Characterization of ATP-induced cell death in the GL261 mouse glioma

Elucidating the molecular basis of ATP-induced cell death in breast cancer: Construction of a robust prognostic model

BACKGROUND

Breast cancer is a multifaceted and formidable disease with profound public health implications. Cell demise mechanisms play a pivotal role in breast cancer pathogenesis, with ATP-triggered cell death attracting mounting interest for its unique specificity and potential therapeutic pertinence.

AIM

To investigate the impact of ATP-induced cell death (AICD) on breast cancer, enhancing our understanding of its mechanism.

METHODS

The foundational genes orchestrating AICD mechanisms were extracted from the literature, underpinning the establishment of a prognostic model. Simultaneously, a microRNA (miRNA) prognostic model was constructed that mirrored the gene-based prognostic model. Distinctions between high- and low-risk cohorts within mRNA and miRNA characteristic models were scrutinized, with the aim of delineating common influence mechanisms, substantiated through enrichment analysis and immune infiltration assessment.

RESULTS

The mRNA prognostic model in this study encompassed four specific mRNAs: P2X purinoceptor 4, pannexin 1, caspase 7, and cyclin 2. The miRNA prognostic model integrated four pivotal miRNAs: hsa-miR-615-3p, hsa-miR-519b-3p, hsa-miR-342-3p, and hsa-miR-324-3p. B cells, CD4+ T cells, CD8+ T cells, endothelial cells, and macrophages exhibited inverse correlations with risk scores across all breast cancer subtypes. Furthermore, Kyoto Encyclopedia of Genes and Genomes analysis revealed that genes differentially expressed in response to mRNA risk scores significantly enriched 25 signaling pathways, while miRNA risk scores significantly enriched 29 signaling pathways, with 16 pathways being jointly enriched.

CONCLUSION

Of paramount significance, distinct mRNA and miRNA signature models were devised tailored to AICD, both potentially autonomous prognostic factors. This study's elucidation of the molecular underpinnings of AICD in breast cancer enhances the arsenal of potential therapeutic tools, offering an unparalleled window for innovative interventions. Essentially, this paper reveals the hitherto enigmatic link between AICD and breast cancer, potentially leading to revolutionary progress in personalized oncology.

Targeting ENPP1 depletion may be a promising therapeutic strategy for treating oral squamous cell carcinoma via cytotoxic autophagy-related apoptosis

ENPP1 depletion closely related with modulation immunotherapy of several types of cancer. However, the role of ENPP1 correlation with autophagy in oral squamous cell carcinoma (OSCC) pathogenesis remain unknown. In this study, effects of ENPP1 on OSCC cells in vitro were examined by cell proliferation assay, transwell chamber assay, flow cytometry analysis and shRNA technique. Cellular key proteins related to cell autophagy and apoptosis were evaluated by Western blot and immunofluorescent staining. Moreover, functions of ENPP1 on OSCC process were observed in nude mouse model. We reported that overexpression of ENPP1 promote the growth of OSCC cell xenografts in nude mouse model. In contrast, ENPP1 downregulation significantly inhibits OSCC cancer growth and induces apoptosis both in vitro and in vivo, which are preceded by cytotoxic autophagy. ENPP1downregulation induces a robust accumulation of autophagosomes, increases LC3B-II and decreases SQSTM1/p62 in ENPP1-shRNA-treated cells and xenografts. Mechanistic studies show that ENPP1 downregulation increases PRKAA1 phosphorylation leading to ULK1 activation. AMPK-inhibition abrogates ENPP1 downregulation-induced ULK1-activation, LC3B-turnover and SQSTM1/p62-degradation while AMPK-activation potentiates it's effects. Collectively, these data uncover that ENPP1 downregulation induces autophagic cell death in OSCC cancer, which may provide a potential therapeutic target for the treatment of OSCC.

Adenosine triphosphate induced cell death: Mechanisms and implications in cancer biology and therapy

Adenosine triphosphate (ATP) induced cell death (AICD) is a critical cellular process that has garnered substantial scientific interest for its profound relevance to cancer biology and to therapeutic interventions. This comprehensive review unveils the intricate web of AICD mechanisms and their intricate connections with cancer biology. This review offers a comprehensive framework for comprehending the multifaceted role of AICD in the context of cancer. This is achieved by elucidating the dynamic interplay between systemic and cellular ATP homeostasis, deciphering the intricate mechanisms governing AICD, elucidating its intricate involvement in cancer signaling pathways, and scrutinizing validated key genes. Moreover, the exploration of AICD as a potential avenue for cancer treatment underscores its essential role in shaping the future landscape of cancer therapeutics.

ATP-induced cell death: a novel hypothesis for osteoporosis

ATP-induced cell death has emerged as a captivating realm of inquiry with profound ramifications in the context of osteoporosis. This study unveils a paradigm-shifting hypothesis that illuminates the prospective involvement of ATP-induced cellular demise in the etiology of osteoporosis. Initially, we explicate the morphological attributes of ATP-induced cell death and delve into the intricacies of the molecular machinery and regulatory networks governing ATP homeostasis and ATP-induced cell death. Subsequently, our focus pivots towards the multifaceted interplay between ATP-induced cellular demise and pivotal cellular protagonists, such as bone marrow-derived mesenchymal stem cells, osteoblasts, and osteoclasts, accentuating their potential contributions to secondary osteoporosis phenotypes, encompassing diabetic osteoporosis, glucocorticoid-induced osteoporosis, and postmenopausal osteoporosis. Furthermore, we probe the captivating interplay between ATP-induced cellular demise and alternative modalities of cellular demise, encompassing apoptosis, autophagy, and necroptosis. Through an all-encompassing inquiry into the intricate nexus connecting ATP-induced cellular demise and osteoporosis, our primary goal is to deepen our comprehension of the underlying mechanisms propelling this malady and establish a theoretical bedrock to underpin the development of pioneering therapeutic strategies.

P2X7 receptor promotes migration and invasion of non-small cell lung cancer A549 cells through the PI3K/Akt pathways

It has been demonstrated that the ATP-gated ion channel P2X7 receptor is involved in tumor progression and plays an important role in regulating tumor cell growth, invasion, migration and angiogenesis. However, P2X7 receptors have been relatively poorly studied in non-small cell lung cancer (NSCLC) cells. Therefore, the aim of this study was to investigate the effects of P2X7 receptor on A549 cells (NSCLC cell line) migration and invasion and to reveal the molecular mechanisms mediated by it. We detected the expression and function of P2X7 receptor in A549 cells. The effects and mechanisms of P2X7 receptor on A549 cells migration, invasion, and epithelial-mesenchymal transition were detected in vitro and in vivo. The results showed P2X7 receptor expressed by A549 cells had ion channel and macropore formation function. In addition, activation of P2X7 receptor by adenosine triphosphate (ATP) or 2'(3')-O-(4-Benzoylbenzoyl)-adenosine-5'-triphosphate (BzATP) promoted Epithelial-mesenchymal transition (EMT), migration and invasion of A549 cells, which was attenuated by treatment of cells with P2X7 receptor antagonist A438079 and Oxidized ATP. Furthermore, activation of P2X7 receptor increased phosphorylated protein kinase B (p-Akt) levels, and the phosphatidylinositol-tris-phosphate kinase 3 (PI3K)/protein kinase B (Akt) inhibitor LY294002 blocked migration and invasion of A549 cells induced by ATP or BzATP. At the same time, in vivo results showed that P2X7 receptor could also promote EMT and PI3K/Akt expression in transplanted tumors. Our study indicated that P2X7 receptor promotes A549 cells migration and invasion through the PI3K/Akt signaling pathway, suggesting that P2X7 receptor may be a potential therapeutic target for NSCLC.

