The purinergic receptor P2X7 as a modulator of viral vector-mediated antigen cross-presentation

Introduction

Modified Vaccinia Virus Ankara (MVA) is a safe vaccine vector inducing long- lasting and potent immune responses. MVA-mediated CD8+T cell responses are optimally induced, if both, direct- and cross-presentation of viral or recombinant antigens by dendritic cells are contributing.

Methods

To improve the adaptive immune responses, we investigated the role of the purinergic receptor P2X7 (P2RX7) in MVA-infected feeder cells as a modulator of cross-presentation by non-infected dendritic cells. The infected feeder cells serve as source of antigen and provide signals that help to attract dendritic cells for antigen take up and to license these cells for cross-presentation.

Results

We demonstrate that presence of an active P2RX7 in major histocompatibility complex (MHC) class I (MHCI) mismatched feeder cells significantly enhanced MVA-mediated antigen cross-presentation. This was partly regulated by P2RX7-specific processes, such as the increased availability of extracellular particles as well as the altered cellular energy metabolism by mitochondria in the feeder cells. Furthermore, functional P2RX7 in feeder cells resulted in a delayed but also prolonged antigen expression after infection.

Discussion

We conclude that a combination of the above mentioned P2RX7-depending processes leads to significantly increased T cell activation via cross- presentation of MVA-derived antigens. To this day, P2RX7 has been mostly investigated in regards to neuroinflammatory diseases and cancer progression. However, we report for the first time the crucial role of P2RX7 for antigen- specific T cell immunity in a viral infection model.

Bitter Taste Receptor Agonist Denatonium Inhibits Stemness Characteristics in Hematopoietic Stem/Progenitor Cells

Bone marrow microenvironmental stimuli profoundly impact hematopoietic stem cell fate and biology. As G protein-coupled receptors, the bitter taste receptors (TAS2Rs) are key in transmitting extracellular stimuli into an intracellular response, within the oral cavity but also in extraoral tissues. Their expression in the bone marrow (BM)-derived cells suggests their involvement in sensing the BM microenvironmental fluctuation. In the present study, we demonstrated that umbilical cord blood (UCB)-derived CD34+ cells express fully functional TAS2Rs along with the signal transduction cascade components and their activation by the prototypical agonist, denatonium benzoate, significantly modulated genes involved in stemness maintenance and regulation of cell trafficking. The activation of these specific pathways was confirmed in functional in vitro experiments. Denatonium exposure exerted an antiproliferative effect on UCB-derived CD34+ cells, mainly affecting the most undifferentiated progenitor frequency. It also reduced their clonogenicity and repopulating potential in vitro. In addition, the TAS2R signaling activation impaired the UCB-derived CD34+ cell trafficking, mainly reducing the migration toward the chemoattractant agent CXCL12 and modulating the expression of the adhesion molecules CD62L, CD49d, and CD29. In conclusion, our results in UCB-derived CD34+ cells expand the observation of TAS2R expression in the setting of BM-resident cells and shed light on the role of TAS2Rs in the extrinsic regulation of hematopoietic stem cell functions.

The P2X7 Receptor in Oncogenesis and Metastatic Dissemination: New Insights on Vesicular Release and Adenosinergic Crosstalk

The tumor niche is an environment rich in extracellular ATP (eATP) where purinergic receptors have essential roles in different cell subtypes, including cancer, immune, and stromal cells. Here, we give an overview of recent discoveries regarding the role of probably the best-characterized purinergic receptor in the tumor microenvironment: P2X7. We cover the activities of the P2X7 receptor and its human splice variants in solid and liquid cancer proliferation, dissemination, and crosstalk with immune and endothelial cells. Particular attention is paid to the P2X7-dependent release of microvesicles and exosomes, their content, including ATP and miRNAs, and, in general, P2X7-activated mechanisms favoring metastatic spread and niche conditioning. Moreover, the emerging role of P2X7 in influencing the adenosinergic axis, formed by the ectonucleotidases CD39 and CD73 and the adenosine receptor A2A in cancer, is analyzed. Finally, we cover how antitumor therapy responses can be influenced by or can change P2X7 expression and function. This converging evidence suggests that P2X7 is an attractive therapeutic target for oncological conditions.

