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

Purinergic pathways and their clinical use in the treatment of acute myeloid leukemia

Despite the use of various therapies such as hematopoietic stem cell transplantation and chimeric antigen receptor T cell therapy (CAR-T), the prognosis of patients with acute myeloid leukemia (AML) is still generally poor. However, immunotherapy is currently a hot topic in the treatment of hematological tumors. Extracellular adenosine triphosphate (ATP) can be converted to adenosine diphosphate (ADP) via CD39, and ADP can be converted to adenosine via CD73, which can bind to P1 and P2 receptors to exert immunomodulatory effects. Research on the mechanism of the purinergic signaling pathway can provide a new direction for the treatment of AML, and inhibitors of this signaling pathway have been discovered by several researchers and gradually applied in the clinic. In this paper, the mechanism of the purinergic signaling pathway and its clinical application are described, revealing a new target for the treatment of AML and subsequent improvement in patient prognosis.

Expression Pattern of Purinergic Signaling Components in Colorectal Cancer Cells and Differential Cellular Outcomes Induced by Extracellular ATP and Adenosine

The purine nucleotide adenosine triphosphate (ATP) is known for its fundamental role in cellular bioenergetics. However, in the last decades, different works have described emerging functions for ATP, such as that of a danger signaling molecule acting in the extracellular space on both tumor and stromal compartments. Beside its role in immune cell signaling, several studies have shown that high concentrations of extracellular ATP can directly or indirectly act on cancer cells. Accordingly, it has been reported that purinergic receptors are widely expressed in tumor cells. However, their expression pattern is often associated with contradictory cellular outcomes. In this work, we first investigated gene expression profiles through "RNA-Sequencing" (RNA Seq) technology in four colorectal cancer (CRC) cell lines (HT29, LS513, LS174T, HCT116). Our results demonstrate that CRC cells mostly express the A2B, P2X4, P2Y1, P2Y2 and P2Y11 purinergic receptors. Among these, the P2Y1 and P2Y2 coding genes are markedly overexpressed in all CRC cells compared to the HCEC-1CT normal-like colonic cells. We then explored the cellular outcomes induced by extracellular ATP and adenosine. Our results show that in terms of cell death induction extracellular ATP is consistently more active than adenosine against CRC, while neither compound affected normal-like colonic cell survival. Intriguingly, while for the P2Y2 receptor pharmacological inhibition completely abolished the rise in cytoplasmic Ca²⁺ observed after ATP exposure in all CRC cell lines, Ca²⁺ mobilization only impacted the cellular outcome for HT29. In contrast, non-selective phosphodiesterase inhibition completely abolished the effects of extracellular ATP on CRC cells, suggesting that cAMP and/or cGMP levels might determine cellular outcome. Altogether, our study provides novel insights into the characterization of purinergic signaling in CRC.

Monocyte-derived and M1 macrophages from ankylosing spondylitis patients released higher TNF-α and expressed more IL1B in response to BzATP than macrophages from healthy subjects

Macrophages participate in the pathogenesis of ankylosing spondylitis (AS) by producing inflammatory cytokines. Extracellular adenosine triphosphate (eATP), released during cell stress, acts through purinergic receptors (P2XR and P2YR) and induces inflammatory responses. We investigated the effect of 2'(3')-O-(4-benzoyl benzoyl) ATP (BzATP) (a prototypic agonist of P2X7R) on the production of inflammatory cytokines in both monocyte-generated (M2-like) and M1 macrophages from patients and controls. Macrophages were differentiated from isolated periphery-monocytes (n = 14 in each group) by macrophage colony-stimulating factor (M-CSF). Using LPS and IFN-γ, macrophages were skewed toward M1 type and were treated with BzATP. Gene expression and protein release of IL-1β, IL-23, and TNF-α were evaluated by real-time PCR and ELISA methods respectively before and after treatment. BzATP significantly increased the protein release of TNF-α and the expression of TNFA and IL1B in monocyte-generated macrophages. Besides, BzATP treatment significantly upregulated IL1B expression, reduced TNFA and IL23A expression, and TNF-α release in M1 macrophages from both groups. Monocyte-generated and M1 macrophages from AS patients released higher TNF-α and expressed more IL1B in response to the same concentration of BzATP treatment respectively. Based on our results, AS macrophages were more sensitive to BzATP treatment and responded more intensively. Besides, the diverse effects of BzATP on monocyte-derived and M1 macrophages in our study may represent the differed inflammatory properties of these two groups of macrophages in response to eATP in the body.

