P2X7 receptor antagonism in the treatment of cancers

P2X7 receptors: a bibliometric review from 2002 to 2023

For many years, there has been ongoing research on the P2X7 receptor (P2X7R). A comprehensive, systematic, and objective evaluation of the scientific output and status of P2X7R will be instrumental in guiding future research directions. This study aims to present the status and trends of P2X7R research from 2002 to 2023. Publications related to P2X7R were retrieved from the Web of Science Core Collection database. Quantitative analysis and visualization tools were Microsoft Excel, VOSviewer, and CiteSpace software. The analysis content included publication trends, literature co-citation, and keywords. 3282 records were included in total, with the majority of papers published within the last 10 years. Based on literature co-citation and keyword analysis, neuroinflammation, neuropathic pain, gastrointestinal diseases, tumor microenvironment, rheumatoid arthritis, age-related macular degeneration, and P2X7R antagonists were considered to be the hotspots and frontiers of P2X7R research. Researchers will get a more intuitive understanding of the status and trends of P2X7R research from this study.

Purinergic P2X7R as a potential target for pancreatic cancer

Pancreatic cancer is one of the deadliest types of cancer, with a death rate nearly equal to the incidence. The P2X7 receptor (P2X7R) is a kind of extracellular adenosine triphosphate (ATP)-gated ion channel with special permeability, which exists in most tissues of human body and mediates inflammation-related signaling pathways and immune signal transduction after activation. P2X7R is also present on the surface of several tumor cells and is involved in tumor growth and progression. P2X7R expression in pancreatic cancer has also been identified in recent studies. Activation of P2X7R in pancreatic cancer can support the proliferation of pancreatic stellate cells, participate in protein interactions, and mediate ERK1/2, IL-6/STAT3, hCAP-18/LL-37, PI3K/AKT signaling pathways to promote pancreatic cancer progression. Inhibitors targeting P2X7R can inhibit the development of pancreatic cancer and are expected to be used in clinical therapy. Therefore, P2X7R is promising as a potential therapeutic target for pancreatic cancer. This article reviews the progress of research on P2X7R in pancreatic cancer.

Quantitative phosphoproteomics reveals ectopic ATP synthase on mesenchymal stem cells to promote tumor progression via ERK/c-Fos pathway activation

The tumor microenvironment (TME), which comprises cellular and noncellular components, is involved in the complex process of cancer development. Emerging evidence suggests that mesenchymal stem cells (MSCs), one of the vital regulators of the TME, foster tumor progression through paracrine secretion. However, the comprehensive phosphosignaling pathways that are mediated by MSC-secreting factors have not yet been fully established. In this study, we attempt to dissect the MSC-triggered mechanism in lung cancer using quantitative phosphoproteomics. A total of 1958 phosphorylation sites are identified in lung cancer cells stimulated with MSC-conditioned medium. Integrative analysis of the identified phosphoproteins and predicted kinases demonstrates that MSC-conditioned medium functionally promotes the proliferation and migration of lung cancer via the ERK/phospho-c-Fos-S374 pathway. Recent studies have reported that extracellular ATP accumulates in the TME and stimulates the P2X7R on the cancer cell membrane via purinergic signaling. We observe that ectopic ATP synthase is located on the surface of MSCs and excreted extracellular ATP into the lung cancer microenvironment to trigger the ERK/phospho-c-Fos-S374 pathway, which is consistent with these previous findings. Our results suggest that ectopic ATP synthase on the surface of MSCs releases extracellular ATP into the TME, which promotes cancer progression via activation of the ERK/phospho-c-Fos-S374 pathway.

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Mesenchymal stem cells (MSCs) enhance lung cancer development through extracellular factor secretion.

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Phosphoproteomics discover MSCs-regulated phosphosignaling in the lung cancer.

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Ectopic ATP synthase on MSCs surface produces ATP into the tumor microenvironment.

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MSC-secreted extracellular ATP mediates the phosphorylation of the ERK/c-Fos axis.

Physalin pool from Physalis angulata L. leaves and physalin D inhibit P2X7 receptor function in vitro and acute lung injury in vivo

P2X7 receptor promotes inflammatory response and neuropathic pain. New drugs capable of impairing inflammation and pain-reducing adverse effects extracted from plant extracts have been studied. Physalis angulate L. possesses traditional uses and exhibits antiparasitic, anti-inflammatory, antimicrobial, antinociceptive, antimalarial, antileishmanial, immunosuppressive, antiasthmatic. diuretic, and antitumor activities. The most representative phytochemical constituents identified with medicinal importance are the physalins and withanolides. However, the mechanism of anti-inflammatory action is scarce. Although some physalins and withanolides subtypes have anti-inflammatory activity, only four physalins subtypes (B, D, F, and G) have further studies. Therefore, we evaluated the crude ethanolic extract enriched with physalins B, D, F, and G from P. angulata leaves, a pool containing the physalins B, D, F, G, and the physalins individually, as P2X7 receptor antagonists. For this purpose, we evaluated ATP-induced dye uptake, macroscopic currents, and interleukin 1-β (IL-1β) in vitro. The crude extract and pool dose-dependently inhibited P2X7 receptor function. Thus, physalin B, D, F, and G individually evaluated for 5'-triphosphate (ATP)-induced dye uptake assay, whole-cell patch-clamp, and cytokine release showed distinct antagonist levels. Physalin D displayed higher potency and efficacy than physalin B, F, and G for all these parameters. In vivo mice model as ATP-induced paw edema was potently inhibited for physalin D, in contrast to physalin B, F, and G. ATP and lipopolysaccharide (LPS)-induced pleurisy in mice were reversed for physalin D treatment. Molecular modeling and computational simulation predicted the intermolecular interactions between the P2X7 receptor and physalin derivatives. In silico results indicated physalin D and F as a potent allosteric P2X7 receptor antagonist. These data confirm physalin D as a promisor source for developing a new P2X7 receptor antagonist with anti-inflammatory action.

P2X Receptors: Potential Therapeutic Targets for Symptoms Associated With Lung Cancer - A Mini Review

Symptoms associated with lung cancer mainly consist of cancer-associated pain, cough, fatigue, and dyspnea. However, underlying mechanisms of lung cancer symptom clusters remain unclear. There remains a paucity of effective treatment to ameliorate debilitating symptoms and improve the quality of life of lung cancer survivors. Recently, extracellular ATP and its receptors have attracted increasing attention among researchers in the field of oncology. Extracellular ATP in the tumor microenvironment is associated with tumor cell metabolism, proliferation, and metastasis by driving inflammation and neurotransmission via P2 purinergic signaling. Accordingly, ATP gated P2X receptors expressed on tumor cells, immune cells, and neurons play a vital role in modulating tumor development, invasion, progression, and related symptoms. P2 purinergic signaling is involved in the development of different lung cancer-related symptoms. In this review, we summarize recent findings to illustrate the role of P2X receptors in tumor proliferation, progression, metastasis, and lung cancer- related symptoms, providing an outline of potential anti-neoplastic activity of P2X receptor antagonists. Furthermore, compared with opioids, P2X receptor antagonists appear to be innovative therapeutic interventions for managing cancer symptom clusters with fewer side effects.

