Extracellular ATP is increased by release of ATP-loaded microparticles triggered by nutrient deprivation

Rationale: Caloric restriction improves the efficacy of anti-cancer therapy. This effect is largely dependent on the increase of the extracellular ATP concentration in the tumor microenvironment (TME). Pathways for ATP release triggered by nutrient deprivation are largely unknown.

Methods: The extracellular ATP (eATP) concentration was in vivo measured in the tumor microenvironment of B16F10-inoculated C57Bl/6 mice with the pmeLuc probe. Alternatively, the pmeLuc-TG-mouse was used. Caloric restriction was in vivo induced with hydroxycitrate (HC). B16F10 melanoma cells or CT26 colon carcinoma cells were in vitro exposed to serum starvation to mimic nutrient deprivation. Energy metabolism was monitored by Seahorse. Microparticle release was measured by ultracentrifugation and by Nanosight.

Results: Nutrient deprivation increases eATP release despite the dramatic inhibition of intracellular energy synthesis. Under these conditions oxidative phosphorylation was dramatically impaired, mitochondria fragmented and glycolysis and lactic acid release were enhanced. Nutrient deprivation stimulated a P2X7-dependent release of ATP-loaded, mitochondria-containing, microparticles as well as of naked mitochondria.

Conclusions: Nutrient deprivation promotes a striking accumulation of eATP paralleled by a large release of ATP-laden microparticles and of naked mitochondria. This is likely to be a main mechanism driving the accumulation of eATP into the TME.

Modulation of Cell Energy Metabolism by the P2X7 Receptor

For many years the P2X7 receptor (P2X7R) was considered the prototypic cytolytic receptor due to its ability to cause dramatic changes in plasma membrane permeability, eventually leading to cell death. However, later studies revealed that controlled P2X7R activation has beneficial effects on cell metabolism and nowadays our perception of the physiological role of this receptor has radically changed. Some of the biochemical pathways underlying the trophic effect of the P2X7R are being unveiled, thus disclosing an unanticipated role of P2X7Rs in mitochondrial and glycolytic metabolism. We provide here an update of the effects of the P2X7R on cell energy metabolism.

Structure, function and techniques of investigation of the P2X7 receptor (P2X7R) in mammalian cells

The P2X7 receptor [P2X7R or P2RX7 in National Center for Biotechnology Information (NCBI) gene nomenclature] is a member of the P2X receptor (P2XR) subfamily of P2 receptors (P2Rs). The P2X7R is an extracellular ATP-gated ion channel with peculiar permeability properties expressed by most cell types, mainly in the immune system, where it has a leading role in cytokine release, oxygen radical generation, T lymphocyte differentiation and proliferation. A role in cancer cell growth and tumor progression has also been demonstrated. These features make the P2X7R an appealing target for drug development in inflammation and cancer. The functional P2X7R, recently (partially) crystallized and 3-D solved, is formed by the assembly of three identical subunits (homotrimer). The P2X7R is preferentially permeable to small cations (Ca2+, Na+, K+), and in most (but not all) cell types also to large positively charged molecules of molecular mass up to 900Da. Permeability to negatively charged species of comparable molecular mass (e.g., Lucifer yellow) is debated. Several highly selective P2X7R pharmacological blockers have been developed over the years, thus providing powerful tools for P2X7R studies. Biophysical properties and coupling to several different physiological responses make the P2X7R amenable to investigation by electrophysiology and cell biology techniques, which allow its identification and characterization in many different cell types and tissues. A careful description of the physiological features of the P2X7R is a prerequisite for an effective therapeutic development. Here we describe the most common techniques to asses P2X7R functions, including patch-clamp, intracellular calcium measurements, and membrane permeabilization to large fluorescent dyes in a selection of different cell types. In addition, we also describe common toxicity assays used to verify the effects of P2X7R stimulation on cell viability.

Amyloid β-dependent mitochondrial toxicity in mouse microglia requires P2X7 receptor expression and is prevented by nimodipine

Previous data from our laboratory show that expression of the P2X7 receptor (P2X7R) is needed for amyloid β (Aβ)-stimulated microglia activation and IL-1β release in vitro and in vivo. We also showed that Aβ-dependent stimulation is inhibited by the dihydropyridine nimodipine at an intracellular site distal to the P2X7R. In the present study, we used the N13 microglia cell line and mouse primary microglia from wt and P2rx7-deleted mice to test the effect of nimodipine on amyloid β (Aβ)-dependent NLRP3 inflammasome expression and function, and on mitochondrial energy metabolism. Our data show that in microglia Aβ causes P2X7R-dependent a) NFκB activation; b) NLRP3 inflammasome expression and function; c) mitochondria toxicity; and these changes are fully inhibited by nimodipine. Our study shows that nimodipine is a powerful blocker of cell damage caused by monomeric and oligomeric Aβ, points to the mitochondria as a crucial target, and underlines the permissive role of the P2X7R.

P2X7 targeting inhibits growth of human mesothelioma

Malignant pleural mesothelioma (MPM) is an aggressive tumor refractory to anti-blastic therapy. MPM cells show several genetic and biochemical defects, e.g. overexpression of oncogenes, downregulation of onco-suppressor genes, dysregulation of microRNA, or alteration of intracellular Ca²⁺ homeostasis and of apoptosis. No information is as yet available on purinergic signalling in this tumor. Signalling via the P2×7 (P2RX7 or P2×7R) purinergic receptor is attracting increasing attention as a pathway involved in cancer cell death or proliferation. In this report we show that the P2×7R is expressed by three MPM cell lines established from MPM patients but not by mesothelial cells from healthy subjects (healthy mesothelial cells, HMCs). MPM cell proliferation was inhibited by in vitro incubation in the presence of selective P2×7R antagonists, as well as by stimulation with the P2×7R agonist BzATP. Systemic administration of the selective P2×7R blocker AZ10606120 inhibited in vivo growth of MPM tumors whether implanted subcutaneously (s.c.) or intraperitoneally (i.p.). Our findings suggest that the P2×7R might be a novel target for the therapy of mesothelioma.

The P2X7 receptor directly interacts with the NLRP3 inflammasome scaffold protein

The P2X7 receptor (P2X7R) is a known and powerful activator of the NOD-like receptor (NLR)P3 inflammasome; however, the underlying pathways are poorly understood. Thus, we investigated the molecular mechanisms involved. The effect of P2X7R expression and activation on NLRP3 expression and recruitment was investigated by Western blot, RT-PCR, coimmunoprecipitation, and confocal microscopy in microglial mouse cell lines selected for reduced P2X7R expression and in primary cells from P2X7R(-/-) C57BL/6 mice. We show here that P2X7R activation by ATP (EC₅₀ = 1 mM) or benzoyl-ATP (EC₅₀ = 300 μM) and P2X7R down-modulation caused a 2- to 8-fold up-regulation of NLRP3 mRNA in mouse N13 microglial cells. Moreover, NLRP3 mRNA was also up-regulated in primary microglial and macrophage cells from P2X7R(-/-) mice. Confocal microscopy and immunoprecipitation assays showed that P2X7R and NLRP3 closely interacted at discrete subplasmalemmal sites. Finally, P2X7R stimulation caused a transient (3-4 min) cytoplasmic Ca(2+) increase localized to small (2-3 µm wide) discrete subplasmalemmal regions. The Ca(2+) increase drove P2X7R recruitment and a 4-fold increase in P2X7R/NLRP3 association within 1-2 min. These data show a close P2X7R and NLRP3 interaction and highlight the role of P2X7R in the localized cytoplasmic ion changes responsible for both NLRP3 recruitment and activation.