CaSR is required for ischemia-induced proliferation and differentiation of white matter progenitor cells from neonatal rats

This study was designed to investigate whether calcium-sensing receptor (CaSR) could induce immture white matter progenitor cells proliferation and differentiation into oligodendrocyte(OL) precursor cells after oxygen-glucose deprivation (OGD) in vitro. Progenitor cells of immature white matter originating from five-day-old newborn rats were divided into control, OGD, control + CaSR silencing, OGD + CaSR silencing, control + adenosine triphosphate magnesium chloride (ATP-MgCl2) and OGD + ATP-MgCl2 groups. Immunofluorescence, real-time RT-PCR, gene silencing, Hoechst 33342/propidium iodide (PI) and Flow cytometry tests were used to examine the proliferation, differentiation and survival of the white matter progenitor cells in the different treatment groups. The results showed that normal immature white matter progenitor cells have certain ability of self-proliferation and differentiation in vitro. OGD could further induce progenitor cells proliferation and differentiation into O4 + OL precursor cells by activating CaSR, but OGD also induced more necrosis and apoptosis of newborn cells and less MBP + OL formation. The addition of ATP-MgCl2 as an activating agent of CaSR further promoted cell proliferation and differentiation both under normal and OGD conditions and reduced OGD-induced apoptosis and necrosis, while CaSR silenced resulted in minimal cell proliferation, differentiation and survival. This study suggests that CaSR plays an important role in the induction of immature white matter progenitor cells proliferation and differentiation into OL precursor cells after OGD, which may provide a new angle to further study whether CaSR initiates the intrinsic repair potential of immature white matter after ischemia in vivo.

Damage/Danger Associated Molecular Patterns (DAMPs) Modulate Chlamydia pecorum and C. trachomatis Serovar E Inclusion Development In Vitro

Persistence, more recently termed the chlamydial stress response, is a viable but non-infectious state constituting a divergence from the characteristic chlamydial biphasic developmental cycle. Damage/danger associated molecular patterns (DAMPs) are normal intracellular components or metabolites that, when released from cells, signal cellular damage/lysis. Purine metabolite DAMPs, including extracellular ATP and adenosine, inhibit chlamydial development in a species-specific manner. Viral co-infection has been shown to reversibly abrogate Chlamydia inclusion development, suggesting persistence/chlamydial stress. Because viral infection can cause host cell DAMP release, we hypothesized DAMPs may influence chlamydial development. Therefore, we examined the effect of extracellular ATP, adenosine, and cyclic AMP exposure, at 0 and 14 hours post infection, on C. pecorum and C. trachomatis serovar E development. In the absence of de novo host protein synthesis, exposure to DAMPs immediately post or at 14 hours post infection reduced inclusion size; however, the effect was less robust upon 14 hours post infection exposure. Additionally, upon exposure to DAMPs immediately post infection, bacteria per inclusion and subsequent infectivity were reduced in both Chlamydia species. These effects were reversible, and C. pecorum exhibited more pronounced recovery from DAMP exposure. Aberrant bodies, typical in virus-induced chlamydial persistence, were absent upon DAMP exposure. In the presence of de novo host protein synthesis, exposure to DAMPs immediately post infection reduced inclusion size, but only variably modulated chlamydial infectivity. Because chlamydial infection and other infections may increase local DAMP concentrations, DAMPs may influence Chlamydia infection in vivo, particularly in the context of poly-microbial infections.