Exploring CD39 and CD73 Expression as Potential Biomarkers in Prostate Cancer

Prostate cancer (PC) is the most diagnosed tumor in males and ranks as the second leading cause of male mortality in the western world. The CD39 and CD73 enzymes play a crucial role in cancer regulation by degrading nucleotides and forming nucleosides. This study aimed to investigate the expression of the CD39 and CD73 enzymes as potential therapeutic targets for PC. The initial part of this study retrospectively analyzed tissue samples from 23 PC patients. Using the TissueFAXSTM cytometry platform, we found significantly higher levels of CD39-labeling its intensity compared to CD73. Additionally, we observed a correlation between the Gleason score and the intensity of CD39 expression. In the prospective arm, blood samples were collected from 25 patients at the time of diagnosis and after six months of treatment to determine the expression of CD39 and CD73 in the serum extracellular vesicles (EVs) and to analyze nucleotide hydrolysis. Notably, the expression of CD39 in the EVs was significantly increased compared to the CD73 and/or combined CD39/CD73 expression levels at initial collection. Furthermore, our results demonstrated positive correlations between ADP hydrolysis and the transurethral resection and Gleason score. Understanding the role of ectonucleotidases is crucial for identifying new biomarkers in PC.

Purine and purinergic receptors in health and disease

Purines and purinergic receptors are widely distributed throughout the human body. Purine molecules within cells play crucial roles in regulating energy metabolism and other cellular processes, while extracellular purines transmit signals through specific purinergic receptors. The ubiquitous purinergic signaling maintains normal neural excitability, digestion and absorption, respiratory movement, and other complex physiological activities, and participates in cell proliferation, differentiation, migration, and death. Pathological dysregulation of purinergic signaling can result in the development of various diseases, including neurodegeneration, inflammatory reactions, and malignant tumors. The dysregulation or dysfunction of purines and purinergic receptors has been demonstrated to be closely associated with tumor progression. Compared with other subtypes of purinergic receptors, the P2X7 receptor (P2X7R) exhibits distinct characteristics (i.e., a low affinity for ATP, dual functionality upon activation, the mediation of ion channels, and nonselective pores formation) and is considered a promising target for antitumor therapy, particularly in patients with poor response to immunotherapy This review summarizes the physiological and pathological significance of purinergic signaling and purinergic receptors, analyzes their complex relationship with tumors, and proposes potential antitumor immunotherapy strategies from tumor P2X7R inhibition, tumor P2X7R overactivation, and host P2X7R activation. This review provides a reference for clinical immunotherapy and mechanism investigation.

P2X7 receptor antagonism by AZ10606120 significantly reduced in vitro tumour growth in human glioblastoma

Glioblastomas are highly aggressive and deadly brain tumours, with a median survival time of 14-18 months post-diagnosis. Current treatment modalities are limited and only modestly increase survival time. Effective therapeutic alternatives are urgently needed. The purinergic P2X7 receptor (P2X7R) is activated within the glioblastoma microenvironment and evidence suggests it contributes to tumour growth. Studies have implicated P2X7R involvement in a range of neoplasms, including glioblastomas, although the roles of P2X7R in the tumour milieu remain unclear. Here, we report a trophic, tumour-promoting role of P2X7R activation in both patient-derived primary glioblastoma cultures and the U251 human glioblastoma cell line, and demonstrate its inhibition reduces tumour growth in vitro. Primary glioblastoma and U251 cell cultures were treated with the specific P2X7R antagonist, AZ10606120 (AZ), for 72 h. The effects of AZ treatment were also compared to cells treated with the current first-line chemotherapeutic drug, temozolomide (TMZ), and a combination of both AZ and TMZ. P2X7R antagonism by AZ significantly depleted glioblastoma cell numbers compared to untreated cells, in both primary glioblastoma and U251 cultures. Notably, AZ treatment was more effective at tumour cell killing than TMZ. No synergistic effect between AZ and TMZ was observed. AZ treatment also significantly increased lactate dehydrogenase release in primary glioblastoma cultures, suggesting AZ-induced cellular cytotoxicity. Our results reveal a trophic role of P2X7R in glioblastoma. Importantly, these data highlight the potential for P2X7R inhibition as a novel and effective alternative therapeutic approach for patients with lethal glioblastomas.

P2RX7 promotes osteosarcoma progression and glucose metabolism by enhancing c-Myc stabilization

BACKGROUND

Osteosarcoma is the most common malignant tumor in bone and its prognosis has reached a plateau in the past few decades. Recently, metabolic reprogramming has attracted increasing attention in the field of cancer research. In our previous study, P2RX7 has been identified as an oncogene in osteosarcoma. However, whether and how P2RX7 promotes osteosarcoma growth and metastasis through metabolic reprogramming remains unexplored.

METHODS

We used CRISPR/Cas9 genome editing technology to establish P2RX7 knockout cell lines. Transcriptomics and metabolomics were performed to explore metabolic reprogramming in osteosarcoma. RT-PCR, western blot and immunofluorescence analyses were used to determine gene expression related to glucose metabolism. Cell cycle and apoptosis were examined by flowcytometry. The capacity of glycolysis and oxidative phosphorylation were assessed by seahorse experiments. PET/CT was carried out to assess glucose uptake in vivo.

RESULTS

We demonstrated that P2RX7 significantly promotes glucose metabolism in osteosarcoma via upregulating the expression of genes related to glucose metabolism. Inhibition of glucose metabolism largely abolishes the ability of P2RX7 to promote osteosarcoma progression. Mechanistically, P2RX7 enhances c-Myc stabilization by facilitating nuclear retention and reducing ubiquitination-dependent degradation. Furthermore, P2RX7 promotes osteosarcoma growth and metastasis through metabolic reprogramming in a predominantly c-Myc-dependent manner.

CONCLUSIONS

P2RX7 plays a key role in metabolic reprogramming and osteosarcoma progression via increasing c-Myc stability. These findings provide new evidence that P2RX7 might be a potential diagnostic and/or therapeutic target for osteosarcoma. Novel therapeutic strategies targeting metabolic reprogramming appear to hold promise for a breakthrough in the treatment of osteosarcoma.

Alternatively Spliced Isoforms of the P2X7 Receptor: Structure, Function and Disease Associations

The P2X7 receptor (P2X7R) is an ATP-gated membrane ion channel that is expressed by multiple cell types. Following activation by extracellular ATP, the P2X7R mediates a broad range of cellular responses including cytokine and chemokine release, cell survival and differentiation, the activation of transcription factors, and apoptosis. The P2X7R is made up of three P2X7 subunits that contain specific domains essential for the receptor’s varied functions. Alternative splicing produces P2X7 isoforms that exclude one or more of these domains and assemble in combinations that alter P2X7R function. The modification of the structure and function of the P2X7R may adversely affect cellular responses to carcinogens and pathogens, and alternatively spliced (AS) P2X7 isoforms have been associated with several cancers. This review summarizes recent advances in understanding the structure and function of AS P2X7 isoforms and their associations with cancer and potential role in modulating the inflammatory response.

