Adenosine induces ATP release via an inositol 1,4,5-trisphosphate signaling pathway in MDCK cells

ATP is released into extracellular space as an autocrine/paracrine molecule by mechanical stress and pharmacological-receptor activation. Released ATP is partly metabolized by ectoenzymes to adenosine. In the present study, we found that adenosine causes ATP release in Madin-Darby canine kidney cells. This release was completely inhibited by CPT (an A1 receptor antagonist), U-73122 (a phospholipase C inhibitor), 2-APB (an inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) receptor blocker), thapsigargin (a Ca2+-ATPase inhibitor), and BAPTA/AM (an intracellular Ca2+ chelator), but not by DMPX (an A2 receptor antagonist). However, forskolin, epinephrine, and isoproterenol, inducers of cAMP accumulation, failed to release ATP. Adenosine increased intracellular Ca2+ concentrations that were strongly blocked by CPT, U-73122, 2-APB, and thapsigargin. Moreover, adenosine enhanced accumulations of Ins(1,4,5)P3 that were significantly reduced by U-73122 and CPT. These data suggest that adenosine induces the release of ATP by activating an Ins(1,4,5)P3 sensitive-Ca2+ pathway through the stimulation of A1 receptors.     

Adenosine arrests breast cancer cell motility by A3 receptor stimulation

In neutrophils, adenosine triphosphate (ATP) release and autocrine purinergic signaling regulate coordinated cell motility during chemotaxis. Here, we studied whether similar mechanisms regulate the motility of breast cancer cells. While neutrophils and benign human mammary epithelial cells (HMEC) form a single leading edge, MDA-MB-231 breast cancer cells possess multiple leading edges enriched with A3 adenosine receptors. Compared to HMEC, MDA-MB-231 cells overexpress the ectonucleotidases ENPP1 and CD73, which convert extracellular ATP released by the cells to adenosine that stimulates A3 receptors and promotes cell migration with frequent directional changes. However, exogenous adenosine added to breast cancer cells or the A3 receptor agonist IB-MECA dose-dependently arrested cell motility by simultaneous stimulation of multiple leading edges, doubling cell surface areas and significantly reducing migration velocity by up to 75 %. We conclude that MDA-MB-231 cells, HMEC, and neutrophils differ in the purinergic signaling mechanisms that regulate their motility patterns and that the subcellular distribution of A3 adenosine receptors in MDA-MB-231 breast cancer cells contributes to dysfunctional cell motility. These findings imply that purinergic signaling mechanisms may be potential therapeutic targets to interfere with the motility of breast cancer cells in order to reduce the spread of cancer cells and the risk of metastasis.     

Autocrine regulation of volume-sensitive anion channels in airway epithelial cells by adenosine

The activity of volume-sensitive Cl- channels was studied in human tracheal epithelial cells (9HTEo-) by taurine efflux experiments. The efflux elicited by a hypotonic shock was partially inhibited by adenosine receptor antagonists, by alpha,beta-methyleneadenosine 5'-diphosphate (alphabetaMeADP), an inhibitor of the 5'-ectonucleotidase, and by adenosine deaminase. On the other hand, dipyridamole, a nucleoside transporter inhibitor, increased the swelling-induced taurine efflux. Extracellular ATP and adenosine increased taurine efflux by potentiating the effect of hypotonic shock. alphabetaMeADP strongly inhibited the effect of extracellular ATP but not that of adenosine. These results suggest that anion channel activation involves the release of intracellular ATP, which is then degraded to adenosine by specific ectoenzymes. Adenosine then binds to purinergic receptors, causing the activation of the channels. To directly demonstrate ATP efflux, cells were loaded with [3H]AMP, and the release of radiolabeled molecules was analyzed by high performance liquid chromatography. During hypotonic shock, cell supernatants showed the presence of ATP, ADP, and adenosine. alphabetaMeADP inhibited adenosine formation and caused the appearance of AMP. Under hypotonic conditions, elevation of intracellular Ca2+ by ionomycin caused an increase of ATP and adenosine in the extracellular solution. Our results demonstrate that volume-sensitive anion channels are regulated with an autocrine mechanism involving swelling-induced ATP release and then hydrolysis to adenosine.     

