Extracellular ATP a New Player in Cancer Metabolism: NSCLC Cells Internalize ATP In Vitro and In Vivo Using Multiple Endocytic Mechanisms

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.

Nitrogen Trapping as a Therapeutic Strategy in Tumors with Mitochondrial Dysfunction

Under conditions of inherent or induced mitochondrial dysfunction, cancer cells manifest overlapping metabolic phenotypes, suggesting that they may be targeted via a common approach. Here, we use multiple oxidative phosphorylation (OXPHOS)-competent and incompetent cancer cell pairs to demonstrate that treatment with α-ketoglutarate (aKG) esters elicits rapid death of OXPHOS-deficient cancer cells by elevating intracellular aKG concentrations, thereby sequestering nitrogen from aspartate through glutamic-oxaloacetic transaminase 1 (GOT1). Exhaustion of aspartate in these cells resulted in immediate depletion of adenylates, which plays a central role in mediating mTOR inactivation and inhibition of glycolysis. aKG esters also conferred cytotoxicity in a variety of cancer types if their cell respiration was obstructed by hypoxia or by chemical inhibition of the electron transport chain (ETC), both of which are known to increase aspartate and GOT1 dependencies. Furthermore, preclinical mouse studies suggested that cell-permeable aKG displays a good biosafety profile, eliminates aspartate only in OXPHOS-incompetent tumors, and prevents their growth and metastasis. This study reveals a novel cytotoxic mechanism for the metabolite aKG and identifies cell-permeable aKG, either by itself or in combination with ETC inhibitors, as a potential anticancer approach. SIGNIFICANCE: These findings demonstrate that OXPHOS deficiency caused by either hypoxia or mutations, which can significantly increase cancer virulence, renders tumors sensitive to aKG esters by targeting their dependence upon GOT1 for aspartate synthesis. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/17/3492/F1.large.jpg.

The pervasiveness of macropinocytosis in oncological malignancies

In tumour cells, macropinocytosis functions as an amino acid supply route and supports cancer cell survival and proliferation. Initially demonstrated in oncogenic KRAS-driven models of pancreatic cancer, macropinocytosis triggers the internalization of extracellular proteins via discrete endocytic vesicles called macropinosomes. The incoming protein cargo is targeted for lysosome-dependent degradation, causing the intracellular release of amino acids. These protein-derived amino acids support metabolic fitness by contributing to the intracellular amino acid pools, as well as to the biosynthesis of central carbon metabolites. In this way, macropinocytosis represents a novel amino acid supply route that tumour cells use to survive the nutrient-poor conditions of the tumour microenvironment. Macropinocytosis has also emerged as an entry mechanism for a variety of nanomedicines, suggesting that macropinocytosis regulation in the tumour setting can be harnessed for the delivery of anti-cancer therapeutics. A slew of recent studies point to the possibility that macropinocytosis is a pervasive feature of many different tumour types. In this review, we focus on the role of this important uptake mechanism in a variety of cancers and highlight the main molecular drivers of macropinocytosis in these malignancies. This article is part of the Theo Murphy meeting issue 'Macropinocytosis'.

3D bimetallic Au/Pt nanoflowers decorated needle-type microelectrode for direct in situ monitoring of ATP secreted from living cells

Adenosine triphosphate (ATP) plays a crucial role in energy metabolism and extracellular purinergic signaling. A 3D bimetallic Au/Pt nanoflowers decorated ATP microelectrode biosensor prepared by facile and effective template-free electrodeposition was firstly reported, realizing local detection of cellular ATP secretion. The ATP biosensor was developed by co-immobilization of glucose oxidase and hexokinase, exhibiting long-term stability (79.39 ± 9.15% of its initial value remained after 14 days at 4 °C) and high selectivity with a limit of detection down to 2.5 μM (S/N = 3). The resulting ATP biosensor was then used for direct in situ monitoring of ATP secreted from living cells (PC12) with the stimulation of high K⁺ solutions. The obtained current was about 21.6 ± 3.4 nA (N = 6), corresponding to 12.2 ± 2.8 μM ATP released from cells, right in the micromolar range and consistent with the suggested levels. The 3D bimetallic Au/Pt nanoflowers possess excellent catalytic activity and large electroactive surface area, contributing to enzymatic activity preservation and long-term stability. This work provides a promising platform for long-time monitoring of other neurotransmitters and secretions in cellular glycolysis and apoptosis processes in the future.

