Faina Barer

Differential Effect of Adenosine on Tumor and Normal Cell Growth: Focus on the A3 Adenosine Receptor

Adenosine is an ubiquitous nucleoside present in all body cells. It is released from metabolically active or stressed cells and subsequently acts as a regulatory molecule through binding to specific A1, A2A, A2B and A3 cell surface receptors. The synthesis of agonists and antagonists to the adenosine receptors and their cloning enabled the exploration of their physiological functions. As nearly all cells express specific adenosine receptors, adenosine serves as a physiological regulator and acts as a cardioprotector, neuroprotector, chemoprotector, and as an immunomodulator. At the cellular level, activation of the receptors by adenosine initiates signal transduction mechanisms through G‐protein associated receptors. Adenosines unique characteristic is to differentially modulate normal and transformed cell growth, depending upon its extracellular concentration, the expression of adenosine cell surface receptors, and the physiological state of the target cell. Stimulatio n of cell proliferation following incubation with adenosine has been demonstr ated in a variety of normal cells in the range of low micromolar con centrations, including mesangial and thymocyte cells, Swiss mouse 3T3 fibroblasts, and bone marrow cells. Induction of apoptosis in tumor or normal cells was shown at higher adenosine concentrations (<100 μM) such as in leukemia HL‐60, lymphoma U‐937, A431 epidermoid cells, and GH3 tumor pituitary cell lines. It was further noted that the A3 adenosine receptor (A3AR) plays a key role in the inhibitory and stimulatory growth activities of adenosine. Modulation of the A3AR was found to affect cell growth either positively or negatively depending on the concentration of the agonist, similar to the effect described for adenosine. At nanomolar concentrations, the A3AR agonists possess dual activity, i.e., anti‐proliferative activity toward tumor cells and stimulator y effect on bone marrow cells. In vivo, these agonists exerted anti‐cancer effects, and when given in combination with chemotherapy, they enhanced the chemotherapeutic index and acted as chemoprotective agents. Taken together, activation of the A3AR, by minute concentrations of its natural ligand or synthetic agonists, may serve as a new approach for cancer therapy. J. Cell. Physiol. 186:19–23, 2001.

The A3 adenosine receptor is highly expressed in tumor versus normal cells: potential target for tumor growth inhibition.

Purpose: A3 adenosine receptor (A3AR) activation was shown to inhibit the growth of various tumor cells via the down-regulation of nuclear factor κB and cyclin D1. To additionally elucidate whether A3AR is a specific target, a survey of its expression in tumor versus adjacent normal cells was conducted. Experimental Design: A3AR mRNA expression in various tumor tissues was tested in paraffin-embedded slides using reverse transcription-PCR analysis. A comparison with A3AR expression in the relevant adjacent normal tissue or regional lymph node metastasis was performed. In addition, A3AR protein expression was studied in fresh tumors and was correlated with that of the adjacent normal tissue. Results: Reverse transcription-PCR analysis of colon and breast carcinoma tissues showed higher A3AR expression in the tumor versus adjacent non-neoplastic tissue or normal tissue. Additional analysis revealed that the lymph node metastasis expressed even more A3AR mRNA than the primary tumor tissue. Protein analysis of A3AR expression in fresh tumors derived from colon (n = 40) or breast (n = 17) revealed that 61% and 78% had higher A3AR expression in the tumor versus normal adjacent tissue, respectively. The high A3AR expression level in the tumor tissues was associated with elevated nuclear factor κB and cyclin D1 levels. High A3AR mRNA expression was also demonstrated in other solid tumor types. Conclusions: Primary and metastatic tumor tissues highly express A3AR indicating that high receptor expression is a characteristic of solid tumors. These findings and our previous data suggest A3AR as a potential target for tumor growth inhibition.

An agonist to the A3 adenosine receptor inhibits colon carcinoma growth in mice via modulation of GSK-3 beta and NF-kappa B.