Emerging roles of purinergic signaling in anti-cancer therapy resistance

Cancer is a complex disease with a rapid growing incidence and often characterized by a poor prognosis. Although impressive advances have been made in cancer treatments, resistance to therapy remains a critical obstacle for the improvement of patients outcome. Current treatment approaches as chemo-, radio-, and immuno-therapy deeply affect the tumor microenvironment (TME), inducing an extensive selective pressure on cancer cells through the activation of the immune system, the induction of cell death and the release of inflammatory and damage-associated molecular patterns (DAMPS), including nucleosides (adenosine) and nucleotides (ATP and ADP). To survive in this hostile environment, resistant cells engage a variety of mitigation pathways related to metabolism, DNA repair, stemness, inflammation and resistance to apoptosis. In this context, purinergic signaling exerts a pivotal role being involved in mitochondrial function, stemness, inflammation and cancer development. The activity of ATP and adenosine released in the TME depend upon the repertoire of purinergic P2 and adenosine receptors engaged, as well as, by the expression of ectonucleotidases (CD39 and CD73) on tumor, immune and stromal cells. Besides its well established role in the pathogenesis of several tumors and in host–tumor interaction, purinergic signaling has been recently shown to be profoundly involved in the development of therapy resistance. In this review we summarize the current advances on the role of purinergic signaling in response and resistance to anti-cancer therapies, also describing the translational applications of combining conventional anticancer interventions with therapies targeting purinergic signaling.

A2A Receptor Contributes to Tumor Progression in P2X7 Null Mice

ATP and adenosine are key constituents of the tumor niche where they exert opposite and complementary roles. ATP can be released in response to cell damage or actively released by tumor cells and subsequently degraded into adenosine, which accumulates within the tumor microenvironment. Notably, while ATP promotes immune eradicating responses mainly via the P2X7 receptor (P2X7R), extracellular adenosine acts as a potent immune suppressor and facilitates neovascularization thanks to the A2A receptor (A2AR). To date, studies exploring the interplay between P2X7R and A2AR in the tumor microenvironment are as yet missing. Here, we show that, in C57/bl6 P2X7 null mice inoculated with B16-F10 melanoma cells, several pro-inflammatory cytokines, including interleukin 1 beta (IL-1β), tumor necrosis factor alpha (TNF-α), interleukin 6 (IL-6), interleukin 12 (IL-12), interleukin 17 (IL-17), interferon gamma (IFN-γ) were significantly decreased, while the immune suppressant transforming growth factor beta (TGF-β) was almost three-fold increased. Interestingly, tumors growing in P2X7-null mice upregulated tumor-associated and splenic A2AR, suggesting that immunosuppression linked to lack of the P2X7R might depend upon A2AR overexpression. Immunohistochemical analysis showed that tumor cells’ A2AR expression was increased, especially around necrotic areas, and that vascular endothelial growth factor (VEGF) and the endothelial marker CD31 were upregulated. A2AR antagonist SCH58261 treatment reduced tumor growth similarly in the P2X7 wild type or null mice strain. However, SCH58261 reduced VEGF only in the P2X7 knock out mice, thus supporting the hypothesis of an A2AR-mediated increase in vascularization observed in the P2X7-null host. SCH58261 administration also significantly reduced intratumor TGF-β levels, thus supporting a key immune suppressive role of A2AR in our model. Altogether, these results indicate that in the absence of host P2X7R, the A2AR favors tumor growth via immune suppression and neovascularization. This study shows a novel direct correlation between P2X7R and A2AR in oncogenesis and paves the way for new combined therapies promoting anti-cancer immune responses and reducing tumor vascularization.

Administration of P2X7 Receptor Blockers in Oncological Experimental Models

The tumor microenvironment is rich in components that strongly influence cancer cell survival. One of the pivotal molecules present at the tumor bed is ATP, which has an essential role in promoting cancer proliferation and metastasis and immune responses via its receptor P2X7. Several studies have proved the efficacy of P2X7 pharmacological blockade in inhibiting primary and metastatic tumor growth in preclinical models. Here we describe the experimental procedures that we optimized to test P2X7 roles in carcinogenesis by antagonist administration. Special attention is paid to their concentrations and routes of administration. The depicted in vitro models include cell count and viability assays, which are useful to test P2X7 roles in cell proliferation and vitality, and the soft agar colony formation test that allows investigation of the transforming and invading abilities of tumor cells. We also describe systemic and intramass administration of P2X7 blockers in murine models of melanoma and leukemia. Both xenotransplant and syngeneic experimental tumor models are detailed.

The P2RX7B splice variant modulates osteosarcoma cell behaviour and metastatic properties

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P2RX7B expression confers a survival advantage in TE85+P2RX7B and MNNG-HOS+P2RX7B OS cell lines.

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P2RX7B expression reduced cell adhesion and activation promoted invasion and migration in vitro.

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MNNG-HOS+P2RX7B tumours in vivo exhibited ectopic bone formation that A740003 reduced.

