Growth inhibition of HL-60 cells by extracellular ATP: concentration-dependent involvement of a P2 receptor and adenosine generation

Extracellular Adenine Nucleotides and Adenosine Modulate the Growth and Survival of THP-1 Leukemia Cells

A new approach to improve the effectiveness of acute myeloid leukemia (AML) treatment is to use the properties of purinergic signaling molecules secreted into the bone marrow milieu in response to leukemic cell growth. Therefore, our study aimed to evaluate the effects of extracellular adenine nucleotides and adenosine on the growth and death parameters in the leukemic THP-1 cell line. Cells were exposed to ATP, ADP, AMP, adenosine and nonhydrolyzable analogues of ATP and ADP (ATPγS and ADPβS) in a 1–1000 μM broad concentration range. The basal mRNA expression of the P1 and P2 receptors was evaluated by real-time PCR. Changes in the processes of cell growth and death were assessed by flow cytometry analysis of proliferation, cell cycle and apoptosis. Chemotaxis toward stromal cell-derived factor-1 (SDF-1) was performed using the modified Boyden chamber assay, and chemokine receptor type 4 (CXCR4) surface expression was quantified by flow cytometry. We indicated several antileukemic actions. High micromolar concentrations (100–1000 μM) of extracellular adenine nucleotides and adenosine inhibit the growth of cells by arresting the cell cycle and/or inducing apoptosis. ATP is characterized by the highest potency and widest range of effects, and is responsible for the cell cycle arrest and the apoptosis induction. Compared to ATP, the effect of ADP is slightly weaker. Adenosine mostly has a cytotoxic effect, with the induction of apoptosis. The last studied nucleotide, AMP, demonstrated only a weak cytotoxic effect without affecting the cell cycle. In addition, cell migration towards SDF-1 was inhibited by low micromolar concentrations (10 μM). One of the reasons for this action of ATPγS and adenosine was a reduction in CXCR4 surface expression, but this only partially explains the mechanism of antimigratory action. In summary, extracellular adenine nucleotides and adenosine inhibit THP-1 cell growth, cause death of cells and modulate the functioning of the SDF-1/CXCR4 axis. Thus, they negatively affect the processes that are responsible for the progression of AML and the difficulties in AML treatment.

Purinergic signalling and cancer

Receptors for extracellular nucleotides are widely expressed by mammalian cells. They mediate a large array of responses ranging from growth stimulation to apoptosis, from chemotaxis to cell differentiation and from nociception to cytokine release, as well as neurotransmission. Pharma industry is involved in the development and clinical testing of drugs selectively targeting the different P1 nucleoside and P2 nucleotide receptor subtypes. As described in detail in the present review, P2 receptors are expressed by all tumours, in some cases to a very high level. Activation or inhibition of selected P2 receptor subtypes brings about cancer cell death or growth inhibition. The field has been largely neglected by current research in oncology, yet the evidence presented in this review, most of which is based on in vitro studies, although with a limited amount from in vivo experiments and human studies, warrants further efforts to explore the therapeutic potential of purinoceptor targeting in cancer.

The role of P2X7 receptor in ATP-mediated human leukemia cell death: calcium influx-independent

Activation of the P2X7 receptor leads to a rapid, bidirectional flux of cations, causing broad range of biological responses including cytotoxicity. However, the mechanism of P2X7-mediated cytotoxicity remains largely unexplored. In our previous study, the lack of P2X7-mediated calcium response under normal conditions was found in P2X7(+) hematopoietic cell lines. In this study, the P2X7-mediated cytotoxicity in different type of cells (P2X7(-), P2X7(+) with calcium response, and P2X7(+) without calcium response) was investigated. Our results showed that P2X7 agonists, adenosine 5'-triphosphate (ATP) or 2',3'-O-(4 benzoylbenzoyl)-ATP, dose-dependently reduced the cell viability in all P2X7(+) cells tested, including J6-1, LCL, and Namalva cells which are negative for P2X7-mediated calcium response, although these effects were lower than those observed in KG1a cells which has normal P2X7 functions. The cytotoxic effect could be blocked by P2X7 antagonists, oxidized ATP and 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine. In addition, externalization of phosphatidylserine could be detected in a time-dependent manner and apoptotic morphological changes could be observed after the activation of P2X7 receptor in J6-1 cells. Furthermore, P2X7-mediated pore formation could be detected in KG1a and J6-1 cells under low-ionic conditions, but not under low-divalent conditions. These effects could not be observed in P2X7(-) Ramos cells. These results suggested that P2X7 receptor-mediated cytotoxic effects may occur independent of calcium response.