Feed forward cycle of hypotonic stress-induced ATP release, purinergic receptor activation, and growth stimulation of prostate cancer cells

ATP is released in many cell types upon mechanical strain, the physiological function of extracellular ATP is largely unknown, however. Here we report that ATP released upon hypotonic stress stimulated prostate cancer cell proliferation, activated purinergic receptors, increased intracellular [Ca(2+)](i), and initiated downstream signaling cascades that involved MAPKs ERK1/2 and p38 as well as phosphatidylinositol 3-kinase (PI3K). MAPK activation, the calcium response as well as induction of cell proliferation upon hypotonic stress were inhibited by preincubation with the ATP scavenger apyrase, indicating that hypotonic stress-induced signaling pathways are elicited by released ATP. Hypotonic stress increased prostaglandin E(2) (PGE(2)) synthesis. Consequently, ATP release was inhibited by antagonists of PI3K (LY294002 and wortmannin), phospholipase A(2) (methyl arachidonyl fluorophosphonate (MAFP)), cyclooxygenase-2 (COX-2) (indomethacin, etodolac, NS398) and 5,8,11,14-eicosatetraynoic acid (ETYA), which are involved in arachidonic acid metabolism. Furthermore, ATP release was abolished in the presence of the adenylate cyclase (AC) inhibitor MDL-12,330A, indicating regulation of ATP-release by cAMP. The hypotonic stress-induced ATP release was significantly blunted when the ATP-mediated signal transduction cascade was inhibited on different levels, i.e. purinergic receptors were blocked by suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), the Ca(2+) response was inhibited upon chelation of intracellular Ca(2+) by 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), and ERK1,2 as well as p38 were inhibited by UO126 and SB203580, respectively. In summary our data demonstrate that hypotonic stress initiates a feed forward cycle of ATP release and purinergic receptor signaling resulting in proliferation of prostate cancer cells.     

Colocalization of ATP release sites and ecto-ATPase activity at the extracellular surface of human astrocytes

Extracellular ATP and other nucleotides function as autocrine and paracrine signaling factors in many tissues. Recent studies suggest that P2 nucleotide receptors and ecto-nucleotidases compete for a limited pool of endogenously released nucleotides within cell surface microenvironments that are functionally segregated from the bulk extracellular compartment. To test this hypothesis, we have used luciferase-based methods to continuously record extracellular ATP levels in monolayers of human 1321N1 astrocytoma cells under resting conditions, during stimulation of Ca2+-mobilizing receptors for thrombin or acetylcholine, and during mechanical stimulation by hypotonic stress. Soluble luciferase was utilized as an indicator of ATP levels within the bulk extracellular compartment, whereas a chimeric protein A-luciferase, adsorbed to antibodies against a glycosylphosphatidylinositol-anchored plasma membrane protein, was used as a spatially localized probe of ATP levels at the immediate extracellular surface. Significant accumulation of ATP in the bulk extracellular compartment, under either resting (1-2 nm ATP) or stimulated (10-80 nm ATP) conditions, was observed only when endogenous ecto-ATPase activity was pharmacologically inhibited by the poorly metabolizable analog, betagamma-methylene ATP. In contrast, accumulation of submicromolar ATP in the cell surface microenvironment was readily measured even in the absence of ecto-ATPase inhibition suggesting that the spatially colocalized luciferase could effectively compete with endogenous ecto-ATPases for released ATP. Other experiments revealed a critical role for elevated cytosolic [Ca2+] in the ATP release mechanism triggered by thrombin or muscarinic receptors but not in basal ATP release or release stimulated by hypotonic stress. These observations suggest that ATP release sites are colocalized with ecto-ATPases at the astrocyte cell surface. This colocalization may act to spatially restrict the actions of released ATP as a paracrine or autocrine mediator of cell-to-cell signaling.     