Nimodipine inhibits IL-1β release stimulated by amyloid β from microglia

BACKGROUND AND PURPOSE

There is growing evidence that inflammation plays a major role in the pathogenesis of neural damage caused by deposition of amyloid β (Aβ) in the brain. Nimodipine has received attention as a drug that might improve learning and reduce cognitive deficits in Alzheimer's disease, but the mechanism of action is poorly known. In this study, we tested the hypothesis that nimodipine inhibited Aβ-stimulated IL-1β release from microglia.

EXPERIMENTAL APPROACH

Cultures of N13 microglia cells or primary mouse microglia were treated with nimodipine, and intracellular accumulation and release of IL-1β in response to Aβ or to the P2 receptor agonists ATP and benzoyl ATP (BzATP) were measured. Accumulation of IL-1β was measured in vivo after intrahippocampal inoculation of Aβ in the absence or presence of nimodipine. The effect of nimodipine on Aβ-triggered cytotoxicity was also investigated.

KEY RESULTS

We show here that nimodipine dose-dependently inhibited Aβ-stimulated IL-1β synthesis and release from primary microglia and microglia cell lines. Furthermore, nimodipine also inhibited Aβ-induced IL-1βin vivo accumulation at concentrations known to be reached in the CNS. Finally, nimodipine protected microglia from Aβ-dependent cytotoxicity.

CONCLUSION AND IMPLICATIONS

These data suggest that alleviation of symptoms of Alzheimer's disease following nimodipine administration might be due to an anti-inflammatory effect and point to a novel role for nimodipine as a centrally acting anti-inflammatory drug.

Trophic activity of a naturally occurring truncated isoform of the P2X7 receptor

P2X7 is the largest member of the P2X subfamily of purinergic receptors. A typical feature is the carboxyl tail, which allows formation of a large pore. Recently a naturally occurring truncated P2X7 splice variant, isoform B (P2X7B), has been identified. Here we show that P2X7B expression in HEK293 cells, a cell type lacking endogenous P2X receptors, mediated ATP-stimulated channel activity but not plasma membrane permeabilization, raised endoplasmic reticulum Ca(2+) content, activated the transcription factor NFATc1, increased the cellular ATP content, and stimulated growth. In addition, P2X7B-transfected HEK293 cells (HEK293-P2X7B), like most tumor cells, showed strong soft agar-infiltrating ability. When coexpressed with full-length P2X7 (P2X7A), P2X7B coassembled with P2X7A into a heterotrimer and potentiated all known responses mediated by this latter receptor. P2X7B mRNA was found to be widely distributed in human tissues, especially in the immune and nervous systems, and to a much higher level than P2X7A. Finally, P2X7B expression was increased on mitogenic stimulation of peripheral blood lymphocyte. Altogether, these data show that P2X7B is widely expressed in several human tissues, modulates P2X7A functions, participates in the control of cell growth, and may help understand the role of the P2X7 receptor in the control of normal and cancer cell proliferation.

Activation of microglia by amyloid {beta} requires P2X7 receptor expression

Extracellular ATP is a mediator of intercellular communication and a danger signal. Release of this and other nucleotides modulates microglia responses via P2Y and P2X receptors, among which the P2X(7) subtype stands out for its proinflammatory activity and for up-regulation in a transgenic model of Alzheimer disease and in brains from Alzheimer disease patients. Here we show that amyloid beta (Abeta) triggered increases in intracellular Ca(2+) ([Ca(2+)](i)), ATP release, IL-1beta secretion, and plasma membrane permeabilization in microglia from wild-type but not from P2X(7)-deleted mice. Likewise, intra-hippocampal injection of Abeta caused a large accumulation of IL-1beta in wild-type but not in P2X(7)(-/-) mice. These observations suggest that Abeta activates a purinergic autocrine/paracrine stimulatory loop of which the P2X(7) receptor is an obligate component. Identification of the P2X(7) receptor as a non-dispensable factor of Abeta-mediated microglia stimulation may open new avenues for the treatment of Alzheimer disease.

P2 purinoceptors in the immune system

Immune cells express plasma membrane receptors for extracellular nucleotides. Both G protein-linked metabotropic and channel-forming ionotropic receptors have been described, although no P2 receptor subtype has been cloned from the immune system thus far. Metabotropic receptors have been described in human B but not T lymphocytes; they have not been found in mouse B and T cells. Ionotropic receptors seem to be ubiquitously expressed in the immune system; however, their functional properties, if not their pharmacology, appear to be different in different immune cells. Human T normal and B leukaemic lymphocytes, human macrophages, mouse B and T lymphocytes, mouse microglial and macrophage cells, and rat mast cells express ionotropic receptors that recognize ATP4- as the preferred ligand, are activated by 3'-O-(4-benzoyl)benzoyl ATP and inhibited by oxidized ATP. The pharmacological profile of ionotropic receptors expressed by different immune cells is similar, but their permeability properties may be different: the pore formed by receptors expressed by macrophages, microglial cells and mast cells is typically permeable to charged molecules of molecular mass up to 900 Da; on the contrary, that expressed by lymphocytes has a molecular cut-off of 200-300 Da. The ionotrpic receptor of immune cells is modulated by inflammatory cytokines (e.g. interleukin [IL]-2 and gamma-interferon) and is also modulated during monocyte to macrophage differentiation. Transient stimulation of the ionotropic receptor of macrophages and microglial cells elicits IL-1 beta release. Sustained activation leads to cell death, either by necrosis or apoptosis, depending on the given cell type.