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Expression of P2RX7B in primary tumour cells increased the propensity to metastasise to the lungs.

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A novel gene axis, FN1/LOX/PDGFB/IGFBP3/BMP4 was downregulated in response to A740003.

Background

Osteosarcoma (OS) is the most common type of primary bone cancer affecting children and adolescents. OS has a high propensity to spread meaning the disease is often incurable and fatal. There have been no improvements in survival rates for decades. This highlights an urgent need for the development of novel therapeutic strategies. Here, we report in vitro and in vivo data that demonstrates the role of purinergic signalling, specifically, the B isoform of the purinergic receptor P2RX7 (P2RX7B), in OS progression and metastasis.

Methods

TE85 and MNNG-HOS OS cells were transfected with P2RX7B. These cell lines were then characterised and assessed for proliferation, cell adhesion, migration and invasion in vitro. We used these cells to perform both paratibial and tail vein injected mouse studies where the primary tumour, bone and lungs were analysed. We used RNA-seq to identify responsive pathways relating to P2RX7B.

Results

Our data shows that P2RX7B expression confers a survival advantage in TE85 + P2RX7B and MNNG-HOS + P2RX7B human OS cell lines in vitro that is minimised following treatment with A740003, a specific P2RX7 antagonist. P2RX7B expression reduced cell adhesion and P2RX7B activation promoted invasion and migration in vitro, demonstrating a metastatic phenotype. Using an in vivo OS xenograft model, MNNG-HOS + P2RX7B tumours exhibited cancer-associated ectopic bone formation that was abrogated with A740003 treatment. A pro-metastatic phenotype was further demonstrated in vivo as expression of P2RX7B in primary tumour cells increased the propensity of tumour cells to metastasise to the lungs. RNA-seq identified a novel gene axis, FN1/LOX/PDGFB/IGFBP3/BMP4, downregulated in response to A740003 treatment.

Conclusion

Our data illustrates a role for P2RX7B in OS tumour growth, progression and metastasis. We show that P2RX7B is a future therapeutic target in human OS.

Corrigendum: Denatonium as a Bitter Taste Receptor Agonist Modifies Transcriptomic Profile and Functions of Acute Myeloid Leukemia Cells

[This corrects the article DOI: 10.3389/fonc.2020.01225.].

P2X7 Receptor in Hematological Malignancies

The P2X7 receptor is an ion channel gated by the nucleotide ATP, known for its role in immune responses and recently emerging as a critical onco-promoting factor. Lymphocytes, myeloid cells, and their precursors were among the first cells proved to express a functional P2X7 receptor; therefore, it is not surprising that lymphoproliferative and myeloproliferative diseases, also known as hematological malignancies, were shown to be related in their insurgence and progression to P2X7 alterations. Here, we overview established and recent literature relating P2X7 with the biological mechanisms underlying leukemias, lymphomas, and multiple myeloma development. Particular attention is paid to studies published in the very recent past correlating P2X7 with ATP concentration in the leukemic microenvironment and P2X7 overexpression to acute myeloid leukemia aggressiveness and response to chemotherapy. The described literature strongly suggests that P2X7 and its genetic variants could be regarded as potential new biomarkers in hematological malignancies and that both P2X7 antagonists and agonists could emerge as new therapeutic tools alone or in combination with traditional chemotherapy.

Differential sensitivity of acute myeloid leukemia cells to daunorubicin depends on P2X7A versus P2X7B receptor expression

Acute myeloid leukemia (AML) is a common adult leukemia often arising from a preexistent myelodysplastic syndrome (MDS). High mortality rates of AML are caused by relapse and chemoresistance; therefore, we analyzed the role of P2X7 receptor (P2X7R) splice variants A and B in AML progression and response to chemotherapy. The expression of P2X7RA and P2X7RB was investigated in samples obtained from MDS and AML untreated subjects or AML patients in relapse or remission after chemotherapy. Both P2X7RA and P2X7RB were overexpressed in AML versus MDS suggesting a disease-promoting function. However, in relapsing patients, P2X7RA was downmodulated, while P2X7RB was upmodulated. Treatment with daunorubicin (DNR), one of the main chemotherapeutics for AML, upregulated P2X7RB expression while reducing P2X7RA mRNA in AML blasts. Interestingly, DNR administration also caused ATP release from AML blasts suggesting that, following chemotherapy, activation of the receptor isoforms via their agonist will be responsible for the differential survival of blasts overexpressing P2X7RA versus P2X7RB. Indeed, AML blasts expressing high levels of P2X7RA were more prone to cell death if exposed to DNR, while those overexpressing P2X7RB were more vital and even protected against DNR toxicity. These data were reproducible also in HEK-293 cells separately expressing P2X7RA and B. P2X7RA facilitation of DNR toxicity was in part due to increased uptake of the drug inside the cell that was lost upon P2X7RB expression. Finally, in an AML xenograft model administration of DNR or the P2X7R antagonist, AZ10606120 significantly reduced leukemic growth and coadministration of the drugs proved more efficacious than single treatment as it reduced both P2X7RA and P2X7RB levels and downmodulated c-myc oncogene. Taken together, our data suggest P2X7RA and P2X7RB as potential prognostic markers for AML and P2X7RB as a therapeutic target to overcome chemoresistance in AML relapsing patients.