A3 adenosine receptor (A3AR) activation with the specific agonist CF101 has been shown to inhibit the development of colon carcinoma growth in syngeneic and xenograft murine models. In the present study, we looked into the effect of CF101 on the molecular mechanisms involved in the inhibition of HCT-116 colon carcinoma in mice. In tumor lesions derived from CF101-treated mice, a decrease in the expression level of protein kinase A (PKA) and an increase in glycogen synthase kinase-3β (GSK-3β) was observed. This gave rise to downregulation of β-catenin and its transcriptional gene products cyclin D1 and c-Myc. Further mechanistic studies in vitro revealed that these responses were counteracted by the selective A3AR antagonist MRS 1523 and by the GSK-3β inhibitors lithium and SB216763, confirming that the observed effects were A3AR and GSK-3β mediated. CF101 downregulated PKB/Akt expression level, resulting in a decrease in the level and DNA-binding capacity of NF-κB, both in vivo and in vitro. Furthermore, the PKA and PKB/Akt inhibitors H89 and Worthmannin mimicked the effect of CF101, supporting their involvement in mediating the response to the agonist. This is the first demonstration that A3AR activation induces colon carcinoma growth inhibition via the modulation of the key proteins GSK-3β and NF-κB.

Evidence for involvement of Wnt signaling pathway in IB-MECA mediated suppression of melanoma cells

The A3 adenosine receptor, A3AR, belongs to the family of Gi proteins, which upon induction, suppresses the formation of cAMP and its downstream effectors. Recent studies have indicated that activation of A3AR by its agonist, IB-MECA, results in growth inhibition of malignant cells. Here we demonstrate the ability of IB-MECA to decrease the levels of protein kinase A, a downstream effector of cAMP, and protein kinase B/Akt in melanoma cells. Examination of glycogen synthase kinase 3β, GSK-3β, whose phosphorylation is controlled by protein kinase A and B, showed a substantial decrease in the levels of its phosphorylated form and an increase in total GSK-3β levels in IB-MECA treated melanoma cells. This observation suggests that the treatment of cells with IB-MECA augments the activity of GSK-3β in the cells. Evaluation of β-catenin, a key component of Wnt signaling pathway which, upon phosphorylation by GSK-3β rapidly ubiquitinates, showed a substantial decrease in its level after IB-MECA treatment. Accordingly, the level of β-catenin responsive cell growth regulatory genes including c-myc and cyclin D1 was severely declined upon treatment of the cells with IB-MECA. These observations which link cAMP to the Wnt signaling pathway provide mechanistic evidence for the involvement of Wnt pathway via its key elements GSK-3β and β-catenin in the anti-tumor activity of A3AR agonists.

Adenosine acts as an inhibitor of lymphoma cell growth: a major role for the A3 adenosine receptor.

In this study, we demonstrated several mechanisms exploring the inhibitory effect of low-dose adenosine on lymphoma cell growth. Adenosine, a purine nucleoside present in plasma and other extracellular fluids, acts as a regulatory molecule, by binding to G-protein associated cell-surface receptors, A1, A2 and A3. Recently we showed that low-dose adenosine released by muscle cells, inhibits tumour cell growth and thus attributes to the rarity of muscle metastases. In the present work, a cytostatic effect of adenosine on the proliferation of the Nb2-11C rat lymphoma cell line was demonstrated. This effect was mediated through the induction of cell cycle arrest in the G0/G1 phase and by decreasing the telomeric signal in these cells. Adenosine was found to exert its antiproliferative effect mainly through binding to its A3 receptor. The cytostatic anticancer activity, mediated through the A3 adenosine receptor, turns it into a potential target for the development of anticancer therapies.

Induction of an antiinflammatory effect and prevention of cartilage damage in rat knee osteoarthritis by CF101 treatment.