The P2X<Subscript>7</Subscript> receptor in retinal ganglion cells: A neuronal model of pressure-induced damage and protection by a shifting purinergic balance

Retinal ganglion cells process the visual signal and transmit it along their axons in the optic nerve to the brain. Molecular, immunohistochemical, and functional analyses indicate that the majority of retinal ganglion cells express the ionotropic P2X(7) receptor. Stimulation of the receptor can lead to a rise in intracellular calcium and cell death, although death does not involve the opening of a large diameter pore. Adenosine acting at A(3) receptors can attenuate the rise in calcium and death accompanying P2X(7) receptor activation, suggesting that dephosphorylation of ATP into adenosine is neuroprotective and that the balance of extracellular purines can influence neuronal survival. Increased intraocular pressure can lead to release of excessive extracellular ATP in the retina and damage ganglion cells by acting on P2X(7) receptors, implicating a role for the receptor in the loss of ganglion cell activity in glaucoma. In summary, the activation of P2X(7) receptors has both physiologic and pathophysiologic implications for ganglion cell function. These characteristics may also provide an insight into the contributions the P2X(7) receptor makes to neurons elsewhere.     

Adenosine uptake is the major effector of extracellular ATP toxicity in human cervical cancer cells

In cervical cancer, HPV infection and disruption of mechanisms involving cell growth, differentiation, and apoptosis are strictly linked with tumor progression and invasion. Tumor microenvironment is ATP and adenosine rich, suggesting a role for purinergic signaling in cancer cell growth and death. Here we investigate the effect of extracellular ATP on human cervical cancer cells. We find that extracellular ATP itself has a small cytotoxic effect, whereas adenosine formed from ATP degradation by ectonucleotidases is the main factor responsible for apoptosis induction. The level of P2×7 receptor seemed to define the main cytotoxic mechanism triggered by ATP, since ATP itself eliminated a small subpopulation of cells that express high P2×7 levels, probably through its activation. Corroborating these data, blockage or knockdown of P2×7 only slightly reduced ATP cytotoxicity. On the other hand, cell viability was almost totally recovered with dipyridamole, an adenosine transporter inhibitor. Moreover, ATP-induced apoptosis and signaling—p53 increase, AMPK activation, and PARP cleavage—as well as autophagy induction were also inhibited by dipyridamole. In addition, inhibition of adenosine conversion into AMP also blocked cell death, indicating that metabolization of intracellular adenosine originating from extracellular ATP is responsible for the main effects of the latter in human cervical cancer cells.     

Molecular mechanism of nucleotide-induced primary granule release in human neutrophils: role for the P2Y2 receptor

Nucleotides are released during vascular injury from activated platelets and broken cells, which could stimulate human neutrophils. In this study, we characterized the P2Y receptors and investigated the functional effects of extracellular nucleotides on human neutrophils. Pharmacological characterization using selective agonists and pertussis toxin revealed that human neutrophils express only functional P2Y₂ receptors. However, P2Y₂ receptor agonists ATP or uridine triphosphate (UTP) caused intracellular Ca²⁺ increases in isolated human neutrophils with an EC₅₀ of 1 µM but failed to cause release of primary granules from human neutrophils. ATP and UTP were equally potent in causing elastase release from human neutrophils in the presence of exogenous soluble fibrinogen, whereas ADP and UDP were without effect. We investigated whether nucleotides depend on generated arachidonic acid metabolites to cause degranulation. However, phenidone and MK-886, inhibitors of the 5-lipoxygenase pathway, failed to block nucleotide-induced intracellular calcium mobilization and elastase release. ATP and UTP caused activation of p38 MAPK and ERK1/2 in human neutrophils. In addition, the inhibitors of the MAPK pathway, SB-203580 and U-0126, inhibited nucleotide-induced elastase release. We conclude that fibrinogen is required for nucleotide-induced primary granule release from human neutrophils through the P2Y₂ receptor without a role for arachidonic acid metabolites. Both ERK1/2 and p38 MAPK play an important role in nucleotide-induced primary granule release from human neutrophils. elastase release; fibrinogen; extracellular nucleotides; uridine triphosphate; adenosine triphosphate     

Extracellular ATP protects pancreatic duct epithelial cells from alcohol-induced damage through P2Y<Subscript>1</Subscript> receptor-cAMP signal pathway

Extracellular adenosine-5′-triphosphate (ATP) regulates cell death and survival of neighboring cells. The detailed effects are diverse depending on cell types and extracellular ATP concentration. We addressed the effect of ATP on ethanol-induced cytotoxicity in epithelial cells, the cell type that experiences the highest concentrations of alcohol. Using pancreatic duct epithelial cells (PDEC), we found that a micromolar range of ATP reverses all intracellular toxicity mechanisms triggered by exceptionally high doses of ethanol and, thus, improves cell viability dramatically. Out of the many purinergic receptors expressed in PDEC, the P2Y₁ receptor was identified to mediate the protective effect, based on pharmacological and siRNA assays. Activation of P2Y₁ receptors increased intracellular cyclic adenosine monophosphate (cAMP). The protective effect of ATP was mimicked by forskolin and 8-Br-cAMP but inhibited by a protein kinase A (PKA) inhibitor, H-89. Finally, ATP reverted leakiness of PDEC monolayers induced by ethanol and helped to maintain epithelial integrity. We suggest that purinergic receptors reduce extreme alcohol-induced cell damage via the cAMP signal pathway in PDEC and some other types of cells.     