Antagonism of the ATP-gated P2X7 receptor: a potential therapeutic strategy for cancer

The P2X receptor 7 (P2X7R) is a plasma membrane receptor sensing extracellular ATP associated with a wide variety of cellular functions. It is most commonly expressed on immune cells and is highly upregulated in a number of human cancers where it can play a trophic role in tumorigenesis. Activation of this receptor leads to the formation of a non-selective cation channel, which has been associated with several cellular functions mediated by the PI3K/Akt pathway and protein kinases. Due to its broad range of functions, the receptor represents a potential therapeutic target for a number of cancers. This review describes the range of mechanisms associated with P2X7R activation in cancer settings and highlights the potential of targeted inhibition of P2X7R as a therapy. It also describes in detail a number of key P2X7R antagonists currently in pre-clinical and clinical development, including oxidised ATP, Brilliant Blue G (BBG), KN-62, KN-04, A740003, A438079, GSK1482160, CE-224535, JNJ-54175446, JNJ-55308942, and AZ10606120. Lastly, it summarises the in vivo studies and clinical trials associated with the use and development of these P2X7R antagonists in different disease contexts.

A perspective on new immune adjuvant principles: Reprogramming inflammatory states to permit clearance of cancer cells and other age-associated cellular pathologies

Aging entails the accumulation of neoantigens comprised of aggregated, oxidized, mutated and misfolded biomolecules, including advanced-glycation end projects (AGEs). There is evidence that the immune system can recognize and clear cells fouled by these molecular debris, which contribute to the emergence of cancer and other major age-associated diseases such as atherogenic and neurodegenerative disorders. However, this process may become increasingly inefficient with aging, perhaps in part because of an insufficiency of adjuvant signals normally associated with infection that can program productive inflammatory states and properly orient the immune system toward regenerative healing. Here we propose conceptual foundations for exploring a small set of infection-associated molecules as potential immune adjuvants to reprogram non-productive inflammatory states in aging tissues, and to improve the clearance of cellular pathologies that engender age-associated disease. The proposed adjuvant classes include a subset of D-amino acids used by bacteria to disrupt biofilms; nucleoside derivatives of N6-methyladenine, which functions at the core of bacterial dam restriction systems; and derivatives of the galactosyl trisaccharide α-Gal, which invokes the hyperacute response in primates. These foreign amino acids, nucleosides and sugar molecules are generally rare or absent in humans, except in association with infections by bacteria, protists or nematodes. A rationale for exploration of these candidate adjuvant principles and their chemical derivatives is discussed in terms of their use in generalized strategies to improve the prevention or treatment of cancer and other age-associated diseases, as negative modifiers of aging.

P2X7 receptor: a critical regulator and potential target for breast cancer

Breast cancer is currently the most common cancer and the leading cause of cancer death among women worldwide. Advanced breast cancer is prone to metastasis, and there is currently no drug to cure metastatic breast cancer. The purinergic ligand-gated ion channel 7 receptor is an ATP-gated nonselective cation channel receptor and is involved in signal transduction, growth regulation, cytokine secretion, and tumor cell development. Recent studies have shown that upregulation of the P2X7 receptor in breast cancer can mediate AKT signaling pathways, Ca2 þ-activated SK3 potassium channels, and EMT and regulate the secretion of small extracellular vesicles to promote breast cancer invasion and migration, which are affected by factors such as hypoxia and ATP. In addition, studies have shown that microRNAs can bind to the 3' untranslated region of the P2X7 receptor, which affects the occurrence and development of breast cancer by upregulating and downregulating P2X7 receptor expression. Studies have shown that new P2X7 receptor inhibitors, such as emodin and Uncaria tomentosa, can inhibit P2X7 receptor-mediated breast cancer invasion and are expected to be used clinically. This article reviews the research progress on the relationship between the P2X7 receptor and breast cancer to provide new ideas and a basis for clinical diagnosis and treatment.

P2X7 receptor and the NLRP3 inflammasome: Partners in crime

Adenosine triphosphate (ATP) is a molecule that on one hand plays a central role in cellular energetics and which on the other is a ubiquitous signaling molecule when released into the extracellular media. Extracellular ATP accumulates in inflammatory environments where it acts as a damage-associated molecular pattern and activates the purinergic P2X receptor 7 (P2X7) in immune cells. P2X7 receptor activation induces the formation of the nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing 3 (NLRP3) inflammasome and the activation of the inflammatory caspase-1. Caspase-1 causes an inflammatory type of cell death called pyroptosis through the release of pro-inflammatory cytokines and intracellular content. Consequently, intense research efforts have been devoted to the design of novel anti-inflammatory therapies, focusing in particular on the P2X7 receptor and the NLRP3 pathway and the introduction of new blocking molecules in early phase clinical trials.

Purinergic Signaling in Pancreas-From Physiology to Therapeutic Strategies in Pancreatic Cancer

The purinergic signaling has an important role in regulating pancreatic exocrine secretion. The exocrine pancreas is also a site of one of the most serious cancer forms, the pancreatic ductal adenocarcinoma (PDAC). Here, we explore how the network of purinergic and adenosine receptors, as well as ecto-nucleotidases regulate normal pancreatic cells and various cells within the pancreatic tumor microenvironment. In particular, we focus on the P2X7 receptor, P2Y2 and P2Y12 receptors, as well as A2 receptors and ecto-nucleotidases CD39 and CD73. Recent studies indicate that targeting one or more of these candidates could present new therapeutic approaches to treat pancreatic cancer. In pancreatic cancer, as much as possible of normal pancreatic function should be preserved, and therefore physiology of purinergic signaling in pancreas needs to be considered.

Evaluation of P2X7 Receptor Function in Tumor Contexts Using rAAV Vector and Nanobodies (AAVnano)

Adenosine triphosphate (ATP) represents a danger signal that accumulates in injured tissues, in inflammatory sites, and in the tumor microenvironment. Extracellular ATP is known to signal through plasma membrane receptors of the P2Y and P2X families. Among the P2X receptors, P2X7 has attracted increasing interest in the field of inflammation as well as in cancer. P2X7 is expressed by immune cells and by most malignant tumor cells where it plays a crucial yet complex role that remains to be clarified. P2X7 activity has been associated with production and release of pro-inflammatory cytokines, modulation of the activity and survival of immune cells, and the stimulation of proliferation and migratory properties of tumor cells. Hence, P2X7 plays an intricate role in the tumor microenvironment combining beneficial and detrimental effects that need to be further investigated. For this, we developed a novel methodology termed AAVnano based on the use of Adeno-associated viral vectors (AAV) encoding nanobodies targeting P2X7. We discuss here the advantages of this tool to study the different functions of P2X7 in cancer and other pathophysiological contexts.

P2X7 Receptor Promotes Mouse Mammary Cancer Cell Invasiveness and Tumour Progression, and Is a Target for Anticancer Treatment

The P2X7 receptor is an ATP-gated cation channel with a still ambiguous role in cancer progression, proposed to be either pro- or anti-cancerous, depending on the cancer or cell type in the tumour. Its role in mammary cancer progression is not yet defined. Here, we show that P2X7 receptor is functional in highly aggressive mammary cancer cells, and induces a change in cell morphology with fast F-actin reorganization and formation of filopodia, and promotes cancer cell invasiveness through both 2- and 3-dimensional extracellular matrices in vitro. Furthermore, P2X7 receptor sustains Cdc42 activity and the acquisition of a mesenchymal phenotype. In an immunocompetent mouse mammary cancer model, we reveal that the expression of P2X7 receptor in cancer cells, but not in the host mice, promotes tumour growth and metastasis development, which were reduced by treatment with specific P2X7 antagonists. Our results demonstrate that P2X7 receptor drives mammary tumour progression and represents a pertinent target for mammary cancer treatment.

