carbamoyladenosine (CL-IB-MECA) induced the release of CCL2. Specific ligands for adenosine A1 or A2 receptors did not affect CCL2 release. Furthermore, CL-IB-MECA-induced CCL2 synthesis was inhibited by adenosine A3 receptor antagonists. These results show that stimulation of adenosine A3 receptors in astrocytes induced the release of CCL2, thus supporting the assumption that adenosine receptors in glial cells regulate the synthesis of neuroprotective substances.

Recent developments in the field of A2A and A3 adenosine receptor antagonists

In the last years adenosine receptors have been extensively studied, and mainly at present we understand the importance of A(2A) and A(3) adenosine receptors. A(2A) selective adenosine receptors antagonists are promising new drugs for the treatment of Parkinson's disease, while A(3) selective adenosine receptors antagonists have been postulated as novel anti-inflammatory and antiallergic agents; recent studies also indicated a possible employment of these derivatives as antitumour agents. Lately different classes of compounds have been identified as potent A(2A) and A(3) antagonists. In this article we report the past and present efforts which led to development of more potent and selective A(2A) and A(3) antagonists. Our group has mainly worked on the pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine nucleus both as A(2A) and A(3) antagonists, aiming to improve the affinity, selectivity and the hydrophilic profile. In fact, we have synthesised several compounds endowed with high affinity and selectivity versus A(2A) adenosine receptors, as 2, 2a-c (K(i)A(2A)=0.12-0.19 nM), or A(3) adenosine receptors, as 4p (K(i)A(3)=0.01 nM) and 4q (K(i)A(3)=0.04 nM).

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.

Synthesis, biological properties, and molecular modeling investigation of the first potent, selective, and water-soluble human A(3) adenosine receptor antagonist

A new, highly potent, selective, and water-soluble antagonist of the hA(3) adenosine receptor was synthesized and tested in binding and functional assays. Compound 4 (5-[[(4-pyridyl)amino]carbonyl]amino-8-methyl-2-(2-furyl)-pyrazolo[4,3-e]1,2,4-triazolo[1,5-c]pyrimidine hydrochloride) displayed high water solubility (15 mM) and the highest affinity (K(i) = 0.01 nM) and selectivity for the hA(3) versus A(1), A(2A), and A(2B) receptors (>10000-fold) ever reported. A Schild analysis of the antagonism by 4 of agonist-induced inhibition of cAMP production in CHO cells expressing the hA(3) receptor indicated a K(B) value of 0.20 nM.

Synthesis, biological activity, and molecular modeling investigation of new pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine derivatives as human A(3) adenosine receptor antagonists

A new series of pyrazolotriazolopyrimidines bearing different substitutions on the phenylcarbamoyl moieties at the N5 position, being highly potent and selective human A(3) adenosine receptor antagonists, is described. The compounds represent an extension and an improvement of our previous work on this class of compounds (J. Med. Chem. 1999, 42, 4473-4478; J. Med. Chem. 2000, 43, 4768-4780). All the synthesized compounds showed A(3) adenosine receptor affinity in the subnanomolar range and high levels of selectivity in radioligand binding assays at the human A(1), A(2A), A(2B), and A(3) adenosine receptors. In particular, the effect of the substitution and its position on the phenyl ring have been studied. From binding data, it is evident that the unsubstituted derivatives on the phenyl ring (e.g., compound 59, hA(3) = 0.16 nM, hA(1)/hA(3) = 3713, hA(2A)/hA(3) = 2381, hA(2B)/hA(3) = 1388) showed the best profile in terms of affinity and selectivity at the human A(3) adenosine receptors. The introduction of a sulfonic acid moiety at the para position on the phenyl ring was attempted in order to design water soluble derivatives. However, this substitution led to a dramatic decrease of affinity at all four adenosine receptor subtypes. A computer-generated model of the human A(3) receptor was built and analyzed to better interpret these results, demonstrating that steric control, in particular at the para position on the phenyl ring, plays a fundamental role in the receptor interaction. Some of the synthesized compounds proved to be full antagonists in a specific functional model, where the inhibition of cAMP-generation by IB-MECA was measured in membranes of CHO cells stably transfected with the human A(3) receptor with IC(50) values in the nanomolar range, with a statistically significative linear relationship with the binding data.