Purinergic receptor-mediated effects of adenosine 5'-triphosphate in urological malignant diseases

Adenosine 5'-triphosphate (ATP) mediates a variety of biological functions and has been shown to play a physiological role in almost every system in the body. In the genito-urinary system, extracellular ATP has been shown to play a functional role in several different capacities, ranging from nociception in the ureter and bladder, to erectile dysfunction via its action on different 'purinergic receptors'. Discovery of the trophic effects of ATP has led to a surge in interest in this signalling system in various malignancies. To date five P2 receptor subtypes have been implicated in the growth inhibition of cancer cells, namely P2X5, P2X7, P2Y1, P2Y2 and P2Y11. Limited data are available on urological malignancies. ATP induces its anti-neoplastic effect primarily via purinergic receptor-mediated apoptosis via calcium-independent pathways, and this has been confirmed in vitro and in vivo. Studies have highlighted functional roles for the P2X5 and/or P2Y11 receptors in both hormone refractory prostate cancer and high-grade bladder cancer, although the contributory effect of pro-apoptotic P2X7 receptors remains unclear. Clinical trials have shown intravenous ATP successfully attenuates a range of systemic symptoms associated with advanced malignancies. This raises the possibility that selective targeting of specific aberrant pathways may allow for treatment of advanced primary malignancies and their systemic effects.

Atypical PKCzeta is involved in RhoA-dependent mitogenic signaling by the P2Y12 receptor in C6 cells

When nucleotide hydrolysis is prevented, agonists of the P2Y(12) receptor enhance the proliferation of C6 glioma cells by RhoA-dependent, protein kinase C (PKC)-dependent activation of the extracellular signal-regulated kinase (ERK) pathway [Claes P, Grobben B, Van Kolen K, Roymans D & Slegers H (2001) Br J Pharmacol134, 402-408; Grobben B, Claes P, Van Kolen K, Roymans D, Fransen P, Sys SU & Slegers H (2001) J Neurochem78, 1325-1338]. In this study, we show that ERK1/2 phosphorylation was not affected by transfection of the cells with the Gbetagamma-subunit-scavenging adrenergic receptor kinase peptide [betaARK1-(495-689)] or with Rap1GAPII, indicating that P2Y(12) receptor stimulation enhances ERK1/2 phosphorylation by G(i)alpha subunit-mediated signaling independently of Rap1 activation. Inhibition of the RhoA downstream effector Rho-associated coiled-coil-containing kinase (ROCK) with Y-27632 did not affect the P2Y(12) receptor-induced increase in ERK1/2 phosphorylation but abrogated the mitogenic response. Involvement of growth factor receptor transactivation in the signaling towards ERK phosphorylation could be ruled out by the lack of an effect of PP2, AG1024, AG1296 or SU1498, inhibitors of Src, insulin-like growth factor receptor, platelet-derived growth factor receptor and vascular endothelial growth factor receptor kinase activity, respectively. Experiments with bisindolylmaleimide I and IX indicated the requirement of PKC activity. Classical and novel PKC isoforms could be excluded by treatment of the cells with Gö6976 and calphostin C, whereas addition of a myristoylated PKCzeta pseudosubstrate inhibitor completely abolished P2Y(12) receptor-induced ERK1/2 activation. Moreover, coimmunoprecipitation experiments revealed PKCzeta/Raf1 and PKCzeta/ERK association, indicating the involvement of PKCzeta. From the data presented, we can conclude that the P2Y(12) receptor enhances cell proliferation by a G(i)alpha-dependent, RhoA-dependent PKCzeta/Raf1/MEK/ERK pathway that requires activation of ROCK, which is not involved in ERK1/2 signaling.