Intracellular ATP Concentration Contributes to the Cytotoxic and Cytoprotective Effects of Adenosine

Extracellular adenosine (Ade) interacts with cells by two pathways: by activating cell surface receptors at nanomolar/micromolar concentrations; and by interfering with the homeostasis of the intracellular nucleotide pool at millimolar concentrations. Ade shows both cytotoxic and cytoprotective effects; however, the underlying mechanisms remain unclear. In the present study, the effects of adenosine-mediated ATP on cell viability were investigated. Adenosine treatment was found to be cytoprotective in the low intracellular ATP state, but cytotoxic under the normal ATP state. Adenosine-mediated cytotoxicity and cytoprotection rely on adenosine-derived ATP formation, but not via the adenosine receptor pathway. Ade enhanced proteasome inhibition-induced cell death mediated by ATP generation. These data provide a new pathway by which adenosine exerts dual biological effects on cell viability, suggesting an important role for adenosine as an ATP precursor besides the adenosine receptor pathway.     

Ecto-Nucleoside Triphosphate Diphosphohydrolase 2 Modulates Local ATP-Induced Calcium Signaling in Human HaCaT Keratinocytes

Keratinocytes are the major building blocks of the human epidermis. In many physiological and pathophysiological conditions, keratinocytes release adenosine triphosphate (ATP) as an autocrine/paracrine mediator that regulates cell proliferation, differentiation, and migration. ATP receptors have been identified in various epidermal cell types; therefore, extracellular ATP homeostasis likely determines its long-term, trophic effects on skin health. We investigated the possibility that human keratinocytes express surface-located enzymes that modulate ATP concentration, as well as the corresponding receptor activation, in the pericellular microenvironment. We observed that the human keratinocyte cell line HaCaT released ATP and hydrolyzed extracellular ATP. Interestingly, ATP hydrolysis resulted in adenosine diphosphate (ADP) accumulation in the extracellular space. Pharmacological inhibition by ARL 67156 or gene silencing of the endogenous ecto-nucleoside triphosphate diphosphohydrolase (NTPDase) isoform 2 resulted in a 25% reduction in both ATP hydrolysis and ADP formation. Using intracellular calcium as a reporter, we found that although NTPDase2 hydrolyzed ATP and generated sustainable ADP levels, only ATP contributed to increased intracellular calcium via P2Y2 receptor activation. Furthermore, knocking down NTPDase2 potentiated the nanomolar ATP-induced intracellular calcium increase, suggesting that NTPDase2 globally attenuates nucleotide concentration in the pericellular microenvironment as well as locally shields receptors in the vicinity from being activated by extracellular ATP. Our findings reveal an important role of human keratinocyte NTPDase2 in modulating nucleotide signaling in the extracellular milieu of human epidermis.     

Kinetics of ATP release and cell volume regulation of hyposmotically challenged goldfish hepatocytes

In most animal cells, hypotonic swelling is followed by a regulatory volume decrease (RVD) thought to prevent cell death. In contrast, goldfish hepatocytes challenged with hypotonic medium (180 mosM, HYPO) increase their volume 1.7 times but remain swollen and viable for at least 5 h. Incubation with ATPgammaS (an ATP analog) in HYPO triggers a 42% volume decrease. This effect is concentration dependent (K(1/2) = 760 nM) and partially abolished by P2 receptor antagonists (64% inhibition). A similar induction of RVD is observed with ATP, UTP, and UDP, whereas adenosine inhibits RVD. Goldfish hepatocytes release more than 500 nM ATP during the first minutes of HYPO with no induction of RVD. The fact that similar concentrations of ATPgammaS did trigger RVD could be explained by showing that ATPgammaS induced ATP release. Finally, we observed that in a very small extracellular volume, hepatocytes do show a 56% RVD. This response was diminished by P2 receptor antagonists (73%) and increased (73%) when the extracellular ATP hydrolysis was inhibited 72%. Using a mathematical model, we predict that during the first 2 min of HYPO exposure the extracellular [ATP] is mainly governed by ATP diffusion and by both nonlytic and lytic ATP release, with almost no contribution from ecto-ATPase activity. We show that goldfish hepatocytes under standard HYPO (large volume) do not display RVD unless this is triggered by the addition of micromolar concentrations of nucleotides. However, under very low assay volumes, sufficient endogenous extracellular [ATP] can build up to induce RVD.     