Extracellular ATP promotes breast cancer invasion and chemoresistance via SOX9 signaling

Our previous research demonstrated that extracellular adenosine 5'-triphosphate (ATP) could promote breast cancer cell invasion. However, the impact of extracellular ATP on chemoresistance and the mechanisms behind ATP pro-invasion and pro-chemoresistance remain unclear. Here we aimed to determine the molecules or signaling pathways involved. cDNA microarray was performed to identify the differentially expressed genes before and after ATP treatment. As a result, Sex-determining region Y-box 9 (SOX9) was up-regulated after ATP treatment in breast cancer cells. In vitro invasion and migration assays demonstrated that knocking down SOX9 attenuated ATP-driven invasive capability. Mass spectrometry and co-IP revealed that SOX9 interacted with Janus kinase 1 (JAK1). Afterward, IL-6-JAK1-STAT3 signaling was demonstrated to promote SOX9 expression and invasion following ATP treatment. Notably, ATP-IL-6-SOX9 signaling was shown to stimulate chemoresistance in breast cancer cells. ChIP assays identified some potential SOX9 target genes, among which carcinoembryonic antigen-related cell adhesion molecule 5/6 (CEACAM5/6) was demonstrated to mediate ATP pro-invasive function, while ATP-binding cassette subfamily B member 1 (ABCB1) and ATP-binding cassette subfamily G member 2 (ABCG2) mediated ATP-driven chemoresistance. In addition, SOX9-knockdown and apyrase (an ATP hydrolase)-treated MDA-MB-231 cells illustrated decreased tumor growth and enhanced drug sensitivity in nude mice. In vitro spheroid formation assays also proved the significance of ATP-SOX9 in mediating chemoresistance. Moreover, molecules involved in ATP-SOX9 signaling were up-regulated in human breast carcinoma specimens and were associated with poor prognosis. Altogether, SOX9 signaling is vital in ATP-driven invasion and chemoresistance, which may serve as a potential target for breast cancer therapies.

Role of the P2X7 receptor in in vitro and in vivo glioma tumor growth

Human glioblastoma cells are strikingly refractory to ATP-stimulated, P2X7 receptor (P2X7R)-mediated cytotoxicity. To elucidate the mechanistic basis of this feature, we investigated P2X7R-dependent responses in wild type and P2X7R-transfected U138 cells. Mouse GL261 glioma cells were used as an additional control. Here, we report that wild type U138 glioma cells expressed the P2X7R to very low level. Contrary to human U138 cells, mouse GL261 cells showed strong P2X7R expression and P2X7R-dependent responses. Transfection of wild type P2RX7 into U138 cells fully restored P2X7R-dependent responses. P2RX7 transfection conferred a negligible in vitro growth advantage to U138 cells, while strongly accelerated in vivo growth. In silico analysis showed that the P2RX7 gene is seldom mutated in specimens from glioblastoma multiforme (GBM) patients. These observations suggest that the P2X7R might be an important receptor promoting GBM growth.

Inflammasome inhibitors: promising therapeutic approaches against cancer

Inflammation has long been accepted as a key component of carcinogenesis. During inflammation, inflammasomes are potent contributors to the activation of inflammatory cytokines that lead to an inflammatory cascade. Considering the contributing role of inflammasomes in cancer progression, inflammasome inhibitors seem to have a promising future in cancer treatment and prevention. Here, we summarize the structures and signaling pathways of inflammasomes and detail some inflammasome inhibitors used to treat various forms of cancer, which we expect to be used in novel anticancer approaches. However, the practical application of inflammasome inhibitors is limited in regard to specific types of cancer, and the associated clinical trials have not yet been completed. Therefore, additional studies are required to explore more innovative and effective medicines for future clinical treatment of cancer.

Purinergic P2X7 Receptor: A Cation Channel Sensitive to Tumor Microenvironment

Objective:

Current review was conducted to check involvement of P2X7R use in cancer treatment.

Methods:

We review the most recent patents focused on the use of P2X7R in the treatment of cancer.

Results:

P2X7R is an intriguing purinergic receptor that plays different roles in tumor progression.

Conclusion:

Powerful strategies able to selectively interfere with its expression and function should reveal helpful in the development of new anti-cancer therapies.

Ectonucleotidase Expression on Human Amnion Epithelial Cells: Adenosinergic Pathways and Dichotomic Effects on Immune Effector Cell Populations

This study investigates the mechanism(s) underlying the immunoregulatory activities of placenta-derived human amnion epithelial cells (hAEC). The working hypothesis is that NAD⁺ and ATP, along with ectoenzymes involved in their metabolism, play a significant role in hAEC-mediated immune regulation. Proof of principle of the hypothesis was obtained by analyzing the interactions between hAEC and the main human leukocyte populations. The results obtained indicate that hAEC constitutively express a unique combination of functional ectoenzymes, driving the production of adenosine (ADO) via canonical (CD39, CD73) and alternative (CD38, CD203a/PC-1, CD73) pathways. Further, the picture is completed by the observation that hAEC express A1, A2a, and A2b ADO receptors as well as ADO deaminase, the enzyme involved in ADO catabolism. The contribution of the purinergic mediator to immunomodulation was confirmed by exposing in vitro different immune effector cells to the action of primary hAECs. B cells showed an enhanced proliferation and diminished spontaneous apoptosis when in contact with hAEC. T cell proliferation was partially inhibited by hAEC through ADO production, as confirmed by using specific ectoenzyme inhibitors. Further, hAEC induced an expansion of both T and B regulatory cells. Last, hAEC inhibited NK cell proliferation. However, the involvement of ADO-producing ectoenzymes is less apparent in this context. In conclusion, hAEC exert different in vitro immunoregulatory effects, per se, as a result of interactions with different populations of immune effector cells. These results support the view that hAEC are instrumental for regenerative medicine as well as in therapeutic applications for immune-related diseases.

Ion Channels in Cancer: Are Cancer Hallmarks Oncochannelopathies?

Genomic instability is a primary cause and fundamental feature of human cancer. However, all cancer cell genotypes generally translate into several common pathophysiological features, often referred to as cancer hallmarks. Although nowadays the catalog of cancer hallmarks is quite broad, the most common and obvious of them are 1) uncontrolled proliferation, 2) resistance to programmed cell death (apoptosis), 3) tissue invasion and metastasis, and 4) sustained angiogenesis. Among the genes affected by cancer, those encoding ion channels are present. Membrane proteins responsible for signaling within cell and among cells, for coupling of extracellular events with intracellular responses, and for maintaining intracellular ionic homeostasis ion channels contribute to various extents to pathophysiological features of each cancer hallmark. Moreover, tight association of these hallmarks with ion channel dysfunction gives a good reason to classify them as special type of channelopathies, namely oncochannelopathies. Although the relation of cancer hallmarks to ion channel dysfunction differs from classical definition of channelopathies, as disease states causally linked with inherited mutations of ion channel genes that alter channel's biophysical properties, in a broader context of the disease state, to which pathogenesis ion channels essentially contribute, such classification seems absolutely appropriate. In this review the authors provide arguments to substantiate such point of view.

P2RX7 Purinoceptor as a Therapeutic Target-The Second Coming?