Mechanisms of adenosine-induced cytotoxicity and their clinical and physiological implications

Extracellular ATP (ATPo) and adenosine are cytotoxic to several cancer cell lines, suggesting their potential use for anticancer therapy. Adenosine causes cytotoxicity, either when added exogenously or when generated from ATPo hydrolysis, via mechanisms which are not mutually exclusive and which involve, adenosine receptor activation, pyrimidine starvation and/or increases in intracellular S-adenosylhomocysteine: S-adenosylmethionine ratio. Given that adenosine also appears to protect against cytotoxicity via mechanisms including immunity against damage by oxygen free radicals, an understanding of the contribution of adenosine to ATPo-induced cytotoxicity is thus crucial, when considering any potential therapeutic use for these compounds. However, such an understanding has been largely hindered by the fact that many studies have not focused enough on the possibility that both ATPo and adenosine may mediate cytotoxicity in the same system. Such studies can benefit from use a range of ATPo concentrations when assessing the contribution of adenosine to ATPo-induced cytotoxicity. Whilst future molecular and pharmacological studies are needed to establish the nature of the cytotoxic adenosine receptor, it is possible that more than just one adenosine receptor type is involved and that the cytotoxic receptor(s) type is more likely to have a low affinity for adenosine. Activation of the adenosine receptor(s) would thus lead to cytotoxicity only at relatively high adenosine concentrations, while lower adenosine concentrations mediate non-cytotoxic physiological effects.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Physiological and pathophysiological functions of the ecto-nucleotide pyrophosphatase/phosphodiesterase family

The ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) multigene family contains five members. NPP1-3 are type II transmembrane metalloenzymes characterized by a similar modular structure composed of a short intracellular domain, a single transmembrane domain and an extracellular domain containing a conserved catalytic site. The short intracellular domain of NPP1 has a basolateral membrane-targeting signal while NPP3 is targeted to the apical surface of polarized cells. NPP4-5 detected by database searches have a predicted type I membrane orientation but have not yet been functionally characterized. E-NPPs have been detected in almost all tissues often confined to specific substructures or cell types. In some cell types, NPP1 expression is constitutive or can be induced by TGF-beta and glucocorticoids, but the signal transduction pathways that control expression are poorly documented. NPP1-3 have a broad substrate specificity which may reflect their role in a host of physiological and biochemical processes including bone mineralization, calcification of ligaments and joint capsules, modulation of purinergic receptor signalling, nucleotide recycling, and cell motility. Abnormal NPP expression is involved in pathological mineralization, crystal depositions in joints, invasion and metastasis of cancer cells, and type 2 diabetes. In this review we summarize the present knowledge on the structure and the physiological and biochemical functions of E-NPP and their contribution to the pathogenesis of diseases.

Agonists of the P2Y(AC)-receptor activate MAP kinase by a ras-independent pathway in rat C6 glioma

We have previously shown that an ecto-NPPase modulates the ATP- and ADP-mediated P2Y(AC)-receptor activation in rat C6 glioma. In the present study, 2MeSADP and Ap(3)A induced no detectable PI turnover and were identified as specific agonists of the P2Y(AC)-receptor with EC(50) values of 250 +/- 37 pM and 1 +/- 0.5 microM, respectively. P2Y(AC)-receptor stimulation increased MAP kinase (ERK1/2) activation that returned to the basal level 4 h after stimulation and was correlated with a gradual desensitization of the P2Y(AC)-purinoceptor. The purinoceptor antagonists DIDS and RB2 blocked MAP kinase activation. An IP(3)-independent Ca(2+)-influx was observed after P2Y(AC)-receptor activation. Inhibition of this influx by Ca(2+)-chelation, did not affect MAP kinase activation. Pertussis toxin, toxin B, selective PKC-inhibitors and a specific MEK-inhibitor inhibited the 2MeSADP- and Ap(3)A-induced MAP kinase activation. In addition, transfection with dominant negative RhoA(Asn19) rendered C6 cells insensitive to P2Y(AC)-receptor-mediated MAP kinase activation whereas dominant negative ras was without effect. Immunoprecipitation experiments indicated a significant increase in the phosphorylation of raf-1 after P2Y(AC)-receptor activation. We may conclude that P2Y(AC)-purinoceptor agonists activate MAP kinase through a G(i)-RhoA-PKC-raf-MEK-dependent, but ras- and Ca(2+)-independent cascade.