O-GlcNAcylation of PGK1 coordinates glycolysis and TCA cycle to promote tumor growth

Many cancer cells display enhanced glycolysis and suppressed mitochondrial metabolism. This phenomenon, known as the Warburg effect, is critical for tumor development. However, how cancer cells coordinate glucose metabolism through glycolysis and the mitochondrial tricarboxylic acid (TCA) cycle is largely unknown. We demonstrate here that phosphoglycerate kinase 1 (PGK1), the first ATP-producing enzyme in glycolysis, is reversibly and dynamically modified with O-linked N-acetylglucosamine (O-GlcNAc) at threonine 255 (T255). O-GlcNAcylation activates PGK1 activity to enhance lactate production, and simultaneously induces PGK1 translocation into mitochondria. Inside mitochondria, PGK1 acts as a kinase to inhibit pyruvate dehydrogenase (PDH) complex to reduce oxidative phosphorylation. Blocking T255 O-GlcNAcylation of PGK1 decreases colon cancer cell proliferation, suppresses glycolysis, enhances the TCA cycle, and inhibits tumor growth in xenograft models. Furthermore, PGK1 O-GlcNAcylation levels are elevated in human colon cancers. This study highlights O-GlcNAcylation as an important signal for coordinating glycolysis and the TCA cycle to promote tumorigenesis.

Low DMT1 Expression Associates With Increased Oxidative Phosphorylation and Early Recurrence in Hepatocellular Carcinoma

BACKGROUND

Despite a high rate of recurrences, long-term survival can be achieved after the resection of hepatocellular carcinoma (HCC) with effective local treatment. Discovery of adverse prognostic variables to identify patients with high risk of recurrence could improve the management of HCC. Accumulating evidence showing a link between carcinogenesis and increased expression of iron import proteins and intracellular iron prompted us to investigate a role of divalent metal-ion transporter-1 (DMT1) that binds and regulates a variety of divalent metals in HCC.

MATERIALS AND METHODS

Clinical and gene expression data from RNA seq in 369 HCC patients were obtained from The Cancer Genome Atlas. Disease-free survival was compared between DMT1 high- and low-expressing tumors, and gene set enrichment analysis was conducted.

RESULTS

Patients with lower expression of DMT1 exhibited significantly worse disease-free survival compared with the DMT1 high group (P = 0.044), notably in advanced-stage patients (P = 0.008). DMT1 expression did not differ in etiologies, stages, and differentiation status of HCC. Interestingly, DMT1 expression levels inversely associated with cellular respiratory function in HCC. Furthermore, gene set enrichment analysis revealed that metabolism-related gene sets such as glycolysis, oxidative phosphorylation, and reactive oxygen species pathway were significantly enriched in the DMT1 low-expressing HCC.

CONCLUSIONS

Low DMT1 expression associates with increased oxidative phosphorylation as well as glycolysis and identifies early recurrence in HCC patients after surgical treatment.

Apoptotic lysosomal proton sponge effect in tumor tissue by cationic gold nanorods

Despite the lysosomal "proton sponge hypothesis" being considered to be an additional factor for stimulating the cellular toxicity of nanoparticle-based drug delivery systems, a clear relationship between the massive influx of calcium ions and the proton sponge effect, both of which are associated with cancer cell apoptosis, has still not been elucidated. Cetrimonium bromide (CTAB: cationic quaternary amino group based) gold nanorods possessed a more effective electric surface charge for inducing the lysosomal proton sponge effect than anionic gold nanoparticles. In this aspect, identifying released cytoplasmic Cl^-, arising from the ruptured lysosomal compartment, in the cytoplasm is critical for supporting the "proton sponge hypothesis". This study clarified that the burst release of Cl^-, as a result of lysosomal swelling by CTAB gold nanorods, stimulates the transient receptor potential channels melastatin 2 (TRPM2) channels, and subsequently induces a massive Ca²⁺ influx, which independently increases apoptosis of cancer cells. Although the previous concept of elevated cancer apoptosis acting through the proton sponge effect is unclear, this study supports the evidence that a massive Ca²⁺ influx mediated in response to a burst release of Cl^- significantly influenced cytotoxicity of cancer cells in tumor tissues.

