Extracellular ATP induces cytokine expression and apoptosis through P2X7 receptor in murine mast cells

Soybean lectin-triggered IL-6 secretion induces autophagy to kill intracellular mycobacteria through P2RX7 dependent activation of the JAK2/STAT3/Mcl-1 pathway

Cytokine therapy and cytokine-mediated autophagy have been used as prominent host-directed therapy (HDT) approaches to restrain M. tb growth in the host cell. In the present study, we have dissected the anti-tubercular activity of Soybean lectin (SBL) through cytokine-mediated autophagy induction in differentiated THP-1 (dTHP-1) cells. A significant increase in IL-6 expression was observed in both uninfected and mycobacteria infected dTHP-1 cells through the P2RX7 mediated pathway via PI3K/Akt/CREB-dependent signalling after SBL treatment. Inhibition of IL-6 level using IL-6 neutralizing antibody or associated signalling significantly enhanced the mycobacterial load in SBL-treated dTHP-1 cells. Further, autocrine signalling of IL-6 through its receptor-induced Mcl-1 expression activated autophagy via JAK2/STAT3 pathway, and inhibition of this pathway affected autophagy. Finally, blocking the IL-6-regulated autophagy through NSC 33994 (a JAK2 inhibitor) or S63845 (an Mcl-1 inhibitor) led to a notable increase in intracellular mycobacterial growth in SBL-treated cells. Taken together, these results indicate that SBL interacts with P2RX7 to regulate PI3K/Akt/CREB network to release IL-6 in dTHP-1 cells. The released IL-6, in turn, activates the JAK2/STAT3/Mcl-1 pathway upon interaction with IL-6Rα to modulate autophagy that ultimately controls mycobacterial growth in macrophages.

Triazoles with inhibitory action on P2X7R impaired the acute inflammatory response in vivo and modulated the hemostatic balance in vitro and ex vivo

OBJECTIVE

The present study aimed to investigate five triazole compounds as P2X7R inhibitors and evaluate their ability to reduce acute inflammation in vivo.

MATERIAL

The synthetic compounds were labeled 5e, 8h, 9i, 11, and 12.

TREATMENT

We administered 500 ng/kg triazole analogs in vivo, (1-10 µM) in vitro, and 1000 mg/kg for toxicological assays.

METHODS

For this, we used in vitro experiments, such as platelet aggregation, in vivo experiments of paw edema and peritonitis in mice, and in silico experiments.

RESULTS

The tested substances 5e, 8h, 9i, 11, and 12 produced a significant reduction in paw edema. Molecules 5e, 8h, 9i, 11, and 12 inhibited carrageenan-induced peritonitis. Substances 5e, 8h, 9i, 11, and 12 showed an anticoagulant effect, and 5e at a concentration of 10 µM acted as a procoagulant. All derivatives, except for 11, had pharmacokinetic, physicochemical, and toxicological properties suitable for substances that are candidates for new drugs. In addition, the ADMET risk assessment shows that derivatives 8h, 11, 5e, and 9i have high pharmacological potential. Finally, docking tests indicated that the derivatives have binding energies comparable to the reference antagonist with a competitive inhibition profile.

CONCLUSIONS

Together, the results indicate that the molecules tested as antagonist drugs of P2X7R had anti-inflammatory action against the acute inflammatory response.

Physiologic roles of P2 receptors in leukocytes

Since their discovery in the 1970s, purinergic receptors have been shown to play key roles in a wide variety of biologic systems and cell types. In the immune system, purinergic receptors participate in innate immunity and in the modulation of the adaptive immune response. In particular, P2 receptors, which respond to extracellular nucleotides, are widely expressed on leukocytes, causing the release of cytokines and chemokines and the formation of inflammatory mediators, and inducing phagocytosis, degranulation, and cell death. The activity of these receptors is regulated by ectonucleotidases-expressed in these same cell types-which regulate the availability of nucleotides in the extracellular environment. In this article, we review the characteristics of the main purinergic receptor subtypes present in the immune system, focusing on the P2 family. In addition, we describe the physiologic roles of the P2 receptors already identified in leukocytes and how they can positively or negatively modulate the development of infectious diseases, inflammation, and pain.

Analysis of Spatial and Temporal Distribution of Purinergic P2 Receptors in the Mouse Hippocampus

ATP and other nucleotides are important glio-/neurotransmitters in the central nervous system. They bind to purinergic P2X and P2Y receptors that are ubiquitously expressed in various brain regions modulating various physiological and pathophysiological processes. P2X receptors are ligand-gated ion channels mediating excitatory postsynaptic responses whereas P2Y receptors are G protein-coupled receptors mediating slow synaptic transmission. A variety of P2X and P2Y subtypes with distinct neuroanatomical localization provide the basis for a high diversity in their function. There is increasing evidence that P2 receptor signaling plays a prominent role in learning and memory and thus, in hippocampal neuronal plasticity. Learning and memory are time-of-day-dependent. Moreover, extracellular ATP shows a diurnal rhythm in rodents. However, it is not known whether P2 receptors have a temporal variation in the hippocampus. This study provides a detailed systematic analysis on spatial and temporal distribution of P2 in the mouse hippocampus. We found distinct spatial and temporal distribution patterns of the P2 receptors in different hippocampal layers. The temporal distribution of P2 receptors can be segregated into two large time domains, the early to mid-day and the mid to late night. This study provides an important basis for understanding dynamic P2 purinergic signaling in the hippocampal glia/neuronal network.

Hypermethylation downregulates P2X7 receptor expression in astrocytoma

The present study investigated the altered expression of p2X purinoceptor (P2X₇R) in astrocytoma. Reverse transcription-quantitative polymerase chain reaction and western blot analysis were used to determine the P2X₇R expression in glioblastoma (GBM) and surrounding normal brain tissue. DNA methylation levels of P2X₇R gene promoter in GBM were analyzed using a Sequenom MassARRAY^® System. Immunohistochemistry (IHC) was used to detect the expression of P2X₇R in astrocytoma at different malignancy grades, including diffuse astrocytoma, anaplastic astrocytoma and GBM. P2X₇R mRNA and protein were significantly decreased in GBM compared with normal brain tissues. IHC results showed a negative correlation between P2X₇R expression and tumor grade. The decreased P2X₇R expression was mostly attributed to hypermethylation of its promoter. Therefore, P2X₇R was found to perform an important role in tumorigenesis and progression of astrocytoma.

Silencing P2X7 receptor downregulates the expression of TCP-1 involved in lymphoma lymphatic metastasis

P2X₇R is an ATP-gated cation channel that participates in cell proliferation and apoptosis. TCP-1 assists with the protein folding. According to our previous research, the P2X₇R has a potential role in P388D1 lymphoid neoplasm cells dissemination to peripheral lymph nodes. In order to make a further exploration about the probable mechanism, the lymph nodes which metastasized by P2X₇R-silenced P388D1 cells or non-silenced cells were analyzed by 2DE and a MALDI-TOF-based proteomics approach. In the 64 proteins which were differentially expressed between two groups, TCP-1 was found to be significantly decreased in P2X₇R shRNA group compared to controls. This correlation was also found in subsequent experiments in vivo and in vitro. The positive correlation between P2X₇R and TCP-1 was also proved in both lymphoma and benign lymphadenopathy tissues from patients. It indicates that TCP-1 may be a crucial downstream molecular of P2X₇R and plays a novel role in lymphoid neoplasm metastasis.