Effects of Exogenous ATP on Melanoma Growth and Tumor Metabolism in C57BL/6 Mice

Altered energy metabolism (glucose, lipid, amino acid) is a hallmark of cancer growth that provides the theoretical basis for the development of metabolic therapies as cancer treatments. ATP is one of the major biochemical constituents of the tumor microenvironment. ATP promotes tumor progression or suppression depending on various factors, including concentration and tumor type. Here we evaluated the antitumor effect of extracellular ATP on melanoma and the potential underlying mechanisms. A subcutaneous tumor model in mice was used to investigate the antitumor effects of ATP. Major lymphocyte cell changes and intratumoral metabolic changes were assessed. Metabolomic analysis (1H nuclear magnetic resonance spectroscopy) was performed on tumor samples. We measured the activities of lactate dehydrogenase A (LDHA) and LDHB in the excised tumors and serum and found that ATP and its metabolites affected the proliferation of and LDHA activity in B16F10 cells, a murine melanoma cell line. In addition, treatment with ATP dose-dependently reduced tumor size in melanoma-bearing mice. Moreover, flow cytometry analysis demonstrated that the antitumor effect of ATP was not achieved through changes in T-cell or B-cell subsets. Metabolomics analysis revealed that ATP treatment simultaneously reduced multiple intratumoral metabolites related to energy metabolism as well as serum and tumor LDHA activities. Furthermore, both ATP and its metabolites significantly suppressed both tumor cell proliferation and LDHA activity in the melanoma cell line. Our results in vivo and in vitro indicate that exogenous ATP inhibits melanoma growth in association with altered intratumoral metabolism.

Inhibition of ATP hydrolysis as a key regulator of temozolomide resistance and migratory phenotype of glioblastoma cells

Glioblastoma (GBM) is the most lethal among malignant gliomas. The tumor invasiveness and therapy-resistance are important clinical hallmarks. Growing evidence emphasizes the purinergic signaling contributing to tumor growth. Here we exposed a potential role of extracellular ATPase activity as a key regulator of temozolomide cytotoxicity and the migration process in GBM cells. The inhibition of ATP hydrolysis was able to improve the impact of temozolomide, causing arrest mainly in S and G2 phases of the cell cycle, leading M059J and U251 cells to apoptosis. In addition to eradicating GBM cells, ATP hydrolysis exhibited a potential to modulate the invasive phenotype and the expression of proteins involved in cell migration and epithelial-to-mesenchymal-like transition in a 3D culture model. Finally, we suggest the ATPase activity as a key target to decline temozolomide resistance and the migratory phenotype in GBM cells.

Irradiation causes senescence, ATP release, and P2X7 receptor isoform switch in glioblastoma

Glioblastoma (GBM) is the most lethal brain tumor in adults. Radiation, together with temozolomide is the standard treatment, but nevertheless, relapse occurs in nearly all cases. Understanding the mechanisms underlying radiation resistance may help to find more effective therapies. After radiation treatment, ATP is released into the tumor microenvironment where it binds and activates purinergic P2 receptors, mainly of the P2X7 subtype. Two main P2X7 splice variants, P2X7A and P2X7B, are expressed in most cell types, where they associate with distinct biochemical and functional responses. GBM cells widely differ for the level of P2X7 isoform expression and accordingly for sensitivity to stimulation with extracellular ATP (eATP). Irradiation causes a dramatic shift in P2X7 isoform expression, with the P2X7A isoform being down- and the P2X7B isoform up-modulated, as well as extensive cell death and overexpression of stemness and senescence markers. Treatment with P2X7 blockers during the post-irradiation recovery potentiated irradiation-dependent cytotoxicity, suggesting that P2X7B activation by eATP generated a trophic/growth-promoting stimulus. Altogether, these data show that P2X7A and B receptor isoform levels are inversely modulated during the post-irradiation recovery phase in GBM cells.

P2X7 receptor: the regulator of glioma tumor development and survival

P2X7 is an ionotropic nucleotide receptor, forming the cation channel upon ATP stimulation. It can also function as a large membrane pore as well as transmit ATP-dependent signal without forming a channel at all. P2X7 activity in somatic cells is well-known, but remains poorly studied in glioma tumors. The current paper presents the comprehensive study of P2X7 activity in C6 and glioma cell line showing the wide range of effects the receptor has on glioma biology. We observed that P2X7 stimulation boosts glioma cell proliferation and increases cell viability. P2X7 activation promoted cell adhesion, mitochondria depolarization, and reactive oxygen species overproduction in C6 cells. P2X7 receptor also influenced glioma tumor growth in vivo via activation of pro-survival signaling pathways and ATP release. Treatment with Brilliant Blue G, a selective P2X7 antagonist, effectively inhibited glioma tumor development; decreased the expression of negative prognostic cancer markers pro-survival and epithelial-mesenchymal transition (EMT)-related proteins; and modulated the immune response toward glioma tumor in vivo. Finally, pathway-specific enrichment analysis of the microarray data from human patients also showed an upregulation of P2X7 receptor in gliomas from grades I to III. The presented results shed more light on the role of P2X7 receptor in the biology of this disease.

Mechanisms of Invasion in Glioblastoma: Extracellular Matrix, Ca2+ Signaling, and Glutamate

Glioblastoma (GBM) is the most common and malignant form of primary brain tumor with a median survival time of 14–16 months in GBM patients. Surgical treatment with chemotherapy and radiotherapy may help increase survival by removing GBM from the brain. However, complete surgical resection to eliminate GBM is almost impossible due to its high invasiveness. When GBM cells migrate to the brain, they interact with various cells, including astrocytes, neurons, endothelial cells, and the extracellular matrix (ECM). They can also make their cell body shrink to infiltrate into narrow spaces in the brain; thereby, they can invade regions of the brain and escape from surgery. Brain tumor cells create an appropriate microenvironment for migration and invasion by modifying and degrading the ECM. During those processes, the Ca²⁺ signaling pathway and other signaling cascades mediated by various ion channels contribute mainly to gene expression, motility, and invasion of GBM cells. Furthermore, GBM cells release glutamate, affecting migration via activation of ionotropic glutamate receptors in an autocrine manner. This review focuses on the cellular mechanisms of glioblastoma invasion and motility related to ECM, Ca²⁺ signaling, and glutamate. Finally, we discuss possible therapeutic interventions to inhibit invasion by GBM cells.

Immortalization of Mesenchymal Stromal Cells by TERT Affects Adenosine Metabolism and Impairs their Immunosuppressive Capacity

Mesenchymal stromal cells (MSCs) are promising candidates for cell-based therapies, mainly due to their unique biological properties such as multipotency, self-renewal and trophic/immunomodulatory effects. However, clinical use has proven complex due to limitations such as high variability of MSCs preparations and high number of cells required for therapies. These challenges could be circumvented with cell immortalization through genetic manipulation, and although many studies show that such approaches are safe, little is known about changes in other biological properties and functions of MSCs. In this study, we evaluated the impact of MSCs immortalization with the TERT gene on the purinergic system, which has emerged as a key modulator in a wide variety of pathophysiological conditions. After cell immortalization, MSCs-TERT displayed similar immunophenotypic profile and differentiation potential to primary MSCs. However, analysis of gene and protein expression exposed important alterations in the purinergic signaling of in vitro cultured MSCs-TERT. Immortalized cells upregulated the CD39/NTPDase1 enzyme and downregulated CD73/NT5E and adenosine deaminase (ADA), which had a direct impact on their nucleotide/nucleoside metabolism profile. Despite these alterations, adenosine did not accumulate in the extracellular space, due to increased uptake. MSCs-TERT cells presented an impaired in vitro immunosuppressive potential, as observed in an assay of co-culture with lymphocytes. Therefore, our data suggest that MSCs-TERT have altered expression of key enzymes of the extracellular nucleotides/nucleoside control, which altered key characteristics of these cells and can potentially change their therapeutic effects in tissue engineering in regenerative medicine.