Abstract 4946: Bitter taste receptors system is expressed and functional in both HSCs and leukemic cells

Acute Myeloid Leukemia (AML) is a clonal disease developing from a rare population of leukemic stem cells. Alongside the identification of disease-specific alleles harbored by AML clones, the contribution that cell-extrinsic factors have in AML generation and persistence by influencing AML cell genomic landscape and therapy-resistance is gaining increasing interest. In the cross-talk between AML cells and their microenvironment, several membrane receptors sense the external changes by triggering intracellular signals. Among these, the largest group belongs to the family of G protein-couple receptors (GPCRs). Bitter taste receptors (T2Rs) comprise 25 distinct members of the GPCR family. Initially described in the oral cavity, T2Rs are actually widely expressed in different tissues and in various cancer cell types. In the present work, we showed that AML cells expressed fully functional T2R subtypes. Their activation substantially modified the AML cell transcriptomic profile and deregulated relevant cellular processes, resulting in inhibited clonogenic capacity, proliferation and cell motility and induced apoptosis. In addition, T2R stimulation with agonist altered mitochondrial bioenergetic capacity and made AML cells more prone to oxidative and metabolic stress. Given the effect of T2R activation on crucial AML cell function, we tested the therapeutical potential of targeting T2Rs. Interestingly, we observed that T2R stimulation had a synergistic effect with cytarabine, reducing leukemia cell viability and allowing to reach high toxicity using lower doses of chemotherapic agent. Then we analyzed T2Rs expression and function on normal hematopoietic stem cells (HSCs). HSCs expressed several T2R subtypes with some differences compare to AML cells. T2Rs activation didn't affect HSCs viability and motility, conversely to AML cells, but significantly inhibited clonogenic capacity, by reducing the frequency of precursors. Our data may suggest a role for microenvironment “bitter” molecules in regulating normal and leukemic hematopoiesis. Notably, many common drugs, such as antibiotics, chloroquine, haloperidol, procainamide, are bitter tasting and are thus effective ligands for T2Rs. For this reason, they could exhibit an off-target effect in T2R expressing cells, including leukemic and normal hematopoietic cells. Our results may have implications in understanding the off-target actions of diverse drugs and could reveal potential new therapeutic targets

Citation Format: Valentina Salvestrini, Valentina Pensato, Marilena Ciciarello, Giorgia Simonetti, Samantha Bruno, Martina Pazzaglia, Elena De Marchi, Dorian Forte, Stefania Orecchioni, Giovanni Martinelli, Francesco Bertolini, Simon Mendez-Ferrer, Elena Adinolfi, Michele Cavo, Antonio Curti. Bitter taste receptors system is expressed and functional in both HSCs and leukemic cells [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4946.

Denatonium as a Bitter Taste Receptor Agonist Modifies Transcriptomic Profile and Functions of Acute Myeloid Leukemia Cells

The contribution of cell-extrinsic factors in Acute Myeloid Leukemia (AML) generation and persistence has gained interest. Bitter taste receptors (TAS2Rs) are G protein-coupled receptors known for their primary role as a central warning signal to induce aversion toward noxious or harmful substances. Nevertheless, the increasing amount of evidence about their extra-oral localization has suggested a wider function in sensing microenvironment, also in cancer settings. In this study, we found that AML cells express functional TAS2Rs. We also highlighted a significant association between the modulation of some TAS2Rs and the poor-prognosis AML groups, i.e., TP53- and TET2-mutated, supporting a potential role of TAS2Rs in AML cell biology. Gene expression profile analysis showed that TAS2R activation with the prototypical agonist, denatonium benzoate, significantly modulated a number of genes involved in relevant AML cellular processes. Functional assay substantiated molecular data and indicated that denatonium reduced AML cell proliferation by inducing cell cycle arrest in G0/G1 phase or induced apoptosis via caspase cascade activation. Moreover, denatonium exposure impaired AML cell motility and migratory capacity, and inhibited cellular respiration by decreasing glucose uptake and oxidative phosphorylation. In conclusion, our results in AML cells expand the observation of cancer TAS2R expression to the setting of hematological neoplasms and shed light on a role of TAS2Rs in the extrinsic regulation of leukemia cell functions.