Multiple P2X receptors are involved in the modulation of apoptosis in human mesangial cells: evidence for a role of P2X4

Apoptosis, a normal event in renal tissue homeostasis, has been considered as a major mechanism for either resolution of glomerular hypercellularity in glomerulonephritis or loss of cellularity and progression to glomerulosclerosis in chronic renal disease. This study was aimed at investigating the role of extracellular ATP (eATP) in mediating apoptosis in human mesangial cells (HMC) and identifying the subtype(s) of purinergic receptors involved. eATP, but not uridin-5'-triphosphate (UTP), caused dose-dependent modifications of cellular morphology, as assessed by contrast-phase microscopy, and late apoptosis, as measured by Annexin V/propidium iodide-based flow cytometry and caspase-3 activation. Both phenomena were prevented by the P2X antagonist oxidized-ATP. 2', 3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (BzATP) was less effective than ATP, whereas 1[N,O-bis (5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl] -4-phenylpiperazine (KN62), a selective inhibitor of human P2X(7), prevented morphological changes but potentiated apoptosis induced by BzATP. P2X(7) was barely expressed in HMC and showed a relatively scarce functional activity, as assessed by monitoring nucleotide-induced intracellular calcium surge and plasma membrane depolarization by Fura-2/AM and bis[1,3-diethylthiobarbiturate]trimethineoxonal uptake, respectively. These data indicated a negligible role of P2X(7) in eATP-mediated apoptosis and pointed to the involvement of other P2X receptor(s). Molecular and inhibitor studies suggested a main role for P2X(4) receptor in nucleotide-induced apoptosis in HMC, indicating a relevant role for purinergic signaling in regulating death rate in these cells.

Modulation of P2X7 receptor functions by polymyxin B: crucial role of the hydrophobic tail of the antibiotic molecule

BACKGROUND AND PURPOSE

P2X7 is a membrane receptor for extracellular ATP which is highly expressed in dendritic cells, macrophages and microglia where it mediates pro-inflammatory responses. The antibiotic polymyxin B, which binds to and neutralizes the toxic residue of bacterial lipopolysaccharide, greatly amplifies cellular responses mediated by the P2X7 receptor. However, the molecular mechanism involved is so far unknown.

EXPERIMENTAL APPROACH

We investigated the effects of polymyxin B and polymyxin B nonapeptide (PMBN) which is the deacylated amino derivative of polymyxin B lacking the N-terminal fatty amino acid 6-methylheptanoic/octanoic-Dab residue, in human macrophages and HEK293 cells stably expressing the human P2X7 receptor (HEK293-hP2X7). Differences between the two antibiotics were assessed by monitoring the following: nucleotide-induced cytoplasmic free Ca2+ concentration changes, plasma membrane permeability changes, lactate dehydrogenase activity, cell morphology changes. Western blot and microscopic analyses of P2X7GFP-expressing cells were also performed.

KEY RESULTS

In contrast to polymyxin B, the polymyxin B nonapeptide was unable to potentiate: a) the ATP-induced Ca2+ increase, b) pore formation and consequently ATP-mediated plasma membrane permeabilization; c) ATP-dependent cytotoxicity. Moreover, in contrast to polymyxin B, polymyxin B nonapeptide did not affect aggregation of the P2X7 receptor subunits and it did not potentiate P2X7-dependent cell fusion.

CONCLUSIONS AND IMPLICATIONS

The effects of polymyxin B depended on the presence of its N-terminal fatty amino acid 6-methylheptanoic/octanoic-Dab residue as deletion of this residue abolished polymyxin B-dependent modulation of ATP-triggered responses. These findings are important in the search for allosteric modulators of the P2X7 receptor.

Involvement of the purinergic P2X7 receptor in the formation of multinucleated giant cells

Multinucleated giant cells (MGC), a hallmark of chronic inflammatory reactions, remain an enigma of cell biology. There is evidence implicating the purinergic P2X7 receptor in the fusion process leading to MGC. To investigate this, we used HEK 293 cells stably transfected with either 1) the full-length rat P2X7 receptor (P2X7 cells), 2) a rat P2X7 receptor lacking the C-terminal domain (P2X7TC), or 3) a mock vector, and rat alveolar macrophages (MA) expressing the native receptor. P2X7 cells cultured in serum-free medium formed increased numbers of MGC and displayed a higher fusion index compared with mock transfectants. Stimulation of P2X7 pore-forming activity in P2X7 cells by polymyxin B (PMB) further increased significantly the formation of MGC. Conversely, blockers of P2X-receptors including oxidized ATP, brilliant blue G, and pyridoxal phosphate-6-azophenyl-2'-4'-disulfonic acid inhibited significantly MGC formation in both unstimulated and PMB-stimulated P2X7-transfected cells. In contrast, cells transfected with the truncated P2X7TC were devoid of pore-forming activity, did not respond to PMB stimulation, and failed to form enhanced numbers of MGC, thus behaving as mock transfectants. As found for P2X7-transfected cells, PMB also potentiated dose-dependently the formation of multinucleated MA by rat alveolar MA. Pretreatment with oxidized ATP abrogated the PMB stimulatory effects. Together, these data demonstrate unequivocally the participation of P2X7 receptor in the process of MGC formation. Our study also provides evidence suggesting that stimulation of the P2X7 receptor pathway in MA may mediate increased formation of MGC during chronic inflammatory reactions.

Stimulation of P2 receptors causes release of IL-1beta-loaded microvesicles from human dendritic cells

Dendritic cells (DCs) are professional antigen-presenting cells that initiate the immune response by activating T lymphocytes. DCs express plasma membrane receptors for extracellular nucleotides named P2 receptors (P2Rs). Stimulation of P2Rs in these cells is known to cause chemotaxis, cytokine release, and cell death and to modulate LPS-dependent differentiation. Here we show that stimulation of the P2X(7) receptor subtype (P2X(7)R) causes fast microvesicle shedding from DC plasma membrane. Vesicle release occurs from both immature and mature DCs; however, only vesicles from mature DCs, due to their previous exposure to LPS, contain IL-1beta. Microvesicles, whether from immature or mature DCs, also contain caspase-1 and -3 and cathepsin D. They also express the P2X(7)R in addition to other P2Rs and known markers of immune cells such as major histocompatibility complex II (MHC II) and CD39. Activation of the P2X(7)R by extracellular ATP causes IL-1beta release from the vesicle lumen. Previous studies demonstrated that high extracellular K(+) inhibits IL-1beta processing and release; here we show that high ionic strength reduces microvesicle shedding when compared with a low ionic strength medium but strongly increases microvesicle IL-1beta loading.

The P2X7 receptor sustains the growth of human neuroblastoma cells through a substance P-dependent mechanism

P2X(7) is a receptor for extracellular nucleotides expressed by different normal cell types. P2X(7) triggering may result in stimulation of cell proliferation or induction of apoptosis depending on the level of activation. P2X(7) expression and function in B-cell chronic lymphocytic leukemia has been shown to correlate with disease severity. Here, we have asked the question of whether P2X(7) is expressed and functional in neuroblastoma, a pediatric tumor of neuroectodermal origin. P2X(7) was detected both in primary neuroblastoma tumors and in neuroblastoma cell lines. In the latter cells, P2X(7) stimulation by ATP was found to trigger (a) increased intracellular calcium fluxes, (b) plasma membrane depolarization, and (c) formation of a nonselective plasma membrane permeable pore. In contrast to the usual response typically observed in the majority of cell types, P2X(7) in vitro stimulation did not induce caspase-3 activation or apoptosis of neuroblastoma cells but rather supported their proliferation. Growth stimulation was partially due to substance P release from nucleotide-activated neuroblastoma cells. Therefore, neuroblastoma cells seem to have molded P2X(7) function to their advantage in two ways (i.e., by silencing P2X(7) proapoptotic activity and by coupling P2X(7) stimulation to release of locally acting trophic factors).