Novel Functions of CD147 in the Mitochondria Exacerbates Melanoma Metastasis

Melanoma is an aggressive form of skin cancer characterized by rapid invasion and metastasis. CD147 is known to be functioning in cell invasion. In this study, we showed that CD147 was translocated from the cell membrane to the mitochondria in advanced melanoma. Melanoma patients with CD147 localized to the mitochondria confer a worse prognosis. The mitochondrial CD147 levels are correlated with the invasion potential of various melanoma cell lines as well as mitochondrial energy metabolism. Depletion of CD147 decreased the activity of mitochondrial complex V. STRING analysis for protein-protein interaction networks (PPIN) in CD147-depleted melanoma cells showed that mitochondrial proteins HSP60 and ATP5B, a subunit of mitochondrial complex V, were node proteins. HSP60 upregulation was correlated with a worse prognosis of melanoma patients. Co-immunoprecipitation (Co-IP) assay indicates that CD147 interacts with HSP60. These data suggested that mitochondrial CD147 may prompt HSP60 to activate ATP5B, thereby promoting the mitochondrial aerobic oxidation and the invasive abilities of melanoma cells. Correlation analysis of the data acquired from patients was helpful to draw a 5-year survival curve for patients who screened positive and negative for mitochondrial CD147. This study unravels the function of CD147 in tumor invasion and highlights it as a potential tumor therapeutic target.

Cancer Cell Fitness Is Dynamic

Several phenotypes that impact the capacity of cancer cells to survive and proliferate are dynamic. Here we used the number of cells in colonies as an assessment of fitness and devised a novel method called Dynamic Fitness Analysis (DynaFit) to measure the dynamics in fitness over the course of colony formation. DynaFit is based on the variance in growth rate of a population of founder cells compared with the variance in growth rate of colonies with different sizes. DynaFit revealed that cell fitness in cancer cell lines, primary cancer cells, and fibroblasts under unhindered growth conditions is dynamic. Key cellular mechanisms such as ERK signaling and cell-cycle synchronization differed significantly among cells in colonies after 2 to 4 generations and became indistinguishable from randomly sampled cells regarding these features. In the presence of cytotoxic agents, colonies reduced their variance in growth rate when compared with their founder cell, indicating a dynamic nature in the capacity to survive and proliferate in the presence of a drug. This finding was supported by measurable differences in DNA damage and induction of senescence among cells of colonies. The presence of epigenetic modulators during the formation of colonies stabilized their fitness for at least four generations. Collectively, these results support the understanding that cancer cell fitness is dynamic and its modulation is a fundamental aspect to be considered in comprehending cancer cell biology and its response to therapeutic interventions. SIGNIFICANCE: Cancer cell fitness is dynamic over the course of the formation of colonies. This dynamic behavior is mediated by asymmetric mitosis, ERK activity, cell-cycle duration, and DNA repair capacity in the absence or presence of a drug.

The P2X7 purinergic receptor: a potential therapeutic target for lung cancer

PURPOSE

Purinergic P2X7 receptor (P2X7R) is a gated ion channel for which adenosine triphosphate (ATP) is a ligand. Activated P2X7R is widely expressed in a variety of immune cells and tissues and is involved in a variety of physiological and pathological processes. Studies have confirmed that P2X7R is involved in the regulation of tumor cell growth, stimulating cell proliferation or inducing apoptosis. Recent studies have found that P2X7R is abnormally expressed in lung cancer and is closely related to the carcinogenesis and development of lung cancer. In this paper, we comprehensively describe the structure, function, and genetic polymorphisms of P2X7R. In particular, the role and therapeutic potential of P2X7R in lung cancer are discussed to provide new targets and new strategies for the treatment and prognosis of clinical lung cancer.

METHODS

The relevant literature on P2X7R and lung cancer from PubMed databases is reviewed in this article.

RESULTS

P2X7R regulates the function of lung cancer cells by activating multiple intracellular signaling pathways (such as the JNK, Rho, HMGB1 and EMT pathways), thereby affecting cell survival, growth, invasion, and metastasis and patient prognosis. Targeting P2X7R with inhibitors effectively suppresses the growth and metastasis of lung cancer cells.

CONCLUSION

In summary, P2X7R is expected to become a potential target for the treatment of lung cancer, and more clinical research is needed in the future to explore the effectiveness of P2X7R antagonists as treatments.

The P2X7 Receptor: Central Hub of Brain Diseases

The P2X7 receptor is a cation channel activated by high concentrations of adenosine triphosphate (ATP). Upon long-term activation, it complexes with membrane proteins forming a wide pore that leads to cell death and increased release of ATP into the extracellular milieu. The P2X7 receptor is widely expressed in the CNS, such as frontal cortex, hippocampus, amygdala and striatum, regions involved in neurodegenerative diseases and psychiatric disorders. Despite P2X7 receptor functions in glial cells have been extensively studied, the existence and roles of this receptor in neurons are still controversially discussed. Regardless, P2X7 receptors mediate several processes observed in neuropsychiatric disorders and brain tumors, such as activation of neuroinflammatory response, stimulation of glutamate release and neuroplasticity impairment. Moreover, P2X7 receptor gene polymorphisms have been associated to depression, and isoforms of P2X7 receptors are implicated in neuropsychiatric diseases. In view of that, the P2X7 receptor has been proposed to be a potential target for therapeutic intervention in brain diseases. This review discusses the molecular mechanisms underlying P2X7 receptor-mediated signaling in neurodegenerative diseases, psychiatric disorders, and brain tumors. In addition, it highlights the recent advances in the development of P2X7 receptor antagonists that are able of penetrating the central nervous system.

Involvement of P2X7 Receptors in the Osteogenic Differentiation of Mesenchymal Stromal/Stem Cells Derived from Human Subcutaneous Adipose Tissue

The ionotropic P2X7 receptor (P2X7R) is involved in bone homeostasis but its role in osteogenesis is controversial. Thus, we investigated the expression of P2X7R and the effects exerted by its modulation in mesenchymal stromal cells from human subcutaneous adipose tissue (S-ASCs), which have potential therapeutic application in bone regenerative medicine. We found that undifferentiated S-ASCs expressed P2X7R and its functional splice variants P2X7AR and P2X7BR. Cell stimulation by P2X7R agonist BzATP (100 μM) neither modified proliferation nor caused membrane pore opening while increasing intracellular Ca²⁺ levels and migration. The P2X7R antagonist A438079 reversed these effects. However, 25-100 μM BzATP, administered to S-ASCs undergoing osteogenic differentiation, dose-dependently decreased extracellular matrix mineralization and expression of osteogenic transcription factors Runx2, alkaline phosphatase and osteopontin. These effects were not coupled to cell proliferation reduction or to cell death increase, but were associated to decrease in P2X7AR and P2X7BR expression. In contrast, expression of P2X7R, especially P2X7BR isoform, significantly increased during the osteogenic process. Noteworthy, the antagonist A438079, administered alone, at first restrained cell differentiation, enhancing it later. Accordingly, A438079 reversed BzATP effects only in the second phase of S-ASCs osteogenic differentiation. Apyrase, a diphosphohydrolase converting ATP/ADP into AMP, showed a similar behavior. Altogether, findings related to A438079 or apyrase effects suggest an earlier and prevailing pro-osteogenic activity by endogenous ATP and a later one by adenosine derived from endogenous ATP metabolism. Conversely, P2X7R pharmacological stimulation by BzATP, mimicking the effects of high ATP levels occurring during tissue injuries, depressed receptor expression/activity impairing MSC osteogenic differentiation.