Detection of Extracellular ATP in the Tumor Microenvironment, Using the pmeLUC Biosensor

ATP is one of the main components of the tumor microenvironment, where it affects cell growth, tumor progression and antitumor immune response. The development of the pmeLUC probe, a luciferase engineered to be expressed on the outer facet of the plasma membrane, allowed real-time measurement of extracellular ATP in vitro and in vivo systems, among which the tumor microenvironment. Here we describe the experimental procedures to measure extracellular ATP levels in the tumor microenvironment of three different cancer models generated by the implant of pmeLUC-expressing tumor cells into the appropriate mice strain: ACN human neuroblastoma (nude/nude mice host), WEHI-3B murine leukemia (BALB/c host), and B16F10 murine melanoma (C57Bl/6 host). The procedure to obtain stable expression of pmeLUC in different cell types and methods for the measurement of extracellular ATP with pmeLUC in vitro are also described.

The P2X7 Receptor 489C>T Gain of Function Polymorphism Favors HHV-6A Infection and Associates With Female Idiopathic Infertility

The P2X7 receptor (P2X7R) is an ATP-gated ion channel known for its proinflammatory activity. Despite its participation in host defense against pathogens, the role played in viral infections, notably those caused by herpes viruses, has been seldom studied. Here we investigated the effect of P2X7R expression on human herpes virus 6 A (HHV-6A) infection of P2X7R-expressing HEK293 cells. We show that functional P2X7R increases while its blockade decreases viral load. Interestingly, HHV-6A infection was enhanced in HEK293 cells transfected with P2X7R cDNA bearing the gain of function 489C>T SNP (rs208294, replacing a histidine for tyrosine at position 155). The P2X7R 489C>T polymorphism correlated with HHV-6A infection also in a cohort of 50 women affected with idiopathic infertility, a condition previously shown to correlate with HHV-6A infection. None of the infertile women infected by HHV-6A was homozygote for 489CC genotype, while on the contrary HHV-6A infection significantly associated with the presence of the rs208294 allele. Levels of soluble human leukocyte antigen G (sHLA-G), a factor promoting embryo implant, measured in uterine flushings negatively correlated with the 489TT genotype and HHV-6A infection, while proinflammatory cytokines interleukins 1α (IL-1α), 1β (IL-1β), and 8 (IL-8) positively correlated with both the 489T allele presence and viral infection. Taken together these data point to the P2X7R as a new therapeutic target to prevent HHV-6A infection and the associated infertility.

The P2X7 receptor modulates immune cells infiltration, ectonucleotidases expression and extracellular ATP levels in the tumor microenvironment

In the tumor microenvironment (TME) ATP and its receptor P2X7 exert a pivotal influence on cancer growth and tumor-host interactions. Here we analyzed the different effect of P2X7 genetic deficiency versus its antagonism on response against P2X7-expressing implanted tumors. We focused on immune cell expression of ATP degrading enzymes CD39 and CD73 and in vivo measured TME's ATP. The immune infiltrate of tumors growing in P2X7 null mice shows a decrease in CD8+ cells and an increased number of Tregs, overexpressing the fitness markers OX40, PD-1, and CD73. A similar Treg phenotype is also present in the spleen of tumor-bearing P2X7 null mice and it is paralleled by a decrease in proinflammatory cytokines and an increase in TGF-β. Differently, systemic administration of the P2X7 blocker A740003 in wild-type mice left unaltered the number of tumor-infiltrating CD8+ and Treg lymphocytes but increased CD4+ effector cells and decreased their expression of CD39 and CD73. P2X7 blockade did not affect spleen immune cell composition or ectonucleotidase expression but increased circulating INF-γ. Augmented CD73 in P2X7 null mice was mirrored by a decrease in TME ATP concentration and nucleotide reduced secretion from immune cells. On the contrary, TME ATP levels remained unaltered upon P2X7 antagonism, owing to release of ATP from cancerous cells and diminished ectonucleotidase expression by CD4+ and dendritic cells. These data point at P2X7 receptor as a key determinant of TME composition due to its combined action on immune cell infiltrate, ectonucleotidases, and ATP release.