Expression of the potential therapeutic target CXXC5 in primary acute myeloid leukemia cells - high expression is associated with adverse prognosis as well as altered intracellular signaling and transcriptional regulation

The CXXC5 gene encodes a transcriptional activator with a zinc-finger domain, and high expression in human acute myeloid leukemia (AML) cells is associated with adverse prognosis. We now characterized the biological context of CXXC5 expression in primary human AML cells. The global gene expression profile of AML cells derived from 48 consecutive patients was analyzed; cells with high and low CXXC5 expression then showed major differences with regard to extracellular communication and intracellular signaling. We observed significant differences in the phosphorylation status of several intracellular signaling mediators (CREB, PDK1, SRC, STAT1, p38, STAT3, rpS6) that are important for PI3K-Akt-mTOR signaling and/or transcriptional regulation. High CXXC5 expression was also associated with high mRNA expression of several stem cell-associated transcriptional regulators, the strongest associations being with WT1, GATA2, RUNX1, LYL1, DNMT3, SPI1, and MYB. Finally, CXXC5 knockdown in human AML cell lines caused significantly increased expression of the potential tumor suppressor gene TSC22 and genes encoding the growth factor receptor KIT, the cytokine Angiopoietin 1 and the selenium-containing glycoprotein Selenoprotein P. Thus, high CXXC5 expression seems to affect several steps in human leukemogenesis, including intracellular events as well as extracellular communication.

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.

Antiproliferative effects of selective adenosine receptor agonists and antagonists on human lymphocytes: evidence for receptor-independent mechanisms

The effects of standard adenosine receptor (AR) agonists and antagonists on the proliferation of human T lymphocytes, unstimulated and phytohemagglutinin-stimulated human peripheral blood lymphocytes (PBL), and Jurkat T cells were investigated. Real-time PCR measurements confirmed the presence of all four AR subtypes on the investigated cells, although at different expression levels. A2A ARs were predominantly expressed in PBL and further upregulated upon stimulation, while malignant Jurkat T cells showed high expression levels of A1, A2A, and A2B ARs. Cell proliferation was measured by [(3)H]-thymidine incorporation assays. Several ligands, including the subtype-selective agonists CPA (A1), BAY60-6583 (A2B), and IB-MECA (A3), and the antagonists PSB-36 (A1), MSX-2 (A2A), and PSB-10 (A3) significantly inhibited cell proliferation at micromolar concentrations, which were about three orders of magnitude higher than their AR affinities. In contrast, further investigated AR ligands, including the agonists NECA (nonselective) and CGS21680 (A2A), and the antagonists preladenant (SCH-420814, A2A), PSB-1115 (A2B), and PSB-603 (A2B) showed no or only minor effects on lymphocyte proliferation. The anti-proliferative effects of the AR agonists could not be blocked by the corresponding antagonists. The non-selective AR antagonist caffeine stimulated phytohemagglutinin-activated PBL with an EC50 value of 104 μM. This is the first study to compare a complete set of commonly used AR ligands for all subtypes on lymphocyte proliferation. Our results strongly suggest that these compounds induce an inhibition of lymphocyte proliferation and cell death through AR-independent mechanisms.

Adenosine induces cell cycle arrest and apoptosis via cyclinD1/Cdk4 and Bcl-2/Bax pathways in human ovarian cancer cell line OVCAR-3

Adenosine is a regulatory molecule with widespread physiological effects in almost every cells and acts as a potent regulator of cell growth. Adenosine has been shown to inhibit cell growth and induce apoptosis in the several cancer cells via caspase activation and Bcl-2/Bax pathway. The present study was designed to understand the mechanism underlying adenosine-induced apoptosis in the OVCAR-3 human ovarian cancer cells. MTT viability, BrdU and cell counting assays were used to study the cell proliferation effect of adenosine in presence of adenosine deaminase inhibitor and the nucleoside transporter inhibitor. Cell cycle analysis, propidium iodide and annexin V staining, caspase-3 activity assay, cyclinD1, Cdk4, Bcl-2 and Bax protein expressions were assessed to detect apoptosis. Adenosine significantly inhibited cell proliferation in a concentration-dependent manner in OVCAR-3 cell line. Adenosine induced cell cycle arrest in G0/G1 phase via Cdk4/cyclinD1-mediated pathway. Adenosine induced apoptosis, which was determined by Annexin V-FITC staining and increased sub-G1 population. Moreover, down-regulation of Bcl-2 protein expression, up-regulation of Bax protein expression and activation of caspase-3 were observed in response to adenosine treatment. The results of this study suggest that extracellular adenosine induced G1 cell cycle arrest and apoptosis in ovarian cancer cells via cyclinD1/ Cdk4 and Bcl-2/Bax pathways and caspase-3 activation. These data might suggest that adenosine could be used as an agent for the treatment of ovarian cancer.