OBJECTIVE Studies have suggested that rheumatoid arthritis (RA) and osteoarthritis (OA) share common characteristics. The highly selective A(3) adenosine receptor agonist CF101 was recently defined as a potent antiinflammatory agent for the treatment of RA. The purpose of this study was to examine the effects of CF101 on the clinical and pathologic manifestations of OA in an experimental animal model. METHODS OA was induced in rats by monosodium iodoacetate, and upon disease onset, oral treatment with CF101 (100 microg/kg given twice daily) was initiated. The A(3) adenosine receptor antagonist MRS1220 (100 microg/kg given twice daily) was administered orally, 30 minutes before CF101 treatment. The OA clinical score was monitored by knee diameter measurements and by radiographic analyses. Histologic analyses were performed following staining with hematoxylin and eosin, Safranin O-fast green, or toluidine blue, and histologic changes were scored according to a modified Mankin system. Signaling proteins were assayed by Western blotting; apoptosis was detected via immunohistochemistry and TUNEL analyses. RESULTS CF101 induced a marked decrease in knee diameter and improved the changes noted on radiographs. Administration of MRS1220 counteracted the effects of CF101. CF101 prevented cartilage damage, osteoclast/osteophyte formation, and bone destruction. In addition, CF101 markedly reduced pannus formation and lymphocyte infiltration. Mechanistically, CF101 induced deregulation of the NF-kappaB signaling pathway, resulting in down-regulation of tumor necrosis factor alpha. Consequently, CF101 induced apoptosis of inflammatory cells that had infiltrated the knee joints; however, it prevented apoptosis of chondrocytes. CONCLUSION CF101 deregulated the NF-kappaB signaling pathway involved in the pathogenesis of OA. CF101 induced apoptosis of inflammatory cells and acted as a cartilage protective agent, which suggests that it would be a suitable candidate drug for the treatment of OA.

Adenosine Acts as a Chemoprotective Agent by Stimulating G-CSF Production: A Role for A1 and A3 Adenosine Receptors

Adenosine, a ubiquitous nucleoside, is released into the extracellular environment from metabolically active or stressed cells. It binds to cells through specific A1, A2A, A2B, and A3 G‐protein–associated cell‐surface receptors, thus acting as a signal‐transduction molecule by regulating the levels of adenylyl cyclase and phospholipase C. In this study, we showed that adenosine stimulates the proliferation of murine bone marrow cells in vitro. Pharmacological studies, using antagonists to the adenosine receptors, revealed that this activity was mediated through the binding of adenosine to its A1 and A3 receptors. This result was further corroborated by showing that the two selective A1 and A3 receptor agonists, N‐cyclopentyladenosine (CPA) and 1‐deoxy‐1‐[6‐[[(3‐iodophenyl)methyl]amino]‐9H‐purin‐9‐yl]‐N‐methyl‐β‐D‐ribofuranuronamide (IB‐MECA) respectively, induced bone marrow cell proliferation in a manner similar to adenosine. Adenosines interaction with its A1 and A3 receptors induced G‐CSF production, which led to its stimulatory effect on bone marrow cells. These results were confirmed in vivo when we demonstrated that low‐dose adenosine (0.25 mg/kg) acted as a chemoprotective agent. When administered after chemotherapy, it restored the number of leukocytes and neutrophils to normal levels, compared with the decline in these parameters after chemotherapy alone. It is suggested that low‐dose adenosine, already in clinical use, may also be applied as a chemoprotective agent. J. Cell. Physiol. 183:393–398, 2000.

The A3 adenosine receptor agonist CF502 inhibits the PI3K, PKB/Akt and NF-κB signaling pathway in synoviocytes from rheumatoid arthritis patients and in adjuvant-induced arthritis rats

The A(3) adenosine receptor (A(3)AR) is over-expressed in inflammatory cells and was defined as a target to combat inflammation. Synthetic agonists to this receptor, such as IB-MECA and Cl-IB-MECA, exert an anti-inflammatory effect in experimental animal models of adjuvant- and collagen-induced arthritis. In this study we present a novel A(3)AR agonist, CF502, with high affinity and selectivity at the human A(3)AR. CF502 induced a dose dependent inhibitory effect on the proliferation of fibroblast-like synoviocytes (FLS) via de-regulation of the nuclear factor-kappa B (NF-kappaB) signaling pathway. Furthermore, CF502 markedly suppressed the clinical and pathological manifestations of adjuvant-induced arthritis (AIA) in a rat experimental model when given orally at a low dose (100 microg/kg). As is typical of other G-protein coupled receptors, the A(3)AR expression level was down-regulated shortly after treatment with agonist CF502 in paw and in peripheral blood mononuclear cells (PBMCs) derived from treated AIA animals. Subsequently, a decrease in the expression levels of protein kinase B/Akt (PKB/Akt), IkappaB kinase (IKK), I kappa B (IkappaB), NF-kappaB and tumor necrosis factor-alpha (TNF-alpha) took place. In addition, the expression levels of glycogen synthase kinase-3 beta (GSK-3beta), beta-catenin, and poly(ADP-ribose)polymerase (PARP), known to control the level and activity of NF-kappaB, were down-regulated upon treatment with CF502. Taken together, CF502 inhibits FLS growth and the inflammatory manifestations of arthritis, supporting the development of A(3)AR agonists for the treatment of rheumatoid arthritis.