Role of the P2X7 receptor in tumor-associated inflammation

Inflammation is constantly associated to cancer. Malignant tumors often develop at sites of chronic inflammation, and inflammation promotes tumor progression. But, at the same time, inflammation is crucial for anti-tumor immune response. Many factors are responsible for this 'Dr Jekyll/Mr Hyde' roles of inflammation, among which one that is attracting increasing attention is the P2X7 receptor (P2X7R). This receptor is expressed by most malignant tumors and widely diffused in innate and adaptive immune cells, where it supports proliferation, chemotaxis, growth factor, and cytokine release. P2X7R-targeting may offer novel avenues for anti-cancer therapeutic intervention, but might also impair host anti-tumor responses. This short review highlights recent findings on the dual role of the P2X7R in cancer-associated inflammation.

Extracellular ATP and P2 purinergic signalling in the tumour microenvironment

Modulation of the biochemical composition of the tumour microenvironment is a new frontier of cancer therapy. Several immunosuppressive mechanisms operate in the milieu of most tumours, a condition that makes antitumour immunity ineffective. One of the most potent immunosuppressive factors is adenosine, which is generated in the tumour microenvironment owing to degradation of extracellular ATP. Accruing evidence over the past few years shows that ATP is one of the major biochemical constituents of the tumour microenvironment, where it acts at P2 purinergic receptors expressed on both tumour and host cells. Stimulation of P2 receptors has different effects depending on the extracellular ATP concentration, the P2 receptor subtype engaged and the target cell type. Among P2 receptors, the P2X purinergic receptor 7 (P2X7R) subtype appears to be a main player in host-tumour cell interactions. Preclinical studies in several tumour models have shown that P2X7R targeting is potentially a very effective anticancer treatment, and many pharmaceutical companies have now developed potent and selective small molecule inhibitors of P2X7R. In this Review, we report on the multiple mechanisms by which extracellular ATP shapes the tumour microenvironment and how its stimulation of host and tumour cell P2 receptors contributes to determining tumour fate.

The P2X7 receptor: A main player in inflammation

Damage associated molecular patterns (DAMPs) are intracellular molecules released from infected or injured cells to activate inflammatory and reparatory responses. One of the most ancient and conserved DAMPs is extracellular ATP that exerts its phlogistic activity mainly through activation of the P2X7 receptor (P2X7R). The P2X7R is an ATP gated ion channel, expressed by most immune cells, including the monocyte-derived cell lineages, T and B lymphocytes and their precursors. Here we give an overview of recent and established literature on the role of P2X7R in septic and sterile inflammation. P2X7R ability in restraining intracellular bacteria and parasite infection by modulation of the immune response are described, with particular focus on Mycobacteria and Plasmodium. Emerging literature on the role of P2X7 in viral infections such as HIV-1 is also briefly covered. Finally, we describe the numerous intracellular pathways related to inflammation and activated by the P2X7R, including the NLRP3 inflammasome, NF-kB, NFAT, GSK3β and VEGF, and discuss the involvement of P2X7R in chronic diseases. The possible therapeutic applications of P2X7R antagonists are also described.

ATP Release from Chemotherapy-Treated Dying Leukemia Cells Elicits an Immune Suppressive Effect by Increasing Regulatory T Cells and Tolerogenic Dendritic Cells

Chemotherapy-induced immunogenic cell death can favor dendritic cell (DC) cross-priming of tumor-associated antigens for T cell activation thanks to the release of damage-associated molecular patterns, including ATP. Here, we tested the hypothesis that in acute myeloid leukemia (AML), ATP release, along with its well-known immune stimulatory effect, may also contribute to the generation of an immune suppressive microenvironment. In a cohort of AML patients, undergoing combined daunorubicin and cytarabine chemotherapy, a population of T regulatory cells (Tregs) with suppressive phenotype, expressing the immune checkpoint programmed cell death protein 1 (PD-1), was significantly increased. Moving from these results, initial in vitro data showed that daunorubicin was more effective than cytarabine in modulating DC function toward Tregs induction and such difference was correlated with the higher capacity of daunorubicin to induce ATP release from treated AML cells. DCs cultured with daunorubicin-treated AML cells upregulated indoleamine 2,3-dioxygenase 1 (IDO1), which induced anti-leukemia Tregs. These data were confirmed in vivo as daunorubicin-treated mice show an increase in extracellular ATP levels with increased number of Tregs, expressing PD-1 and IDO1⁺CD39⁺ DCs. Notably, daunorubicin failed to induce Tregs and tolerogenic DCs in mice lacking the ATP receptor P2X7. Our data indicate that ATP release from chemotherapy-treated dying cells contributes to create an immune suppressive microenvironment in AML.