Extracellular and macropinocytosis internalized ATP work together to induce epithelial-mesenchymal transition and other early metastatic activities in lung cancer

Background

Extracellular ATP (eATP) was shown to induce epithelial-mesenchymal transition (EMT), a very important early process in metastasis, in cancer cells via purinergic receptor signaling. However, the exact induction mechanisms are far from fully known. We previously described that eATP is internalized by cancer cells in vitro and in vivo by macropinocytosis in human non-small cell lung cancer A549 and other cancer cells, drastically elevates intracellular ATP levels, enhances cell proliferation and resistance to anticancer drugs. In this study, we tested the hypothesis that eATP and macropinocytosis-internalized eATP also induces EMT and other early steps of metastasis.

Methods

Floating cells, fencing, and transwell assays were used to show that ATP induces cell detachment, new colony formation, migration and invasion in human A549 and other lung cancer cells. Western blots were used to detect ATP-induced changes in EMT-related proteins; Confocal microscopy was used to demonstrate ATP-induced metastasis-related cell morphological changes. Inhibitors and siRNA knockdowns were used to determine P2X7's involvement in the ATP-induced EMT. CRISPR-Cas9 knockout of the SNX5 gene was used to identify macropinocytosis' roles in EMT and cancer cell growth both in vitro and in vivo. Student t-test and one-way ANOVA were used to determine statistical significance, P < 0.05 was considered significant.

Results

eATP potently induces expression of matrix metallopeptidases (MMPs), and detachment, EMT, migration, and invasion of lung cancer cells. The induction was independent of TGF-β and semi-independent of P2X7 activation. eATP performs these functions not only extracellularly, but also intracellularly after being macropinocytically internalized to further enhance P2X7-mediated EMT, filopodia formation and other early steps of metastasis. The knockout of macropinocytosis-associated SNX5 gene significantly reduces macropinocytosis, slows down tumor growth, and changes tumor morphology in nude mice.

Conclusions

Collectively, these results show that eATP's functions in these processes not only from outside of cancer cells but also inside after being macropinocytotically internalized. These findings shed light on eATP's initiator and effector roles in almost every step in early metastasis, which calls for rethinking and rebalancing energy equations of intracellular biochemical reactions and the Warburg effect, and identifies eATP and macropinocytosis as novel targets for potentially slowing down EMT and preventing metastasis.

Role of coenzymes in cancer metabolism

Cancer is a heterogeneous set of diseases characterized by the rewiring of cellular signaling and the reprogramming of metabolic pathways to sustain growth and proliferation. In past decades, studies were focused primarily on the genetic complexity of cancer. Recently, increasing number of studies have discovered several mutations among metabolic enzymes in different tumor cells. Most of the enzymes are regulated by coenzymes, organic cofactors, that function as intermediate carrier of electrons or functional groups that are transferred during the reaction. However, the precise role of cofactors is not well elucidated. In this review, we discuss several metabolic enzymes associated to cancer metabolism rewiring, whose inhibition may represent a therapeutic target. Such enzymes, upon expression or inhibition, may impact also the coenzymes levels, but only in few cases, it was possible to direct correlate coenzymes changes with a specific enzyme. In addition, we also summarize an up-to-date information on biological role of some coenzymes, preclinical and clinical studies, that have been carried out in various cancers and their outputs.

Drug resistance and combating drug resistance in cancer

Cancer is the second leading cause of death in the US. Current major treatments for cancer management include surgery, cytotoxic chemotherapy, targeted therapy, radiation therapy, endocrine therapy and immunotherapy. Despite the endeavors and achievements made in treating cancers during the past decades, resistance to classical chemotherapeutic agents and/or novel targeted drugs continues to be a major problem in cancer therapies. Drug resistance, either existing before treatment (intrinsic) or generated after therapy (acquired), is responsible for most relapses of cancer, one of the major causes of death of the disease. Heterogeneity among patients and tumors, and the versatility of cancer to circumvent therapies make drug resistance more challenging to deal with. Better understanding the mechanisms of drug resistance is required to provide guidance to future cancer treatment and achieve better outcomes. In this review, intrinsic and acquired resistance will be discussed. In addition, new discoveries in mechanisms of drug resistance will be reviewed. Particularly, we will highlight roles of ATP in drug resistance by discussing recent findings of exceptionally high levels of intratumoral extracellular ATP as well as intracellular ATP internalized from extracellular environment. The complexity of drug resistance development suggests that combinational and personalized therapies, which should take ATP into consideration, might provide better strategies and improved efficacy for fighting drug resistance in cancer.