Role of the purinergic signaling in epilepsy

Adenine nucleotides and adenosine are signaling molecules that activate purinergic receptors P1 and P2. Activation of A1 adenosine receptors has an anticonvulsant action, whereas activation of A2A receptors might initiate seizures. Therefore, a significant limitation to the use of A1 receptor agonists as drugs in the CNS might be their peripheral side effects. The anti-epileptic activity of adenosine is related to its increased concentration outside the cell. This increase might result from the inhibition of the equilibrative nucleoside transporters (ENTs). Moreover, the implantation of implants or stem cells into the brain might cause slow and persistent increases in adenosine concentrations in the extracellular spaces of the brain. The role of adenosine in seizure inhibition has been confirmed by results demonstrating that in patients with epilepsy, the adenosine kinase (ADK) present in astrocytes is the only purine-metabolizing enzyme that exhibits increased expression. Increased ADK activity causes intensified phosphorylation of adenosine to 5'-AMP, which therefore lowers the adenosine level in the extracellular spaces. These changes might initiate astrogliosis and epileptogenesis, which are the manifestations of epilepsy. Seizures might induce inflammatory processes and vice versa. Activation of P2X7 receptors causes intensified release of pro-inflammatory cytokines (IL-1β and TNF-α) and activates metabolic pathways that induce inflammatory processes in the CNS. Therefore, antagonists of P2X7 and the interleukin 1β receptor might be efficient drugs for recurring seizures and prolonged status epilepticus. Inhibitors of ADK would simultaneously inhibit the seizures, prevent the astrogliosis and epileptogenesis processes and prevent the formation of new epileptogenic foci. Therefore, these drugs might become beneficial seizure-suppressing drugs.

Co-Expression of Wild-Type P2X7R with Gln460Arg Variant Alters Receptor Function

The P2X7 receptor is a member of the P2X family of ligand-gated ion channels. A single-nucleotide polymorphism leading to a glutamine (Gln) by arginine (Arg) substitution at codon 460 of the purinergic P2X7 receptor (P2X7R) has been associated with mood disorders. No change in function (loss or gain) has been described for this SNP so far. Here we show that although the P2X7R-Gln460Arg variant per se is not compromised in its function, co-expression of wild-type P2X7R with P2X7R-Gln460Arg impairs receptor function with respect to calcium influx, channel currents and intracellular signaling in vitro. Moreover, co-immunoprecipitation and FRET studies show that the P2X7R-Gln460Arg variant physically interacts with P2X7R-WT. Specific silencing of either the normal or polymorphic variant rescues the heterozygous loss of function phenotype and restores normal function. The described loss of function due to co-expression, unique for mutations in the P2RX7 gene so far, explains the mechanism by which the P2X7R-Gln460Arg variant affects the normal function of the channel and may represent a mechanism of action for other mutations.

Ectonucleotidases and adenosine deaminase activity in laying hens naturally infected by Salmonella Gallinarum and their effects on the pathogenesis of the disease

Salmonella Gallinarum is the etiologic agent of fowl typhoid that affects chickens and turkeys causing egg production drops, infertility, lower hatchability, high mortality, and as a consequence severe economic losses to the poultry industry. The alterations in NTPDase and adenosine deaminase (ADA) activities have been demonstrated in several inflammatory conditions; however, there are no data in the literature associated with this infection. Thus, the aim of this study was to evaluate the activities of NTPDase, 5'nucleotidase, and ADA in serum and hepatic tissue of laying hens naturally infected by Salmonella Gallinarum. Liver and serum samples were collected of 27 laying hens (20 S. Gallinarum infected and 7 uninfected). NTPDase and 5'-nucleotidase activities in serum were increased (P < 0.001) in infected animals to hydrolysis of substrate ATP, ADP and AMP. In addition, it was observed decreased (P < 0.001) in ADA activity in serum of laying hens naturally infected by S. Gallinarum; as well as increased (P < 0.001) ADA activity in liver tissue of infected laying hens. Histopathological analyses revealed that S. Gallinarum caused fibrinoid necrosis in liver and spleen associated with infiltrates of heterophils, macrophages, lymphocytes, and plasma cells. Considering that NTPDase and ADA are involved in the cell-mediated immunity, this study suggests that activities of these enzymes could be important biomarkers to determine the severity of inflammatory and immune responses in salmonellosis, contributing to clarify the pathogenesis of the disease.

Rhein antagonizes P2X7 receptor in rat peritoneal macrophages

P2X7 receptor plays important roles in inflammation and immunity, and thereby it serves as a potential therapeutic target for inflammatory diseases. Rhein, an anthraquinone derivative, exhibits significant anti-inflammatory and immunosuppressive activities in therapy. However, the underlying mechanisms are largely unclear. Here, we aimed to investigate the effects of rhein on P2X7 receptor-mediated responses in vitro. In HEK293 cells expressing rat P2X7 receptor, we first found that rhein concentration-dependently blocked ATP-induced cytosolic calcium concentration ([Ca(2+)]c) elevation and pore formation of the plasma membrane, two hallmarks of the P2X7 receptor activation. These two inhibitory effects of rhein were also observed in rat peritoneal macrophages. Furthermore, rhein counteracted macrophage phagocytosis attenuation and suppressed reactive oxygen species (ROS) production triggered by ATP/BzATP. Meanwhile, rhein reduced ATP/BzATP-induced IL-1β release in lipopolysaccharide-activated macrophages. Prolonged application of ATP caused macrophage apoptosis, while the presence of rhein suppressed this cell cytotoxicity. Such ATP/BzATP-induced cellular reactions were also inhibited by a well-known rat P2X7 receptor antagonist, brilliant blue G, in a similar way to rhein. Together, our results demonstrate that rhein inhibit ATP/BzATP-induced [Ca(2+)]c increase, pore formation, ROS production, phagocytosis attenuation, IL-1β release and cell apoptosis by antagonizing the P2X7 receptor in rat peritoneal macrophages.

Silencing of MUC8 by siRNA increases P2Y₂-induced airway inflammation

Mucin hypersecretion and overproduction are frequent manifestations of respiratory disease. Determining the physiological function of airway mucin is presently considered more important than identifying the relevant signaling pathways. The lack of a full-length human mucin 8 (MUC8) cDNA sequence has hindered the generation of a Muc8 knockout mouse line. Thus, the precise physiological functions of MUC8 are unclear. Herein, we investigated the function of MUC8 using a small-interfering RNA (siRNA)-mediated genetic silencing approach in human airway epithelial cells. Herein, intracellular IL-1α production was stimulated by an ATP/P2Y2 complex. While ATP/P2Y₂ increased IL-1α secretion in a time-dependent manner, treatment with P2Y₂-specific siRNA significantly decreased IL-1α secretion. Moreover, ATP increased P2Y₂-mediated upregulation of MUC8 expression; however, IL-1α significantly decreased the extent to which ATP/P2Y₂ upregulated MUC8 expression. Interestingly, treatment with MUC8-specific siRNA decreased the production of anti-inflammatory cytokines (TGF-β and IL-1 receptor antagonist) and increased the production of inflammatory cytokines (IL-1α and IL-6) in our system. In addition, siRNA-mediated knockdown of MUC8 expression dramatically increased the secretion of inflammatory chemokines and resulted in an approximately threefold decrease in cell chemotaxis. We propose that MUC8 may function as an anti-inflammatory mucin that participates in inflammatory response by attracting immune cells/cytokines to the site of inflammation. Our results provide new insight into the physiological function of MUC8 and enhance our understanding of mucin overproduction during airway inflammation.

