Oncogene (2010) 29, 5346–5358 & 2010 Macmillan Publishers Limited All rights reserved 0950-9232/10 www.nature.com/onc REVIEW Extracellular adenosine triphosphate and adenosine in cancer J Stagg and MJ Smyth Cancer Immunology Program, Sir Donald and Lady Trescowthick Laboratories, Peter MacCallum Cancer Centre, East Melbourne, Victoria, Australia Adenosine triphosphate (ATP) is actively released in the mulated the hypothesis of purinergic neurotransmission extracellular environment in response to tissue damage (Burnstock, 1972). Burnstock’s hypothesis that ATP and cellular stress. Through the activation of P2X and could be released by cells to perform intercellular P2Y receptors, extracellular ATP enhances tissue repair, signaling was initially met with skepticism, as it seemed promotes the recruitment of immune phagocytes and unlikely that a molecule that acts as an intracellular dendritic cells, and acts as a co-activator of NLR family, source of energy would also function as an extracellular pyrin domain-containing 3 (NLRP3) inflammasomes. messenger. Nevertheless, Burnstock pursued his work The conversion of extracellular ATP to adenosine, in and, together with Che Su and John Bevan, reported contrast, essentially through the enzymatic activity of the that ATP was also released from sympathetic nerves ecto-nucleotidases CD39 and CD73, acts as a negative- during stimulation (Su et al., 1971). Three decades later, feedback mechanism to prevent excessive immune responses. following the cloning and characterization of ATP and Here we review the effects of extracellular ATP and adenosine adenosine cell surface receptors, purinergic signaling is a on tumorigenesis. First, we summarize the functions of well-established concept and constitutes an expanding extracellular ATP and adenosine in the context of tumor field of research in health and disease, including cancer immunity. Second, we present an overview of the immuno- (Burnstock, 2007). Although early studies focused on suppressive and pro-angiogenic effects of extracellular the role of purinergic receptors in neurotransmission, it adenosine. Third, we present experimental evidence that soon became obvious that extracellular ATP and extracellular ATP and adenosine receptors are expressed adenosine have important roles in another system by tumor cells and enhance tumor growth. Finally, we discuss requiring efficient cell-to-cell communication: the recent studies, including our own work, which suggest that immune system. Over the last decade, there has been therapeutic approaches that promote ATP-mediated activa- increasing interest not only in the effects of extracellular tion of inflammasomes, or inhibit the accumulation of tumor- purines and pyrimidines on inflammatory responses, derived extracellular adenosine, may constitute effective new but also on general biological pathways, such as cell means to induce anticancer activity. survival, proliferation, differentiation and motility. Oncogene (2010) 29, 5346–5358; doi:10.1038/onc.2010.292; It is becoming increasingly clear that the release of published online 26 July 2010 extracellular purines and pyrimidines represents a ubiquitous means of intercellular communication Keywords: immunosuppression; adenosine; used by different cell types, involved in various ecto-nucleotidases; inflammasome; ATP biological processes and conserved throughout evolu- tion, as evidenced by the discovery of ATP receptors in invertebrates (Fountain et al., 2007) and plants (Kim et al., 2006). Introduction Extracellular ATP in immunity For many scientists in the 1960s, cells could not possibly release a molecule as fundamental as adenosine tripho- ATP receptors sphate (ATP). Owing to its established role in the Krebs In 1978, Burnstock proposed two types of purinergic cycle, there was considerable skepticism to the notion receptors: P1 receptors selective for adenosine and that ATP—and purines in general—could exert extra- P2 receptors selective for ATP and ADP. Some P2 cellular function. In 1970, Burnstock et al. described the receptors additionally bind UTP or UDP (Burnstock, release of extracellular ATP as a transmitter substance by 2006). In 1985, a pharmacological approach was non-adrenergic inhibitory nerves, and later in 1972, for- proposed to distinguish between two types of P2 receptors: ionotropic P2X and metabotropic P2Y Correspondence: Dr J Stagg or Professor MJ Smyth, Cancer receptors. Currently, four subtypes of P1 receptors Immunology Program, Sir Donald and Lady Trescowthick Labora- (A1, A2A, A2B and A3), seven subtypes of P2X tories, Peter MacCallum Cancer Centre, Locked Bag 1, A’Beckett receptors and eight subtypes of P2Y receptors have Street, East Melbourne, Victoria 8006, Australia. E-mails: [email protected] or [email