Extracellular vesicles in the tumor microenvironment: old stories, but new tales

Mammalian cells synthesize and release heterogeneous extracellular vesicles (EVs) which can be generally recognized as subclasses including exosomes, microvesicles (MVs), and apoptotic bodies (ABs), each differing in their biogenesis, composition and biological functions from others. EVs can originate from normal or cancer cells, transfer bioactive cargoes to both adjacent and distant sites, and orchestrate multiple key pathophysiological events such as carcinogenesis and malignant progression. Emerging as key messengers that mediate intercellular communications, EVs are being paid substantial attention in various disciplines including but not limited to cancer biology and immunology. Increasing lines of research advances have revealed the critical role of EVs in the establishment and maintenance of the tumor microenvironment (TME), including sustaining cell proliferation, evading growth suppression, resisting cell death, acquiring genomic instability and reprogramming stromal cell lineages, together contributing to the generation of a functionally remodeled TME. In this article, we present updates on major topics that document how EVs are implicated in proliferative expansion of cancer cells, promotion of drug resistance, reprogramming of metabolic activity, enhancement of metastatic potential, induction of angiogenesis, and escape from immune surveillance. Appropriate and insightful understanding of EVs and their contribution to cancer progression can lead to new avenues in the prevention, diagnosis and treatment of human malignancies in future medicine.

GATA Binding Protein 3 Boosts Extracellular ATP Hydrolysis and Inhibits Metastasis of Breast Cancer by Up-regulating Ectonucleoside Triphosphate Diphosphohydrolase 3

Despite remarkable advancements in our understanding of breast cancer, it remains the leading cause of cancer deaths in women. Distant recurrence and metastasis is the main reason for death due to breast cancer. It is well recognized that the GATA binding protein 3 (GATA3), a transcription factor, is a tumor suppressor in breast cancer. To date, the mechanistic molecular details of GATA3 remain elusive, because, as a transcription factor, it is not a direct executor in physiological and pathological processes. Here, we demonstrate that GATA3 reduces the ATP level in the breast cancer microenvironment and inhibits breast cancer metastasis by up-regulating ectonucleoside triphosphate diphosphohydrolase 3 (ENTPD3). The extracellular ATP concentration is significantly higher in tumor tissues than in normal tissues and promotes the migration of cancer cells from the primary site. ENTPD3 hydrolyzes ATP in tumor microenvironment and suppresses breast cancer metastasis. Furthermore, ENTPD3 inhibits epithelial-to-mesenchymal transition, a key program responsible for the development of metastatic disease. These findings provide novel insights into the tumor suppressor activity of GATA3.

Extracellular vesicles and energy metabolism

Glycolytic enzymes are among the most frequently identified proteins in proteomics of exosomes/extracellular vesicles. This review brings up the possibility that exosomes/extracellular vesicles during their life-time in bodily fluids power important energy-consuming functions by glycolytic conversion of glucose or fructose into ATP. It was seen that prostasomes (exosomes of the prostate) could produce ATP by glycolysis and that the produced ATP quickly was consumed by adjacent prostasomal ATPases. The glycolytic ATP production appeared to be coupled to self-sustaining energy requirements. It will also be discussed how a failure in this machinery (lowered activity of ATPases) with a resultant polluting leakage of extracellular ATP could affect cancer development.

Alpl prevents bone ageing sensitivity by specifically regulating senescence and differentiation in mesenchymal stem cells

Mutations in the liver/bone/kidney alkaline phosphatase (Alpl) gene cause hypophosphatasia (HPP) and early-onset bone dysplasia, suggesting that this gene is a key factor in human bone development. However, how and where Alpl acts in bone ageing is largely unknown. Here, we determined that ablation of Alpl induces prototypical premature bone ageing characteristics, including bone mass loss and marrow fat gain coupled with elevated expression of p16INK4A (p16) and p53 due to senescence and impaired differentiation in mesenchymal stem cells (MSCs). Mechanistically, Alpl deficiency in MSCs enhances ATP release and reduces ATP hydrolysis. Then, the excessive extracellular ATP is, in turn, internalized by MSCs and causes an elevation in the intracellular ATP level, which consequently inactivates the AMPKα pathway and contributes to the cell fate switch of MSCs. Reactivating AMPKα by metformin treatment successfully prevents premature bone ageing in Alpl+/- mice by improving the function of endogenous MSCs. These results identify a previously unknown role of Alpl in the regulation of ATP-mediated AMPKα alterations that maintain MSC stemness and prevent bone ageing and show that metformin offers a potential therapeutic option.