Purinergic signalling and immune cells

This review article provides a historical perspective on the role of purinergic signalling in the regulation of various subsets of immune cells from early discoveries to current understanding. It is now recognised that adenosine 5'-triphosphate (ATP) and other nucleotides are released from cells following stress or injury. They can act on virtually all subsets of immune cells through a spectrum of P2X ligand-gated ion channels and G protein-coupled P2Y receptors. Furthermore, ATP is rapidly degraded into adenosine by ectonucleotidases such as CD39 and CD73, and adenosine exerts additional regulatory effects through its own receptors. The resulting effect ranges from stimulation to tolerance depending on the amount and time courses of nucleotides released, and the balance between ATP and adenosine. This review identifies the various receptors involved in the different subsets of immune cells and their effects on the function of these cells.

Conductance simulation of the purinergic P2X2, P2X4, and P2X7 ionic channels using a combined Brownian dynamics and molecular dynamics approach

This paper investigates the application of an original combined approach of molecular and Brownian dynamic methods with quantum chemistry calculations for modeling the process of conductance of ion channels using purinergic P2X family receptors P2X2, P2X4, and P2X7 as a case study. A simplified model of the ionic channel in the lipid bilayer has been developed. A high level of conductance (30 pS) of P2X2 ionic channel together with the key role of Asp349 in forming the selectivity filter of P2X2 has been shown by using this approach. Calculated P2X2 permeability to monovalent cations Li(+), Na(+), and K(+) conforms to the free diffusion coefficient of these ions, which shows the low selectivity of P2X2 ionic channel.

Emodin regulates bone remodeling by inhibiting osteoclastogenesis and stimulating osteoblast formation

Bone remodeling, a physiological process in which new bone is formed by osteoblasts and the preexisting bone matrix is resorbed by osteoclasts, is vital for the maintenance of healthy bone tissue in adult humans. Imbalances in this process can cause various pathological conditions, including osteoporosis. Emodin, a naturally occurring anthraquinone derivative found in Asian herbal medicines, has numerous beneficial pharmacologic effects, including anticancer and antidiabetic activities. However, the effect of emodin on the regulation of osteoblast and osteoclast activity has not yet been investigated. We show here that emodin is a potential target for osteoporosis therapeutics, as treatment with this agent enhances osteoblast differentiation and bone growth and suppresses osteoclast differentiation and bone resorption. In this study, emodin suppressed receptor activator of nuclear factor-κB (NF-κB) ligand (RANKL)-induced osteoclast differentiation of bone marrow macrophages (BMMs) and the bone-resorbing activity of mature osteoclasts by inhibiting RANKL-induced NF-κB, c-Fos, and NFATc1 expression. Emodin also increased ALP, Alizarin Red-mineralization activity, and the expression of osteoblastogenic gene markers, such as Runx2, osteocalcin (OCN), and ALP in mouse calvarial primary osteoblasts, as well as activated the p38-Runx2 pathway, which enhanced osteoblast differentiation. Moreover, mice treated with emodin showed marked attenuation of lipopolysaccharide (LPS)-induced bone erosion and increased bone-forming activity in a mouse calvarial bone formation model based on micro-computed tomography and histologic analysis of femurs. Our findings reveal a novel function for emodin in bone remodeling, and highlight its potential for use as a therapeutic agent in the treatment of osteoporosis that promotes bone anabolic activity and inhibits osteoclast differentiation.

Emodin inhibits ATP-induced IL-1β secretion, ROS production and phagocytosis attenuation in rat peritoneal macrophages via antagonizing P2X₇ receptor

CONTEXT

Previous in vitro studies have demonstrated that emodin (1,3,8-trihydroxy-6-methyl-anthraquinone), an anthraquinone derivative from the rhizome of Rheum palmatum L., can inhibit the activation of P2X₇ receptors (P2X₇R) as a potential antagonist. However, the effects of emodin on P2X₇R-related inflammatory processes remain unclear.

OBJECTIVE

This study aimed to investigate the effects of emodin on different inflammation responses of macrophages induced by ATP, the natural ligand of P2X₇R.

MATERIALS AND METHODS

Rat peritoneal macrophages were treated with millimolar ATP and emodin (0.1, 0.3, 1, 3, 10 µM) or brilliant blue G (BBG, 0.1, 1, 10 µM). Cytosolic Ca²⁺ concentration ([Ca²⁺]c) was detected by fluorescent Ca²⁺ imaging. Interleukin-1β (IL-1β) release was measured by rat IL-1β ELISA kits. Reactive oxygen species (ROS) generation was examined by dihydroethidium (DHE) fluorescent staining. Phagocytic activity was tested by neutral red uptake assay.

RESULTS

We found that the [Ca²⁺](c) increase evoked by ATP (5 mM) was inhibited by emodin, in a dose-dependent manner with IC₅₀ of 0.5 μM. Furthermore, emodin reduced the IL-1β release induced by ATP (2 mM) in lipopolysaccharide (LPS)-activated macrophages, with an IC₅₀ of 1.6 μM. Emodin also strongly suppressed the ROS production and phagocytosis attenuation triggered by ATP (1 mM), with IC₅₀ values of 1 μM and 0.7 μM, respectively. Besides, BBG, a specific antagonist of P2X₇R, exhibited similar suppressive effects on these inflammation responses.

CONCLUSION

These results showed the inhibitory effects of emodin on ATP-induced [Ca²⁺](c) increase, IL-1β release, ROS production and phagocytosis attenuation in rat peritoneal macrophages, by inhibiting the activation of P2X₇R.

Purinergic receptor P2X₇: a novel target for anti-inflammatory therapy

Purinergic receptors, also known as purinoceptors, are ligand gated membrane ion channels involved in many cellular functions. Among all identified purinergic receptors, P2X₇ subform is unique since it induces the caspase activity, cytokine secretion, and apoptosis. The distribution of P2X₇ receptors, and the need of high concentration of ATP required to activate this receptor exhibited its ability to function as 'danger' sensor associated with tissue inflammation and damage. Further, the modulation of other signalling pathways associated with P2X₇ has also been proposed to play an important role in the control of macrophage functions and inflammatory responses, especially towards lipopolysaccharides. Experimentally, researchers have also observed the decreased severity of inflammatory responses in P2X₇ receptor expressing gene (P2RX₇) knockout (KO) phenotypes. Therefore, newly developed potent antagonists of P2X₇ receptor would serve as novel therapeutic agents to combat various inflammatory conditions. In this review article, we tried to explore various aspects of P2X₇ receptors including therapeutic potential, and recent discoveries and developments of P2X₇ receptor antagonists.