The A3 adenosine receptor as a new target for cancer therapy and chemoprotection

Adenosine, a purine nucleoside, acts as a regulatory molecule, by binding to specific G-protein-coupled A(1), A(2A), A(2B), and A(3) cell surface receptors. We have recently demonstrated that adenosine induces a differential effect on tumor and normal cells. While inhibiting in vitro tumor cell growth, it stimulates bone marrow cell proliferation. This dual activity was mediated through the A3 adenosine receptor. This study showed that a synthetic agonist to the A3 adenosine receptor, 2-chloro-N(6)-(3-iodobenzyl)-adenosine-5'-N-methyl-uronamide (Cl-IB-MECA), at nanomolar concentrations, inhibited tumor cell growth through a cytostatic pathway, i.e., induced an increase number of cells in the G0/G1 phase of the cell cycle and decreased the telomeric signal. Interestingly, Cl-IB-MECA stimulates murine bone marrow cell proliferation through the induction of granulocyte-colony-stimulating factor. Oral administration of Cl-IB-MECA to melanoma-bearing mice suppressed the development of melanoma lung metastases (60.8 +/- 6.5% inhibition). In combination with cyclophosphamide, a synergistic anti-tumor effect was achieved (78.5 +/- 9.1% inhibition). Furthermore, Cl-IB-MECA prevented the cyclophosphamide-induced myelotoxic effects by increasing the number of white blood cells and the percentage of neutrophils, demonstrating its efficacy as a chemoprotective agent. We conclude that A3 adenosine receptor agonist, Cl-IB-MECA, exhibits systemic anticancer and chemoprotective effects.

Pharmacological and biochemical characterization of A3 adenosine receptors in Jurkat T cells

1. The present work was devoted to the study of A3 adenosine receptors in Jurkat cells, a human leukemia line. 2. The A3 subtype was found by means of RT-PCR experiments and characterized by using 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 K(D) of 1.9+/-0.2 nM and B(max) of 1.3+/-0.1 pmol mg(-1) of protein. 3. The pharmacological profile of [3H]-MRE 3008F20 binding on Jurkat cells was established using typical adenosine ligands which displayed a rank order of potency typical of the A3 subtype. 4. Thermodynamic data indicated that [3H]-MRE 3008F20 binding to A3 subtype in Jurkat cells was entropy- and enthalpy-driven, according with that found in cells expressing the recombinant human A3 subtype. 5. In functional assays the high affinity A3 agonists Cl-IB-MECA and IB-MECA were able to inhibit cyclic AMP accumulation and stimulate Ca(2+) release from intracellular Ca(2+) pools followed by Ca(2+) influx. 6. The presence of the other adenosine subtypes was investigated in Jurkat cells. A1 receptors were characterized using [3H]-DPCPX binding with a K(D) of 0.9+/-0.1 nM and B(max) of 42+/-3 fmol mg(-1) of protein. A2A receptors were studied with [3H]-SCH 58261 binding and revealed a K(D) of 2.5+/-0.3 nM and a B(max) of 1.4+/-0.2 pmol mg(-1) of protein. 7. In conclusion, by means of the first antagonist radioligand [3H]-MRE 3008F20 we could demonstrate the existence of functional A3 receptors on Jurkat cells.

P2 receptors in regulation of renal microvascular function

In the last 10-15 years, interest in the physiological role of P2 receptors has grown rapidly. Cellular, tissue, and organ responses to P2 receptor activation have been described in numerous in vivo and in vitro models. The purpose of this review is to provide an update of the recent advances made in determining the involvement of P2 receptors in the control of renal hemodynamics and the renal microcirculation. Special attention will be paid to work published in the last 5-6 years directed at understanding the role of P2 receptors in the physiological control of renal microvascular function. Several investigators have begun to evaluate the effects of P2 receptor activation on renal microvascular function across several species. In vivo and in vitro evidence consistently supports the hypothesis that P2 receptor activation by locally released extracellular nucleotides influences microvascular function. Extracellular nucleotides selectively influence preglomerular resistance without having an effect on postglomerular tone. P2 receptor inactivation blocks autoregulatory behavior whereas responsiveness to other vasoconstrictor agonists is retained. P2 receptor stimulation activates multiple intracellular signal transduction pathways in preglomerular smooth muscle cells and mesangial cells. Renal microvascular cells and mesangial cells express multiple subtypes of P2 receptors; however, the specific role each plays in regulating vascular and mesangial cell function remains unclear. Accordingly, the results of studies performed to date provide strong support for the hypothesis that P2 receptors are important contributors to the physiological regulation of renal microvascular and/or glomerular function.