P2X7 receptor antagonism inhibits tumour growth in human high-grade gliomas

Gliomas, the most common primary brain cancer, are highly infiltrative and extremely difficult to treat. Despite advancements, current treatment is limited, with patients surviving for a median of 14-15 months post-diagnosis. Previous research has demonstrated the upregulation of a purinergic receptor, P2X7R, in human gliomas. P2X7R is expressed on both glioma cells and microglia within the glioma microenvironment. It is hypothesized that P2X7R contributes to tumour growth and proliferation via immune-mediated mechanisms involving tumour cells and surrounding microglia. We sought to elucidate the role of P2X7R in a human glioblastoma cell line (U251) and on surgically resected human glioma samples. We treated U251 and human glioma cultures for 72 h with P2X7R antagonists, Brilliant Blue G (BBG), oxidized ATP (oATP) and AZ10606120. Cell counting via fluorescence confocal microscopy was conducted to assess tumour proliferation. We observed no significant reductions in tumour cell numbers following P2X7R antagonism with BBG (20 μM) and oATP (250 μM) in both U251 cells and human glioma samples. Interestingly, there was a significant reduction in tumour cell number in both U251 cells (p = 0.0156) and human glioma samples (p = 0.0476) treated with varying concentrations of AZ10606120. When compared with the conventional chemotherapeutic agent, temozolomide, AZ10606120 was also found to more effectively inhibit tumour proliferation in U251 cells (p < 0.0001). Our pilot results demonstrate a potential trophic role of P2X7R where its inhibition by AZ10606120, a potent antagonist, hinders glioma growth directly or through the inactivation of microglia. This sheds new light on P2X7R as a therapeutic target for human gliomas.

P2X7 in Cancer: From Molecular Mechanisms to Therapeutics

P2X7 is a transmembrane receptor expressed in multiple cell types including neurons, dendritic cells, macrophages, monocytes, B and T cells where it can drive a wide range of physiological responses from pain transduction to immune response. Upon activation by its main ligand, extracellular ATP, P2X7 can form a nonselective channel for cations to enter the cell. Prolonged activation of P2X7, via high levels of extracellular ATP over an extended time period can lead to the formation of a macropore, leading to depolarization of the plasma membrane and ultimately to cell death. Thus, dependent on its activation state, P2X7 can either drive cell survival and proliferation, or induce cell death. In cancer, P2X7 has been shown to have a broad range of functions, including playing key roles in the development and spread of tumor cells. It is therefore unsurprising that P2X7 has been reported to be upregulated in several malignancies. Critically, ATP is present at high extracellular concentrations in the tumor microenvironment (TME) compared to levels observed in normal tissues. These high levels of ATP should present a survival challenge for cancer cells, potentially leading to constitutive receptor activation, prolonged macropore formation and ultimately to cell death. Therefore, to deliver the proven advantages for P2X7 in driving tumor survival and metastatic potential, the P2X7 macropore must be tightly controlled while retaining other functions. Studies have shown that commonly expressed P2X7 splice variants, distinct SNPs and post-translational receptor modifications can impair the capacity of P2X7 to open the macropore. These receptor modifications and potentially others may ultimately protect cancer cells from the negative consequences associated with constitutive activation of P2X7. Significantly, the effects of both P2X7 agonists and antagonists in preclinical tumor models of cancer demonstrate the potential for agents modifying P2X7 function, to provide innovative cancer therapies. This review summarizes recent advances in understanding of the structure and functions of P2X7 and how these impact P2X7 roles in cancer progression. We also review potential therapeutic approaches directed against P2X7.

Introduction to Purinergic Signalling in the Brain

ATP is a cotransmitter with glutamate, noradrenaline, GABA, acetylcholine and dopamine in the brain. There is a widespread presence of both adenosine (P1) and P2 nucleotide receptors in the brain on both neurons and glial cells. Adenosine receptors play a major role in presynaptic neuromodulation, while P2X ionotropic receptors are involved in fast synaptic transmission and synaptic plasticity. P2Y G protein-coupled receptors are largely involved in presynaptic activities, as well as mediating long-term (trophic) signalling in cell proliferation, differentiation and death during development and regeneration. Both P1 and P2 receptors participate in neuron-glial interactions. Purinergic signalling is involved in control of cerebral vascular tone and remodelling and has been implicated in learning and memory, locomotor and feeding behaviour and sleep. There is increasing interest in the involvement of purinergic signalling in the pathophysiology of the CNS, including trauma, ischaemia, epilepsy, neurodegenerative diseases, neuropsychiatric and mood disorders, and cancer, including gliomas.

Purinergic Signaling in Glioma Progression

Among the pathological alterations that give tumor cells invasive potential, purinergic signaling is emerging as an important component. Studies performed in in vitro, in vivo and ex vivo glioma models indicate that alterations in the purinergic signaling are involved in the progression of these tumors. Gliomas have low expression of all E-NTPDases, when compared to astrocytes in culture. Nucleotides induce glioma proliferation and ATP, although potentially neurotoxic, does not evoke cytotoxic action on the majority of glioma cells in culture. The importance of extracellular ATP for glioma pathobiology was confirmed by the reduction in glioma tumor size by apyrase, which degrades extracellular ATP to AMP, and the striking increase in tumor size by over-expression of an ecto-enzyme that degrades ATP to ADP, suggesting the effect of extracellular ATP on the tumor growth depends on the nucleotide produced by its degradation. The participation of purinergic receptors on glioma progression, particularly P2X₇, is involved in the resistance to ATP-induced cell death. Although more studies are necessary, the purinergic signaling, including ectonucleotidases and receptors, may be considered as future target for glioma pharmacological or gene therapy.

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.

P2RX7 functions as a putative biomarker of gastric cancer and contributes to worse prognosis

IMPACT STATEMENT

The mechanism of gastric cancer is highly complex, accompanied by a variety of genetic abnormalities. It is of great significance to elucidate the pathogenesis of gastric cancer, find its markers and therapeutic targets in the fight against this fatal disease. In this study, we identified P2RX7 as a putative target of gastric cancer, which was overexpressed in gastric cancer tissues and had relationship with worse prognosis. We also elucidated the roles of P2RX7 on the growth and metastasis of gastric cancer cells, and explored the relationship between it and ERK1/2 pathway, Akt pathway, and epithelial-mesenchymal transition. Our findings begin to offer useful insights into the mechanism of gastric cancer progression and provide clues to novel therapy strategies.

The role of microglia and P2X7 receptors in gliomas

Gliomas are the most prevalent tumours of the central nervous system and present with high morbidity and mortality. The most common and most aggressive form of glioma is glioblastoma multiforme, of which patients have a median survival time of only 12 to 15 months. Current treatment options are limited and have a small impact on clinical outcome and prognosis. There is accumulating evidence that microglia, the immunocompetent cells of the central nervous system, and the purinergic P2X7 receptor (P2X7R) may contribute to tumour progression and pathology. Importantly, P2X7R on both tumour cells and infiltrating microglia is overexpressed in animal and human glioma cultures. Factors released by glioma cells and P2X7R activation recruit microglia into the largely immunosuppressive tumour microenvironment where they have been demonstrated to contribute to either tumour proliferation or tumour suppression. It is likely that P2X7R mediates a range of microglia effector functions in the glioma setting, potentially increasing tumour growth and proliferation. This review evaluates current evidence on the roles of microglia and P2X7R in glioma pathogenesis. Understanding the nature, mechanisms and outcomes of microglia and P2X7R activation in gliomas is necessary for the development of more therapies with increased efficacy and specificity.