Physiological roles and potential therapeutic applications of the P2X7 receptor in inflammation and pain

The P2X7 receptor (P2X7R) is a nonselective cation channel that is activated by extracellular ATP and triggers the secretion of several proinflammatory substances, such as IL-1β, IL-18, TNF-α, and nitric oxide. Recently, several preclinical studies have demonstrated that this receptor participates in inflammation and pain mechanisms. Taken together, these results indicate that P2X7R is a promising pharmacological target, and compounds that modulate the function of this receptor show potential as new anti-inflammatory medicines. In this review, we discuss aspects of P2X7R pharmacology and the participation of this protein in inflammation and pain and provide an overview of some promising compounds that have been tested as antagonists of P2X7R, with clinical applicability.

Purinergic signaling in inflammatory cells: P2 receptor expression, functional effects, and modulation of inflammatory responses

Extracellular ATP and related nucleotides promote a wide range of pathophysiological responses via activation of cell surface purinergic P2 receptors. Almost every cell type expresses P2 receptors and/or exhibit regulated release of ATP. In this review, we focus on the purinergic receptor distribution in inflammatory cells and their implication in diverse immune responses by providing an overview of the current knowledge in the literature related to purinergic signaling in neutrophils, macrophages, dendritic cells, lymphocytes, eosinophils, and mast cells. The pathophysiological role of purinergic signaling in these cells include among others calcium mobilization, actin polymerization, chemotaxis, release of mediators, cell maturation, cytotoxicity, and cell death. We finally discuss the therapeutic potential of P2 receptor subtype selective drugs in inflammatory conditions.

Purinergic signaling in inflammatory renal disease

Extracellular purines have a role in renal physiology and adaption to inflammation. However, inflammatory renal disease may be mediated by extracellular purines, resulting in renal injury. The role of purinergic signaling is dependent on the concentrations of extracellular purines. Low basal levels of purines are important in normal homeostasis and growth. Concentrations of extracellular purines are significantly elevated during inflammation and mediate either an adaptive role or propagate local inflammation. Adenosine signaling mediates alterations in regional renal blood flow by regulation of the renal microcirculation, tubulo-glomerular feedback, and tubular transport of sodium and water. Increased extracellular ATP and renal P2 receptor-mediated inflammation are associated with various renal diseases, including hypertension, diabetic nephropathy, and glomerulonephritis. Experimental data suggests P2 receptor deficiency or receptor antagonism is associated with amelioration of antibody-mediated nephritis, suggesting a pathogenic (rather than adaptive) role of purinergic signaling. We discuss the role of extracellular nucleotides in adaptation to ischemic renal injury and in the pathogenesis of inflammatory renal disease.

Possible involvement of TLRs and hemichannels in stress-induced CNS dysfunction via mastocytes, and glia activation

In the central nervous system (CNS), mastocytes and glial cells (microglia, astrocytes and oligodendrocytes) function as sensors of neuroinflammatory conditions, responding to stress triggers or becoming sensitized to subsequent proinflammatory challenges. The corticotropin-releasing hormone and glucocorticoids are critical players in stress-induced mastocyte degranulation and potentiation of glial inflammatory responses, respectively. Mastocytes and glial cells express different toll-like receptor (TLR) family members, and their activation via proinflammatory molecules can increase the expression of connexin hemichannels and pannexin channels in glial cells. These membrane pores are oligohexamers of the corresponding protein subunits located in the cell surface. They allow ATP release and Ca²⁺ influx, which are two important elements of inflammation. Consequently, activated microglia and astrocytes release ATP and glutamate, affecting myelinization, neuronal development, and survival. Binding of ligands to TLRs induces a cascade of intracellular events leading to activation of several transcription factors that regulate the expression of many genes involved in inflammation. During pregnancy, the previous responses promoted by viral infections and other proinflammatory conditions are common and might predispose the offspring to develop psychiatric disorders and neurological diseases. Such disorders could eventually be potentiated by stress and might be part of the etiopathogenesis of CNS dysfunctions including autism spectrum disorders and schizophrenia.

Natural Products as a Source for New Anti-Inflammatory and Analgesic Compounds through the Inhibition of Purinergic P2X Receptors

Natural products have reemerged in traditional medicine as a potential source of new molecules or phytomedicines to help with health disorders. It has been established that members of the P2X subfamily, ATP-gated ion channels, are crucial to the inflammatory process and pain signalization. As such, several preclinical studies have demonstrated that P2X2R, P2X3R, P2X4R and P2X7R are promising pharmacological targets to control inflammatory and pain disorders. Several studies have indicated that natural products could be a good source of the new specific molecules needed for the treatment of diseases linked to inflammation and pain disorders through the regulation of these receptors. Herein, we discuss and give an overview of the applicability of natural products as a source to obtain P2X receptors (P2XR) selective antagonists for use in clinical treatment, which require further investigation.

The second transmembrane domain of P2X7 contributes to dilated pore formation

Activation of the purinergic receptor P2X7 leads to the cellular permeability of low molecular weight cations. To determine which domains of P2X7 are necessary for this permeability, we exchanged either the C-terminus or portions of the second transmembrane domain (TM2) with those in P2X1 or P2X4. Replacement of the C-terminus of P2X7 with either P2X1 or P2X4 prevented surface expression of the chimeric receptor. Similarly, chimeric P2X7 containing TM2 from P2X1 or P2X4 had reduced surface expression and no permeability to cationic dyes. Exchanging the N-terminal 10 residues or C-terminal 14 residues of the P2X7 TM2 with the corresponding region of P2X1 TM2 partially restored surface expression and limited pore permeability. To further probe TM2 structure, we replaced single residues in P2X7 TM2 with those in P2X1 or P2X4. We identified multiple substitutions that drastically changed pore permeability without altering surface expression. Three substitutions (Q332P, Y336T, and Y343L) individually reduced pore formation as indicated by decreased dye uptake and also reduced membrane blebbing in response to ATP exposure. Three others substitutions, V335T, S342G, and S342A each enhanced dye uptake, membrane blebbing and cell death. Our results demonstrate a critical role for the TM2 domain of P2X7 in receptor function, and provide a structural basis for differences between purinergic receptors.

ATP induces protein arginine deiminase 2-dependent citrullination in mast cells through the P2X7 purinergic receptor

Posttranslational modifications regulate physiology either by directly modulating protein function or by impacting immune recognition of self-proteins. Citrullination is a posttranslational modification formed by the conversion of arginine residues into the citrulline amino acid by protein arginine deiminase (PAD) family members. We have identified mast cells as a major source of the PAD2 enzyme. Activation of the P2X7 purinergic receptor (P2X7) by the inflammatory "danger" signal ATP induces PAD2 activity and robust protein citrullination. P2X7-mediated activation of PAD2 is sensitive to p38 MAPK and protein kinase C inhibitors, and PAD2 regulates the expression of the TNFR2, Adamts-9, and Rab6b transcripts in mast cells. Further, the PAD2 enzyme and its citrullinated substrate proteins are released from mast cells on activation with ATP. PAD2 expression is closely linked with inflammation in rheumatoid arthritis (RA) synovial tissue, and PAD2 and citrullinated proteins are found in the synovial fluid of RA patients. In addition, RA is associated with the development of autoantibodies to citrullinated self-proteins. Our results suggest that P2X7 activation of mast cells may play a role in inflammation by providing PAD2 and PAD2 substrates access to the extracellular space.