Methotrexate enhances the anti-inflammatory effect of CF101 via up-regulation of the A 3 adenosine receptor expression

Methotrexate (MTX) exerts an anti-inflammatory effect via its metabolite adenosine, which activates adenosine receptors. The A3 adenosine receptor (A3AR) was found to be highly expressed in inflammatory tissues and peripheral blood mononuclear cells (PBMCs) of rats with adjuvant-induced arthritis (AIA). CF101 (IB-MECA), an A3AR agonist, was previously found to inhibit the clinical and pathological manifestations of AIA. The aim of the present study was to examine the effect of MTX on A3AR expression level and the efficacy of combined treatment with CF101 and MTX in AIA rats. AIA rats were treated with MTX, CF101, or both agents combined. A3AR mRNA, protein expression and exhibition were tested in paw and PBMC extracts from AIA rats utilizing immunohistochemistry staining, RT-PCR and Western blot analysis. A3AR level was tested in PBMC extracts from patients chronically treated with MTX and healthy individuals. The effect of CF101, MTX and combined treatment on A3AR expression level was also tested in PHA-stimulated PBMCs from healthy individuals and from MTX-treated patients with rheumatoid arthritis (RA). Combined treatment with CF101 and MTX resulted in an additive anti-inflammatory effect in AIA rats. MTX induced A2AAR and A3AR over-expression in paw cells from treated animals. Moreover, increased A3AR expression level was detected in PBMCs from MTX-treated RA patients compared with cells from healthy individuals. MTX also increased the protein expression level of PHA-stimulated PBMCs from healthy individuals. The increase in A3AR level was counteracted in vitro by adenosine deaminase and mimicked in vivo by dipyridamole, demonstrating that receptor over-expression was mediated by adenosine. In conclusion, the data presented here indicate that MTX induces increased A3AR expression and exhibition, thereby potentiating the inhibitory effect of CF101 and supporting combined use of these drugs to treat RA.

CF102 an A3 Adenosine Receptor Agonist Mediates Anti-Tumor and Anti-Inflammatory Effects in the Liver

The Gi protein‐associated A3 adenosine receptor (A3AR) is a member of the adenosine receptor family. Selective agonists at the A3AR, such as CF101 and CF102 were found to induce anti‐inflammatory and anti‐cancer effects. In this study, we examined the differential effect of CF102 in pathological conditions of the liver. The anti‐inflammatory protective effect of CF101 was tested in a model of liver inflammation induced by Concanavalin A (Con. A) and the anti‐cancer effect of CF102 was examined in vitro and in a xenograft animal model utilizing Hep‐3B hepatocellular carcinoma (HCC) cells. The mechanism of action was explored by following the expression levels of key signaling proteins in the inflamed and tumor liver tissues, utilizing Western blot (WB) analysis. In the liver inflammation model, CF102 (100 µg/kg) markedly reduced the secretion of serum glutamic oxaloacetic transaminase and serum glutamic pyruvic transaminase in comparison to the vehicle‐treated group. Mechanistically, CF102 treatment decreased the expression level of phosphorylated glycogen synthase kinase‐3β, NF‐κB, and TNF‐α and prevented apoptosis in the liver. This was demonstrated by decreased expression levels of Fas receptor (FasR) and of the pro‐apoptotic proteins Bax and Bad in liver tissues. In addition, CF102‐induced apoptosis of Hep‐3B cells both in vitro and in vivo via de‐regulation of the PI3K‐NF‐κB signaling pathway, resulting in up‐regulation of pro‐apoptotic proteins. Taken together, CF102 acts as a protective agent in liver inflammation and inhibits HCC tumor growth. These results suggest that CF102 through its differential effect is a potential drug candidate to treat various pathological liver conditions. J. Cell. Physiol. 226: 2438–2447, 2011.