Intracellular P2X receptors as novel calcium release channels and modulators of osmoregulation in Dictyostelium

P2X receptors are calcium permeable ligand-gated ion channels activated by ATP. Their role as cell surface receptors for extracellular ATP released physiologically by mammalian cells is well established. However, the cellular function of P2X receptor subtypes that populate the membranes of intracellular compartments is not defined. An initial report described how intracellular P2X receptors control the function of the contractile vacuole, an osmoregulatory organelle in Dictyostelium and other protists, and that genetic disruption of P2X receptors severely impaired cell volume control during hypotonic stress. However, later studies refuted a functional role of intracellular P2X receptors in Dictyostelium. Here we provide evidence that the discrepancies reported between the studies are due to the laboratory strain of Dictyostelium employed, which display different phenotypes in response to hypotonic stress and a varied dependency upon P2X receptors for osmoregulation. We use the recent discovery that intracellular P2X receptors are novel calcium release channels to provide some mechanistic insight in an effort to explain why the strain variance may exist.     

Bi-functional effects of ATP/P2 receptor activation on tumor necrosis factor-alpha release in lipopolysaccharide-stimulated astrocytes

Neuroinflammation is associated with a variety of CNS pathologies. Levels of tumor necrosis factor-alpha (TNF-alpha), a major proinflammatory cytokine, as well as extracellular ATP, are increased following various CNS insults. Here we report on the relationship between ATP/P2 purinergic receptor activation and lipopolysaccharide (LPS)-induced TNF-alpha release from primary cultures of rat cortical astrocytes. Using ELISA, we confirmed that treatment with LPS stimulated the release of TNF-alpha in a concentration and time dependent manner. ATP treatment alone had no effect on TNF-alpha release. LPS-induced TNF-alpha release was attenuated by 1 mm ATP, a concentration known to activate P2X7 receptors. Consistent with this, 3'-O-(4-Benzoyl)benzoyl-ATP (BzATP), a P2X7 receptor agonist, also attenuated LPS-induced TNF-alpha release. This reduction in TNF-alpha release was not due to loss of cell viability. Adenosine and 2-chloroadenosine were ineffective, suggesting that attenuation of LPS-induced TNF-alpha release by ATP was not due to ATP breakdown and subsequent activation of adenosine/P1 receptors. Interestingly, treatment of astrocyte cultures with 10 microm or 100 microm ATP potentiated TNF-alpha release induced by a submaximal concentration of LPS. UTP and 2methylthioADP (2-MeSADP), P2Y receptor agonists, also enhanced this LPS-induced TNF-alpha release. Our observations demonstrate opposing effects of ATP/P2 receptor activation on TNF-alpha release, i.e. P2X receptor activation attenuates, whereas P2Y receptor activation potentiates TNF-alpha release in LPS-stimulated astrocytes. These observations suggest a mechanism whereby astrocytes can sense the severity of damage in the CNS via ATP release from damaged cells and can modulate the TNF-alpha mediated inflammatory response depending on the extracellular ATP concentration and corresponding type of astrocyte ATP/P2 receptor activated.     

Dual regulation of arachidonic acid release by P2U purinergic receptors in dibutyryl cyclic AMP-differentiated HL60 cells.