The His155Tyr (489C>T) single nucleotide polymorphism of P2RX7 gene confers an enhanced function of P2X7 receptor in immune cells from patients with rheumatoid arthritis

We assessed the possible association between several single nucleotide polymorphisms (SNP) of P2RX7 gene with systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). We determined the function of P2X7 receptor and the frequency of the 489C>T, 1096C>G, and 1513A>C SNP of P2RX7 gene in 111 and 122 patients with SLE and RA, and 98 healthy subjects. We found no significant association between the SNPs studied and SLE or RA. We also detected that lymphocytes from SLE and RA patients with the 489C>T SNP showed a higher ethidium bromide uptake in response to ATP than wild type or 1096C>G/1513A>C subjects. In addition, cells from RA patients and the 489C>T genotype, showed higher [Ca(2+)]i responses to ATP. Our data indicate that the 489C>T SNP of P2RX7 gene confers an enhanced function of this receptor in patients with RA, which may contribute to the pathogenesis of this condition.

Increased NTPDase activity in lymphocytes during experimental sepsis

We investigated in rats induced to sepsis the activity of ectonucleoside triphosphate diphosphohydrolase (NTPDase; CD39; E.C. 3.6.1.5), an enzyme involved in the modulation of immune responses. After 12 hours of surgery, lymphocytes were isolated from blood and NTPDase activity was determined. It was also performed the histology of kidney, liver, and lung. The results demonstrated an increase in the hydrolysis of adenosine-5′-triphosphate (ATP) (P < 0.01), but no changes regarding adenosine-5′-monophosphate (ADP) hydrolysis (P > 0.05). Histological analysis showed several morphological changes in the septic group, such as vascular congestion, necrosis, and infiltration of mononuclear cells. It is known that the intracellular milieu contains much more ATP nucleotides than the extracellular. In this context, the increased ATPasic activity was probably induced as a dynamic response to clean up the elevated ATP levels resulting from cellular death.

An ATP signalling pathway in plant cells: extracellular ATP triggers programmed cell death in Populus euphratica

We elucidated the extracellular ATP (eATP) signalling cascade active in programmed cell death (PCD) using cell cultures of Populus euphratica. Millimolar amounts of eATP induced a dose- and time-dependent reduction in viability, and the agonist-treated cells displayed hallmark features of PCD. eATP caused an elevation of cytosolic Ca(2+) levels, resulting in Ca(2+) uptake by the mitochondria and subsequent H(2) O(2) accumulation. P. euphratica exhibited an increased mitochondrial transmembrane potential, and cytochrome c was released without opening of the permeability transition pore over the period of ATP stimulation. Moreover, the eATP-induced increase of intracellular ATP, essential for the activation of caspase-like proteases and subsequent PCD, was found to be related to increased mitochondrial transmembrane potential. NO is implicated as a downstream component of the cytosolic Ca(2+) concentration but plays a negligible role in eATP-stimulated cell death. We speculate that ATP binds purinoceptors in the plasma membrane, leading to the induction of downstream intermediate signals, as the proposed sequence of events in PCD signalling was terminated by the animal P2 receptor antagonist suramin.

Mast cell function and death in Trypanosoma cruzi infection

Although the roles of mast cells (MCs) are essential in many inflammatory and fibrotic diseases, their role in Trypanosoma cruzi-induced cardiomyopathy is unexplored. In this study, we treated infected CBA mice with cromolyn, an MC stabilizer, and observed much greater parasitemia and interferon-γ levels, higher mortality, myocarditis, and cardiac damage. Although these data show that MCs are important in controlling acute infection, we observed MC apoptosis in the cardiac tissue and peritoneal cavity of untreated mice. In the heart, pericardial mucosal MC die, perhaps because of reduced amounts of local stem cell factor. Using RT-PCR in purified cardiac MCs, we observed that infection induced transcription of P2X(7) receptor and Fas, two molecules reportedly involved in cell death and inflammatory regulation. In gld/gld mice (FasL(-/-)), apoptosis of cardiac, but not peritoneal, MCs was decreased. Conversely, infection of P2X(7)(-/-) mice led to reduced peritoneal, but not cardiac, MC death. These data illustrate the immunomodulatory role played by MCs in T. cruzi infection and the complexity of molecular interactions that control inflammatory pathways in different tissues and compartments.

Receptor localization, native tissue binding and ex vivo occupancy for centrally penetrant P2X7 antagonists in the rat

BACKGROUND AND PURPOSE

The P2X7 receptor is implicated in inflammation and pain and is therefore a potential target for therapeutic intervention. Here, the development of a native tissue radioligand binding, localization and ex vivo occupancy assay for centrally penetrant P2X7 receptor antagonists is described.

EXPERIMENTAL APPROACH

Autoradiography studies using the P2X7 antagonist radioligand [³H]-A-804598 were carried out in rat brain and spinal cord. Subsequent in vitro binding and ex vivo occupancy assays were performed using rat cortex homogenate.

KEY RESULTS

P2X7 expression was shown to be widespread throughout the rat brain, and in the grey matter of the spinal cord. In binding assays in rat cortex homogenate, ∼60% specific binding was achieved at equilibrium. In kinetic binding assays, k(on) and k(off) values of 0.0021·min⁻¹·nM⁻¹ and 0.0070·min⁻¹ were determined, and the K(d) derived from kinetic measurements was consistent with that derived from saturation analysis. Novel P2X7 antagonists inhibited the binding of [³H]-A-804598 to rat cortex P2X7 receptors with K(i) values of <40 nM. In an ex vivo occupancy assay, a P2X7 antagonist dosed orally to rats caused a concentration-dependent inhibition of the specific binding of [³H]-A-804598 to rat cortex.

CONCLUSIONS AND IMPLICATIONS

The present study describes the development of an assay that allows localization of P2X7 receptors, the measurement of the binding affinity of P2X7 receptor antagonists in native tissue, and provides a means of determining central P2X7 receptor occupancy. These assays could form an important part of a P2X7 drug discovery programme.

C terminus of the P2X7 receptor: treasure hunting

P2X receptor (P2XR) is a family of the ATP-gated ion channel family and can permeabilize the plasma membrane to small cations such as potassium, sodium, and calcium, resulting in cellular depolarization. There are seven P2XR that have been described and cloned, with 45% identity in amino acid sequence. Each P2X receptors has two transmembrane domains that are separated by an extracellular loop and an intracellular N and C terminus. Unlike the other P2X receptors, the P2X7R has a larger C terminus with an extra 200 amino acid residues compared with the other receptors. The C terminus of the P2X7R has been implicated in regulating receptor function including signaling pathway activation, cellular localization, protein-protein interactions, and post-translational modification (PTM). In the present review, we discuss the role of the P2X7R C terminus in regards to receptor function, describe the specific domains and motifs found therein and compare the C terminus sequence with others proteins to discover predicted domains or sites of PTM.