P2X7 Receptor as a Therapeutic Target

P2X7 receptor is an ATP-gated cation channel that upon agonist interaction leads to cellular influx of Na(+) and Ca(2+) and efflux of K(+). P2X7 is expressed by a wide variety of cells and its activation mediates a large number of biological processes like inflammation, neuromodulation, cell death or cell proliferation and it has been associated to related pathological conditions including infectious, inflammatory, autoimmune, neurological, and musculoskeletal disorders and, in the last years, to cancer. This chapter describes structural features of P2X7, chemical properties of its agonist, antagonist, and allosteric modulators and summarizes recent advances on P2X7 receptor as therapeutic target in the aforementioned diseases. We also give an overview on recent literature suggesting that P2X7 single-nucleotide polymorphisms could be exploited as diagnostic biomarkers for the development of tailored therapies.

Emerging roles of P2X receptors in cancer

Tumor microenvironment composition strongly conditions cancer growth and progression, acting not only at cancer itself but also modifying its interactions with immune, endothelial and nervous cells. Extracellular ATP and its receptors recently gained increasing attention in the oncological field. ATP accumulates in cancer milieu through spontaneous release, tumor necrosis or chemotherapy exerting a trophic activity on cancer cells, modulating the cross talk among tumor, and surrounding tissues. Accordingly, ATP gated P2X receptors emerged as central players in tumor development, invasion, progression and related symptoms. Indeed, P2X receptors are expressed and are functional not only on tumor cells but also in immune-infiltrate and nearby neurons. In this review, we summarize recent findings on P2X receptors role in tumor cell differentiation, bioenergetics, angiogenesis, metastasis and associated pain, giving an outline of the potential anti-neoplastic activity of receptor agonists and antagonists.

Methods to monitor and compare mitochondrial and glycolytic ATP production

ATP is commonly considered as the main energy unit of the cell and participates in a variety of cellular processes. Thus, intracellular ATP concentrations rapidly vary in response to a wide variety of stimuli, including nutrients, hormones, cytotoxic agents, and hypoxia. Such alterations not necessarily affect cytosolic and mitochondrial ATP to similar extents. From an oncological perspective, this is particularly relevant in the course of tumor progression as well as in the response of cancer cells to therapy. In normal cells, mitochondrial oxidative phosphorylation (OXPHOS) is the predominant source of ATP. Conversely, many cancer cells exhibit an increased flux through glycolysis irrespective of oxygen tension. Assessing the relative contribution of glycolysis and OXPHOS to intracellular ATP production is fundamental not only for obtaining further insights into the peculiarities and complexities of oncometabolism but also for developing therapeutic and diagnostic tools. Several techniques have been developed to measure intracellular ATP levels including enzymatic methods based on hexokinase, glucose-6-phosphate dehydrogenase, and firefly luciferase. Here, we summarize conventional methods for measuring intracellular ATP levels and we provide a detailed protocol based on cytosol- and mitochondrion-targeted variants of firefly luciferase to determine the relative contribution of glycolysis and OXPHOS to ATP synthesis.

The mitochondrial permeability transition pore is a dispensable element for mitochondrial calcium efflux

The mitochondrial permeability transition pore (mPTP) has long been known to have a role in mitochondrial calcium (Ca(2+)) homeostasis under pathological conditions as a mediator of the mitochondrial permeability transition and the activation of the consequent cell death mechanism. However, its role in the context of mitochondrial Ca(2+) homeostasis is not yet clear. Several studies that were based on PPIF inhibition or knock out suggested that mPTP is involved in the Ca(2+) efflux mechanism, while other observations have revealed the opposite result. The c subunit of the mitochondrial F1/FO ATP synthase has been recently found to be a fundamental component of the mPTP. In this work, we focused on the contribution of the mPTP in the Ca(2+) efflux mechanism by modulating the expression of the c subunit. We observed that forcing mPTP opening or closing did not impair mitochondrial Ca(2+) efflux. Therefore, our results strongly suggest that the mPTP does not participate in mitochondrial Ca(2+) homeostasis in a physiological context in HeLa cells.