P2Y(AC)(-)-receptor agonists enhance the proliferation of rat C6 glioma cells through activation of the p42/44 mitogen-activated protein kinase

1. Extracellularly added P(1),P(3)-di(adenosine-5') triphosphate (Ap(3)A), P(1),P(4)-di(adenosine-5') tetraphosphate (Ap(4)A), ATP, ADP, AMP and adenosine are growth inhibitory for rat C6 glioma cells. Analysis of nucleotide hydrolysis and the use of nucleotidase inhibitors demonstrated that the latter inhibition is due to hydrolysis of the nucleotides to adenosine. 2. Agonists of the P2Y(AC)(-)-receptor enhance the growth of C6 cells if their hydrolysis to adenosine is inhibited by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). In these conditions, the potency to stimulate cell growth parallels the ranking of the receptor agonists, i.e. 2-methylthioadenosine-5'-diphosphate (2MeSADP)>Ap(3)A>Ap(4)A. ATP and ADP are still hydrolysed in the presence of PPADS and have no proliferative effect on C6 cells. 3. The enhanced growth is due to a P2Y(AC)(-)-receptor-mediated activation of p42/44 mitogen-activated protein kinase (MAPK) as shown by immunoblotting and protein kinase assays for active MAPK and the use of the MAPK/extracellular signal-regulated kinase kinase (MEK) inhibitor PD98059. 4. The UTP-induced enhancement of the growth of C6 cells is due to activation of MAPK by a PPADS sensitive nucleotide receptor. 5. In conclusion, the effect of nucleotides on the growth of C6 cells is determined by ecto-nucleotidases and by activation of nucleotide receptors. Hydrolysis of nucleotides to adenosine induces growth inhibition while inhibition of the hydrolysis of agonists of the P2Y(AC)(-)-receptor enhances cell growth by activation of MAPK.

By releasing ADP, Acanthamoeba castellanii causes an increase in the cytosolic free calcium concentration and apoptosis in wish cells

The role played by soluble molecules that may participate in acanthamoebal cytopathogenicity has yet to be fully characterized. We demonstrate here that Acanthamoeba castellanii trophozoites constitutively release ADP in the medium. Cell-free supernatants prepared from A. castellanii, by interaction with specific P(2y2) purinoceptors expressed on the Wish cell membrane, caused a biphasic rise in [Ca(2+)](i), extensive cell membrane blebbing, cytoskeletal disorganization, and the breakdown of nuclei. Cell damage induced by amoebic supernatants was blocked by the P(2y2) inhibitor Suramin. The same results were found in Wish cells exposed to purified ADP. These findings suggest that pathogenic free-living A. castellanii may have a cytopathic effect on human epithelial cells through ADP release, by a process that begins with a rise of cytosolic free-calcium concentration, and culminates in apoptosis.

Biphasic cytotoxic mechanism of extracellular ATP on U-937 human histiocytic leukemia cells: involvement of adenosine generation

Since extracellular ATP can exhibit cytotoxic activity in vivo and in vitro, its application has been proposed as an alternative anticancer therapy. In this study we investigated the mechanisms of ATP-induced cytotoxicity in a human leukemic cell line (U-937). ATP added as a single dose exceeding 50 microM was cytostatic or even cytotoxic for U-937 cells. Interestingly, growth inhibition by ATP (50-3500 microM) showed a biphasic dose response. Up to 800 microM, ATP was cytotoxic in a dose-dependent manner (EC(50) 90 microM). In a range between 800 and 2500 microM, cell count was markedly higher despite the higher ATP concentrations. The cytotoxic effect of ATP could be antagonized by addition of uridine as a pyrimidine source and, alternatively, by addition of the nucleoside transmembrane inhibitor dipyridamole. The apoptosis-inducing adenosine A(3) receptor was not involved in measurable quantities, since (1) adenosine did not lead to an elevation of intracellular calcium levels, and (2) an unselective A(1-3) antagonist (ULS-II-80) could not abrogate the cytotoxic effect. Experiments monitoring extracellular nucleotide metabolism confirmed the assumption that the long-term production and continuous uptake of adenosine, which is extracellularly generated by degradation of ATP, led to an intracellular nucleotide imbalance with pyrimidine starvation. The biphasic dose response to higher ATP concentrations could be explained by the rapid degradation of lower ATP concentrations (300 microM) to adenosine by serum-derived enzymes, whereas higher concentrations (900 microM) only produced small amounts of adenosine due to forward inhibition of AMP hydrolysis by prolonged high ADP levels. FACS analysis revealed that at lower adenosine concentrations (300 microM) a reversible G(1) phase arrest of the cell cycle was induced, whereas higher concentrations (1000 microM) triggered apoptosis. Considering ATP as a potential cytostatic drug, our data have important implications concerning metabolic interactions of administered nucleotides.