Dangerous attraction: phagocyte recruitment and danger signals of apoptotic and necrotic cells

Tissue homeostasis in metazoa requires the rapid and efficient clearance of dying cells by professional or semi-professional phagocytes. Impairment of this finely regulated, fundamental process has been implicated in the development of autoimmune diseases, such as systemic lupus erythematosus. Various studies have provided us a detailed understanding of the interaction between dying cells and phagocytes as well as the current concept that apoptotic cell removal leads to a non- or anti-inflammatory response, whereas necrotic cell removal stimulates a pro-inflammatory reaction. In contrast, our knowledge about the soluble factors released from dying cells is rather limited, although meanwhile it is generally accepted that not only the dying cell itself but also the substances liberated during cell death contribute to the process of corpse clearance and the subsequent immune response. This review article is intended as an up-to-date survey over attraction and danger signals of apoptotic, primary and secondary necrotic cells, their function as chemoattractants in phagocyte recruitment, additional effects on the immune system, and the receptors, which are engaged in this scenario.

Extracellular high dosages of adenosine triphosphate induce inflammatory response and insulin resistance in rat adipocytes

Adenosine triphosphate (ATP), an important signaling molecule, participates in various pathophysiological processes via the activation of purinergic-receptors. Recent studies have shown that the expression and function of purinergic-receptors (P2-receptors) could be altered in diabetic or hyperinsulinemia conditions. To characterize the effect of ATP on insulin signaling, we treated primary rat adipocytes with varied concentrations of ATP. The pre-treatment led to impaired insulin signaling, i.e., blunted phosphorylation in Insulin Receptor Substrate-1 (IRS-1) tyrosine and Protein Kinase B (PKB) Ser473 in response to insulin treatment, when ATP concentration reached 1mM. We then observed that ATP dose-dependently reduced the level of IκB, a negative regulator of inflammatory response. Consistently, IRS-1 Ser307 phosphorylation in response to insulin treatment, a site for inflammatory pathway to interfere insulin signaling, was enhanced by ATP. Furthermore, effects of ATP on insulin signaling and IκB content were blocked by P2-receptor inhibition. Finally, insulin-stimulated glucose uptake was impaired by ATP in adipocytes but not in the L6 muscle cells. This study therefore shows for the first time the involvement of ATP-evoked P2-receptor activation in mediating the inflammatory response and the generation of insulin resistance in adipocytes.

Inhibition of ATP-induced macrophage death by emodin via antagonizing P2X7 receptor

Emodin (1,3,8-trihydroxy-6-methylanthraquinone), an anthraquinone derivative from Rheum officinale Baill, exhibits anti-inflammatory and immunosuppressive activities, however, the underlying mechanisms are not fully understood. This study examined the effects of emodin on ATP-evoked responses in rat peritoneal macrophages and in human embryonic kidney 293 cells (HEK293) heterologously expressing the cloned rat P2X7 receptor. Emodin reduced macrophage death induced by millimolar ATP in a concentration-dependent manner with the half of maximal inhibition values (IC50) of 0.2 microM. It also strongly inhibited ATP-induced dye uptake or pore formation, a hallmark property associated with P2X7 receptor activation, and 2',3'-O-(benzoyl-4-benzoyl)-ATP (BzATP) induced increases in intracellular Ca2+ concentrations in macrophages with an IC50 of 0.5 microM. Furthermore, emodin significantly suppressed BzATP-evoked currents in P2X7 receptor expressing HEK293 cells with an IC50 of 3.4 microM. Taken together, these results provide compelling evidence for a novel action of emodin as a P2X7 receptor antagonist, which may underlie its anti-inflammatory and immunosuppressive activities.

Induction of IL-13 production and upregulation of gene expression of protease activated receptors in P815 cells by IL-6

Interleukin (IL)-6 is a multifunctional cytokine which has been showed to induce up-regulated expression of Fc epsilon RI receptor and histamine production in mast cells. However, little is known of its effects on Th2 cytokine secretion and protease activated receptor (PAR) expression in mast cells. In the present study, we examined potential influence of IL-6 on IL-13, IL-4 and IL-10 release from P815 cells and PAR expression on P815 cells by using flow cytometry analysis, quantitative real-time PCR, ELISA and cellular activation of signaling ELISA (CASE) techniques. The results showed that IL-6 induced up to 1.8-fold increase in IL-13, but not IL-4 or IL-10 release from P815 cells, and FSLLRY-NH(2) did not affect IL-6 induced IL-13 release. Tryptase elicited 2.0-fold increase in IL-13 release from P815 cells, which can be inhibited by IL-6. IL-6 elicited the up-regulated expression of PAR-1, PAR-2, PAR-3 and PAR-4 mRNAs, but had little effects on expression of PAR proteins. U0126, PD98059 and LY204002 abolished IL-6 induced IL-13 release when they were preincubated with P815 cells, indicating ERK and Akt cell signaling pathways may be involved in the event. In conclusion, IL-6 can stimulate IL-13 release from mast cells through an ERK and Akt cell signaling pathway dependent, but PAR independent mechanism.

P2 receptor-mediated signaling in mast cell biology

Mast cells are widely recognized as effector cells of allergic inflammatory reactions. They contribute to the pathogenesis of different chronic inflammatory diseases, wound healing, fibrosis, thrombosis/fibrinolysis, and anti-tumor immune responses. In this paper, we summarized the role of P2X and P2Y receptors in mast cell activation and effector functions. Mast cells are an abundant source of ATP which is stored in their granules and secreted upon activation. We discuss the contribution of mast cells to the extracellular ATP release and to the maintenance of extracellular nucleotides pool. Recent publications highlight the importance of purinergic signaling for the pathogenesis of chronic airway inflammation. Therefore, the role of ATP and P2 receptors in allergic inflammation with focus on mast cells was analyzed. Finally, ATP functions as mast cell autocrine/paracrine factor and as messenger in intercellular communication between mast cells, nerves, and glia in the central nervous system.

Identification and characterization of a novel variant of the human P2X(7) receptor resulting in gain of function

The P2X(7) receptor exhibits significant allelic polymorphism in humans, with both loss and gain of function variants potentially impacting on a variety of infectious and inflammatory disorders. At least five loss-of-function polymorphisms (G150R, R307Q, T357S, E496A, and I568N) and two gain-of-function polymorphisms (H155Y and Q460R) have been identified and characterized to date. In this study, we used RT-PCR cloning to isolate and characterize P2X(7) cDNA clones from human PBMCs and THP-1 cells. A previously unreported variant with substitutions of V80M and A166G was identified. When expressed in HEK293 cells, this variant exhibited heightened sensitivity to the P2X(7) agonist (BzATP) relative to the most frequent allele, as shown by pore formation measured by fluorescent dye uptake into cells. Mutational analyses showed that A166G alteration was critical for the gain-of-function change, while V80M was not. Full-length variants with multiple previously identified nonsynonymous SNPs (H155Y, H270R, A348T, and E496A) were also identified. Distinct functional phenotypes of the P2X(7) variants or mutants constructed with multiple polymorphisms were observed. Gain-of-function variations (A166G or H155Y) could not rescue the loss-of-function E496A polymorphism. Synergistic effects of the gain-of-function variations were also observed. We also identified the A348T alteration as a weak gain-of-function variant. Thus, these results identify the new gain-of-function variant A166G and demonstrate that multiple-gene polymorphisms contribute to functional phenotypes of the human P2X(7) receptor. Furthermore, the results demonstrate that the C-terminal of the cysteine-rich domain 1 of P2X(7) is critical for regulation of P2X(7)-mediated pore formation.

