Stefania Merighi

Adenosine receptors and cancer.

Adenosine is a ubiquitous signaling molecule whose physiological functions are mediated by its interaction with four G-protein-coupled receptor subtypes, termed A(1), A(2A), A(2B) and A(3). As a result of increased metabolic rates, this nucleoside is released from a variety of cells throughout the body in concentrations that can have a profound impact on vasculature and immunoescape. However, as high concentrations of adenosine have been reported in cancer tissues, it also appears to be implicated in the growth of tumors. Thus, full characterisation of the role of adenosine in tumor development, by addressing the question of whether adenosine receptors are present in cancer tissues, and, if so, which receptor subtype mediates its effects in cancer growth, is a vital research goal. To this end, this review focuses on the most relevant aspects of adenosine receptor subtype activation in tumors reported so far. Although all adenosine receptors now have an increasing number of recognised biological roles in tumors, it seems that the A(2A) and A(3) subtypes are the most promising as regards drug development. In particular, activation of A(2A) receptors leads to immunosuppressive effects, which decreases anti-tumoral immunity and thereby encourages tumor growth. Due to this behavior, the addition of A(2A) antagonists to cancer immunotherapeutic protocols has been suggested as a way of enhancing tumor immunotherapy. Interestingly, the safety of such compounds has already been demonstrated in trials employing A(2A) antagonists in the treatment of Parkinsons disease. As for A(3) receptors, the effectiveness of their agonists in several animal tumor models has led to the introduction of these molecules into a programme of pre-clinical and clinical trials. Paradoxically, A(3) receptor antagonists also appear to be promising candidates in human cancer treatment of regimes. Clearly, research in this still field is still in its infancy, with several important and challenging issues remaining to be addressed, although purine scientists do seem to be getting closer to their goal: the incorporation of adenosine ligands into drugs with the ability to save lives and improve human health.

Caffeine inhibits adenosine-induced accumulation of hypoxia-inducible factor-1alpha, vascular endothelial growth factor, and interleukin-8 expression in hypoxic human colon cancer cells.

Frequent coffee consumption has been associated with a reduced risk of colorectal cancer in a number of case-control studies. Coffee is a leading source of methylxanthines, such as caffeine. The induction of vascular endothelial growth factor (VEGF) and interleukin-8 (IL-8) is an essential feature of tumor angiogenesis, and the hypoxia-inducible factor-1 (HIF-1) transcription factor is known to be a key regulator of this process. In this study, we investigated the effects of caffeine on HIF-1 protein accumulation and on VEGF and IL-8 expression in the human colon cancer cell line HT29 under hypoxic conditions. Our results show that caffeine significantly inhibits adenosine-induced HIF-1α protein accumulation in cancer cells. We show that HIF-1α and VEGF are increased through A3 adenosine receptor stimulation, whereas the effects on IL-8 are mediated via the A2B subtype. Pretreatment of cells with caffeine significantly reduces adenosine-induced VEGF promoter activity and VEGF and IL-8 expression. The mechanism of caffeine seems to involve the inhibition of the extracellular signal-regulated kinase 1/2 (ERK1/2), p38, and Akt, leading to a marked decrease in adenosine-induced HIF-1α accumulation, VEGF transcriptional activation, and VEGF and IL-8 protein accumulation. From a functional perspective, we observe that caffeine also significantly inhibits the A3 receptor-stimulated cell migration of colon cancer cells. Conditioned media prepared from colon cells treated with an adenosine analog increased human umbilical vein endothelial cell migration. These data provide evidence that adenosine could modulate the migration of colon cancer cells by an HIF-1α/VEGF/IL-8-dependent mechanism and that caffeine has the potential to inhibit colon cancer cell growth.