Effects of extracellular nucleotides and nucleosides on prostate carcinoma cells

1. The purpose of this work was to characterize the receptors involved in the action of nucleotides on the human prostate carcinoma cell lines LNCaP, PC-3 and DU145. 2. Northern blotting revealed the presence of P2Y(2), P2Y(6) and P2Y(11) messengers in the three cell lines. P2Y(1) mRNA was only observed in the DU145 cells. In both PC-3 and DU145 cells, ATP and UTP stimulated inositol phosphate accumulation in an equipotent, equiactive and non-additive way, suggesting the involvement of P2Y(2) receptors. ATP also increased cyclic AMP, but this effect is likely to result from degradation into adenosine and activation of A(2) receptor. A(2) receptor activation led to a synergistic enhancement of prostate-specific antigen secretion induced by vasoactive intestinal peptide. 3. RT - PCR experiments detected the expression of the P2X(4) and P2X(5) receptors in the DU145 cells and the P2X(4), P2X(5) and P2X(7) receptors in the PC-3 cells. The calcium influx induced by BzATP confirmed the functional expression of P2X receptors. 4. ATP inhibited the growth of PC-3 and DU145 cells. This effect was mimicked neither by UTP nor by adenosine, indicating that it does not result from phospholipase C or adenylyl cyclase activation. On the contrary, in PC-3 cells, BzATP reproduced the effect of ATP, which was associated to a moderate decrease of proliferation and an increase of apoptosis. In DU145 cells, ATP was more potent than BzATP and growth inhibition was mainly associated with necrosis. We suggest that P2X receptors might be involved in the inhibition by nucleotides of prostate carcinoma cell growth.

Adenosine acts as an inhibitor of lymphoma cell growth: a major role for the A3 adenosine receptor

In this study, we demonstrated several mechanisms exploring the inhibitory effect of low-dose adenosine on lymphoma cell growth. Adenosine, a purine nucleoside present in plasma and other extracellular fluids, acts as a regulatory molecule, by binding to G-protein associated cell-surface receptors, A1, A2 and A3. Recently we showed that low-dose adenosine released by muscle cells, inhibits tumour cell growth and thus attributes to the rarity of muscle metastases. In the present work, a cytostatic effect of adenosine on the proliferation of the Nb2-11C rat lymphoma cell line was demonstrated. This effect was mediated through the induction of cell cycle arrest in the G0/G1 phase and by decreasing the telomeric signal in these cells. Adenosine was found to exert its antiproliferative effect mainly through binding to its A3 receptor. The cytostatic anticancer activity, mediated through the A3 adenosine receptor, turns it into a potential target for the development of anticancer therapies.

Rapid up-regulation of P2Y messengers during granulocytic differentiation of HL-60 cells

HL-60 cells are human promyelocytic cells expressing two ATP receptors: the P2Y(2) and P2Y(11) subtypes. Our Northern blotting experiments have shown that P2Y(2) and P2Y(11) messengers were up-regulated in these cells, rapidly and independently of protein synthesis, following treatment with granulocytic differentiating agents such as retinoic acid, dimethylsulfoxide, granulocyte-colony stimulating factor, dibutyryl cyclic AMP and ATP. AR-C67085 and adenosine 5'-O-(3-thiotriphosphate), two potent agonists of the recombinant P2Y(11) receptor, increased intracellular cAMP concentration in HL-60 cells more potently than ATP itself. These observations support the conclusion that the effect of ATP on HL-60 cell differentiation is mediated by the P2Y(11) receptor.