Purine metabolism regulates DNA repair and therapy resistance in glioblastoma

Evaluation of Proton-Induced DNA Damage in 3D-Engineered Glioblastoma Microenvironments

Glioblastoma (GBM) is a devastating cancer of the brain with an extremely poor prognosis. For this reason, besides clinical and preclinical studies, novel in vitro models for the assessment of cancer response to drugs and radiation are being developed. In such context, three-dimensional (3D)-engineered cellular microenvironments, compared to unrealistic two-dimensional (2D) monolayer cell culture, provide a model closer to the in vivo configuration. Concerning cancer treatment, while X-ray radiotherapy and chemotherapy remain the current standard, proton beam therapy is an appealing alternative as protons can be efficiently targeted to destroy cancer cells while sparing the surrounding healthy tissue. However, despite the treatment's compelling biological and medical rationale, little is known about the effects of protons on GBM at the cellular level. In this work, we designed novel 3D-engineered scaffolds inspired by the geometry of brain blood vessels, which cover a vital role in the colonization mechanisms of GBM cells. The architectures were fabricated by two-photon polymerization (2PP), cultured with U-251 GBM cells and integrated for the first time in the context of proton radiation experiments to assess their response to treatment. We employed Gamma H2A.X as a fluorescent biomarker to identify the DNA damage induced in the cells by proton beams. The results show a higher DNA double-strand breakage in 2D cell monolayers as compared to cells cultured in 3D. The discrepancy in terms of proton radiation response could indicate a difference in the radioresistance of the GBM cells or in the rate of repair kinetics between 2D cell monolayers and 3D cell networks. Thus, these biomimetic-engineered 3D scaffolds pave the way for the realization of a benchmark tool that can be used to routinely assess the effects of proton therapy on 3D GBM cell networks and other types of cancer cells.

miR-10b-5p-mediated upregulation of PIEZO1 predicts poor prognosis and links to purine metabolism in breast cancer

BACKGROUND

Increasing evidence has reported the critical roles of PIEZO1 in organism. However, the knowledge of PIEZO1 in human cancers is still inadequate.

METHODS

In silico analyses and experimental validation were performed to analyze PIEZO1's expression, prognostic values and potential upstream/downstream mechanism in breast cancer.

RESULTS

PIEZO1 was significantly overexpressed in human breast cancer cell lines and tissue samples. PIEZO1 expression was statistically positively associated with malignant progression and short survival of breast cancer. miR-10b-5p downregulation was partially responsible for PIEZO1 upregulation in breast cancer. Further exploration revealed that PIEZO1 might exert its function by regulating purine metabolism (especially GUK1, POLD1 and APRT) in breast cancer.

CONCLUSIONS

Collectively, the current study elucidated an important role of PIEZO1 in breast cancer and providing key clues for identifying PIEZO1 as a therapeutic target and prognostic biomarker in breast cancer.

A Novel Risk Score Model Based on Eleven Extracellular Matrix-Related Genes for Predicting Overall Survival of Glioma Patients

Gliomas are the most common lethal primary brain tumors with variable survival outcomes for patients. The extracellular matrix (ECM) is linked with clinical prognosis of glioma patients, but it is not commonly used as a clinical indicator. Herein, we investigated changes in ECM-related genes (ECMRGs) via analyzing the transcriptional data of 938 gliomas from TCGA and CGGA datasets. Based on least absolute shrinkage and selection operator (LASSO) Cox regression analysis, a 11-ECMRG signature that is strongly linked with overall survival (OS) in glioma patients was identified. This signature was characterized by high-risk and low-risk score patterns. We found that the patients in the high-risk group are significantly linked with malignant molecular features and worse outcomes. Univariate and multivariate Cox regression analyses suggested that the signature is an independent indicator for glioma prognosis. The prediction accuracy of the signature was verified through time-dependent receiver operating characteristic (ROC) curves and calibration plots. Further bioinformatics analyses implied that the ECMRG signature is strongly associated with the activation of multiple oncogenic and metabolic pathways and immunosuppressive tumor microenvironment in gliomas. In addition, we confirmed that the high-risk score is an indicator for a therapy-resistant phenotype. In addition to bioinformatics analyses, we functionally verified the oncogenic role of bone morphogenetic protein 1 (BMP1) in gliomas in vitro.