The P2RX7 receptor is a unique member of a family of extracellular ATP (eATP)-gated ion channels expressed in immune cells, where its activation triggers the inflammatory cascade. Therefore, P2RX7 has been long investigated as a target in the treatment of infectious and inflammatory diseases. Subsequently, P2RX7 signaling has been documented in other physiological and pathological processes including pain, CNS and psychiatric disorders and cancer. As a result, a range of P2RX7 antagonists have been developed and trialed. Interestingly, the recent crystallization of mammalian and chicken receptors revealed that most widely-used antagonists may bind a unique allosteric site. The availability of crystal structures allows rational design of improved antagonists and modeling of binding sites of the known or presumed inhibitors. However, several unanswered questions limit the cogent development of P2RX7 therapies. Firstly, this receptor functions as an ion channel, but its chronic stimulation by high eATP causes opening of the non-selective large pore (LP), which can trigger cell death. Not only the molecular mechanism of LP opening is still not fully understood but its function(s) are also unclear. Furthermore, how can tumor cells take advantage of P2RX7 for growth and spread and yet survive overexpression of potentially cytotoxic LP in the eATP-rich environment? The recent discovery of the feedback loop, wherein the LP-evoked release of active MMP-2 triggers the receptor cleavage, provided one explanation. Another mechanism might be that of cancer cells expressing a structurally altered P2RX7 receptor, devoid of the LP function. Exploiting such mechanisms should lead to the development of new, less toxic anticancer treatments. Notably, targeted inhibition of P2RX7 is crucial as its global blockade reduces the immune and inflammatory responses, which have important anti-tumor effects in some types of malignancies. Therefore, another novel approach is the synthesis of tissue/cell specific P2RX7 antagonists. Progress has been aided by the development of p2rx7 knockout mice and new conditional knock-in and knock-out models are being created. In this review, we seek to summarize the recent advances in our understanding of molecular mechanisms of receptor activation and inhibition, which cause its re-emergence as an important therapeutic target. We also highlight the key difficulties affecting this development.

Negative selection in tumor genome evolution acts on essential cellular functions and the immunopeptidome

Background

Natural selection shapes cancer genomes. Previous studies used signatures of positive selection to identify genes driving malignant transformation. However, the contribution of negative selection against somatic mutations that affect essential tumor functions or specific domains remains a controversial topic.

Results

Here, we analyze 7546 individual exomes from 26 tumor types from TCGA data to explore the portion of the cancer exome under negative selection. Although we find most of the genes neutrally evolving in a pan-cancer framework, we identify essential cancer genes and immune-exposed protein regions under significant negative selection. Moreover, our simulations suggest that the amount of negative selection is underestimated. We therefore choose an empirical approach to identify genes, functions, and protein regions under negative selection. We find that expression and mutation status of negatively selected genes is indicative of patient survival. Processes that are most strongly conserved are those that play fundamental cellular roles such as protein synthesis, glucose metabolism, and molecular transport. Intriguingly, we observe strong signals of selection in the immunopeptidome and proteins controlling peptide exposition, highlighting the importance of immune surveillance evasion. Additionally, tumor type-specific immune activity correlates with the strength of negative selection on human epitopes.

Conclusions

In summary, our results show that negative selection is a hallmark of cell essentiality and immune response in cancer. The functional domains identified could be exploited therapeutically, ultimately allowing for the development of novel cancer treatments.

Electronic supplementary material

The online version of this article (10.1186/s13059-018-1434-0) contains supplementary material, which is available to authorized users.

Kinin and Purine Signaling Contributes to Neuroblastoma Metastasis

Bone marrow metastasis occurs in approximately 350,000 patients that annually die in the U.S. alone. In view of the importance of tumor cell migration into the bone marrow, we have here investigated effects of various concentrations of stromal cell-derived factor-1 (SDF-1), bradykinin- and ATP on bone marrow metastasis. We show for first time that bradykinin augmented chemotactic responsiveness of neuroblastoma cells to SDF-1 and ATP concentrations, encountered under physiological conditions. Bradykinin upregulated VEGF expression, increased metalloproteinase activity and induced adhesion of neuroblastoma cells. Bradykinin augmented SDF-1-induced intracellular Ca²⁺ mobilization as well as resensitization and expression of ATP-sensing P2X7 receptors. Bradykinin treatment resulted in higher gene expression levels of the truncated P2X7B receptor compared to those of the P2X7A full-length isoform. Bradykinin as pro-metastatic factor induced tumor proliferation that was significantly decreased by P2X7 receptor antagonists; however, the peptide did not enhance cell death nor P2X7A receptor-related pore activity, promoting neuroblastoma growth. Furthermore, immunodeficient nude/nude mice transplanted with bradykinin-pretreated neuroblastoma cells revealed significantly higher metastasis rates compared to animals injected with untreated cells. In contrast, animals receiving Brilliant Blue G, a P2X7 receptor antagonist, did not show any specific dissemination of neuroblastoma cells to the bone marrow and liver, and metastasis rates were drastically reduced. Our data suggests correlated actions of kinins and purines in neuroblastoma dissemination, providing novel avenues for clinic research in preventing metastasis.

Lack of chemopreventive effects of P2X7R inhibitors against pancreatic cancer

Pancreatic cancer (PC) is an almost uniformly lethal disease with inflammation playing an important role in its progression. Sustained stimulation of purinergic receptor P2X7 drives induction of NLRP inflammasome activation. To understand the role of P2X7 receptor and inflammasome, we performed transcriptomic analysis of p48^Cre/+-LSL-Kras^G12D/+ mice pancreatic tumors by next generation sequencing. Results showed that P2X7R's key inflammasome components, IL-1β and caspase-1 are highly expressed (p < 0.05) in pancreatic tumors. Hence, to target P2X7R, we tested effects of two P2X7R antagonists, A438079 and AZ10606120, on pancreatic intraepithelial neoplasms (PanINs) and their progression to PC in p48^Cre/+-LSL-Kras^G12D/+ mice. Following dose optimization studies, for chemoprevention efficacy, six-week-old p48^Cre/+-LSL-Kras^G12D/+ mice (24–36/group) were fed modified AIN-76A diets containing 0, 50 or 100 ppm A438079 and AZ10606120 for 38 weeks. Pancreata were collected, weighed, and evaluated for PanINs and PDAC. Control diet-fed male mice showed 50% PDAC incidence. Dietary A438079 and AZ10606120 showed 60% PDAC incidence. A marginal increase of PanIN 3 (carcinoma in-situ) was observed in drug-treated mice. Importantly, the carcinoma spread in untreated mice was 24% compared to 43–53% in treatment groups. Reduced survival rates were observed in mice exposed to P2X7R inhibitors. Both drugs showed a decrease in caspase-3, caspase-1, p21 and Cdc25c. Dietary A438079 showed modest inhibition of P2X7R, NLRP3, and IL-33, whereas AZ10606120 had no effects. In summary, targeting the P2X7R pathway by A438079 and AZ10606120 failed to show chemopreventive effects against PC and slightly enhanced PanIN progression to PDAC. Hence, caution is needed while treating high-risk individuals with P2X7R inhibitors.

The P2X7 receptor forms a dye-permeable pore independent of its intracellular domain but dependent on membrane lipid composition

The P2X7 receptor mediates extracellular ATP signaling implicated in the development of devastating diseases such as chronic pain and cancer. Activation of the P2X7 receptor leads to opening of the characteristic dye-permeable membrane pore for molecules up to ~900 Da. However, it remains controversial what constitutes this peculiar pore and how it opens. Here we show that the panda receptor, when purified and reconstituted into liposomes, forms an intrinsic dye-permeable pore in the absence of other cellular components. Unexpectedly, we found that this pore opens independent of its unique C-terminal domain. We also found that P2X7 channel activity is facilitated by phosphatidylglycerol and sphingomyelin, but dominantly inhibited by cholesterol through direct interactions with the transmembrane domain. In combination with cell-based functional studies, our data suggest that the P2X7 receptor itself constitutes a lipid-composition dependent dye-permeable pore, whose opening is facilitated by palmitoylated cysteines near the pore-lining helix.