Functional evidence for the expression of P2X1, P2X4 and P2X7 receptors in human lung mast cells

BACKGROUND AND PURPOSE

P2X receptors are widely expressed in cells of the immune system with varying functions. This study sought to characterize P2X receptor expression in the LAD2 human mast cell line and human lung mast cells (HLMCs).

EXPERIMENTAL APPROACH

Reverse transcriptase polymerase chain reaction (RT-PCR) and patch clamp studies were used to characterize P2X expression in mast cells using a range of pharmacological tools.

KEY RESULTS

RT-PCR revealed P2X1, P2X4 and P2X7 transcripts in both cell types; mRNA for P2X6 was also detected in LAD2 cells. Under whole-cell patch clamp conditions, rapid application of ATP (1-1000 microM) to cells clamped at -60 mV consistently evoked inward currents in both types of cells. Brief application of ATP (1 s) evoked a rapidly desensitizing P2X1-like current in both cell types. This current was also elicited by alphabetamethylene ATP (10 microM, 94% cells, n= 31) and was antagonized in LAD2 cells by NF 449 (1 microM) and pyridoxal phosphate-6-azo(benzene-2,4-disulphonic acid) (1-10 microM). A P2X7-like non-desensitizing current in response to high concentrations of ATP (1-5 mM) was also seen in both cell types (96% LAD2, n= 24; 54% HLMCs, n= 24) which was antagonized by AZ11645373 (1 microM). P2X7-like responses were also evoked in LAD2 cells by 2'(3')-0-(4-benzoylbenzoyl)ATP (300 microM). A P2X4-like current was evoked by 100 microM ATP (80% LAD2, n= 10; 21% HLMCs, n= 29), the amplitude and duration of which was potentiated by ivermectin (3 microM).

CONCLUSION AND IMPLICATIONS

Our data confirmed the presence of functional P2X1, P2X4 and P2X7 receptors in LAD2 cells and HLMCs.

Functional decreases in P2X7 receptors are associated with retinoic acid-induced neuronal differentiation of Neuro-2a neuroblastoma cells

Neuro-2a (N2a) cells are derived from spontaneous neuroblastoma of mouse and capable to differentiate into neuronal-like cells. Recently, P2X7 receptor has been shown to sustain growth of human neuroblastoma cells but its role during neuronal differentiation remains unexamined.We characterized the role of P2X7 receptors in the retinoic acid (RA)-differentiated N2a cells. RA induced N2a cells differentiation into neurite bearing and neuronal specific proteins, microtubule-associated protein 2 (MAP2) and neuronal specific nuclear protein (NeuN), expressing neuronal-like cells. Interestingly, the RA-induced neuronal differentiation was associated with decreases in the expression and function of P2X7 receptors. Functional inhibition of P2X7 receptors by P2X7 receptor selective antagonists, 5'-triphosphate, periodate-oxidized 2',3'-dialdehyde ATP (oATP), brilliant blue G (BBG) or A438079 induced neurite outgrowth. In addition, RA and oATP treatment stimulated the expression of neuron-specific class III beta-tubulin (TuJ1), and knockdown of P2X7 receptor expression by siRNA induced neurite outgrowth. To elucidate the possible mechanism, we found the levels of basal intracellular Ca2+ concentrations ([Ca2+]i) were decreased in either RA- or oATP-differentiated or P2X7receptor knockdown N2a cells. Simply cultured N2a cells in low Ca2+ medium induced a 2-fold increase in neurite length. Treatment of N2a cells with ATP hydrolase apyrase and the P2X7 receptors selective antagonist oATP or BBG decreased cell viability and cell number. Nevertheless, oATP but not BBG decreased cell proliferation and cell cycle progression. These results suggest for the first time that decreases in expression/function of P2X7 receptors are involved in neuronal differentiation.We provide additional evidence shown that the ATP release-activated P2X7 receptor is important in maintaining cell survival of N2a neuroblastoma cells.

Mast cells mediate neutrophil recruitment and vascular leakage through the NLRP3 inflammasome in histamine-independent urticaria

Urticarial rash observed in cryopyrin-associated periodic syndrome (CAPS) caused by nucleotide-binding oligomerization domain-leucine-rich repeats containing pyrin domain 3 (NLRP3) mutations is effectively suppressed by anti-interleukin (IL)-1 treatment, suggesting a pathophysiological role of IL-1beta in the skin. However, the cellular mechanisms regulating IL-1beta production in the skin of CAPS patients remain unclear. We identified mast cells (MCs) as the main cell population responsible for IL-1beta production in the skin of CAPS patients. Unlike normal MCs that required stimulation with proinflammatory stimuli for IL-1beta production, resident MCs from CAPS patients constitutively produced IL-1beta. Primary MCs expressed inflammasome components and secreted IL-1beta via NLRP3 and apoptosis-associated speck-like protein containing a caspase recruitment domain when stimulated with microbial stimuli known to activate caspase-1. Furthermore, MCs expressing disease-associated but not wild-type NLRP3 secreted IL-1beta and induced neutrophil migration and vascular leakage, the histological hallmarks of urticarial rash, when transplanted into mouse skin. Our findings implicate MCs as IL-1beta producers in the skin and mediators of histamine-independent urticaria through the NLRP3 inflammasome.

Expression of purinergic receptors and modulation of P2X7 function by the inflammatory cytokine IFNgamma in human epithelial cells

The cervical epithelial cell line, HeLa, is one of the oldest and most commonly used cell lines in cell biology laboratories. Although a truncated P2X(7) receptor has recently been identified in HeLa cells, the expression of other purinergic receptors or the function of the P2X(7) protein has not been characterized. We here show that HeLa cells express transcripts for most P2X and P2Y purinergic receptors. Treatment of cells with ATP or other P2X(7) agonists does not stimulate cell death, but can induce atypical calcium fluxes and ion currents. Cervical epithelial cells represent an important target for sexually-transmitted pathogens and are commonly exposed to pro-inflammatory cytokines such as IFNgamma. Stimulation of HeLa cells with IFNgamma upregulates expression of P2X(7) mRNA and full-length protein, modifies ATP-dependent calcium fluxes, and renders the cells sensitive to ATP-induced apoptosis, which can be blocked by a P2X(7) antagonist. IFNgamma treatment also increased dramatically the sensitivity of the intestinal epithelial cell line, HCT8, to ATP-induced apoptosis. Significantly, IFNgamma also stimulated P2X(7) expression on human intestinal tissues. Responses to other purinergic receptor ligands suggest that HeLa cells may also express functional P2Y(1), P2Y(2) and P2Y(6) receptors, which could be relevant for modulating ion homeostasis in the cells.