Foxo3a targets mitochondria during guanosine 5'-triphosphate guided erythroid differentiation

Evidence is emerging that Foxo family proteins serve as biochemical signal integrators in complex signaling networks mediating and modulating diverse cellular functions. Herein, we report that besides the well-established function of Foxo3a as a transcriptional regulator of multiple target genes in nucleus, a substantial fraction of Foxo3a translocates to mitochondria leading to disruption of mitochondrial membrane potential, release of cytochrome c and caspase activation during erythroid differentiation mediated by guanosine 5'-triphosphate (GTP). In fact, non transcriptional role of Foxo3a in mitochondria was achieved through the protein-protein interaction with pro-apoptotic protein Bax and its translocation to mitochondrial membrane. Furthermore, our results revealed that mitochondrial localization of Foxo3a was modulated by intracellular GTP content which is sensed by PKC signaling element. Collectively, our findings provided insight into a novel Foxo3a mechanism in leukemia cells which led to engagement of cells in the maturation pathway.

Adenosine induces cell-cycle arrest and apoptosis in androgen-dependent and -independent prostate cancer cell lines, LNcap-FGC-10, DU-145, and PC3

BACKGROUND

Adenosine has been shown to inhibit cell growth and induce apoptosis in the several cancer cells via intrinsic and extrinsic pathway. The present study was designed to understand the mechanism underlying adenosine-induced apoptosis in the DU-145, PC3, and LNcap-FGC10 human prostate cancer cells.

METHODS

To observe cell viability and proliferation, MTT assay, cell counting, and BrdU assay were carried out in DU-145, PC3, and LNcap-FGC10 cells. Apoptosis was assessed with the analysis of cell cycle, Hoechst 33258 staining, propidium iodide and annexin-V staining, reactive oxygen species (ROS) formation, mitochondrial membrane potential (ΔΨM) measurement, caspase-3 activity assay, Bcl-2 and Bax protein expression. Moreover, the expression of adenosine receptors and the effects of adenosine receptor (A(1) , A(2a) , and A(3) ) antagonists were examined.

RESULT

Adenosine significantly reduced cell proliferation in a dose-dependent manner in DU-145, PC3, and LNcap-FGC10 cell lines. Adenosine induced arrest in the cell-cycle progression in G0/G1 phase through Cdk4/cyclinD1-mediated pathway. Adenosine induced apoptosis, which was determined by morphological changes and increased sub-G1 population. Furthermore, increase of ROS, loss of MMP, activation of caspase-3, and down-regulation of Bcl-2 expression was observed. A(1) , A(2a) , A(2b) , and A(3) adenosine receptors mRNA are expressed in the cell lines. Moreover, adenosine-induced apoptosis was inhibited by MRS1220, A(3) adenosine receptor antagonist.

CONCLUSION

Our results suggest that adenosine induced apoptosis in prostate cancer cells via the mitochondrial pathway and is related to the adenosine receptors. These data might suggest that adenosine could be used as an agent for the treatment of prostate cancer.

ATP sensitizes H460 lung carcinoma cells to cisplatin-induced apoptosis

Platinum resistance of cancer cells may evolve due to a decrease in intracellular drug accumulation, decreased cell permeability or by an increased deactivation of the drug by glutathione (GSH). The aim of this study was (1) to investigate the effect of adenosine 5'-triphosphate (ATP) on the cytotoxicity of cisplatin in a large cell lung carcinoma cell line (H460), and (2) to examine the potential involvement of increased cisplatin uptake, GSH depletion and pyrimidine starvation by ATP in this effect. H460 cells were harvested and seeded (5% CO(2); 37 degrees C). Subsequently, cells were incubated with medium or ATP followed by an incubation with cisplatin. Cytotoxicity screening was analyzed by the sulforhodamine B (SRB) colorimetric assay, lactate dehydrogenase and caspase-3/7 activity. Pre-incubation for 72h with 0.3 and 3mM ATP strongly enhanced the anti-proliferative potency of cisplatin 2.9- and 7.6-fold, respectively. Moreover, after incubation of H460 cells with 0.3mM ATP the intracellular platinum concentration increased, indicating increased cisplatin uptake by ATP. ATP, despite lowering the LD(50) of cisplatin, did not modulate GSH levels in H460 cells. ATP itself showed a biphasic effect on H460 cell growth: 0.3mM inhibited H460 cell growth via the pyrimidine starvation effect, activation of caspase-3/7 and LDH leakage, while 3mM ATP showed no effect on cell growth. In conclusion, ATP sensitizes the H460 cells to cisplatin-induced apoptosis. The effect of 0.3mM ATP is not due to GSH depletion but involves increased cisplatin uptake and pyrimidine starvation due to ATP conversion to adenosine followed by cellular uptake.