Role of the c subunit of the FO ATP synthase in mitochondrial permeability transition

The term "mitochondrial permeability transition" (MPT) refers to an abrupt increase in the permeability of the inner mitochondrial membrane to low molecular weight solutes. Due to osmotic forces, MPT is paralleled by a massive influx of water into the mitochondrial matrix, eventually leading to the structural collapse of the organelle. Thus, MPT can initiate mitochondrial outer membrane permeabilization (MOMP), promoting the activation of the apoptotic caspase cascade as well as of caspase-independent cell death mechanisms. MPT appears to be mediated by the opening of the so-called "permeability transition pore complex" (PTPC), a poorly characterized and versatile supramolecular entity assembled at the junctions between the inner and outer mitochondrial membranes. In spite of considerable experimental efforts, the precise molecular composition of the PTPC remains obscure and only one of its constituents, cyclophilin D (CYPD), has been ascribed with a crucial role in the regulation of cell death. Conversely, the results of genetic experiments indicate that other major components of the PTPC, such as voltage-dependent anion channel (VDAC) and adenine nucleotide translocase (ANT), are dispensable for MPT-driven MOMP. Here, we demonstrate that the c subunit of the FO ATP synthase is required for MPT, mitochondrial fragmentation and cell death as induced by cytosolic calcium overload and oxidative stress in both glycolytic and respiratory cell models. Our results strongly suggest that, similar to CYPD, the c subunit of the FO ATP synthase constitutes a critical component of the PTPC.

Mitochondrial Ca(2+) and apoptosis

Mitochondria are key decoding stations of the apoptotic process. In support of this view, a large body of experimental evidence has unambiguously revealed that, in addition to the well-established function of producing most of the cellular ATP, mitochondria play a fundamental role in triggering apoptotic cell death. Various apoptotic stimuli cause the release of specific mitochondrial pro-apoptotic factors into the cytosol. The molecular mechanism of this release is still controversial, but there is no doubt that mitochondrial calcium (Ca(2+)) overload is one of the pro-apoptotic ways to induce the swelling of mitochondria, with perturbation or rupture of the outer membrane, and in turn the release of mitochondrial apoptotic factors into the cytosol. Here, we review as different proteins that participate in mitochondrial Ca(2+) homeostasis and in turn modulate the effectiveness of Ca(2+)-dependent apoptotic stimuli. Strikingly, the final outcome at the cellular level is similar, albeit through completely different molecular mechanisms: a reduced mitochondrial Ca(2+) overload upon pro-apoptotic stimuli that dramatically blunts the apoptotic response.

Calcium signaling around Mitochondria Associated Membranes (MAMs)

Calcium (Ca²⁺) homeostasis is fundamental for cell metabolism, proliferation, differentiation, and cell death. Elevation in intracellular Ca²⁺ concentration is dependent either on Ca²⁺ influx from the extracellular space through the plasma membrane, or on Ca²⁺ release from intracellular Ca²⁺ stores, such as the endoplasmic/sarcoplasmic reticulum (ER/SR). Mitochondria are also major components of calcium signalling, capable of modulating both the amplitude and the spatio-temporal patterns of Ca²⁺ signals. Recent studies revealed zones of close contact between the ER and mitochondria called MAMs (Mitochondria Associated Membranes) crucial for a correct communication between the two organelles, including the selective transmission of physiological and pathological Ca²⁺ signals from the ER to mitochondria. In this review, we summarize the most up-to-date findings on the modulation of intracellular Ca²⁺ release and Ca²⁺ uptake mechanisms. We also explore the tight interplay between ER- and mitochondria-mediated Ca²⁺ signalling, covering the structural and molecular properties of the zones of close contact between these two networks.

Protein kinases and phosphatases in the control of cell fate

Protein phosphorylation controls many aspects of cell fate and is often deregulated in pathological conditions. Several recent findings have provided an intriguing insight into the spatial regulation of protein phosphorylation across different subcellular compartments and how this can be finely orchestrated by specific kinases and phosphatases. In this review, the focus will be placed on (i) the phosphoinositide 3-kinase (PI3K) pathway, specifically on the kinases Akt and mTOR and on the phosphatases PP2a and PTEN, and on (ii) the PKC family of serine/threonine kinases. We will look at general aspects of cell physiology controlled by these kinases and phosphatases, highlighting the signalling pathways that drive cell division, proliferation, and apoptosis.

Mitochondrial calcium homeostasis as potential target for mitochondrial medicine

Mitochondria are crucial in different intracellular pathways of signal transduction. Mitochondria are capable of decoding a variety of extracellular stimuli into markedly different intracellular actions, ranging from energy production to cell death. The fine modulation of mitochondrial calcium (Ca²⁺) homeostasis plays a fundamental role in many of the processes involving this organelle. When mitochondrial Ca²⁺ homeostasis is compromised, different pathological conditions can occur, depending on the cell type involved. Recent data have shed light on the molecular identity of the main proteins involved in the handling of mitochondrial Ca²⁺ traffic, opening fascinating and ambitious new avenues for mitochondria-based pharmacological strategies.