Selective Insulin Resistance Affecting Nitric Oxide Release But Not Plasminogen Activator Inhibitor-1 Synthesis in Fibroblasts From Insulin-Resistant Individuals

OBJECTIVE

Insulin activates several processes potentially dangerous for the arterial wall and hyperinsulinemia might be atherogenic. However, other insulin effects are protective for the vessel wall and thus anti-atherogenic. Aim of this study was to investigate whether insulin effects on potentially pro-atherogenic and anti-atherogenic processes were differently affected in cells from insulin-resistant individuals.

METHODS AND RESULTS

We determined insulin effect on nitric oxide (NO) production and plasminogen activator inhibitor (PAI)-1 synthesis in 12 fibroblast strains obtained from skin biopsy samples of 6 insulin-sensitive (IS) (clamp M >7 mg/kg body weight per minute) and 6 insulin-resistant (IR) (clamp M <5 mg/kg body weight per minute) healthy volunteers. Insulin effects on NO release and Akt phosphorylation were significantly impaired in fibroblasts from IR as compared with IS individuals. Conversely, there was not any difference between IR and IS strains in insulin ability to increase PAI-1 antigen levels and, after 24-hour insulin incubation, PAI-1 mRNA increase in IR strains was only slightly less than in IS strains. Insulin ability to induce MAPK activation was also comparable in IR and IS cells.

CONCLUSIONS

We conclude that in cells from IR individuals, insulin action on anti-atherogenic processes, such as NO release, is impaired, whereas the hormone ability to stimulate atherogenic processes, such as PAI-1 release, is preserved.

Purinergic modulation of mesangial extracellular matrix production: role in diabetic and other glomerular diseases

BACKGROUND

Extracellular adenosine triphosphate (ATP) (eATP) mediates several biologic activities via purinergic P2 receptors (P2Rs). This study aimed at (1) evaluating the role of the purinergic system in modulating mesangial extracellular matrix (ECM) and transforming growth factor-beta (TGF-beta) production and (2) its contribution to diabetes-induced mesangial ECM accumulation.

METHODS

Rat mesangial cells were grown in normal glucose (5.5 mmol/L) or high glucose (30 mmol/L) containing media and probed with purinergic agonists and antagonists for the assessment of the expression pattern and function of P2Rs; release of ATP and activity of ectoATPases; and changes in ECM and TGF-beta expression.

RESULTS

Cells cultured in normal glucose and high glucose expressed similar amounts of functional P2Rs of the P2X(2), P2X(3), P2X(4), P2X(5), P2X(7), P2Y(1), P2Y(2), P2Y(4), and P2Y(6) subtypes. Levels of eATP were higher in high glucose vs. normal glucose, with unchanged ectoATPase activity. The ATP-hydrolyzing enzymes hexokinase or apyrase reduced ECM and TGF-beta production from cells grown in high glucose, but not normal glucose. Under both normal glucose and high glucose conditions, ATP and the P2X(7) agonist benzoylbenzoylATP increased dose-dependently ECM and TGF-beta production, whereas the P2Y agonist uridine triphosphate (UTP) produced the opposite effect. The P2X(7) inhibitor oxidized ATP attenuated the ECM and TGF-beta up-regulation induced by ATP and, to a lesser extent, that caused by high glucose. A TGF-beta neutralizing antibody also prevented ATP-induced ECM up-regulation.

CONCLUSION

These data indicate a role for eATP in regulating ECM production via TGF-beta and suggest that P2XRs and P2YRs differentially modulate this process. An increased ATP release induced by hyperglycemia might contribute to mesangial matrix expansion occurring in diabetes.

Extracellular nucleotides are potent stimulators of human hematopoietic stem cells in vitro and in vivo

Although extracellular nucleotides support a wide range of biologic responses of mature blood cells, little is known about their effect on blood cell progenitor cells. In this study, we assessed whether receptors for extracellular nucleotides (P2 receptors [P2Rs]) are expressed on human hematopoietic stem cells (HSCs), and whether activation by their natural ligands, adenosine triphosphate (ATP) and uridine triphosphate (UTP), induces HSC proliferation in vitro and in vivo. Our results demonstrated that CD34(+) HSCs express functional P2XRs and P2YRs of several subtypes. Furthermore, stimulation of CD34(+) cells with extracellular nucleotides caused a fast release of Ca(2+) from intracellular stores and an increase in ion fluxes across the plasma membrane. Functionally, ATP and, to a higher extent, UTP acted as potent early acting growth factors for HSCs, in vitro, because they strongly enhanced the stimulatory activity of several cytokines on clonogenic CD34(+) and lineage-negative CD34(-) progenitors and expanded more primitive CD34(+)-derived long-term culture-initiating cells. Furthermore, xenogenic transplantation studies showed that short-term preincubation with UTP significantly expanded the number of marrow-repopulating HSCs in nonobese diabetic/severe combined immunodeficiency mice. Our data suggest that extracellular nucleotides may provide a novel and powerful tool to modulate HSC functions.

Enhanced P2X7 activity in human fibroblasts from diabetic patients: a possible pathogenetic mechanism for vascular damage in diabetes

OBJECTIVE

We have investigated expression and function of the P2X7 receptor in fibroblasts from healthy subjects and patients with type 2 diabetes.

METHODS AND RESULTS

Fibroblasts were isolated from skin biopsies. P2X7 receptor expression in both cell populations was measured by functional assays, RT-PCR, fluorescence-activated cell sorter, and immunoblotting. We found that fibroblasts from diabetic subjects are characterized by enhanced P2X7-mediated responses as indicated by increased shape changes, microvesiculation, enhanced fibronectin and interleukin 6 secretion, and accelerated apoptosis. These responses were blocked by preincubation with the P2X blockers KN-62, oxidized ATP, or pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid). Furthermore, we also found a higher level of spontaneous fibronectin secretion and of apoptosis in fibroblasts from diabetic compared with healthy subjects. Both higher basal level of fibronectin secretion and spontaneous rate of apoptosis were likely attributable to the increased pericellular concentration of ATP because fibroblasts from diabetic subjects released 3x as much ATP into the supernatants compared with fibroblasts from healthy subjects.

CONCLUSIONS

We conclude that fibroblasts from type 2 diabetes patients are characterized by a hyperactive purinergic loop based either on a higher level of ATP release or on increased P2X7 reactivity.