Mechanisms of AMPK in the maintenance of ATP balance during energy metabolism

AMP-activated protein kinase (AMPK) is a conserved sensor of cellular energy change and is activated by increased AMP/ATP and/or ADP/ATP ratios. AMPK maintains the energy balance by decreasing the ATP-consuming processes such as transcription of synthetic fat genes and rRNA, the translation of ribosomal proteins, synthesis of cholesterol and fatty acid, while the metabolic pathways such as glucose and fatty transport, fatty acid oxidation, autophagy, mitochondrial synthesis and oxidative metabolism are increased to preserve ATP during energy deficiency. Recent advance has demonstrated that AMPK activity has a close association with the initiation and progression in various cancers. Here we review the mechanisms that AMPK controls energy metabolism through regulating ATP synthesis and consumption, and further discuss the deregulation of AMPK in cancers.

Bioenergetics of acquired cisplatin resistant H1299 non-small cell lung cancer and P31 mesothelioma cells

Acquired cisplatin resistance is a common feature of tumours following cancer treatment with cisplatin and also of non-small cell lung cancer (H1299) and mesothelioma (P31) cell lines exposed to cisplatin. To elucidate the cellular basis of acquired cisplatin resistance, a comprehensive bioenergetic analysis was undertaken. We demonstrate that cellular oxygen consumption was significantly decreased in cisplatin resistant cells and that the reduction was primarily due to reduced mitochondrial activity as a result of reduced mitochondrial abundance. The differential mitochondrial abundance was supported by data showing reduced sirtuin 1 (SIRT1), peroxisome-proliferator activator receptor-γ co-activator 1-alpha (PGC1α), sirtuin 3 (SIRT3) and mitochondrial transcription factor A (TFAM) protein expression in resistant cells. Consistent with these data we observed increased reactive oxygen species (ROS) production and increased hypoxia inducible factor 1-alpha (HIF1α) stabilization in cisplatin resistant cells when compared to cisplatin sensitive controls. We also observed an increase in AMP kinase subunit α2 (AMPKα2) transcripts and protein expression in resistant H1299 cells. mRNA expression was also reduced for cisplatin resistant H1299 cells in these genes, however the pattern was not consistent in resistant P31 cells. There was very little change in DNA methylation of these genes, suggesting that the cells are not stably reprogrammed epigenetically. Taken together, our data demonstrate reduced oxidative metabolism, reduced mitochondrial abundance, potential for increased glycolytic flux and increased ROS production in acquired cisplatin resistant cells. This suggests that the metabolic changes are a result of reduced SIRT3 expression and increased HIF-1α stabilization.

Extracellular ATP, as an energy and phosphorylating molecule, induces different types of drug resistances in cancer cells through ATP internalization and intracellular ATP level increase

Cancer cells are able to uptake extracellular ATP (eATP) via macropinocytosis to elevate intracellular ATP (iATP) levels, enhancing their survival in drug treatment. However, the involved drug resistance mechanisms are unknown. Here we investigated the roles of eATP as either an energy or a phosphorylating molecule in general drug resistance mediated by ATP internalization and iATP elevation. We report that eATP increased iATP levels and promoted drug resistance to various tyrosine kinase inhibitors (TKIs) and chemo-drugs in human cancer cell lines of five cancer types. In A549 lung cancer cells, the resistance was downregulated by macropinocytosis inhibition or siRNA knockdown of PAK1, an essential macropinocytosis enzyme. The elevated iATP upregulated the efflux activity of ABC transporters in A549 and SK-Hep-1 cells as well as phosphorylation of PDGFRα and proteins in the PDGFR-mediated Akt-mTOR and Raf-MEK signaling pathways in A549 cells. Similar phosphorylation upregulations were found in A549 tumors. These results demonstrate that eATP induces different types of drug resistance by eATP internalization and iATP elevation, implicating the ATP-rich tumor microenvironment in cancer drug resistance, expanding our understanding of the roles of eATP in the Warburg effect and offering new anticancer drug resistance targets.