The signaling effects of ATP on melanoma-like skin cancer

Melanoma is a type of skin cancer originated by the malignant transformation of melanocytes. Increasing incidence and mortality require efforts focused on studies and research about this cancer. Its microenvironment is rich in extracellular ATP, but there are no studies evaluating the ectonucleotidases and ATP effects on tumor-derived melanoma cells with known amounts of ATP. This way, the objective of this work was to evaluate the purinergic signaling in the pathophysiology of in vivo melanoma and the in vitro effects of ATP signaling. We found increased and effective extracellular ATP hydrolysis in platelets and a significant decrease of extracellular ATP levels and adenosine hydrolysis. In addition, we cultured PBMCs of melanoma patients and used ATP salt with specific concentrations to evaluate its signaling effects. The enzymatic activity analysis revealed that even with higher ATP doses cells metabolize adenine nucleotides less efficiently, and present low ATP, ADP and AMP hydrolytic activity in CM compared to CT cells. In summary, we showed for the first time important data about the purinergic signaling in the pathophysiology of melanoma and ATP signaling exercising immunosuppressive effects. Therefore, as already shown for other tumors, the purinergic signaling should be considered a potential target for melanoma management and treatment and could offer novel therapeutic prospects.

Extracellular ATP is Differentially Metabolized on Papillary Thyroid Carcinoma Cells Surface in Comparison to Normal Cells

The incidence of differentiated thyroid cancer has been increasing. Nevertheless, its molecular mechanisms are not well understood. In recent years, extracellular nucleotides and nucleosides have emerged as important modulators of tumor microenvironment. Extracellular ATP is mainly hydrolyzed by NTPDase1/CD39 and NTPDase2/CD39L1, generating AMP, which is hydrolyzed by ecto-5'-nucleotidase (CD73) to adenosine, a possible promoter of tumor growth and metastasis. There are no studies evaluating the expression and functionality of these ectonucleotidases on normal or tumor-derived thyroid cells. Thus, we investigated the ability of thyroid cancer cells to hydrolyze extracellular ATP generating adenosine, and the expression of ecto-enzymes, as compared to normal cells. We found that normal thyroid derived cells presented a higher ability to hydrolyze ATP and higher mRNA levels for ENTDP1-2, when compared to papillary thyroid carcinoma (PTC) derived cells, which had a higher ability to hydrolyze AMP and expressed CD73 mRNA and protein at higher levels. In addition, adenosine induced an increase in proliferation and migration in PTC derived cells, whose effect was blocked by APCP, a non-hydrolysable ADP analogue, which is an inhibitor of CD73. Taken together, these results showed that thyroid follicular cells have a functional purinergic signaling. The higher expression of CD73 in PTC derived cells might favor the accumulation of extracellular adenosine in the tumor microenvironment, which could promote tumor progression. Therefore, as already shown for other tumors, the purinergic signaling should be considered a potential target for thyroid cancer management and treatment.

The P2X7 Receptor in Inflammatory Diseases: Angel or Demon?

Under physiological conditions, adenosine triphosphate (ATP) is present at low levels in the extracellular milieu, being massively released by stressed or dying cells. Once outside the cells, ATP and related nucleotides/nucleoside generated by ectonucleotidases mediate a high evolutionary conserved signaling system: the purinergic signaling, which is involved in a variety of pathological conditions, including inflammatory diseases. Extracellular ATP has been considered an endogenous adjuvant that can initiate inflammation by acting as a danger signal through the activation of purinergic type 2 receptors-P2 receptors (P2Y G-protein coupled receptors and P2X ligand-gated ion channels). Among the P2 receptors, the P2X7 receptor is the most extensively studied from an immunological perspective, being involved in both innate and adaptive immune responses. P2X7 receptor activation induces large-scale ATP release via its intrinsic ability to form a membrane pore or in association with pannexin hemichannels, boosting purinergic signaling. ATP acting via P2X7 receptor is the second signal to the inflammasome activation, inducing both maturation and release of pro-inflammatory cytokines, such as IL-1β and IL-18, and the production of reactive nitrogen and oxygen species. Furthermore, the P2X7 receptor is involved in caspases activation, as well as in apoptosis induction. During adaptive immune response, P2X7 receptor modulates the balance between the generation of T helper type 17 (Th17) and T regulatory (Treg) lymphocytes. Therefore, this receptor is involved in several inflammatory pathological conditions. In infectious diseases and cancer, P2X7 receptor can have different and contrasting effects, being an angel or a demon depending on its level of activation, cell studied, type of pathogen, and severity of infection. In neuroinflammatory and neurodegenerative diseases, P2X7 upregulation and function appears to contribute to disease progression. In this review, we deeply discuss P2X7 receptor dual function and its pharmacological modulation in the context of different pathologies, and we also highlight the P2X7 receptor as a potential target to treat inflammatory related diseases.

GL261 glioma tumor cells respond to ATP with an intracellular calcium rise and glutamate release

Glioblastoma (GBM) is an aggressive brain cancer with an average survival rate of 15 months. The composition of the GBM tumor microenvironment-its pH, the presence of growth and immune factors, neurotransmitters, and gliotransmitters-plays an important role in GBM pathophysiology and facilitates tumor survival and growth. In particular, GBM tumor cells produce glutamate, which is toxic to healthy tissue and is associated with increased tumor invasion into adjacent brain regions. The conditions that lead to this excitotoxic release of glutamate are not completely understood. Previous studies have demonstrated that extracellular ATP is present at high levels in the tumor microenvironment, and that ATP stimulates the release of glutamate from astrocytes in culture. Here we examine the functional effects of extracellular ATP on the GL261 cell line, a model system for high-grade astrocytomas such as GBM. We show that treatment with ATP leads to an immediate, dose-dependent influx of calcium into the cell that is partially inhibited by an antagonist (o-ATP) of the ionotropic ATP receptor P2X7. In addition, GL261 cells respond to extracellular ATP with a dose-dependent release of glutamate. Consistent with other reports, we find that ATP is toxic to GL261 cells at high concentrations. Together, these results provide insight into the mechanisms responsible for glutamate production by tumor cells and inform future studies that will identify how the GBM tumor microenvironment facilitates tumor invasion into healthy areas of the brain.

Hyperthermia and associated changes in membrane fluidity potentiate P2X7 activation to promote tumor cell death

Extracellular ATP (eATP) accumulation within the tumor microenvironment (TME) has the potential to activate purinergic signaling. The eATP evoked signaling effects bolster antitumor immune responses while exerting direct cytotoxicity on tumor cells and vascular endothelial cells, mediated at least in part through P2X7 receptors. Approaches to augment purinergic signaling in TME e.g. by ectonucleotidase CD39 blockade, and/or boosting P2X7 functional responses, might be used as immunomodulatory therapies in cancer treatment. In this study, we delineated the translatable strategy of hyperthermia to demonstrate impacts on P2X7 responsiveness to eATP. Hyperthermia (40°C) was noted to enhance eATP-mediated cytotoxicity on MCA38 colon cancer cells. Increased membrane fluidity induced by hyperthermia boosted P2X7 functionality, potentiating pore opening and modulating downstream AKT/PRAS40/mTOR signaling events. When combined with cisplatin or mitomycin C, hyperthermia and eATP together markedly potentiate cancer cell death. Our data indicate that clinically tolerable hyperthermia with modulated P2X7-purinergic signaling will boost efficacy of conventional cancer treatments.