Defective P2Y purinergic receptor function: A possible novel mechanism for impaired glucose transport

Extracellular ATP is an ubiquitous mediator that regulates several cellular functions via specific P2 plasma membrane receptors (P2Rs), for which a role in modulating intracellular glucose metabolism has been recently suggested. We have investigated glucose uptake in response to P2Rs stimulation in fibroblasts from type 2 diabetic (T2D) patients and control subjects. P2Rs expression was evaluated by RT-PCR; intracellular calcium release by fluorometry; glucose transporter (GLUT1) translocation by immunoblotting and chemiluminescence; glucose uptake was measured with 2-deoxy-D-[1-(3)H]glucose (2-DOG) and ATP by luminometry. Cells from T2D patients, in contrast to those from healthy controls, showed no increase in glucose uptake after ATP stimulation; extracellular ATP caused, however, a similar GLUT1 recruitment to the plasma membrane in both groups. P2Rs expression did not differ between fibroblasts from diabetic and healthy subjects, but while plasma membrane depolarization, a P2X-mediated response was similar in both groups, no evident intracellular calcium increase was detectable in the cells from the former group. The calcium response in fibroblasts from diabetics was restored by co-incubation with apyrase or hexokinase, suggesting that P2YRs in those cells were normally expressed but chronically desensitised. In support to this finding, fibroblasts from T2D subjects secreted a two-fold larger amount of ATP compared to controls. Pre-treatment with apyrase or hexokinase also restored ATP stimulated glucose uptake in fibroblasts from diabetic subjects. These results suggest that extracellular ATP plays a role in the modulation of glucose transport via GLUT1, and that the P2Y-dependent GLUT1 activation is deficient in fibroblasts from T2D individuals. Our observations may point to additional therapeutic targets for improving glucose utilization in diabetes.

Extracellular ATP causes ROCK I-dependent bleb formation in P2X7-transfected HEK293 cells

The P2X7 ATP receptor mediates the cytotoxic effect of extracellular ATP. P2X7-dependent cell death is heralded by dramatic plasma membrane bleb formation. Membrane blebbing is a complex phenomenon involving as yet poorly characterized intracellular pathways. We have investigated the effect of extracellular ATP on HEK293 cells transfected with the cytotoxic/pore-forming P2X7 receptor. Addition of ATP to P2X7-transfected, but not to wt P2X7-less, HEK293 cells caused massive membrane blebbing within 1-2 min. UTP, a nucleotide incapable of activating P2X7, had no early effects on cell shape and bleb formation. Bleb formation triggered by ATP was reversible and required extracellular Ca2+ and an intact cytoskeleton. Furthermore, it was completely prevented by preincubation with the P2X blocker oxidized ATP. It was recently observed that the ROCK protein is a key determinant of bleb formation. Preincubation of HEK293-P2X7 cells with the ROCK blocker Y-27632 completely prevented P2X7-dependent blebbing. Although ATP triggered cleavage of the ROCK I isoform in P2X7-transfected HEK293 cells, the wide range caspase inhibitor z-VAD-fluoromethylketone had no effect. These observations suggest that P2X7-dependent plasma membrane blebbing depends on the activation of the serine/threonine kinase ROCK I.

Gamma-ray activity in the volcanic islands of the Southern Tyrrhenian Sea

Field gamma-ray spectrometry was used for the quantitative assessment of U, Th and K of rocks of Stromboli, Salina, Filicuidi and Panarea (Aeolian arc of the Southern Tyrrhenian, Italy). The air absorbed dose rate was calculated from radioelement concentrations. For some rocks the gamma-ray spectra were analysed with the three photo-peak methods and the response matrix method, which converts the pulse height distribution into the true incident gamma-ray energy spectrum. The higher values of U (8.2-9.8 ppm) coincide with higher Th (20.6-27.8 ppm) concentrations associated with rocks of shoshonitic composition. The spatial variation in radioelement concentration reflects the geochemical differences among the rocks. The air absorbed dose rate varies from 25 to 215 nGy h(-1). The highest values correspond to outcrops located in the eastern part of Stromboli, where the annual effective dose equivalent reaches a value of 264 microSv.

P2X7 receptor expression in evolutive and indolent forms of chronic B lymphocytic leukemia

Human leukocytes express a receptor for extracellular nucleotides, named P2X7R, that in lymphocytes can either mediate cell death or proliferation, depending on the level of activation. The authors have investigated P2X7R expression and function in 21 patients affected by B-cell chronic lymphocytic leukemia, 13 with an evolutive and 8 with an indolent variant of the disease. Resting cytoplasmic Ca++ concentration was significantly higher in lymphocytes from patients with the evolutive compared with indolent variant. Furthermore, in the former, P2X7R stimulation triggered a Ca++ influx significantly larger. Higher Ca++ influx correlated with an increased P2X7R expression in the lymphocytes from patients with the evolutive form. Finally, incubation in the presence of extracellular adenosine triphosphate decreased spontaneous proliferation of lymphocytes from patients affected with the evolutive variant but had no effects on lymphocytes from patients with the indolent form. These results suggest that expression and function of P2X7R may correlate with the severity of B-cell chronic lymphocytic leukemia.

Natural gamma-radiation in the Aeolian volcanic arc

Pulse-height distributions of gamma-rays, obtained with a field NaI(Tl) scintillation spectrometer in numerous sites of the Lipari and Vulcano islands (Aeolian volcanic arc, Italy), were measured to determine the U, Th and K concentrations of the bedrock and the relative values of the air absorbed dose rate. U is spatially related to both Th and K and the Th/U ratio is on average 3.1-3.5. The magmatic evolution is reflected by the concentration of the three radioelements, as they are more abundant within the more felsic units of the volcanic series. The higher values of U (15.7-20.0 ppm) coincide with higher Th (48.3-65.9 ppm) and K (4.9-6.1%) concentrations associated with rhyolitic rocks of the third cycle (< 50 ky). The air absorbed dose rate varies from 20 to 470 nGy h(-1). The highest values (> 350 nGy h(-1)) are observed on outcrops of rhyolitic obsidian lava flows. The cosmic-ray contribution is also evaluated to estimate the total background radiation dose rate.

A defect in glycogen synthesis characterizes insulin resistance in hypertensive patients with type 2 diabetes

A subgroup of patients with type 2 diabetes shows a clustering of abnormalities such as peripheral insulin resistance, hypertension, and microalbuminuria. To evaluate whether these traits reflect intrinsic disorders of cell function rather than in vivo environmental effects, we studied a group of 7 nondiabetic hypertensive subjects with an altered albumin excretion rate (AER) (HyMA+) and 3 groups of patients with type 2 diabetes: 7 with normal blood pressure and normal AER (DH-MA-), 7 with high blood pressure and normal AER (DH+MA-), and 7 with both high blood pressure and altered AER (DH+MA+). Glucose disposal was measured during an hyperinsulinemic clamp (40 mU. m(2)(-1). min(-1)) with primed deuterated [6.6 (2)H(2)] glucose infusion. In the same subjects, a skin biopsy was performed and the following parameters were investigated: glucose transport (as determined by [(3)H]2-deoxyglucose uptake); glycogen synthase activity (as determined by [(14)C] glucose incorporation from UDP-[U-(14)C] glucose into glycogen); glycogen phosphorylase activity (as measured by the incorporation of [U-(14)C]glucose 1-phosphate into glycogen); and total glycogen content. In vivo glucose disposal was significantly reduced in DH+MA- and DH+MA+, with respect to DH-MA-, HyMA+, and controls. Insulin-stimulated glucose transport was similar in the 3 groups of patients with diabetes. A significant reduction of intracellular glycogen content was observed in DH+MA- and DH+MA+ compared with DH-MA- in both basal and insulin-stimulated conditions, probably because of a major impairment of glycogen synthase activity. Glycogen phosphorylase activity did not show differences between the groups. These results suggest that (1) the combination of type 2 diabetes with hypertension and altered AER is associated with impaired insulin sensitivity, and (2) intrinsic, possibly genetic, factors may account for increased peripheral insulin resistance in hypertensive microalbuminuric patients with type 2 diabetes, pointing to the reduction of glycogen synthase activity as a shared common defect.