Potential role of P2X7R in esophageal squamous cell carcinoma proliferation

Esophageal cancer is an aggressive tumor and is the sixth leading cause of cancer death worldwide. ATP is well known to regulate cancer progression in a variety of models by different mechanisms, including P2X7R activation. This study aimed to evaluate the role of P2X7R in esophageal squamous cell carcinoma (ESCC) proliferation. Our results show that treatment with high ATP concentrations induced a decrease in cell number, cell viability, number of polyclonal colonies, and reduced migration of ESCC. The treatment with the selective P2X7R antagonist A740003 or siRNA for P2X7 reverted this effect in the KYSE450 cell line. In addition, results showed that P2X7R is highly expressed, at mRNA and protein levels, in KYSE450 lineage. Additionally, KYSE450, KYSE30, and OE21 cells express P2X3R, P2X4R, P2X5R, P2X6R, and P2X7R genes. P2X1R is expressed by KYSE30 and KYSE450, and only KYSE450 expresses the P2X2R gene. Furthermore, esophageal cancer cell line KYSE450 presented higher expression of E-NTPDases 1 and 2 and of Ecto-5'-NT/CD73 when compared to normal cells. This cell line also exhibits ATPase, ADPase, and AMPase activity, although in different levels, and the co-treatment of apyrase was able to revert the antiproliferative effects of ATP. Moreover, results showed high immunostaining for P2X7R in biopsies of patients with esophageal carcinoma, indicating the involvement of this receptor in the growth of this type of cancer. The results suggest that P2X7R may be a potential pharmacological target to treat ESCC and can lead us to further investigate the effect of this receptor in cancer cell progression.

Escape from adamantane: Scaffold optimization of novel P2X7 antagonists featuring complex polycycles

The adamantane scaffold, despite being widely used in medicinal chemistry, is not devoid of problems. In recent years we have developed new polycyclic scaffolds as surrogates of the adamantane group with encouraging results in multiple targets. As an adamantane scaffold is a common structural feature in several P2X7 receptor antagonists, herein we report the synthesis and pharmacological evaluation of multiple replacement options of adamantane that maintain a good activity profile. Molecular modeling studies support the binding of the compounds to a site close to the central pore, rather than to the ATP-binding site and shed light on the structural requirements for novel P2X7 antagonists.

Targeting of the P2X7 receptor in pancreatic cancer and stellate cells

The ATP‐gated receptor P2X7 (P2X7R) is involved in regulation of cell survival and has been of interest in cancer field. Pancreatic ductal adenocarcinoma (PDAC) is a deadly cancer and new markers and therapeutic targets are needed. PDAC is characterized by a complex tumour microenvironment, which includes cancer and pancreatic stellate cells (PSCs), and potentially high nucleotide/side turnover. Our aim was to determine P2X7R expression and function in human pancreatic cancer cells in vitro as well as to perform in vivo efficacy study applying P2X7R inhibitor in an orthotopic xenograft mouse model of PDAC. In the in vitro studies we show that human PDAC cells with luciferase gene (PancTu‐1 Luc cells) express high levels of P2X7R protein. Allosteric P2X7R antagonist AZ10606120 inhibited cell proliferation in basal conditions, indicating that P2X7R was tonically active. Extracellular ATP and BzATP, to which the P2X7R is more sensitive, further affected cell survival and confirmed complex functionality of P2X7R. PancTu‐1 Luc migration and invasion was reduced by AZ10606120, and it was stimulated by PSCs, but not by PSCs from P2X7^‐/‐ animals. PancTu‐1 Luc cells were orthotopically transplanted into nude mice and tumour growth was followed noninvasively by bioluminescence imaging. AZ10606120‐treated mice showed reduced bioluminescence compared to saline‐treated mice. Immunohistochemical analysis confirmed P2X7R expression in cancer and PSC cells, and in metaplastic/neoplastic acinar and duct structures. PSCs number/activity and collagen deposition was reduced in AZ10606120‐treated tumours.

What's new?

Pancreatic ductal adenocarcinoma (PDAC) is one the most difficult types of cancer to detect and treat, challenges that could be overcome through the discovery and development of novel markers and therapeutic strategies. Here, the P2X7 receptor, which regulates cell survival, is shown to also support cell proliferation, migration and invasion in human P2X7R‐expressing PDAC cells. Treatment of orthotopic PDAC tumor‐bearing mice with the P2X7R‐specific inhibitor, AZ10606120, resulted in decreased tumor bioluminescence and reductions in pancreatic stellate cells and collagen deposition. Targeting of P2X7R warrants further investigation as a promising therapeutic approach in pancreatic cancer.

Calcium Channels and Associated Receptors in Malignant Brain Tumor Therapy

Malignant brain tumors are highly lethal and aggressive. Despite recent advances in the current therapies, which include the combination of surgery and radio/chemotherapy, the average survival rate remains poor. Altered regulation of ion channels is part of the neoplastic transformation, which suggests that ion channels are involved in cancer. Distinct classes of calcium-permeable channels are abnormally expressed in cancer and are likely involved in the alterations underlying malignant growth. Specifically, cytosolic Ca(2+) activity plays an important role in the regulation of cell proliferation, and Ca(2+) signaling is altered in proliferating tumor cells. A series of previous studies emphasized the importance of the T-type low-voltage-gated calcium channels (VGCC) in different cancer types, including gliomas, and remarkably, pharmacologic inhibition of T-type VGCC caused antiproliferative effects and triggered apoptosis of human glioma cells. Other calcium permeable channels, such as transient receptor potential (TRP) channels, contribute to changes in Ca(2+) by modulating the driving force for Ca(2+) entry, and some TRP channels are required for proliferation and migration in gliomas. Furthermore, recent evidence shows that TRP channels contribute to the progression and survival of the glioblastoma patients. Likewise, the purinergic P2X7 receptor acts as a direct conduit for Ca(2+)-influx and an indirect activator of voltage-gated Ca(2+)-channel. Evidence also shows that P2X7 receptor activation is linked to elevated expression of inflammation promoting factors, tumor cell migration, an increase in intracellular mobilization of Ca(2+), and membrane depolarization in gliomas. Therefore, this review summarizes the recent findings on calcium channels and associated receptors as potential targets to treat malignant gliomas.

P2X7 receptor in skin biology and diseases

The P2X7 receptor is a trimeric ligand-gated cation channel present on immune and other cells. Activation of this receptor by its natural ligand extracellular adenosine triphosphate results in a variety of downstream responses, including the release of pro-inflammatory mediators and cell death. In normal skin, P2X7 is present on keratinocytes, Langerhans cells and fibroblasts, while the presence of this receptor on other cutaneous cells is mainly inferred from studies of equivalent cell types present in other tissues. Mast cells in normal skin however express negligible amounts of P2X7, which can be upregulated in cutaneous disease. This review discusses the potential significance of P2X7 in skin biology, and the role of this receptor in inflammatory skin disorders such as irritant and chronic dermatitis, psoriasis, graft-versus-host disease, as well is in wound healing, transplantation and skin cancer.

G protein-coupled receptors as promising cancer targets

G protein-coupled receptors (GPCRs) regulate an array of fundamental biological processes, such as growth, metabolism and homeostasis. Specifically, GPCRs are involved in cancer initiation and progression. However, compared with the involvement of the epidermal growth factor receptor in cancer, that of GPCRs have been largely ignored. Recent findings have implicated many GPCRs in tumorigenesis, tumor progression, invasion and metastasis. Moreover, GPCRs contribute to the establishment and maintenance of a microenvironment which is permissive for tumor formation and growth, including effects upon surrounding blood vessels, signaling molecules and the extracellular matrix. Thus, GPCRs are considered to be among the most useful drug targets against many solid cancers. Development of selective ligands targeting GPCRs may provide novel and effective treatment strategies against cancer and some anticancer compounds are now in clinical trials. Here, we focus on tumor related GPCRs, such as G protein-coupled receptor 30, the lysophosphatidic acid receptor, angiotensin receptors 1 and 2, the sphingosine 1-phosphate receptors and gastrin releasing peptide receptor. We also summarize their tissue distributions, activation and roles in tumorigenesis and discuss the potential use of GPCR agonists and antagonists in cancer therapy.