Effect of Adenosine on the Growth of Human T-Lymphocyte Leukemia Cell Line MOLT-4

Adenosine has been observed to suppress the growth of MOLT-4 human leukemia cells in vitro. Changes in the cell cycle, especially increased percentage of cells in S phase, prolonged generation time, and induction of apoptosis at higher adenosine concentrations have been found to be responsible for the growth suppression. Dipyridamole, a drug inhibiting the cellular uptake of adenosine, reversed partially but significantly the adenosine-induced growth suppression. It follows from these results that the action of adenosine on the MOLT-4 cells comprises its cellular uptake and intracellular operation. These findings present new data on anticancer efficacy of adenosine.

Diminution in adenine nucleotide hydrolysis by platelets and serum from rats submitted to Walker 256 tumour

Extracellular adenine nucleotide hydrolysis in the circulation is mediated by the action of an NTPDase (CD39, apyrase) and of a 5'-nucleotidase (CD73), presenting as a final product, adenosine. Among other properties described for adenine nucleotides, an anti-cancer activity is suggested, since ATP is considered a cytotoxic molecule in several tumour cell systems. Conversely, some studies demonstrate that adenosine presents a tumour-promoting activity. In this study, we evaluated the pattern of adenine nucleotide hydrolysis by serum and platelets from rats submitted to the Walker 256 tumour model. Extracellular adenine nucleotide hydrolysis by blood serum and platelets obtained from rats at, 6, 10 and 15 days after the subcutaneous Walker 256 tumour inoculation, was evaluated. Our results demonstrate a significant reduction in ATP, ADP and AMP hydrolysis in blood serum at 6, 10 and 15 days after tumour induction. In platelets, a significant reduction in ATP and AMP hydrolysis was observed at 10 and 15 days after tumour induction, while an inhibition of ADP hydrolysis was observed at all times studied. Based on these results, it is possible to suggest a physiologic protection mechanism against the tumoral process in circulation. The inhibition in nucleotide hydrolysis observed probably maintains ATP levels elevated (cytotoxic compound) and, at the same time, reduces the adenosine production (tumour-promoting molecule) in the circulation.

NTPDase and 5' ecto-nucleotidase expression profiles and the pattern of extracellular ATP metabolism in the Walker 256 tumor

In this study, we evaluated the NTPDases and ecto-5'-nucleotidase (CD73) expression profiles and the pattern of adenine nucleotide hydrolysis in rats submitted to the Walker 256 tumor model, 6, 10 and 15 days after the subcutaneous inoculation. Using RT-PCR analysis, we identified mRNA for all of the members of the ecto-nucleoside triphosphate diphosphohydrolase family investigated and a 5'-nucleotidase. By quantitative real-time PCR, Entpd1 (Cd39) and Entpd2 (Cd39L1) and CD73 were identified as the dominant genes expressed by the Walker 256 tumor, at all times studied. Extracellular adenine nucleotide hydrolysis by the Walker 256 tumor was estimated by HPLC analysis. Rapid hydrolysis of extracellular ATP by the tumor cells was observed, leading to the formation of adenosine and inosine in cells obtained from solid tumors at 6 and 10 days after inoculation. Cells obtained from solid tumors at 15 days of growth presented high levels of AMP and presented adenosine as a final product after 90 min of incubation. Results demonstrate that the presence of NTPDases and 5'-nucleotidase enzymes in Walker 256 tumor cells may be important for regulation of the extracellular adenine nucleotides/adenine nucleoside ratio, therefore leading to tumor growth.