Nucleotide receptors: an emerging family of regulatory molecules in blood cells

Nucleotides are emerging as an ubiquitous family of extracellular signaling molecules. It has been known for many years that adenosine diphosphate is a potent platelet aggregating factor, but it is now clear that virtually every circulating cell is responsive to nucleotides. Effects as different as proliferation or differentiation, chemotaxis, release of cytokines or lysosomal constituents, and generation of reactive oxygen or nitrogen species are elicited upon stimulation of blood cells with extracellular adenosine triphosphate (ATP). These effects are mediated through a specific class of plasma membrane receptors called purinergic P2 receptors that, according to the molecular structure, are further subdivided into 2 subfamilies: P2Y and P2X. ATP and possibly other nucleotides are released from damaged cells or secreted via nonlytic mechanisms. Thus, during inflammation or vascular damage, nucleotides may provide an important mechanism involved in the activation of leukocytes and platelets. However, the cell physiology of these receptors is still at its dawn, and the precise function of the multiple P2X and P2Y receptor subtypes remains to be understood.

P2 purinergic receptors of human eosinophils: characterization and coupling to oxygen radical production

Extracellular nucleotides elicit multiple responses in eosinophils but no information on expression of purinergic receptors in these cells is available so far. In the present study we show that human eosinophils express the following P2Y and P2X subtypes: P2Y(1), P2Y(2), P2Y(4), P2Y(6), P2Y(11), and P2X(1), P2X(4), P2X(7), whose stimulation results in intracellular Ca(2+) increase and production of large amounts of reactive oxygen intermediates. These events are stimulated or inhibited, respectively, by P2 receptor agonists or antagonists.

The P2 purinergic receptors of human dendritic cells: identification and coupling to cytokine release

We investigated the expression of purinoceptors in human dendritic cells, providing functional, pharmacological, and biochemical evidence that immature and mature cells express P2Y and P2X subtypes, coupled to increase in the intracellular Ca(2+), membrane depolarization, and secretion of inflammatory cytokines. The ATP-activated Ca(2+) change was biphasic, with a fast release from intracellular stores and a delayed influx across the plasma membrane. A prolonged exposure to ATP was toxic to dendritic cells that swelled, lost typical dendrites, became phase lucent, detached from the substrate, and eventually died. These changes were highly suggestive of expression of the cytotoxic receptor P2X(7), as confirmed by ability of dendritic cells to become permeant to membrane impermeant dyes such as Lucifer yellow or ethidium bromide. The P2X(7) receptor ligand 2',3'-(4-benzoylbenzoyl)-ATP was a better agonist then ATP for Ca(2+) increase and plasma membrane depolarization. Oxidized ATP, a covalent blocker of P2X receptors, and the selective P2X(7) antagonist KN-62 inhibited both permeabilization and Ca(2+) changes induced by ATP. The following purinoceptors were expressed by immature and mature dendritic cells: P2Y(1), P2Y(2), P2Y(5), P2Y(11) and P2X(1), P2X(4), P2X(7). Finally, stimulation of LPS-matured cells with ATP triggered release of IL-1 beta and TNF-alpha. Purinoceptors may provide a new avenue to modulation of dendritic cells function.

High glucose modulates P2X7 receptor-mediated function in human primary fibroblasts

AIMS/HYPOTHESIS

Purinergic receptors are a family of newly characterized plasma membrane molecules involved in several and as yet only partially known cellular functions such as vascular reactivity, apoptosis and cytokine secretion. Little is known about the effect extracellular microenvironment has on their function. Fibroblasts share several features with smooth muscle cells and are an important constituent of the atherosclerotic plaque. Our aim was to evaluate the effect of high glucose concentration on ATP-mediated responses in human fibroblasts.

METHODS

Fibroblasts were obtained by skin biopsies and grown at two different glucose concentrations. We evaluated receptor expression by RT-PCR and immunoblotting and receptor localization by immunofluorescence. Plasma membrane potential and calcium changes were measured by fluorescent indicators. Apoptosis was determined by ethidium bromide staining and caspase-3 activation.

RESULTS

We show that cells grown in a medium with high glucose concentration underwent great ATP-mediated morphological changes, enhanced apoptosis, caspase 3 activation and interleukin-6 release. We identified P2X7 as the main purinergic receptor involved in these responses. Furthermore, high glucose concentration triggered the assembly of P2X7 into ring-like structures located at the periphery of the cells.

CONCLUSION/INTERPRETATION

Given that ATP is frequently released into the extracellular milieu upon cell and tissue damage, secretory exocytosis or activation of plasma membrane transporters, we hypothesize that ATP receptors participate in the pathogenesis of vascular complications of diabetes.

P2X(7) receptor and polykarion formation

Cell fusion is a central phenomenon during the immune response that leads to formation of large elements called multinucleated giant cells (MGCs) of common occurrence at sites of granulomatous inflammation. We have previously reported on the involvement in this event of a novel receptor expressed to high level by mononuclear phagocytes, the purinergic P2X(7) receptor. Herein, we show that blockade of this receptor by a specific monoclonal antibody prevents fusion in vitro. In contrast, cell fusion is stimulated by addition of enzymes that destroy extracellular ATP (i.e., apyrase or hexokinase). Experiments performed with phagocytes selected for high (P2X(7) hyper) or low (P2X(7) hypo) P2X(7) expression show that fusion only occurs between P2X(7) hyper/P2X(7) hyper and not between P2X(7) hyper/P2X(7) hypo or P2X(7) hypo/P2X(7) hypo. During MGCs formation we detected activation of caspase 3, an enzyme that is powerfully stimulated by P2X(7). Finally, we observed that during MGCs formation, the P2X(7) receptor is preferentially localized at sites of cell-to-cell contact. These findings support the hypothesis originally put forward by our group that the P2X(7) receptor participates in multinucleated giant cell formation.