Energy-requiring uptake of prostasomes and PC3 cell-derived exosomes into non-malignant and malignant cells

Epithelial cells lining the prostate acini release, in a regulated manner (exocytosis), nanosized vesicles called prostasomes that belong to the exosome family. Prostate cancer cells have preserved this ability to generate and export exosomes to the extracellular space. We previously demonstrated that human prostasomes have an ATP-forming capacity. In this study, we compared the capacity of extracellular vesicles (EVs) to generate ATP between normal seminal prostasomes and exosomes secreted by PC3 cells (PC3 exosomes), a prostate cancer cell line. Proteomic analyses identified enzymes of the glycolytic chain in both prostasomes and PC3 exosomes, and we found that both of them were capable of generating ATP when supplied with substrates. Notably, the net production of extracellular ATP was low for prostasomes due to a high ATPase activity contrary to an elevated net ATP level for PC3 exosomes because of their low ATPase activity. The uptake of the 2 types of EVs by normal prostate epithelial cells (CRL2221) and prostate cancer cells (PC3) was visualized and measured, demonstrating differential kinetics. Interestingly, this uptake was dependent upon an ongoing glycolytic flux involving extracellular ATP formation by EVs and/or intracellular ATP produced from the recipient cells. We conclude that the internalization of EVs into recipient cells is an energy-requiring process also demanding an active V-ATPase and the capacity of EVs to generate extracellular ATP may play a role in this process.

The P2X7 receptor regulates cell survival, migration and invasion of pancreatic ductal adenocarcinoma cells

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) is presently one of the cancers with the worst survival rates and least effective treatments. Moreover, total deaths due to PDAC are predicted to increase in the next 15 years. Therefore, novel insights into basic mechanism of PDAC development and therapies are needed. PDAC is characterized by a complex microenvironment, in which cancer and stromal cells release different molecules, such as ATP. ATP can be transported and/or exocytosed from active cancer cells and released from dying cells in the necrotic core of the cancer. We hypothesized that one of the ATP receptors, the P2X7 receptor (P2X7R) could be an important player in PDAC behaviour.

METHODS

We determined the expression (real time PCR and Western blot) and localization (immunofluorescence) of P2X7R in human PDAC cell lines (AsPC-1, BxPC-3, Capan-1, MiaPaCa-2, Panc-1) and a "normal" human pancreatic duct epithelial cell line (HPDE). The function of P2X7R in proliferation (BrdU assay), migration (wound assay) and invasion (Boyden chamber with matrigel) was characterized. Furthermore, we studied P2X7R-dependent pore formation (YoPro-1 assay) and cell death (caspase and annexin V / propidium iodide assays).

RESULTS

We found higher expression of P2X7R protein in PDAC compared to HPDE cells. P2X7R had notable disparate effects on PDAC survival. Firstly, high concentrations of ATP or the specific P2X7R agonist, BzATP, had cytotoxic effects in all cell lines, and cell death was mediated by necrosis. Moreover, the P2X7R-pore antagonist, A438079, prevented ATP-induced pore formation and cell death. Second, in basal conditions and with low concentrations of ATP/BzATP, the P2X7R allosteric inhibitor AZ10606120 reduced proliferation in all PDAC cell lines. P2X7R also affected other key characteristics of cancer cell behavior. AZ10606120 reduced cell migration and invasion in PDAC cell lines compared to that of untreated/vehicle-treated control cells, and stimulation with sub-millimolar concentrations of ATP or BzATP substantially increased cell invasion.

CONCLUSIONS

PDAC cell lines overexpress P2X7R and the receptor plays crucial roles in cell survival, migration and invasion. Therefore, we propose that drugs targeting P2X7R could be exploited in therapy of pancreatic cancer.

P2X7 receptor as predictor gene for glioma radiosensitivity and median survival

Glioblastoma multiforme (GBM) is considered the most lethal intracranial tumor and the median survival time is approximately 14 months. Although some glioma cells present radioresistance, radiotherapy has been the mainstay of therapy for patients with malignant glioma. The activation of P2X7 receptor (P2X7R) is responsible for ATP-induced death in various cell types. In this study, we analyzed the importance of ATP-P2X7R pathway in the radiotherapy response P2X7R silenced cell lines, in vivo and human tumor samples. Both glioma cell lines used in this study present a functional P2X7R and the P2X7R silencing reduced P2X7R pore activity by ethidium bromide uptake. Gamma radiation (2Gy) treatment reduced cell number in a P2X7R-dependent way, since both P2X7R antagonist and P2X7R silencing blocked the cell cytotoxicity caused by irradiation after 24h. The activation of P2X7R is time-dependent, as EtBr uptake significantly increased after 24h of irradiation. The radiotherapy plus ATP incubation significantly increased annexin V incorporation, compared with radiotherapy alone, suggesting that ATP acts synergistically with radiotherapy. Of note, GL261 P2X7R silenced-bearing mice failed in respond to radiotherapy (8Gy) and GL261 WT-bearing mice, that constitutively express P2X7R, presented a significant reduction in tumor volume after radiotherapy, showing in vivo that functional P2X7R expression is essential for an efficient radiotherapy response in gliomas. We also showed that a high P2X7R expression is a good prognostic factor for glioma radiosensitivity and survival probability in humans. Our data revealed the relevance of P2X7R expression in glioma cells to a successful radiotherapy response, and shed new light on this receptor as a useful predictor of the sensitivity of cancer patients to radiotherapy and median survival.

P2X7 receptor predicts postoperative cancer-specific survival of patients with clear-cell renal cell carcinoma

The P2X7 receptor, an ATP-gated plasma membrane ion channel, is involved in inflammation, apoptosis and cell proliferation, and thereby plays a crucial role during oncogenic transformation in various malignancies. This study aims to evaluate the impact of P2X7 receptor expression on postoperative cancer-specific survival of patients with clear-cell renal cell carcinoma (ccRCC). A total of 273 patients with ccRCC undergoing nephrectomy at a single institution were retrospectively enrolled in this study, among which 86 patients died of this disease and six patients died of other causes. Clinicopathologic features and cancer-specific survival (CSS) were recorded. P2X7 expression was assessed by immunohistochemistry in clinical specimens. Kaplan–Meier method with log rank test was performed to compare survival curves. Cox regression models were used to evaluate the prognostic values of variables on CSS. Concordance index was calculated to assess prognostic accuracy of prognostic models. Median follow-up period was 90 months (range, 11–120 months). Intratumoral P2X7 expression was significantly lower than peritumoral tissues (P < 0.001). Moreover, high intratumoral P2X7 expression, which was significantly associated with shorten CSS (P < 0.001), high TNM stage (P = 0.038), Fuhrman grade (P = 0.035), SSIGN (stage, size, grade, and necrosis) score (P = 0.021) and University of California Integrated Staging System (UISS) score (P = 0.007), was indicated to be an independent prognostic factor for CSS (hazard ratio [HR], 1.693; P = 0.034). The prognostic accuracy of TNM stage, UISS and SSIGN scoring models was improved when intratumoral P2X7 expression was added. Intratumoral P2X7 expression is a potential independent adverse prognostic indicator for postoperative CSS of patients with ccRCC.