GTP induces S-phase cell-cycle arrest and inhibits DNA synthesis in K562 cells but not in normal human peripheral lymphocytes

Since differentiation therapy is one of the promising strategies for treatment of leukemia, universal efforts have been focused on finding new differentiating agents. In that respect, we used guanosine 5'-triphosphate (GTP) to study its effects on K562 cell line. GTP, at concentrations between 25-200 microM, inhibited proliferation (3-90%) and induced 5-78% increase in benzidine-positive cells after 6-days of treatments of K562 cells. Flow cytometric analyses of glycophorine A (GPA) showed that GTP can induce expression of this marker in more mature erythroid cells in a time- and dose-dependent manner. These effects of GTP were also accompanied with inhibition of DNA synthesis (measured by [3H]-thymidine incorporation) and early S-phase cell cycle arrest by 96 h of exposure. In contrast, no detectable effects were observed when GTP administered to unstimulated human peripheral blood lymphocytes (PBL). However, GTP induced an increase in proliferation, DNA synthesis and viability of mitogen-stimulated PBL cells. In addition, growth inhibition and differentiating effects of GTP were also induced by its corresponding nucleotides GDP, GMP and guanosine (Guo). In heat-inactivated medium, where rapid degradation of GTP via extracellular nucleotidases is slow, the anti-proliferative and differentiating effects of all type of guanine nucleotides (except Guo) were significantly decreased. Moreover, adenosine, as an inhibitor of Guo transporter system, markedly reduced the GTP effects in K562 cells, suggesting that the extracellular degradation of GTP or its final conversion to Guo may account for the mechanism of GTP effects. This view is further supported by the fact that GTP and Guo are both capable of impeding the effects of mycophenolic acid. In conclusion, our data will hopefully have important impact on pharmaceutical evaluation of guanine nucleotides for leukemia treatments.

Guanosine 5'-triphosphate induces differentiation-dependent apoptosis in human leukemia U937 and KG1 cells

AIM

The differentiation capability of guanosine 5'-triphosphate (GTP) was studied using U937 and KG1 cells.

METHODS

Cell cycle was analyzed by PI staining using flow cytometry. Apoptosis was measured by Annexin-V-FITC/PI double staining using flow cytometry. Differentiation was observed by morphological criteria, Wright-Giemsa staining and expression of cell surface markers CD11b and CD14.

RESULTS

Variable GTP concentrations (25-200 micromol/L) at short treatment times (up to 24 h) showed significant anti-proliferative activities among both cell types. However, longer treatment times (up to 72 h) were required to trigger apoptosis. Cell-cycle analyses of the GTP-treated cells indicated an increase in S-phase population by 48 h followed by the appearance of a sub-G(1) peak after 72 h of treatment. The effects of GTP on U937 and KG1 cells were accompanied with differentiation toward monocyte/macrophage lineage. This was evidenced by a sharp increase in the extent of CD11b and CD14 expression after 24 h of exposure to GTP. The viability of both cell types did not significantly change during the first 24 h. However, at longer treatment times (72-96 h), dramatic decreases in both the extent of CD14 expression and the cell viabilities were observed. Simultaneous measurement of apoptosis and CD14 expression in GTP-treated U937 cells indicated that cells with lower CD14 content underwent more apoptosis.

CONCLUSION

These finding may pave the way for further pharmaceutical evaluation of GTP as a suitable differentiating agent for acute myeloblastic leukemia (AML) therapy.

Growth inhibitory effect and apoptosis induced by extracellular ATP and adenosine on human gastric carcinoma cells: involvement of intracellular uptake of adenosine

AIM

To study the growth inhibitory and apoptotic effects of adenosine triphosphate (ATP) and adenosine (ADO) on human gastric carcinoma (HGC)-27 cells in vitro and the mechanisms related to the actions of ATP and ADO.

METHODS

MTT assay was used to determine the reduction of cell viability. The morphological changes of HGC-27 cells induced by ATP or ADO were observed under fluorescence light microscope by acridine orange/ethidium bromide double-stained cells. The internucleosomal fragmentation of genomic DNA was detected by agarose gel electrophoresis. The apoptotic rate and cell-cycle analysis after treatment with ATP or ADO was determined by flow cytometry.