A3 Adenosine Receptor Ligands: History and Perspectives

Adenosine regulates many physiological functions through specific cell membrane receptors. On the basis of pharmacological studies and molecular cloning, four different adenosine receptors have been identified and classified as A1, A2A, A2B, and A3. These adenosine receptors are members of the G‐protein‐coupled receptor family. While adenosine A1 and A2A receptor subtypes have been pharmacologically characterized through the use of selective ligands, the A3 adenosine receptor subtype is presently under study in order to better understand its physio‐pathological functions. Activation of adenosine A3 receptors has been shown to stimulate phospholipase C and D and to inhibit adenylate cyclase. Activation of A3 adenosine receptors also causes the release of inflammatory mediators such as histamine from mast cells. These mediators are responsible for processes such as inflammation and hypotension. It has also been suggested that the A3 receptor plays an important role in brain ischemia, immunosuppression, and bronchospasm in several animal models. Based on these results, highly selective A3 adenosine receptor agonists and/or antagonists have been indicated as potential drugs for the treatment of asthma and inflammation, while highly selective agonists have been shown to possess cardioprotective effects. The updated material related to this field of research has been rationalized and arranged in order to offer an overview of the topic.

Effect of low frequency electromagnetic fields on A2A adenosine receptors in human neutrophils

The present study describes the effect of low frequency, low energy, pulsing electromagnetic fields (PEMFs) on A2A adenosine receptors in human neutrophils. Saturation experiments performed using a high affinity adenosine antagonist [3H]‐ZM 241385 revealed a single class of binding sites in control and in PEMF‐treated human neutrophils with similar affinity (KD=1.05±0.10 and 1.08±0.12 nM, respectively). Furthermore, after 1 h of exposure to PEMFs the receptor density was statistically increased (P<0.01) (Bmax =126±10 and 215±15 fmol mg−1 protein, respectively). The effect of PEMFs was specific to the A2A adenosine receptors. This effect was also intensity, time and temperature dependent. In the adenylyl cyclase assays the A2A receptor agonists, HE‐NECA and NECA, increased cyclic AMP accumulation in untreated human neutrophils with an EC50 value of 43 (40 – 47) and 255 (228 – 284) nM, respectively. The capability of HE‐NECA and NECA to stimulate cyclic AMP levels in human neutrophils was increased (P<0.01) after exposure to PEMFs with an EC50 value of 10(8 – 13) and 61(52 – 71) nM, respectively. In the superoxide anion (O2−) production assays HE‐NECA and NECA inhibited the generation of O2− in untreated human neutrophils, with an EC50 value of 3.6(3.1 – 4.2) and of 23(20 – 27) nM, respectively. Moreover, in PEMF‐treated human neutrophils, the same compounds show an EC50 value of 1.6(1.2 – 2.1) and of 6.0(4.7 – 7.5) nM respectively. These results indicate the presence of significant alterations in the expression and in the functionality of adenosine A2A receptors in human neutrophils treated with PEMFs.

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

The present work characterizes, from a pharmacological and biochemical point of view, adenosine receptors in the human malignant melanoma A375 cell line. 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 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 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 mg−1 of protein. 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. Thermodynamic data indicated that radioligand binding to adenosine receptor subtypes in A375 cells was entropy‐ and enthalpy‐driven. 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.

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

The present work was devoted to the study of A3 adenosine receptors in Jurkat cells, a human leukemia line. 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 KD of 1.9±0.2 nM and Bmax of 1.3±0.1 pmol mg−1 of protein. 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. 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. In functional assays the high affinity A3 agonists Cl‐IB‐MECA and IB‐MECA were able to inhibit cyclic AMP accumulation and stimulate Ca2+ release from intracellular Ca2+ pools followed by Ca2+ influx. The presence of the other adenosine subtypes was investigated in Jurkat cells. A1 receptors were characterized using [3H]‐DPCPX binding with a KD of 0.9±0.1 nM and Bmax of 42±3 fmol mg−1 of protein. A2A receptors were studied with [3H]‐SCH 58261 binding and revealed a KD of 2.5±0.3 nM and a Bmax of 1.4±0.2 pmol mg−1 of protein. In conclusion, by means of the first antagonist radioligand [3H]‐MRE 3008F20 we could demonstrate the existence of functional A3 receptors on Jurkat cells.