Potentiation of temozolomide antitumor effect by purine receptor ligands able to restrain the in vitro growth of human glioblastoma stem cells

Glioblastoma multiforme (GBM), the most common and aggressive brain tumor in humans, comprises a population of stem-like cells (GSCs) that are currently investigated as potential target for GBM therapy. Here, we used GSCs isolated from three different GBM surgical specimens to examine the antitumor activity of purines. Cultured GSCs expressed either metabotropic adenosine P1 and ATP P2Y receptors or ionotropic P2X7 receptors. GSC exposure for 48 h to 10-150 μM ATP, P2R ligand, or to ADPβS or MRS2365, P2Y1R agonists, enhanced cell expansion. This effect was counteracted by the PY1R antagonist MRS2500. In contrast, 48-h treatment with higher doses of ATP or UTP, which binds to P2Y2/4R, or 2'(3')-O-(4-benzoylbenzoyl)-ATP (Bz-ATP), P2X7R agonist, decreased GSC proliferation. Such a reduction was due to apoptotic or necrotic cell death but mostly to growth arrest. Accordingly, cell regrowth and secondary neurosphere formation were observed 2 weeks after the end of treatment. Suramin, nonselective P2R antagonist, MRS1220 or AZ11645373, selective A3R or P2X7R antagonists, respectively, counteracted ATP antiproliferative effects. AZ11645373 also abolished the inhibitory effect of Bz-ATP low doses on GSC growth. These findings provide important clues on the anticancer potential of ligands for A3R, P2Y1R, and P2X7R, which are involved in the GSC growth control. Interestingly, ATP and BzATP potentiated the cytotoxicity of temozolomide (TMZ), currently used for GBM therapy, enabling it to cause a greater and long-lasting inhibitory effect on GSC duplication when readded to cells previously treated with purine nucleotides plus TMZ. These are the first findings identifying purine nucleotides as able to enhance TMZ antitumor efficacy and might have an immediate translational impact.

The expression of P2X₇ receptors in EPCs and their potential role in the targeting of EPCs to brain gliomas

In order to use endothelial progenitor cells (EPCs) as a therapeutic and imaging probe to overcome antiangiogenic resistance for gliomas, how to enhance proliferation and targeting ability of transplanted EPCs is a high priority. Here, we confirmed, for the first time, the expression of P2X7 receptors in rat spleen-derived EPCs. Activation of P2X7 receptors in EPCs by BzATP promoted cells proliferation and migration, rather than apoptosis. In vivo, the homing of transplanted EPCs after long-term suppression of P2X7 receptors by persistent BBG stimulation was evaluated by MRI, immunohistochemistry and flow cytometry. Compared to the group without BBG treatment, less transplanted EPCs homed to gliomas in the group with BBG treatment, especially integrated into the vessels containing tumor-derived endothelial cells in gliomas. Moreover, western blot showed that CXCL1 expression was downregulated in gliomas with BBG treatment, which meant P2X7 receptors suppression inhibited the homing of EPCs to gliomas through down-regulation of CXCLl expression. Further, effects of P2X7 receptors on C6 glioma cells or gliomas were evaluated at the same dose of BzATP or BBG used in EPCs experiments in vitro and in vivo. MTT assay and MRI revealed that P2X7 receptors exerted no significant promoting effect on C6 glioma cells proliferation, gliomas growth and angiogenesis. Taken together, our findings imply the possibility of promoting proliferation and targeting ability of transplanted EPCs to brain gliomas in vivo through P2X7 receptors, which may provide new perspectives on application of EPCs as a therapeutic and imaging probe to overcome antiangiogenic resistance for gliomas.

Prelude to oral microbes and chronic diseases: past, present and future

Associations between oral and systemic health are ancient. Oral opportunistic bacteria, particularly, Porphyromonas gingivalis and Fusobacterium nucleatum, have recently been deviated from their traditional roles as periodontal pathogens and arguably ascended to central players based on their participations in complex co-dependent mechanisms of diverse systemic chronic diseases risk and pathogenesis, including cancers, rheumatoid-arthritis, and diabetes.

Role of ion channels in regulating Ca²⁺ homeostasis during the interplay between immune and cancer cells

Ion channels are abundantly expressed in both excitable and non-excitable cells, thereby regulating the Ca²⁺ influx and downstream signaling pathways of physiological processes. The immune system is specialized in the process of cancer cell recognition and elimination, and is regulated by different ion channels. In comparison with the immune cells, ion channels behave differently in cancer cells by making the tumor cells more hyperpolarized and influence cancer cell proliferation and metastasis. Therefore, ion channels comprise an important therapeutic target in anti-cancer treatment. In this review, we discuss the implication of ion channels in regulation of Ca²⁺ homeostasis during the crosstalk between immune and cancer cell as well as their role in cancer progression.

Understanding the roles of the P2X7 receptor in solid tumour progression and therapeutic perspectives

P2X7 is an intriguing ionotropic receptor for which the activation by extracellular ATP induces rapid inward cationic currents and intracellular signalling pathways associated with numerous physiological processes such as the induction of the inflammatory cascade, the survival and proliferation of cells. In contrast, long-term stimulation of P2X7 is generally associated with membrane permeabilisation and cell death. Recently, P2X7 has attracted great attention in the cancer field, and particularly in the neoplastic transformation and the progression of solid tumours. A growing number of studies were published; however they often appeared contradictory in their results and conclusions. As such, the involvement of P2X7 in the oncogenic process remains unclear so far. The present review aims to discuss the current knowledge and hypotheses on the involvement of the P2X7 receptor in the development and progression of solid tumours, and highlight the different aspects that require further clarification in order to decipher whether P2X7 could be considered as a cancer biomarker or as a target for pharmacological intervention. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

P2X7 protein expression and polymorphism in non-small cell lung cancer (NSCLC)

Background

P2X7, a purinergic receptor, plays important roles in inflammatory diseases, but recently its expression has been found in several tumors, suggesting a potential role as a cancer cell biomarker. Moreover, the relative amount of P2X7 varies among human individuals due to numerous single nucleotide polymorphisms resulting in either a loss- or gain-of-function; the P2X7 gene is highly polymorphic, and polymorphisms in the promoter or coding region may modify its expression or function. A polymorphism in exon 13 of the P2X7 receptor gene at the +1513 position (Glu496Ala substitution, corresponding to SNP rs3751143) has been shown to eradicate the function of this receptor and has been correlated with histological variants and clinical parameters in thyroid cancer. Until now, no data regarding P2X7 expression and polymorphisms in lung cancer have been published; based on these premises, we decided to evaluate the impact of the P2X7 expression and polymorphisms in ninety-seven cases of non-small cell lung cancer (NSCLC).

Results

No significant difference in the genotype frequency of the A1513C polymorphism was found between the two histological variants of NSCLC, adenocarcinoma and squamous cell carcinoma, and no statistically significant associations were observed between P2X7 protein expression and the main clinico-pathological characteristics of the NSCLC patients.

Conclusions

Based on our results, P2X7 expression and polymorphisms seem to have no potential impact in patients with non-small cell lung cancer; however, further studies will surely provide deeper insights regarding the role of this receptor at the clinical level in NSCLC.

Purinergic signalling and cancer

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

Possible role of hemichannels in cancer

In humans, connexins (Cxs) and pannexins (Panxs) are the building blocks of hemichannels. These proteins are frequently altered in neoplastic cells and have traditionally been considered as tumor suppressors. Alteration of Cxs and Panxs in cancer cells can be due to genetic, epigenetic and post-transcriptional/post-translational events. Activated hemichannels mediate the diffusional membrane transport of ions and small signaling molecules. In the last decade hemichannels have been shown to participate in diverse cell processes including the modulation of cell proliferation and survival. However, their possible role in tumor growth and expansion remains largely unexplored. Herein, we hypothesize about the possible role of hemichannels in carcinogenesis and tumor progression. To support this theory, we summarize the evidence regarding the involvement of hemichannels in cell proliferation and migration, as well as their possible role in the anti-tumor immune responses. In addition, we discuss the evidence linking hemichannels with cancer in diverse models and comment on the current technical limitations for their study.