ATP promoted biphasic effects on both basal and fMLP-stimulated arachidonic acid (AA) release in neutrophil-like HL60 cells: stimulation in the micromolar range (EC50 = 3.2 +/- 0.9 microM) and inhibition at higher concentrations (EC50 = 90 +/- 11 microM). ATP also inhibited UTP- and platelet activating factor-stimulated AA release. Only stimulatory effects of ATP on basal or fMLP-stimulated phospholipase C were observed. The inhibitory effect of ATP on AA release was not due to reacylation of released AA, chelation of extracellular Ca2+, cell permeabilization, or changes in the rise of [Ca2+]i induced by agonist. The inhibition was rapid, being detected within 5-15 s. The inhibitory effect of ATP on fMLP-stimulated AA release could be desensitized by pretreatment of the cells with 2 mM ATP, but not 20 microM ATP, the concentration that resulted in maximal release of AA and inositol phosphates. The inhibition by ATP was neither dependent on generation of adenosine by ATP hydrolysis nor the result of direct interaction of ATP with P1 purinergic receptors. Among other nucleotides tested (CTP, GTP, ITP, TTP, XTP, adenosine 5'-(beta,gamma-methylene)triphosphate (AMP-PCP), adenyl-5'-yl imidodiphosphate (AMP-P(NH)P), ADP, adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S), and UTP), only UTP and ATP gamma S displayed biphasic effects with potencies and efficacies almost identical to those of ATP. The other nucleotides only exhibited stimulatory effects (EC50 = 60-300 microM). The results are consistent with a model of dual regulation of AA release by two distinct subtypes of P2U receptors in HL60 cells.     

Mechanisms of ATP release by human trabecular meshwork cells, the enabling step in purinergic regulation of aqueous humor outflow

Our guiding hypothesis is that ecto-enzymatic conversion of extracellular ATP to adenosine activates A(1) adenosine receptors, reducing resistance to aqueous humor outflow and intraocular pressure. The initial step in this purinergic regulation is ATP release from outflow-pathway cells by mechanisms unknown. We measured similar ATP release from human explant-derived primary trabecular meshwork (TM) cells (HTM) and a human TM cell line (TM5). Responses to 21 inhibitors indicated that pannexin-1 (PX1) and connexin (Cx) hemichannels and P2X(7) receptors (P2RX(7) ) were comparably important in modulating ATP release induced by hypotonic swelling, whereas vesicular release was insignificant. Consistent with prior studies of PX1 activity in certain other cells, ATP release was lowered by the reducing agent dithiothreitol. Overexpressing PX1 in HEK293T cells promoted, while partial knockdown (KD) in both HEK293T and TM5 cells inhibited hypotonicity-activated ATP release. Additionally, KD reduced the pharmacologically defined contribution of PX1 and enhanced those of Cx and P2RX(7) . ATP release was also triggered by raising intracellular Ca(2+) activity with ionomycin after a prolonged lag time and was unaffected by the PX1 blocker probenecid, but nearly abolished by P2RX(7) antagonists. We conclude that swelling-stimulated ATP release from human TM cells is physiologically mediated by PX1 and Cx hemichannels and P2X(7) receptors, but not by vesicular release. PX1 appears not to be stimulated by intracellular Ca(2+) in TM cells, but can be modulated by oxidation-reduction state. The P2RX(7) -dependent component of swelling-activated release may be mediated by PX1 hemichannels or reflect apoptotic magnification of ATP release, either through itself and/or hemichannels.     

P2X receptors: New players in cancer pain

Pain is unfortunately a quite common symptom for cancer patients. Normally pain starts as an episodic experience at early cancer phases to become chronic in later stages. In order to improve the quality of life of oncological patients, anti-cancer treatments are often accompanied by analgesic therapies. The P2X receptor are adenosine triphosphate (ATP) gated ion channels expressed by several cells including neurons, cancer and immune cells. Purinergic signaling through P2X receptors recently emerged as possible common pathway for cancer onset/growth and pain sensitivity. Indeed, tumor microenvironment is rich in extracellular ATP, which has a role in both tumor development and pain sensation. The study of the different mechanisms by which P2X receptors favor cancer progression and relative pain, represents an interesting challenge to design integrated therapeutic strategies for oncological patients. This review summarizes recent findings linking P2X receptors and ATP to cancer growth, progression and related pain. Special attention has been paid to the role of P2X2, P2X3, P2X4 and P2X7 in the genesis of cancer pain and to the function of P2X7 in tumor growth and metastasis. Therapeutic implications of the administration of different P2X receptor blockers to alleviate cancer-associated pain sensations contemporarily reducing tumor progression are also discussed.