Live-cell calcium imaging of adherent and non-adherent GL261 cells reveals phenotype-dependent differences in drug responses

BACKGROUND

The tumor-derived GL261 cell line is used as a model for studying glioblastoma and other high-grade gliomas, and can be cultured adherently or as free-floating aggregates known as neurospheres. These different culture conditions give rise to distinct phenotypes, with increased tumorigenicity displayed by neurosphere-cultured cells. An important technique for understanding GL261 pathobiology is live cell fluorescent imaging of intracellular calcium. However, live cell imaging of GL261 neurospheres presents a technical challenge, as experimental manipulations where drugs are added to the extracellular media cause the cells to move during analysis. Here we present a method to immobilize GL261 neurospheres with low melting point agarose for calcium imaging using the fluorescent calcium sensor fura-2.

METHODS

GL261 cells were obtained from the NCI-Frederick Cancer Research Tumor Repository and cultured as adherent cells or induced to form neurospheres by placing freshly trypsinized cells into serum-free media containing fibroblast growth factor 2, epidermal growth factor, and B-27 supplement. Prior to experiments, adherent cells were loaded with fura-2 and cultured on 8-well chamber slides. Non-adherent neurospheres were first loaded with fura-2, placed in droplets onto an 8-well chamber slide, and finally covered with a thin layer of low melting point agarose to immobilize the cells. Ratiometric pseudocolored images were obtained during treatment with ATP, capsaicin, or vehicle control. Cells were marked as responsive if fluorescence levels increased more than 30% above baseline. Differences between treatment groups were tested using Student's t-tests and one-way ANOVA.

RESULTS

We found that cellular responses to pharmacological treatments differ based on cellular phenotype. Adherent cells and neurospheres both responded to ATP with a rise in intracellular calcium. Notably, capsaicin treatment led to robust responses in GL261 neurospheres but not adherent cells.

CONCLUSIONS

We demonstrate the use of low melting point agarose for immobilizing GL261 cells, a method that is broadly applicable to any cell type cultured in suspension, including acutely trypsinized cells and primary tumor cells. Our results indicate that it is important to consider GL261 phenotype (adherent or neurosphere) when interpreting data regarding physiological responses to experimental compounds.

The role of microglial P2X7: modulation of cell death and cytokine release

Background

ATP-gated P2X7 is a non-selective cation channel, which participates in a wide range of cellular functions as well as pathophysiological processes including neuropathic pain, immune response, and neuroinflammation. Despite its abundant expression in microglia, the role of P2X7 in neuroinflammation still remains unclear.

Methods

Primary microglia were isolated from cortices of P0-2 C57BL/6 wild-type or P2X7 knockout (P2X7^−/−) mouse pups. Lipopolysaccharide, lipopolysaccharide plus IFNγ, or IL4 plus IL13 were used to polarize microglia to pro-inflammatory or anti-inflammatory states. P2rx7 expression level in resting or activated mouse and human microglia was measured by RNA-sequencing and quantitative real-time PCR. Microglial cell death was measured by cell counting kit-8 and immunocytochemistry, and microglial secretion in wild-type or P2X7^−/− microglia was examined by Luminex multiplex assay or ELISA using P2X7 agonist BzATP or P2X7 antagonist A-804598. P2X7 signaling was analyzed by Western blot.

Results

First, we confirmed that P2rx7 is constitutively expressed in mouse and human primary microglia. Moreover, P2rx7 mRNA level was downregulated in mouse microglia under both pro- and anti-inflammatory conditions. Second, P2X7 agonist BzATP caused cell death of mouse microglia, while this effect was suppressed either by P2X7 knockout or by A-804598 under both basal and pro-inflammatory conditions, which suggests the mediating role of P2X7 in BzATP-induced microglial cell death. Third, BzATP-induced release of IL1 family cytokines including IL1α, IL1β, and IL18 was blocked in P2X7^−/− microglia or by A-804598 in pro-inflammatory microglia, while the release of other cytokines/chemokines was independent of P2X7 activation. These findings support the specific role of P2X7 in IL1 family cytokine release. Finally, P2X7 activation was discovered to be linked to AKT and ERK pathways, which may be the underlying mechanism of P2X7 functions in microglia.

Conclusions

These results reveal that P2X7 mediates BzATP-induced microglial cell death and specific release of IL1 family cytokines, indicating the important role of P2X7 in neuroinflammation and implying the potential of targeting P2X7 for the treatment of neuroinflammatory disorders.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-017-0904-8) contains supplementary material, which is available to authorized users.

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.

Ratiometric analysis of Acridine Orange staining in the study of acidic organelles and autophagy

Acridine Orange is a cell-permeable green fluorophore that can be protonated and trapped in acidic vesicular organelles (AVOs). Its metachromatic shift to red fluorescence is concentration-dependent and, therefore, Acridine Orange fluoresces red in AVOs, such as autolysosomes. This makes Acridine Orange staining a quick, accessible and reliable method to assess the volume of AVOs, which increases upon autophagy induction. Here, we describe a ratiometric analysis of autophagy using Acridine Orange, considering the red-to-green fluorescence intensity ratio (R/GFIR) to quantify flow cytometry and fluorescence microscopy data of Acridine-Orange-stained cells. This method measured with accuracy the increase in autophagy induced by starvation or rapamycin, and the reduction in autophagy produced by bafilomycin A1 or the knockdown of Beclin1 or ATG7. Results obtained with Acridine Orange, considering R/GFIR, correlated with the conversion of the unlipidated form of LC3 (LC3-I) into the lipidated form (LC3-II), SQSTM1 degradation and GFP-LC3 puncta formation, thus validating this assay to be used as an initial and quantitative method for evaluating the late step of autophagy in individual cells, complementing other methods.

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 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.

Prognostic value of purinergic P2X7 receptor expression in patients with hepatocellular carcinoma after curative resection

The family of type 2 purinergic (P2) receptors, especially P2X7, is responsible for the direct tumor-killing functions of extracellular adenosine triphosphate (ATP), but the precise role of P2X7 in the progression of hepatocellular carcinoma (HCC) remains elusive. This study aims to evaluate prognostic value of P2X7 expression in HCC patients after surgical resection. Expression of P2X7 was assessed by immunohistochemistry in tissue microarrays containing paired tumor and peritumoral liver tissues from 273 patients with HCC who had undergone hepatectomy between 2006 and 2007. Prognostic value of P2X7 expression and clinical outcomes were evaluated. Peritumoral P2X7 expression was significantly higher than intratumoral P2X7 expression. No significant prognostic difference was observed for overall survival for intratumoral P2X7 density, whereas peritumoral P2X7 density indicates unfavorable overall survival in training set and BCLC stage 0-A subset. Besides, peritumoral P2X7 density, which correlated with tumor size, venous invasion, and BCLC stage, was identified as an independent poor prognosticator for overall survival and recurrence-free survival. The association was further validated in validation set. Peritumoral P2X7 is a potential unfavorable prognosticator for overall survival and recurrence free survival in HCC patients after surgical resection. Further external validation and functional analysis should be pursued to evaluate its potential prognostic value and therapeutic significance for HCC patients.

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.

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.