Looking in the Porphyromonas gingivalis cabinet of curiosities: the microbium, the host and cancer association

The past decades of biomedical research have yielded massive evidence for the contribution of the microbiome in the development of a variety of chronic human diseases. There is emerging evidence that Porphyromonas gingivalis, a well-adapted opportunistic pathogen of the oral mucosa and prominent constituent of oral biofilms, best known for its involvement in periodontitis, may be an important mediator in the development of a number of multifactorial and seemingly unrelated chronic diseases, such as rheumatoid arthritis and orodigestive cancers. Orodigestive cancers represent a large proportion of the total malignancies worldwide, and include cancers of the oral cavity, gastrointestinal tract and pancreas. For prevention and/or enhanced prognosis of these diseases, a good understanding of the pathophysiological mechanisms and the interaction between P. gingivalis and host is much needed. With this review, we introduce the currently accumulated knowledge on P. gingivalis's plausible association with cancer as a risk modifier, and present the putative cancer-promoting cellular and molecular mechanisms that this organism may influence in the oral mucosa.

The P2X7 receptor: a key player in immune-mediated bone loss?

Inflammatory diseases are often multiorganic diseases with manifestations not related directly to the primary affected organ. They are often complicated by a generalized bone loss that subsequently leads to osteoporosis and bone fractures. The exact mechanism for the accompanying bone loss is not understood in full detail, but factors such as glucocorticoid treatment, immobilization, malnutrition, and insufficient intake of vitamin D play a role. However, it has become evident that the inflammatory process itself is involved and the resulting bone loss is termed immune-mediated bone loss. It stems from an increase in bone resorption and the pro-inflammatory cytokines tumor necrosis factor alpha and interleukin 1 beta and has been shown to not only mediate the inflammatory response but also to strongly stimulate bone degradation. The purinergic P2X7 receptor is central in the processing of these two cytokines and in the initiation of the inflammatory response, and it is a key molecule in the regulation of both bone formation and bone resorption. The aim of this review is therefore to provide evidence-based novel hypotheses of the role of ATP-mediated purinergic signalling via the P2X7 receptor in immune-mediated bone loss and -osteoporosis.

The activation of P2Y2 receptors increases MCF-7 breast cancer cells migration through the MEK-ERK1/2 signalling pathway

Adenosine 5'-triphosphate (ATP) is found in high concentrations in the extracellular microenvironment of tumours and is postulated to play critical roles in cancer progression. In the present study, we found that stimulation of human MCF-7 breast cancer cells with 30 µM ATP increased their migration by 140 ± 31%, whereas it had minor or no effect on their proliferation. This effect was prevented by the ectonucleotidase apyrase and was antagonized by suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, consistently with the participation of P2 receptors. MCF-7 cells expressed messenger RNA for all known P2Y receptors and for P2X2, P2X4, P2X5, P2X6 and P2X7 receptors. Brief applications (20 s) of external ATP resulted in a 50 pA P2X-like inward current. ATP, but not adenosine diphosphate or uridine diphosphate, increased the intracellular calcium concentration in absence of extracellular calcium, and this effect was prevented by the inhibition of phospholipase C. Uridine triphosphate (UTP) (10 µM) and 2-thio-UTP (10 µM) increased intracellular calcium concentration and cell migration to the same extent as ATP. The UTP-dependent increase in cell migration was absent in cells knocked-down for P2Y2. It was inhibited by MEK inhibitor PD98059. UTP induced a time-dependent phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2), which was prevented by the incubation with PD98059. Taken together, these results highlight the importance of the purinergic signalling in cancer cells and indicate that the activation of P2Y2 receptors enhances breast cancer cells migration through the activation of a MEK-ERK1/2-dependent signalling pathway.

P2X7 receptor activation impairs exogenous MHC class I oligopeptides presentation in antigen presenting cells

Major histocompatibility complex class I (MHC I) on antigen presenting cells (APCs) is a potent molecule to activate CD8⁺ T cells and initiate immunity. P2X7 receptors (P2X7Rs) are present on the plasma membrane of APCs to sense the extracellular danger signal adenosine-5′-triphosphate (ATP). P2X7R activates the inflammasome and the release of IL-1β in macrophages and other immune cells to initiate the inflammatory response. Here we show that P2X7R stimulation by ATP in APCs decreased the amount of MHC I at the plasma membrane. Specific antagonism or genetic ablation of P2X7R inhibited the effects of ATP on levels of cellular MHC I. Furthermore, P2X7R stimulation was able to inhibit activation of CD8⁺ T cells via specific MHC I-oligopeptide complexes. Our study suggests that P2X7R activation on APCs is a novel inhibitor of adaptive CD8⁺ T cell immunity.

Insights into the Molecular Mechanisms Underlying Mammalian P2X7 Receptor Functions and Contributions in Diseases, Revealed by Structural Modeling and Single Nucleotide Polymorphisms

The mammalian P2X7 receptors (P2X7Rs), a member of the ionotropic P2X receptor family with distinctive functional properties, play an important part in mediating extracellular ATP signaling in health and disease. A clear delineation of the molecular mechanisms underlying the key receptor properties, such as ATP-binding, ion permeation, and large pore formation of the mammalian P2X7Rs, is still lacking, but such knowledge is crucial for a better understanding of their physiological functions and contributions in diseases and for development of therapeutics. The recent breakthroughs in determining the atomic structures of the zebrafish P2X4.1R in the closed and ATP-bound open states have provided the long-awaited structural information. The human P2RX7 gene is abundant with non-synonymous single nucleotide polymorphisms (NS-SNPs), which generate a repertoire of human P2X7Rs with point mutations. Characterizations of the NS-SNPs identified in patients of various disease conditions and the resulting mutations have informed previously unknown molecular mechanisms determining the mammalian P2X7R functions and diseases. In this review, we will discuss the new insights into such mechanisms provided by structural modeling and recent functional and genetic linkage studies of NS-SNPs.

Anthraquinone emodin inhibits human cancer cell invasiveness by antagonizing P2X7 receptors

The adenosine 5'-triphosphate (ATP)-gated Ca(2+)-permeable channel P2X7 receptor (P2X7R) is strongly upregulated in many tumors and cancer cells, and has an important role in cancer cell invasion associated with metastases. Emodin (1,3,8-trihydroxy-6-methylanthraquinone) is an anthraquinone derivative originally isolated from Rheum officinale Baill known for decades to possess anticancer properties. In this study, we examined the effects of emodin on P2X7R-dependent Ca(2+) signaling, extracellular matrix degradation, and in vitro and in vivo cancer cell invasiveness using highly aggressive human cancer cells. Inclusion of emodin at doses ≤10 µM in cell culture had no or very mild effect on the cell viability. ATP elicited increases in intracellular Ca(2+) concentration were reduced by 35 and 60% by 1 and 10 µM emodin, respectively. Emodin specifically inhibited P2X7R-mediated currents with an IC50 of 3 µM and did not inhibit the currents mediated by the other human P2X receptors heterologously expressed in human embryonic kidney (HEK293T) cells. ATP-induced increase in gelatinolytic activity, in cancer cell invasiveness in vitro and in cell morphology changes were prevented by 1 µM emodin. Furthermore, such ATP-evoked effects and inhibition by emodin were almost completely ablated in cancer cells transfected with P2X7R-specific small interfering RNA (siRNA) but not with scrambled siRNA. Finally, the in vivo invasiveness of the P2X7R-positive MDA-MB-435s breast cancer cells, assessed using a zebrafish model of micrometastases, was suppressed by 40 and 50% by 1 and 10 µM emodin. Taken together, these results provide consistent evidence to indicate that emodin inhibits human cancer cell invasiveness by specifically antagonizing the P2X7R.