RESULTS

ATP, ADO and the intermediate metabolites, ADP and AMP, and the agonist of purinergic receptors, reduced cell viability of HGC-27 cells at doses of 0.3 and 1.0 mmol/L. The distribution of cell cycle phase and proliferation index (PI) value of HGC-27 cells changed when exposed to ATP or ADO at the concentrations of 0.1, 0.3 and 1 mmol/L for 48 h. ATP and ADO both altered the distribution of cell cycle phase via G0/G1- phase arrest and significantly decreased PI value. Under light microscope, the tumor cells exposed to 0.3 mmol/L ATP or ADO displayed morphological changes of apoptosis; a ladder-like pattern of DNA fragmentation obtained from HGC-27 cells treated with 0.1-1 mmol/L ATP or ADO appeared in agarose gel electrophoresis; ATP and ADO induced the apoptosis of HGC-27 cells in a dose-dependent manner at concentrations between 0.03-1 mmol/L. The maximum apoptotic rate of HGC-27 cells exposed to ATP or ADO for 48 h was 13.53% or 15.9%, respectively. HGC-27 cell death induced by ATP or ADO was significantly inhibited by dipyridamole (10 mmol/L), an inhibitor of adenosine transporter, but was not affected by aminophylline, a broad inhibitor of P1 receptors and pyridoxal-phosphate-6-azophenyl-2, 4-disulphonic acid tetrasodium salt (30 micromol/L), a non-selective antagonist of P2 receptors.

CONCLUSION

Extracellular ATP and ADO reduced the cell viability, arrested cell cycle and induced apoptosis in HGC-27 cell line by intracellular uptake of ADO. One of the main routes of ATP-induced apoptosis in HGC-27 cells is through the breakdown to adenosine.

Molecular mechanisms of adenosine-induced apoptosis in human HepG2 cells

AIM

To investigate effects of adenosine on cell proliferation and apoptosis in human HepG2 cells.

METHODS

HepG2 cells were incubated in the presence of adenosine (0.1-5 mmol/L) for 12-48 h, and the effect of adenosine on cell proliferation was evaluated by using 3-(4,5-dimethyl-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. Hoechst 33342 fluorescent staining, dUTP-fluorescein isothiocyanate (FITC) fluorescence and flow cytometric analysis techniques were used to observe cell apoptosis. The effects of adenosine receptor (A1, A2a, A3 and nonspecific receptor) antagonists (8-cpt, DMPX, MRS1191, and theophylline) and an adenosine transporter protein inhibitor (dipyridamole) on adenosine-induced cell apoptosis were observed. Mitochondrial membrane potential was analyzed using DePsipher fluorescent staining, and caspase activity was detected using a Fluorometric assay kit and a fluorescence microplate reader.

RESULTS

Adenosine significantly reduced cell viability in a dose- and time-dependent manner. The cytotoxicity of adenosine was related to the induction of cell apoptosis. Four adenosine receptor antagonists had no effect on cell apoptosis. However, dipyridamole significantly reduced the percentage of adenosine-induced apoptotic cells from 27.3% to 7.1% (P<0.05). At 48 h after treatment, 3 mmol/L adenosine increased caspase-3 activity 3.5-fold; dipyridamole markedly decreased caspase-3 activity 1.6-fold, and decreased apoptotic cell numbers. When HepG2 cells were treated with 3 mmol/L adenosine, mitochondrial membrane potential and the activity of caspase-8 or -9 remained unchanged.

CONCLUSION

Our results suggest that adenosine-induced apoptosis in HepG2 cells is related to intracellular events rather than cell surface receptors, and that a caspase-3 cascade activation is required, which is not mediated via a mitochondrial pathway.

Inhibitory effects of extracellular adenosine triphosphate on growth of esophageal carcinoma cells

AIM: To study the growth inhibitory effects of ATP on TE-13 human squamous esophageal carcinoma cells in vitro.

METHODS: MTT assay was used to determine the inhibition of proliferation of ATP or adenosine (ADO) on TE-13 cell line. The morphological changes of TE-13 cells induced by ATP or ADO were observed under fluorescence light microscope by acridine orange (AO)/ethidium bromide (EB) double stained cells. The internucleosomal fragmentation of genomic DNA was detected by agarose gel electrophoresis. The apoptotic rate and cell cycle after treatment with ATP or ADO were determined by flow cytometry.