Adenosine receptors in colon carcinoma tissues and colon tumoral cell lines: Focus on the A3 adenosine subtype

Adenosine may affect several pathophysiological processes, including cellular proliferation, through interaction with A1, A2A, A2B, and A3 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 A1, A2A, and A2B receptors were detected, whilst the A3 was the most abundant subtype in both cancer tissues and cells, with a pharmacological profile typical of the A3 subtype. All the receptors were coupled to stimulation/inhibition of adenylyl‐cyclase in cancer cells, with the exception of A1 subtype. Adenosine increased cell proliferation with an EC50 of 3–12 µM 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 A3 agonist 2‐chloro‐N6‐(3‐iodobenzyl)‐N‐methyl‐5′‐carbamoyladenosine (Cl‐IB‐MECA) able to stimulate cell proliferation with an EC50 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 A3 receptors, mediates a tonic proliferative effect. J. Cell. Physiol. 211: 826–836, 2007.

The A3 Adenosine Receptor: History and Perspectives

By general consensus, the omnipresent purine nucleoside adenosine is considered a major regulator of local tissue function, especially when energy supply fails to meet cellular energy demand. Adenosine mediation involves activation of a family of four G protein–coupled adenosine receptors (ARs): A1, A2A, A2B, and A3. The A3 adenosine receptor (A3AR) is the only adenosine subtype to be overexpressed in inflammatory and cancer cells, thus making it a potential target for therapy. Originally isolated as an orphan receptor, A3AR presented a twofold nature under different pathophysiologic conditions: it appeared to be protective/harmful under ischemic conditions, pro/anti-inflammatory, and pro/antitumoral depending on the systems investigated. Until recently, the greatest and most intriguing challenge has been to understand whether, and in which cases, selective A3 agonists or antagonists would be the best choice. Today, the choice has been made and A3AR agonists are now under clinical development for some disorders including rheumatoid arthritis, psoriasis, glaucoma, and hepatocellular carcinoma. More specifically, the interest and relevance of these new agents derives from clinical data demonstrating that A3AR agonists are both effective and safe. Thus, it will become apparent in the present review that purine scientists do seem to be getting closer to their goal: the incorporation of adenosine ligands into drugs with the ability to save lives and improve human health.

Adenosine as a Multi-Signalling Guardian Angel in Human Diseases: When, Where and How Does it Exert its Protective Effects?

The importance of adenosine for human health cannot be overstated. Indeed, this ubiquitous nucleoside is an integral component of ATP, and regulates the function of every tissue and organ in the body. Acting via receptor-dependent and -independent mechanisms [the former mediated via four G-protein-coupled receptors (GPCRs), A1, A2A, A2B, and A3,], it has a significant role in protecting against cell damage in areas of increased tissue metabolism, and combating organ dysfunction in numerous pathological states. Accordingly, raised levels of adenosine have been demonstrated in epilepsy, ischaemia, pain, inflammation, and cancer, in which its behaviour can be likened to that of a guardian angel, even though there are instances in which overproduction of adenosine is pathological. In this review, we condense the current body of knowledge on the issue, highlighting when, where, and how adenosine exerts its protective effects in both the brain and the periphery.

Adenosine and lymphocyte regulation

Adenosine is a potent extracellular messenger that is produced in high concentrations under metabolically unfavourable conditions. Tissue hypoxia, consequent to a compromised cellular energy status, is followed by the enhanced breakdown of ATP leading to the release of adenosine. Through the interaction with A2 and A3 membrane receptors, adenosine is devoted to the restoration of tissue homeostasis, acting as a retaliatory metabolite. Several aspects of the immune response have to be taken into consideration and even though in general it is very important to dampen inflammation, in some circumstances, such as the case of cancer, it is also necessary to increase the activity of immune cells against pathogens. Therefore, adenosine receptors that are defined as ‘sensors–of metabolic changes in the local tissue environment may be very important targets for modulation of immune responses and drugs devoted to regulating the adenosinergic system are promising in different clinical situations.