Laboratory application of NaI(Tl) gamma-ray spectrometry to studies of natural radioactivity in geophysics

We implemented a laboratory apparatus based on a NaI(TI) gamma-ray spectrometer for the assessment of naturally occurring radioactive materials. This apparatus is devoted to the quantitative determination of U, Th and K in rock and soil samples. We analysed the calibration procedures with particular emphasis to factors which affect accuracy and detection limits. A first series of measurements were compared to those obtained by means of high energy gamma-ray spectrometry. A discussion was made on factors restricting the approach based on scintillation spectrometry, such as secular equilibrium and radon encapsulation.

Increased proliferation rate of lymphoid cells transfected with the P2X(7) ATP receptor

Human leukocytes can express the P2X(7) purinergic receptor, an ionic channel gated by extracellular ATP, for which the physiological role is only partially understood. Transfection of P2X(7) cDNA into lymphoid cells that lack this receptor sustains their proliferation in serum-free medium. Increased proliferation of serum-starved P2X(7) transfectants is abolished by the P2X(7) receptor blocker oxidized ATP or by the ATP hydrolase apyrase. Both wild type and P2X(7)-transfected lymphoid cells release large amounts of ATP into the culture medium. These data suggest the operation of an ATP-based autocrine/paracrine loop that supports lymphoid cell growth in the absence of serum-derived growth factors.

ATP receptors and giant cell formation

We have investigated the role of the purinergic P2X7 receptor in the formation of multinucleated giant cells in human monocyte/macrophage cultures stimulated with either concanavalin A or phytohemagglutinin. Macrophage fusion can be blocked by a P2X7-selective pharmacological antagonist or by a mAb directed against the extracellular P2X7 domain. Furthermore, macrophage cell clones expressing high P2X7 levels spontaneously fuse in culture, whereas macrophage clones lacking P2X7 are unable to fuse. Our findings suggest that the newly identified purinergic P2X7 receptor plays a central role in the complex chain of events leading to generation of macrophage-derived giant cells.

Mouse dendritic cells express the P2X7 purinergic receptor: characterization and possible participation in antigen presentation

Immune cells express P2 purinoceptors of the P2Y and P2X subtypes. In the present work, we show that three dendritic cell (DC) lines, D2SC/1, CB1, and FSDC, representative of immature DCs, express the P2X7 (formerly P2Z) receptor, as judged from RT-PCR amplification, reactivity to a specific antiserum, and pharmacological and functional evidence. Receptor expression is higher in FSDC cells, a cell line that is functionally more mature than D2SC/1 and CB1. From the wild-type DC population, we selected cell clones lacking the P2X7R (P2X7less). We also used a P2XR blocker, oxidized ATP, to irreversibly inhibit the P2X7R. Ability of P2X7less FSDCs or of oxidized ATP-inhibited FSDCs to stimulate Ag-specific TH lymphocytes was severely decreased although Ag endocytosis was minimally affected. During coculture with TH lymphocytes, wild-type FSDC secreted large amounts of IL-1beta. Release of this cytokine was reduced in P2X7less DCs. These data show that DCs express the P2X7 purinoceptor and suggest a correlation between P2X7R expression and Ag-presenting activity.

Mouse Dendritic Cells Express the P2X7 Purinergic Receptor: Characterization and Possible Participation in Antigen Presentation

Immune cells express P2 purinoceptors of the P2Y and P2X subtypes. In the present work, we show that three dendritic cell (DC) lines, D2SC/1, CB1, and FSDC, representative of immature DCs, express the P2X7 (formerly P2Z) receptor, as judged from RT-PCR amplification, reactivity to a specific antiserum, and pharmacological and functional evidence. Receptor expression is higher in FSDC cells, a cell line that is functionally more mature than D2SC/1 and CB1. From the wild-type DC population, we selected cell clones lacking the P2X7R (P2X7less). We also used a P2XR blocker, oxidized ATP, to irreversibly inhibit the P2X7R. Ability of P2X7less FSDCs or of oxidized ATP-inhibited FSDCs to stimulate Ag-specific TH lymphocytes was severely decreased although Ag endocytosis was minimally affected. During coculture with TH lymphocytes, wild-type FSDC secreted large amounts of IL-1β. Release of this cytokine was reduced in P2X7less DCs. These data show that DCs express the P2X7 purinoceptor and suggest a correlation between P2X7R expression and Ag-presenting activity.

Human primary fibroblasts in vitro express a purinergic P2X7 receptor coupled to ion fluxes, microvesicle formation and IL-6 release

We have investigated reponses to extracellular ATP in human fibroblasts obtained by skin biopsies. Our data show that these cells express a P2X7 purinergic receptor, as judged by (1) RT-PCR with specific primers, (2) reactivity with a specific anti-P2X7 antiserum, (3) activation by the selective P2X agonist benzoylbenzoylATP and (4) stimulation of transmembrane ion fluxes. Stimulation with benzoylbenzoylATP, and to a lesser extent with ATP, also caused striking morphological changes and increased formation of cytoplasmic microvesicles. These changes were fully reversible upon nucleotide removal. Two known blockers of P2X receptors, oxidised ATP and pyridoxalphosphate-6-azophenyl-2′,4′-disulfonic acid, inhibited the morphological changes fully and the ion fluxes partially. The residual rise in intracellular Ca2+ levels and membrane depolarization observed in the presence of the inhibitors were dependent upon activation of a P2Y-type receptor exhibiting a peculiar pharmacological profile, in that CTP was the preferred agonist. ATP stimulation triggered release of the pro-inflammatory cytokine IL-6 in fibroblasts pre-treated with PMA and bacterial endotoxin. These observations reveal a novel pathway for fibroblast activation and for their recruitment in the inflammatory response.

Tenidap enhances P2Z/P2X7 receptor signalling in macrophages

Tenidap is an anti-inflammatory drug whose mechanism of action is not fully understood. It has been shown to block plasma membrane anion transport and to decrease release of interleukin-1beta, probably via the inhibition of interleukin-1beta converting enzyme. In the present study we showed that: (a) tenidap increases the sensitivity of mouse macrophages to cytotoxic effects mediated by extracellular ATP; (b) tenidap increases lucifer yellow uptake through the macrophage ATP receptor; (c) pretreatment with oxidised ATP, a blocker of the P2Z/P2X7 receptor, inhibits cytotoxicity and lucifer yellow uptake due to the combined effects of ATP and tenidap; (d) macrophages lacking the P2Z/P2X7 receptor are resistant to the synergistic effect of tenidap and ATP. The results suggest that tenidap synergises with extracellular ATP for activation of the P2Z/P2X7 receptor.

Cytolytic P2X purinoceptors

Anecdoctal evidence accumulated over almost 20 years has shown that many different cell types are killed by sustained exposure to high concentrations of extracellular ATP. The plasma membrane receptors involved have been pharmacologically characterized and cloned during the last 3 years, and named purinergic P2X. P2X receptors share an intriguing structural relatedness with Caenorhabditis elegans degenerins and mammalian amiloride-sensitive Na channels (ENaCs). Depending on the ATP dose, length of stimulation and receptor subtype, P2X receptor stimulation may cause necrosis or apoptosis. The intracellular pathways activated are poorly known, but the perturbation in intracellular ion homeostasis clearly plays a major role. ICE proteases (caspases) are also triggered, nonetheless their activation is not requested for ATP-dependent cell death. The physiological meaning of P2X receptor-dependent cytotoxicity is not understood, but an involvement in immune-mediated reactions is postulated.