RESULTS: ATP and ADO produced inhibitory effects on TE-13 cells at the concentration between 0.01 and 1.0 mmol/L. The IC50 of TE-13 cells exposed to ATP or ADO for 48 and 72 h was 0.71 or 1.05, and 0.21 or 0.19 mmol/L, respectively. The distribution of cell cycle phase and proliferation index (PI) value of TE-13 cells changed, when being exposed to ATP or ADO at the concentrations of 0.01, 0.1, and 1 mmol/L for 48 h. ATP and ADO inhibited the cell proliferation by changing the distribution of cell cycle phase via either G0/G1 phase (ATP or ADO, 1 mmol/L) or S phase (ATP, 0.1 mmol/L) arrest. Under light microscope, the tumor cells exposed to 0.3 mmol/L ATP or ADO displayed morphological changes of apoptosis. A ladder-like pattern of DNA fragmentation was obtained from TE-13 cells treated with 0.1-1 mmol/L ATP or ADO in agarose gel electrophoresis. ATP and ADO induced apoptosis of TE-13 cells in a dose-dependent manner at the concentration between 0.03 and 1 mmol/L. The maximum apoptotic rate of TE-13 cells exposed to ATP or ADO for 48 h was 16.63% or 16.9%, respectively.

CONCLUSION: ATP and ADO inhibit cell proliferation, arrest cell cycle, and induce apoptosis of TE-13 cell line.

An adenosine analog (IB-MECA) inhibits anchorage-dependent cell growth of various human breast cancer cell lines

A3 adenosine receptor agonists have been reported to influence cell death and survival. Here we report the effects of an A3 adenosine receptor agonist, IB-MECA, on the cell growth of human breast cancer cell lines, MCF-7 (estrogen receptor positive) and MDA-MB468 (estrogen receptor negative). Therefore, this study was aimed to investigate the expression and possible action of A3 receptor in the human breast cancer cell lines. IB-MECA, at 1-100 microM, resulted in a significant cell growth inhibition (P < 0.05) which reached the maximum at 48 h, in the cell lines. In both cell lines, agonist-induced effects were antagonized by pretreatment with a selective A3 adenosine receptor antagonist, MRS1220. Using RT-PCR method, further confirmation was provided by the presence of mRNA of A3 receptor in the cells. In addition, IB-MECA was able to inhibit forskolin-stimulated cAMP levels, which indicate the functional form of A3 receptor on the cell surface of these breast cancer cell lines. These results suggest that the inhibitory effect of IB-MECA on the growth of human breast cancer cell lines is mediated through activation of A3 adenosine receptor.

2-ClATP exerts anti-tumoural actions not mediated by P2 receptors in neuronal and glial cell lines

We investigated the effects of the ATP analogue and P2 receptor agonist 2-ClATP on growth and survival of different neuronal (PC12, PC12nnr5 and SH-SY5Y) and glial (U87 and U373) cell lines, by the use of direct count of intact nuclei, fluorescence microscopy, fluorescence-activated cell sorter analysis (FACS) and high pressure liquid chromatography (HPLC). 2-ClATP lowered the number of cultured PC12nnr5, SH-SY5Y, U87 and U373 cells to almost 5%, and of PC12 cells to about 35% after 3-4 days of treatment. EC(50) was in the 5-25 microM range, with 2-ClATP behaving as a cytotoxic or cytostatic agent. Analysis of the biological mechanisms demonstrated that pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (P2 receptor antagonist and nucleotidases inhibitor), but not Caffeine or CGS-15493 (P1 receptor antagonists) effectively prevented 2-ClATP-induced toxicity. 2-ClATP metabolic products (2-ClADP, 2-ClAMP, 2-Cladenosine) and new synthesis derivatives (2-CldAMP, 2-Cldadenosine-3',5'-bisphosphate and 2-CldATP) exerted similar cytotoxic actions. Inhibition of both serum nucleotidases and purine nucleoside transporters strongly reduced 2-ClATP-induced cell death, which was conversely increased by the nucleotide hydrolyzing enzyme apyrase. The adenosine kinase inhibitor 5-iodotubericidin totally prevented 2-ClATP or 2-Cladenosine-induced toxicity. In summary, our findings indicate that 2-ClATP exerts either cell cycle arrest or cell death, acting neither on P2 nor on P1 receptors, but being extracellularly metabolized into 2-Cladenosine, intracellularly transported and re-phosphorylated.

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.