protected] been identified (Figure 1). P2Y receptors are subdivided Received 12 May 2010; revised 11 June 2010; accepted 13 June 2010; into five Gq/G11-coupled subtypes (P2Y1, P2Y2, P2Y4, published online 26 July 2010 P2Y6 and P2Y11) and three Gi/o-coupled subtypes Extracellular adenosine triphosphate and adenosine J Stagg and MJ Smyth 5347 Figure 1 Adenosine triphosphate (ATP) and adenosine receptor signaling. Extracellular ATP binds G-protein-coupled P2Y and trimeric ion channel P2X receptors. P2Y receptors are subdivided into Gq/G11-coupled subtypes that activate the phospholipase C and inositol triphosphate pathways, and Gi/o-coupled subtypes that inhibit adenylyl cyclase. P2X7 is an atypical ATP receptor, forming ion channels at low concentrations of ATP, activating NLRP3 inflammasomes by K þ efflux and the recruitment of the pannexin-1 hemichannel, and inducing cell death at high concentrations of ATP. Extracellular adenosine binds Gi/o-coupled A1 and A3 receptors and Gs-coupled A2A and A2B receptors. In contrast to A1 and A3 receptors, A2A and A2B receptors increase intracellular cyclic AMP levels. (P2Y12, P2Y13 and P2Y14). Gq/G11-coupled P2Y recep- 2006) and the processing and secretion of the cytokines tors generally activate the phospholipase C and inositol interleukin-1b (IL-1b) and IL-18, important activators triphosphate pathways, whereas Gi/o-coupled P2Y of innate and adaptive immune responses. The NLRP3 receptors generally inhibit adenylyl cyclase and mod- inflammasome is the best-characterized inflammasome ulate ion channels (Abbracchio et al., 2009). and belongs to the intracellular NOD-like receptor In contrast to G-protein-coupled P2Y receptors, P2X (NLR) family. Together with Toll-like receptors (TLRs) receptors are trimeric cationic channels permeable to and C-type lectins, NLRs scan the extracellular and Na þ ,Kþ and Ca2 þ upon activation. Six homomeric intracellular environment for pathogen-associated mo- (P2X1À5 and P2X7) and six heteromeric (P2X1/2, P2X1/4, lecular patterns and host-derived danger-associated P2X1/5, P2X2/3, P2X2/6 and P2X4/6) receptors have been molecular patterns to alert the immune system (Schro- described. The homomeric P2X7 receptor is an atypical der and Tschopp, 2010). The NLRP3 inflammasome is purinergic receptor with a longer carboxy-terminal activated in response to various danger signals, such as tail and a number of polymorphisms or spliced variants co-activation of P2X7 and TLRs, increased cytosolic (Gunosewoyo et al., 2007; Wu et al., 2009). The DNA levels, monosodium urate crystals, fibrillar activation of P2X7 receptors is highly regulated by amyloid-b peptide and high extracellular glucose levels extracellular levels of ATP. Whereas low ATP concen- (Schroder and Tschopp, 2010). The NLRP3 inflamma- trations activate P2X7 ion channels to become perme- some can also be activated independently of TLR able to small ions, high ATP concentrations result in signaling (Kanneganti et al., 2007). Schroder and pore formation permeable to molecules as large as Tschopp (2010) recently proposed that a unifying 900 kDa, which ultimately causes cell death (Khakh and element in NLRP3 activation might be the generation North, 2006). of reactive oxygen species. The activation of P2X7 receptors and the recruitment of the pannexin-1 membrane pore may also allow NLRP3 agonists to Extracellular ATP and the NLR family, pyrin enter the cells and directly activate the inflammasome domain-containing 3 inflammasome (Kanneganti et al., 2007). In the context of inflammation, the release of ATP by activated monocytes, dying, injured or stressed cells, degranulating platelets or secreted by bacteria them- Extracellular ATP and T helper type 17 cell immune selves, acts as a co-activator of the NLR family, pyrin responses domain-containing 3 (NLRP3) (cryopirin/NALP3) T-helper type 17 cell immune responses are required for inflammasome (Piccini et al., 2008; Netea et al., 2009). the control of infectious agents (Cho et al., 2010) and are The NLRP3 inflammasome is a multiprotein complex involved in the pathogenesis of various autoimmune that triggers caspase-1 activation (Mariathasan et al., diseases, such as multiple sclerosis (Axtell et al., 2010) Oncogene Extracellular adenosine triphosphate and adenosine J Stagg and MJ Smyth 5348 and inflammatory bowel disease (Cho and Weaver, responses. Extracellular ATP, by activation of P2Y 2007). The generation of Th17 cells is controlled by the receptors, has been identified as a potent chemotactic cytokines IL-6, tumor growth factor (TGF)-b and IL-23 stimulus for immature DCs (Idzko et al., 2002). In (Bettelli et al., 2007). ‘Naturally occurring’ IL-17- contrast, mature DCs exposed to ATP have decreased producing CD4 þ T cells are also present in the gut.