Resistance of muscle to tumor metastases: a role for a3 adenosine receptor agonists

Tumor metastases are extremely rare in striated muscles. Lately, we have found that muscle cell conditioned medium (MCM) inhibits the proliferation of various tumor cells while maintaining the growth of normal murine bone marrow cells. This dual activity was confirmed in vivo when the MCM was administered orally, i.e., it inhibited the development of tumor growth in mice and prevented the myelotoxic effects of chemotherapy. Adenosine was found to be one of the active components of MCM, inhibiting tumor cell growth while maintaining bone marrow cell proliferation in vitro. Adenosine is known to act as an important regulatory molecule through its binding to specific G-protein-associated A1, A(2a), A(2b) and A3 cell surface receptors. In distinction from MCM, adenosine did not suppress tumor development in mice and was not active as a chemoprotective agent when administered orally or intravenously. Thus, the in vivo activity of MCM could not be attributed to adenosine. In this study, MCM from which adenosine was enzymatically removed still retained its dual activity that was also found to be mediated through the A3 adenosine receptor (A3AR). This result led to the conclusion that natural agonists to A3AR were responsible for the activity of MCM. We further tested synthetic agonist to the A3AR and demonstrated that it possessed the same in vitro and in vivo activity profile as MCM. Taken together, muscle cells, in addition to adenosine, secrete natural agonists to A3AR. These agonists are stable nondegradable molecules and may contribute to the systemic anticancer and chemoprotective activity exerted by MCM. This group of molecules may account for the rarity of tumor metastases in muscle.

Adenosine inhibits IL-12 and TNF-[alpha] production via adenosine A2a receptor-dependent and independent mechanisms

Interleukin 12 (IL-12) is a crucial cytokine in the regulation of T helper 1 vs. T helper 2 immune responses. In the present study, we investigated the effect of the endogenous purine nucleoside adenosine on the production of IL-12. In mouse macrophages, adenosine suppressed IL-12 production. Although the order of potency of adenosine receptor agonists suggested the involvement of A2a receptors, data obtained with A2a receptor-deficient mice showed that the adenosine suppression of IL-12 and even TNF-alpha production is only partly mediated by A2a receptor ligation. Studies with adenosine receptor antagonists or the adenosine uptake blocker dipyridamole showed that adenosine released endogenously also decreases IL-12. Although adenosine increases IL-10 production, the inhibition of IL-12 production is independent of the increased IL-10. The mechanism of action of adenosine was not associated with alterations of the activation of the p38 and p42/p44 mitogen-activated protein kinases or the phosphorylation of the c-Jun terminal kinase. Adenosine failed to affect steady-state levels of either IL-12 p35 or p40 mRNA, but augmented IL-10 mRNA levels. In summary, adenosine inhibits IL-12 production via various adenosine receptors. These results support the notion that adenosine-based therapies might be useful in certain autoimmune and/or inflammatory diseases.

Purinergic and pyrimidinergic receptors as potential drug targets

In the last decade, the field of purinergic pharmacology has continued to grow as the complexity of the receptor families and the various enzymes involved in purine metabolism have been defined in molecular terms. A major theme that has emerged from these studies is the functional complexity of the interactions between P1 and P2 receptors, based upon the dynamic interrelationship between ATP and adenosine as extracellular signaling molecules. It is now clear that ATP and its degradation products (particularly ADP and adenosine) form a complex cascade for the regulation of cell-to-cell communication that can function to attenuate the consequences of tissue trauma (e.g. ischemia) that involve alterations in cellular energy charge and depletion of ATP stores. In addition to the P2 receptor family, alterations in cellular ATP stores can also affect the function of other receptors, e.g. K(ATP) channels, and mitochondrial function. The discovery of pyrimidine-preferring (UTP/UDP) P2Y receptors has also raised the possibility that the corresponding nucleoside, uracil, may function as a signaling molecule.

Inosine inhibits inflammatory cytokine production by a posttranscriptional mechanism and protects against endotoxin-induced shock

Extracellular purines, including adenosine and ATP, are potent endogenous immunomodulatory molecules. Inosine, a degradation product of these purines, can reach high concentrations in the extracellular space under conditions associated with cellular metabolic stress such as inflammation or ischemia. In the present study, we investigated whether extracellular inosine can affect inflammatory/immune processes. In immunostimulated macrophages and spleen cells, inosine potently inhibited the production of the proinflammatory cytokines TNF-alpha, IL-1, IL-12, macrophage-inflammatory protein-1alpha, and IFN-gamma, but failed to alter the production of the anti-inflammatory cytokine IL-10. The effect of inosine did not require cellular uptake by nucleoside transporters and was partially reversed by blockade of adenosine A1 and A2 receptors. Inosine inhibited cytokine production by a posttranscriptional mechanism. The activity of inosine was independent of activation of the p38 and p42/p44 mitogen-activated protein kinases, the phosphorylation of the c-Jun terminal kinase, the degradation of inhibitory factor kappaB, and elevation of intracellular cAMP. Inosine suppressed proinflammatory cytokine production and mortality in a mouse endotoxemic model. Taken together, inosine has multiple anti-inflammatory effects. These findings, coupled with the fact that inosine has very low toxicity, suggest that this agent may be useful in the treatment of inflammatory/ischemic diseases.