Purines, Purinergic Receptors, and Cancer

Purines were long thought to be restricted to the intracellular compartment, where they are used for energy transactions, nucleic acid synthesis, and amultiplicity of biochemical reactions. However, it is now clear that both adenosine and adenosine triphosphate are (i) abundant biochemical components of the tumor microenvironment, (ii) potent modulators of immune cell responses and cytokine release, and (iii) key players in host–tumor interaction. Moreover, both ATP and adenosine directly affect tumor cell growth. Adenosine is a powerful immunosuppressant (mainly acting at A2A receptors) and a modulator of cell growth (mainly acting at A3 receptors). ATP is a proinflammatory (acting at P2Y1, P2Y2, P2Y4, P2Y6, and P2Y12, and at P2X4 and P2X7 receptors), an immunosuppressant (acting at P2Y11), and a growth-promoting agent (acting at P2Y1, P2Y2, and P2X7 receptors). This complex signaling network generates an array of inhibitory and stimulatory responses that affect immune cell function, tumor growth, and metastatic dissemination. Investigation of purinergic signaling has increased our understanding of the tumor microenvironment and opened new and exciting avenues for the development of novel therapeutics. Cancer Res; 72(21); 5441–7. 2012 AACR. Extracellular Purines: A Heterodox Concept ATP, the key intracellular energy currency, was isolated by Karl Lohmann in 1929 (1). At about the same time, Drury and Szent-Gy€ orgy reported the first effects of extracellular purines on cell responses (2). However, and with hindsight rather surprisingly, the observation of Drury and Szent-Gy€ orgy was neglected for several decades, until the role of extracellular purines as extracellular messengers was brought to light by the seminal contributions of Robert Berne, Bertil Fredholm, and above all Geoff Burnstock (3–5). Indeed, we owe our present understanding of purinergic signaling and the current classification of P2 receptors to Geoff Burnstock's stubborn activity and enthusiasm (6). Although other nucleotides, such as the pyrimidines UTP and UDP, have important extracellular signaling roles, most recent reports have highlighted the involvement of ATP in host–tumor interaction. As for nucleosides, there is no convincing demonstration that other nucleosides besides adenosine participate in cell-to-cell signaling; thus in this review, I concentrate on ATP and adenosine. Adenosine receptors, named P1, comprise 4 members, A1, A2A, A2B, and A3, which couple via different G proteins to phospholipase C and adenylyl cyclase. Other intracellular transduction systems, such as the mitogen-activated protein kinase pathway, can also be triggered (7). Nucleotide receptors, named P2, comprise 2 subfamilies, metabotropic P2Y and ionotropic P2X receptors (8). P2Y receptors can be further subdivided into 2 groups depending on the coupling to specific G proteins: P2Y1, P2Y2, P2Y4, and P2Y6 couple toGq to activate phospholipase Cb, whereas P2Y12, P2Y13, and P2Y14 couple to Gi to inhibit adenylyl cyclase. P2Y11 is peculiar in that it couples to both Gq and Gs and thus triggers an increase in intracellular Ca2þ and in cAMP levels. P2X receptors are cation-selective, ATP-gated plasmamembrane channels made by the assembly of 7 different subunits (P2X1–7) to form trimeric receptors (9). P2X1, 2, 3, 4, 6 subunits can assemble to form homotrimeric or heterotrimeric channels, whereas P2X5 is poorly active as a homomeric complex and seems to be functional mainly as a P2X1/P2X5, P2X2/P2X5, or P2X4/P2X5 heterotrimer (10). Intriguingly, the P2X2/P2X5 heterotrimer exhibits functional features, notably large pore formation, to date thought to be hallmarks of P2X7. The P2X7 subunit, on the other hand, assembles as a homotrimer and, under certain conditions, as a homoexamer (11, 12). For all P2X receptors, the main signal-transducing mechanism seems to be the alteration in the intracellular ion concentration, but in addition, P2X7 has been reported to interact directly with at least 11 different intracellular proteins, among which are heat shock proteins, b-actin, and phosphatidylinositol 4-kinase (13). P2Y and P2X receptors are widely distributed across animal species and ubiquitously present on both excitable and nonexcitablemammalian cells.Within the P2X subfamily, the P2X4 and P2X7 subtypes are expressed to high level by immune cells (14). Participation of P2X4 to immune responses is as yet not well characterized, whereas on the contrary, P2X7 has a role in interleukin-1b (IL-1b) release, Ag presentation, and lymphocyte proliferation and differentiation (15, 16). P2Y and P2X receptors significantly differ in their ligand selectivity, because whereas P2Y receptors recognize a wide range of purinergic agonists, ATP is the only physiologic ligand of P2X receptors so far identified. At P2Y1, P2Y12, and P2Y13 receptors, the Author's Affiliation: Department of Experimental and Diagnostic Medicine, Section of General Pathology, University of Ferrara, Ferrara, Italy CorrespondingAuthor:FrancescoDi Virgilio, Department of Experimental and Diagnostic Medicine, University of Ferrara, Via Borsari 46, 44121 Ferrara, Italy. Phone: 39-0532-455353; Fax: 39-0532-455351; E-mail: [email protected] doi: 10.1158/0008-5472.CAN-12-160