Lymphocytes from P2X7-deficient mice exhibit enhanced P2X7 responses

The purinergic receptor P2X(7) is expressed on immune cells, and its stimulation results in the release of IL-1beta from macrophages. Its absence, as evidenced from the analysis of two independent strains of P2X(7)-deficient mice, results in reduced susceptibility to inflammatory disease, and the molecule is an important, potential therapeutic target in autoimmunity. However, P2X(7) has also been detected in several neuronal cell types, although its function and even its presence in these cells are highly contested, with anti-P2X(7) antibodies staining brain tissue from both strains of P2X(7)(-/-) mice identically to wild-type mice. It has therefore been suggested that neurons express a distinct "P2X(7)-like" protein that has similar antibody recognition epitopes to P2X(7) and some properties of the genuine receptor. In this study, we show that whereas P2X(7) activity is absent from macrophages and dendritic cells in P2X(7)(-/-) animals, T cells from one gene-deficient strain unexpectedly exhibit higher levels of P2X(7) activity than that found in cells from control, unmanipulated C57BL/6 mice. A potential mechanism for this tissue-specific P2X(7) expression in P2X(7)(-/-) animals is discussed, as is the implication that the immune and indeed neuronal functions of P2X(7) may have been underestimated.

Atypical P2X receptor pharmacology in two human osteoblast-like cell lines

BACKGROUND AND PURPOSE

The expression and function of P2X(7) receptors in osteoclasts is well established, but less is known about their role in osteoblast-like cells. A study in P2X(7) receptor knockout mice suggested the involvement of these receptors in bone formation. We have investigated the expression and pharmacology of several P2X receptors in two human osteosarcoma cell lines to see if they could be involved in bone turnover in man.

EXPERIMENTAL APPROACH

Reverse transcriptase-polymerase chain reaction and Western blotting were used to study P2X(2), P2X(4) and P2X(7) receptor expression at mRNA and protein levels, respectively, in human osteoblast-like cells. P2X(7) receptor pharmacology was studied by measuring pore formation in the presence of different agonists and antagonists using the YO-PRO 1 uptake method.

KEY RESULTS

P2X(4) and P2X(7) receptor mRNA and protein were found to be expressed by these cell lines. No evidence was found for P2X(4)/P2X(7) receptor heteropolymerization. 2'-3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (DBzATP) was equipotent to ATP and the antagonists used were either ineffective or weakly blocked pore formation.

CONCLUSIONS AND IMPLICATIONS

This study demonstrates that P2X(4) and P2X(7) receptors are expressed by human osteoblast-like cells. The affinities of the different agonists suggest that the P2X(7) receptor is mainly responsible for pore formation although P2X(4) receptors may also be involved. The low affinity of DBzATP and the weak action of the antagonists support the previously described atypical pharmacology of the P2X(7) receptor in osteoblasts. Targeting the P2X(7) receptor in osteoblasts could represent a promising new treatment for bone diseases such as osteoporosis and rheumatoid arthritis.

Mast cell-expressed orphan receptor CCRL2 binds chemerin and is required for optimal induction of IgE-mediated passive cutaneous anaphylaxis

Mast cells contribute importantly to both protective and pathological IgE-dependent immune responses. We show that the mast cell–expressed orphan serpentine receptor mCCRL2 is not required for expression of IgE-mediated mast cell–dependent passive cutaneous anaphylaxis but can enhance the tissue swelling and leukocyte infiltrates associated with such reactions in mice. We further identify chemerin as a natural nonsignaling protein ligand for both human and mouse CCRL2. In contrast to other “silent” or professional chemokine interreceptors, chemerin binding does not trigger ligand internalization. Rather, CCRL2 is able to bind the chemoattractant and increase local concentrations of bioactive chemerin, thus providing a link between CCRL2 expression and inflammation via the cell-signaling chemerin receptor CMKLR1.

The nucleotide receptor P2RX7 mediates ATP-induced CREB activation in human and murine monocytic cells

Nucleotide receptors serve as sensors of extracellular ATP and are important for immune function. The nucleotide receptor P2RX7 is a cell-surface, ligand-gated cation channel that has been implicated in many diseases, including arthritis, granuloma formation, sepsis, and tuberculosis. These disorders are often exacerbated by excessive mediator release from activated macrophages in the inflammatory microenvironment. Although P2RX7 activation can modulate monocyte/macrophage-induced inflammatory events, the relevant molecular mechanisms are poorly understood. Previous studies suggest that MAPK cascades and transcriptional control via CREB-linked pathways regulate the inflammatory capacity of monocytic cells. As P2RX7 promotes MAPK activation and inflammatory mediator production, we examined the involvement MAPK-induced CREB activation in P2RX7 action. Our data reveal that stimulation of multiple monocytic cell lines with P2RX7 agonists induces rapid CREB phosphorylation. In addition, we observed a lack of nucleotide-induced CREB phosphorylation in RAW 264.7 cells expressing nonfunctional P2RX7 and a gain of nucleotide-induced CREB phosphorylation in human embryonic kidney-293 cells that heterologously express human P2RX7. Furthermore, our results indicate that P2RX7 agonist-induced CREB phosphorylation is partly mediated via Ca(2+) fluxes and the MEK/ERK system. Mechanistic analyses revealed that macrophage stimulation with a P2RX7 agonist induces CREB/CREB-binding protein complex formation, which is necessary for CREB transcriptional activation. Also, we demonstrate that P2RX7 activation induces a known CREB-dependent gene (c-fos) and that dominant-negative CREB constructs attenuate this response. These studies support the idea that P2RX7 stimulation can directly regulate protein expression that is not dependent on costimulation with other immune modulators such as LPS.

Extracellular ATP and cancer: an overview with special reference to P2 purinergic receptors

Purinergic signal transduction mechanisms have been appreciated as a complex intercellular signalling network that plays an important regulatory role in both short- and long-term processes in practically every living cell. One of the most intriguing aspects of the field is the participation of ATP and other purine nucleotides in the determination of cell fate and the way they direct cells towards proliferation, differentiation or apoptosis, thereby possibly taking part in promoting or preventing malignant transformation. In this review, following a very brief introduction to the historical aspects of purinergic signalling and a concise overview of the structure of and signal transduction pathways coupled to P2 purinergic receptors, the current theories concerning the possible ways how extracellular ATP can alter the function of tumour cells and the effectiveness of anticancer therapies are discussed, including pharmacological, nutritional, vasoactive and 'anti-antioxidant' actions of the nucleotide. The effects of ATP on animals inoculated with human tumours and on patients with cancer are looked over next, and then an overview of the literature regarding the expression and presumed functions of P2 purinoceptors on tumour cells in vitro is presented, sorted out according to the relevant special clinical fields. The article is closed by reviewing the latest developments in the diagnostic use of P2 purinergic receptors as tumour markers and prognostic factors, while discussing some of the difficulties and pitfalls of the therapeutic use of ATP analogues.