ATP-mediated cytotoxicity in microglial cells

Microglial cells are known to express purinergic receptors for extracellular ATP of both the P2Y and P2X subtypes. Functional studies have shown that both primary mouse microglial cells and the N9 and N13 microglial cell lines express the pore-forming P2Z/P2X7 receptor. Here we identify the presence of this receptor in N9 and N13 cells with a specific polyclonal Ab and show that microglial cells expressing the P2Z/P2X7 receptor are exquisitively sensitive to ATP-mediated cytotoxicity while clones selected for the lack of this receptor are resistant. Transfection of HEK293 cells with P2X7 (but not P2X2) receptor cDNA confers susceptibility to ATP-mediated cytotoxicity. Morphological and biochemical analysis suggests that ATP-dependent cell death in microglial cells occurs by apoptosis. Finally, microglial cells release ATP via a non-lytic mechanism when activated by bacterial endotoxin, thus suggesting the operation of a purinergic autocrine/paracrine loop.

Spontaneous cell fusion in macrophage cultures expressing high levels of the P2Z/P2X7 receptor

Mouse and human macrophages express a plasma membrane receptor for extracellular ATP named P2Z/P2X7. This molecule, recently cloned, is endowed with the intriguing property of forming an aqueous pore that allows transmembrane fluxes of hydrophylic molecules of molecular weight below 900. The physiological function of this receptor is unknown. In a previous study we reported experiments suggesting that the P2Z/P2X7 receptor is involved in the formation of macrophage-derived multinucleated giant cells (MGCs; Falzoni, S., M. Munerati, D. Ferrari, S. Spisani, S. Moretti, and F. Di Virgilio. 1995. J. Clin. Invest. 95:1207- 1216). We have selected several clones of mouse J774 macrophages that are characterized by either high or low expression of the P2Z/P2X7 receptor and named these clones P2Zhyper or P2Zhypo, respectively. P2Zhyper, but not P2Zhypo, cells grown to confluence in culture spontaneously fuse to form MGCs. As previously shown for human macrophages, fusion is inhibited by the P2Z/P2X7 blocker oxidized ATP. MGCs die shortly after fusion through a dramatic process of cytoplasmic sepimentation followed by fragmentation. These observations support our previous hypothesis that the P2Z/P2X7 receptor is involved in macrophage fusion.

Extracellular ATP triggers IL-1 beta release by activating the purinergic P2Z receptor of human macrophages

Extracellular ATP (ATPe) is known to cause release of processed IL-1 beta from LPS-treated macrophages and microglial cells. IL-1 beta release is fast and thought to be associated with cell death. We have reinvestigated this process to identify 1) the purinergic receptor involved; 2) the relationship to cell death; and 3) pharmacologic agonists or antagonists able to modulate IL-1 beta release. Our data confirm that ATPe is a powerful stimulus for IL-1 beta release from LPS-treated human macrophages; however, we also show that IL-1 beta release is not necessarily associated with cell death, as it occurs at lower ATP concentrations and much earlier than leakage of cytoplasmic markers. The selective purinergic P2Z receptor agonist benzoylbenzoyl ATP was at least one order of magnitude more powerful than ATP, but also had a strong cytotoxic effect. 2-Methylthio-ATP was equipotent as ATPe at the optimal concentration of 1 mM, but markedly inhibitory at higher concentrations. The irreversible P2Z blocker-oxidized ATP completely inhibited ATPe-induced IL-1 beta release. IL-1 beta release also was inhibited by increasing the K+ concentration of the incubation medium. These data suggest that ATPe triggers IL-1 beta via the purinergic P2Z receptor recently shown to be expressed by human macrophages and identified as a new member of the P2X family (P2X7), and provide pharmacologic tools for the modulation of IL-1 beta release in vitro and, possibly, in vivo.

Extracellular ATP triggers IL-1 beta release by activating the purinergic P2Z receptor of human macrophages

Extracellular ATP (ATPe) is known to cause release of processed IL-1 beta from LPS-treated macrophages and microglial cells. IL-1 beta release is fast and thought to be associated with cell death. We have reinvestigated this process to identify 1) the purinergic receptor involved; 2) the relationship to cell death; and 3) pharmacologic agonists or antagonists able to modulate IL-1 beta release. Our data confirm that ATPe is a powerful stimulus for IL-1 beta release from LPS-treated human macrophages; however, we also show that IL-1 beta release is not necessarily associated with cell death, as it occurs at lower ATP concentrations and much earlier than leakage of cytoplasmic markers. The selective purinergic P2Z receptor agonist benzoylbenzoyl ATP was at least one order of magnitude more powerful than ATP, but also had a strong cytotoxic effect. 2-Methylthio-ATP was equipotent as ATPe at the optimal concentration of 1 mM, but markedly inhibitory at higher concentrations. The irreversible P2Z blocker-oxidized ATP completely inhibited ATPe-induced IL-1 beta release. IL-1 beta release also was inhibited by increasing the K+ concentration of the incubation medium. These data suggest that ATPe triggers IL-1 beta via the purinergic P2Z receptor recently shown to be expressed by human macrophages and identified as a new member of the P2X family (P2X7), and provide pharmacologic tools for the modulation of IL-1 beta release in vitro and, possibly, in vivo.

Purinergic modulation of interleukin-1 beta release from microglial cells stimulated with bacterial endotoxin

Microglial cells express a peculiar plasma membrane receptor for extracellular ATP, named P2Z/P2X7 purinergic receptor, that triggers massive transmembrane ion fluxes and a reversible permeabilization of the plasma membrane to hydrophylic molecules of up to 900 dalton molecule weight and eventual cell death (Di Virgilio, F. 1995. Immunol. Today, 16:524-528). The physiological role of this newly cloned (Surprenant, A., F. Rassendren, E. Kawashima, R. A. North and G. Buell, 1996. Science (Wash. DC). 272:735-737) cytolytic receptor is unknown. In vitro and in vivo activation of the macrophage and microglial cell P2Z/P2X7 receptor by exogenous ATP causes a large and rapid release of mature IL-1 beta. In the present report we investigated the role of microglial P2Z/P2X7 receptor in IL-1 beta release triggered by LPS. Our data suggest that LPS-dependent IL-1 beta release involves activation of this purinergic receptor as it is inhibited by the selective P2Z/P2X7 blocker oxidized ATP and modulated by ATP-hydrolyzing enzymes such as apyrase or hexokinase. Furthermore, microglial cells release ATP when stimulated with LPS. LPS-dependent release of ATP is also observed in monocyte-derived human macrophages. It is suggested that bacterial endotoxin activates an autocrine/paracrine loop that drives ATP-dependent IL-1 beta secretion.