Activation of the A3 adenosine receptor affects cell cycle progression and cell growth

Abstract

The A3 adenosine receptor has been implicated in modulation of cell growth. As a first step to the characterization of the underlying mechanisms, we exposed Chinese hamster ovary (CHO) cells transfected with the human A3 receptor (A3R-CHO) to selective A3 receptor ligands. At micromolar concentrations, the A3 agonists N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (IB-MECA) and its 2-chloro derivative Cl-IB-MECA reduced cell number, with no effects on either parental CHO cells (not expressing any adenosine receptor), or CHO cells transfected with the human A1 receptor. Cl-IB-MECA also reduced cell number in the human HEK293 cell line transfected with the human A3 receptor cDNA as opposed to the respective untransfected wild-type cells. In A3R-CHO, agonist-induced effects were antagonized by nanomolar concentrations of A3 antagonists, including the triazoloquinazoline derivative MRS 1220, the dihydropyridine derivative MRS 1191, and the triazolonaphthyridine derivative L-249,313. A3 agonist-induced effects were not due to modulation of cell adhesion, nor to necrosis or apoptosis. Growth curves revealed highly impeded growth, and flow-cytometric analysis showed markedly reduced bromodeoxyuridine incorporation into nuclei. The effect on cell cycle was completely antagonized by MRS1191. Hence, activation of the human A3 receptor in A3R-CHO results in markedly impaired cell cycle progression, suggesting an important role for this adenosine receptor subtype in cell cycle regulation and cell growth.

Antiproliferative effects of selective adenosine receptor agonists and antagonists on human lymphocytes: evidence for receptor-independent mechanisms

“…The next step of this study was therefore aimed at investigating the effects of several nonselective and subtype-selective AR agonists as well as antagonists on the proliferative capacity of these cells, as measured by [ 3 H]thymidine uptake during DNA synthesis. In agreement with the reports showing that adenosine could impair cell proliferation [22,24,26], the present observations demonstrated an increased proliferation of human lymphocytes upon removal of endogenously released adenosine. The addition of ADA, which degrades adenosine to inosine, resulted in a significant stimulation of unstimulated PBL.…”

“…Moreover, we studied the effects of standard AR agonists and antagonists on lymphocyte proliferation. The results obtained from real-time PCR analysis showed expression for all four AR subtypes in the investigated cells, consistent with previous reports [22][23][24]39]. The adenosine A 2A receptor mRNA expression level was higher in PBL than in Jurkat T cells and was further upregulated after stimulation of the cells with PHA, as previously reported [40,41].…”

“…In these cells, as observed in the present study, the abolishment of cell cycle checkpoints (e.g., through caffeine) results in an increased proliferation rate. The results of the present study indicate that the compounds probably do not only induce cell cycle arrest, as previously shown for certain AR ligands in some cancer cell lines [22,36], but inhibit proliferation by additional mechanisms. An indication for this assumption is that unstimulated PBL resting in the G 0 phase were significantly affected and [ 3 H]thymidine uptake was potently reduced after the administration of CPA, BAY60-6583, and IB-MECA as well as PSB-36, MSX-2, and PSB-10.…”

“…Furthermore, adenosine is able to stimulate cell growth and proliferation and/or to induce apoptosis in various normal and tumor cell lines. Proliferative or anti-proliferative effects appear to be dependent on the cell type, the distribution of adenosine receptor subtypes, and their expression on the cell surface [22][23][24][25]. A 3 adenosine receptors seem to play an important role [26][27][28], but recent evidence accumulates that A 2B ARs can be upregulated on malignant cells and are involved in the control of tumor growth and development [16,24,[29][30][31][32].…”

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

“…15 Partial A 3 AR agonists can act as cardioprotective agents. 27 Selective antagonists for the A 3 AR promise to be useful in the regulation of cell growth 28,29 and as anti-asthmatic, 30 cerebroprotective 20,31 and anti-inflammatory agents. 32 Since its discovery in 1991, 12 the development of agonists of the A 3 AR has been an active area of research.…”