Tumor Cells
Adenosine binding to ARs on the surface of cancer cells has a profound impact on their biology. For example, the triggering of A1R (337, 338), A2AR (54, 339, 340), and A3R (339, 341–343) induces a variety of cellular responses that augment cancer cell survival such as AKT and ERK1/2 stimulation, as well as Bad inactivation (342). Additional responses to AR signaling contributing to bolstered cancer cell survival include upregulation of Bcl2 (343), downregulation of p53 (338) and Bax (343) as well as aversion of caspase-9 (343) and caspase-3 (54, 343, 344) activation. Paradoxically, extracellular adenosine has also been demonstrated to cause cancer cell death either by setting off A1R (345, 346), A2AR (341, 347), A2BR (348, 349), and A3R (339, 350–354) or via induction of AMPK activation upon its cellular uptake and subsequent conversion to AMP (345).
Moreover, A1R (337, 355), A2AR (341, 356), A2BR (344, 357–359), and A3R (343, 360) stimulation augments cancer cell proliferation through activation of PLC (356), protein kinase C-delta (PKC-δ) (356), AKT (356, 357), ERK1/2 (356–360), JNK (356, 358), and p38 (358). Furthermore, triggering of the ARs leads to upregulation of cyclins A (343), B (358), D (343, 358), E (337, 343, 358), estrogen receptor-α (355) as well as downregulation of the cell-cycle inhibitors p27 (337) and p21 (343, 358). Surprisingly, though, activation of A2BR (349) and A3R (341, 350, 353, 361–363) has also been reported to result in a potent cytostatic effect.
Motility (358, 359, 364–369) and invasiveness (358, 359, 367, 370) are additional features of cancer cells that are boosted upon engagement of A1R (364, 365), A2AR (366), A2BR (358, 359, 367, 368), and A3R (369, 370). In terms of mechanisms, signaling initiated by these receptors promotes filopodia formation (367) as well as expression of matrix metalloproteases (MMPs) (358, 359, 370) and FXYD5 (359), a cell membrane glycoprotein known to drive metastasis by reducing cell adhesion (371). In contrast, others claim that A3R triggering hinders the motility and invasiveness of cancer cells (372, 373). Finally, A2AR (374), A2BR (369, 375), and A3R (369, 375–377) stimulation on the surface of cancer cells promotes angiogenesis by boosting secretion of the pro-angiogenic factors VEGF (369, 375, 377), IL-8 (369, 375), angiopoietin 2 (376), and erythropoietin (374).
The contrasting consequences of triggering particular ARs, on the survival, proliferation or migration and invasiveness of tumor cells most probably occur due to the heterogeneity between cells and/or experimental settings employed to assess them. For instance, two different cancer cell lines of distinct tissue origin could have profoundly diverse AR expression profiles as well as different ability to transmit/terminate signaling initiated by these receptors. Moreover, they might have different capacity to produce adenosine, which once released into the medium can trigger ARs in an autocrine fashion. Finally, different concentrations used between experiments, as well as limited specificity of the AR agonists/antagonists, probably constitute additional factors contributing to the observed discrepancies.