Adenosine uptake is the major effector of extracellular ATP toxicity in human cervical cancer cells

In cervical cancer, HPV infection and disruption of mechanisms involving cell growth, differentiation, and apoptosis are strictly linked with tumor progression and invasion. Tumor microenvironment is ATP and adenosine rich, suggesting a role for purinergic signaling in cancer cell growth and death. Here we investigate the effect of extracellular ATP on human cervical cancer cells. We find that extracellular ATP itself has a small cytotoxic effect, whereas adenosine formed from ATP degradation by ectonucleotidases is the main factor responsible for apoptosis induction. The level of P2 × 7 receptor seemed to define the main cytotoxic mechanism triggered by ATP, since ATP itself eliminated a small subpopulation of cells that express high P2 × 7 levels, probably through its activation. Corroborating these data, blockage or knockdown of P2 × 7 only slightly reduced ATP cytotoxicity. On the other hand, cell viability was almost totally recovered with dipyridamole, an adenosine transporter inhibitor. Moreover, ATP-induced apoptosis and signaling-p53 increase, AMPK activation, and PARP cleavage-as well as autophagy induction were also inhibited by dipyridamole. In addition, inhibition of adenosine conversion into AMP also blocked cell death, indicating that metabolization of intracellular adenosine originating from extracellular ATP is responsible for the main effects of the latter in human cervical cancer cells.

Ectonucleotidase expression profile and activity in human cervical cancer cell lines

Cervical cancer is the third most frequent cancer in women worldwide. Adenine nucleotide signaling is modulated by the ectonucleotidases that act in sequence, forming an enzymatic cascade. Considering the relationship between the purinergic signaling and cancer, we studied the E-NTPDases, ecto-5'-nucleotidase, and E-NPPs in human cervical cancer cell lines and keratinocytes. We evaluated the expression profiles of these enzymes using RT-PCR and quantitative real-time PCR analysis. The activities of these enzymes were examined using ATP, ADP, AMP, and p-nitrophenyl-5'-thymidine monophosphate (p-Nph-5'-TMP) as substrate, in a colorimetric assay. The extracellular adenine nucleotide hydrolysis was estimated by HPLC analysis. The hydrolysis of all substrates exhibited a linear pattern and these activities were cation-dependent. An interesting difference in the degradation rate was observed between cervical cancer cell lines SiHa, HeLa, and C33A and normal imortalized keratinocytes, HaCaT cells. The mRNA of ecto-5'-nucleotidase, E-NTPDases 5 and 6 were detectable in all cell lines, and the dominant gene expressed was the Entpd 5 enzyme, in SiHa cell line (HPV16 positive). In accordance with this result, a higher hydrolysis activity for UDP and GDP nucleotides was observed in the supernatant of the SiHa cells. Both normal and cancer cells presented activity and mRNAs of members of the NPP family. Considering that these enzymes exert an important catalytic activity, controlling purinergic nucleotide concentrations in tumors, the presence of ectonucleotidases in cervical cancer cells can be important to regulate the levels of extracellular adenine nucleotides, limiting their effects.

Hypoxia Attenuates Purinergic P2X Receptor-Induced Inflammatory Gene Expression in Brainstem Microglia

Hypoxia and increased extracellular nucleotides are frequently coincident in the brainstem. Extracellular nucleotides are potent modulators of microglial inflammatory gene expression via P2X purinergic receptor activation. Although hypoxia is also known to modulate inflammatory gene expression, little is known about how hypoxia or P2X receptor activation alone affects inflammatory molecule production in brainstem microglia, nor how hypoxia and P2X receptor signaling interact when they occur together. In the study reported here, we investigated the ability of a brief episode of hypoxia (2 hours) in the presence and absence of the nonselective P2X receptor agonist 2′(3′)-O-(4-benzoylbenzoyl)adenosine-5′-triphosphate (BzATP) to promote inflammatory gene expression in brainstem microglia in adult rats. We evaluated inducible nitric oxide synthase (iNOS), tumor necrosis factor alpha (TNFα), and interleukin (IL)-6 messenger RNA levels in immunomagnetically isolated brainstem microglia. While iNOS and IL-6 gene expression increased with hypoxia and BzATP alone, TNFα expression was unaffected. Surprisingly, BzATP-induced inflammatory effects were lost after hypoxia, suggesting that hypoxia impairs proinflammatory P2X-receptor signaling. We also evaluated the expression of key P2X receptors activated by BzATP, namely P2X1, P2X4, and P2X7. While hypoxia did not alter their expression, BzATP upregulated P2X4 and P2X7 mRNAs; these effects were ablated in hypoxia. Although both P2X4 and P2X7 receptor expression correlated with increased microglial iNOS and IL-6 levels in microglia from normoxic rats, in hypoxia, P2X7 only correlated with IL-6, and P2X4 correlated only with iNOS. In addition, correlations between P2X7 and P2X4 were lost following hypoxia, suggesting that P2X4 and P2X7 receptor signaling differs in normoxia and hypoxia. Together, these data suggest that hypoxia suppresses P2X receptor-induced inflammatory gene expression, indicating a potentially immunosuppressive role of extracellular nucleotides in brainstem microglia following exposure to hypoxia.

P2X7R suppression promotes glioma growth through epidermal growth factor receptor signal pathway

P2X7 receptor (P2X7R) has been shown to mediate an anticancer effect via apoptosis in different types of cancer. However, whether P2X7R exerts a promoting or suppressive effect on brain glioma is still a controversial issue and its underlying mechanism remains unknown. We showed here that P2X7R suppression exerted a pro-growth effect on glioma through directly promoting cells proliferation and pro-angiogenesis, which was associated with epidermal growth factor receptor (EGFR) signaling. The P2X7R was markedly downregulated by cells exposure to the P2X7R antagonist, brilliant blue G (BBG), moreover, the cells proliferation was enhanced in a dose-dependent manner and the expression of EGFR or p-EGFR protein was significantly upregulated. By constructing C6 cells with reduced expression of P2X7R using shRNA, we also demonstrated strong upregulation in cells proliferation and EGFR/p-EGFR expression. However, this effect of BBG was reversed in the presence of gefitinib or suramin. Magnetic resonance imaging and computed tomography perfusion showed that the BBG or P2X7R shRNA promoted the tumor growth by about 40% and 50%, respectively, and significantly increased angiogenesis. Nissl and Ki-67 staining also confirmed that BBG or P2X7R shRNA notably increased the tumor growth. More importantly, either BBG or P2X7R shRNA could markedly upregulated the expression of EGFR, p-EGFR, HIF-1α and VEGF in glioma cells. In conclusion, P2X7R suppression exerts a promoting effect on glioma growth, which is likely to be related to upregulated EGFR, HIF-1α and VEGF expression. These findings provide important clues to the molecular basis of anticancer effect of targeting purinergic receptors.

Purinergic signaling in glioma progression

Among the pathological alterations that give tumor cells invasive potential, purinergic signaling is emerging as an important component. Studies performed in in vitro, in vivo and ex vivo glioma models indicate that alterations in the purinergic signaling are involved in the progression of these tumors. Gliomas have low expression of all E-NTPDases, when compared to astrocytes in culture. Nucleotides induce glioma proliferation and ATP, although potentially neurotoxic, does not evoke cytotoxic action on the majority of glioma cells in culture. The importance of extracellular ATP for glioma pathobiology was confirmed by the reduction in glioma tumor size by apyrase, which degrades extracellular ATP to AMP, and the striking increase in tumor size by over-expression of an ecto-enzyme that degrades ATP to ADP, suggesting the effect of extracellular ATP on the tumor growth depends on the nucleotide produced by its degradation. The participation of purinergic receptors on glioma progression, particularly P2X(7), is involved in the resistance to ATP-induced cell death. Although more studies are necessary, the purinergic signaling, including ectonucleotidases and receptors, may be considered as future target for glioma pharmacological or gene therapy.