Metabolism drives macrophage heterogeneity in the tumor microenvironment

Tumor-associated macrophages (TAMs) are a major cellular component in the tumor microenvironment (TME). However, the relationship between the phenotype and metabolic pattern of TAMs remains poorly understood. We performed single-cell transcriptome profiling on hepatic TAMs from mice bearing liver metastatic tumors. We find that TAMs manifest high heterogeneity at the levels of transcription, development, metabolism, and function. Integrative analyses and validation experiments indicate that increased purine metabolism is a feature of TAMs with pro-tumor and terminal differentiation phenotypes. Like mouse TAMs, human TAMs are highly heterogeneous. Human TAMs with increased purine metabolism exhibit a pro-tumor phenotype and correlate with poor therapeutic efficacy to immune checkpoint blockade. Altogether, our work demonstrates that TAMs are developmentally, metabolically, and functionally heterogeneous and purine metabolism may be a key metabolic feature of a pro-tumor macrophage population.

Liquid-liquid phase separation drives cellular function and dysfunction in cancer

Cancer is a disease of uncontrollably reproducing cells. It is governed by biochemical pathways that have escaped the regulatory bounds of normal homeostatic balance. This balance is maintained through precise spatiotemporal regulation of these pathways. The formation of biomolecular condensates via liquid-liquid phase separation (LLPS) has recently emerged as a widespread mechanism underlying the spatiotemporal coordination of biological activities in cells. Biomolecular condensates are widely observed to directly regulate key cellular processes involved in cancer cell pathology, and the dysregulation of LLPS is increasingly implicated as a previously hidden driver of oncogenic activity. In this Perspective, we discuss how LLPS shapes the biochemical landscape of cancer cells.

Purine Synthesis Inhibitor L-Alanosine Impairs Mitochondrial Function and Stemness of Brain Tumor Initiating Cells

Glioblastoma (GBM) is a lethal brain cancer exhibiting high levels of drug resistance, a feature partially imparted by tumor cell stemness. Recent work shows that homozygous MTAP deletion, a genetic alteration occurring in about half of all GBMs, promotes stemness in GBM cells. Exploiting MTAP loss-conferred deficiency in purine salvage, we demonstrate that purine blockade via treatment with L-Alanosine (ALA), an inhibitor of de novo purine synthesis, attenuates stemness of MTAP-deficient GBM cells. This ALA-induced reduction in stemness is mediated in part by compromised mitochondrial function, highlighted by ALA-induced elimination of mitochondrial spare respiratory capacity. Notably, these effects of ALA are apparent even when the treatment was transient and with a low dose. Finally, in agreement with diminished stemness and compromised mitochondrial function, we show that ALA sensitizes GBM cells to temozolomide (TMZ) in vitro and in an orthotopic GBM model. Collectively, these results identify purine supply as an essential component in maintaining mitochondrial function in GBM cells and highlight a critical role of mitochondrial function in sustaining GBM stemness. We propose that purine synthesis inhibition can be beneficial in combination with the standard of care for MTAP-deficient GBMs, and that it may be feasible to achieve this benefit without inflicting major toxicity.

Overcoming Radiation Resistance in Gliomas by Targeting Metabolism and DNA Repair Pathways

Gliomas represent a wide spectrum of brain tumors characterized by their high invasiveness, resistance to chemoradiotherapy, and both intratumoral and intertumoral heterogeneity. Recent advances in transomics studies revealed that enormous abnormalities exist in different biological layers of glioma cells, which include genetic/epigenetic alterations, RNA expressions, protein expression/modifications, and metabolic pathways, which provide opportunities for development of novel targeted therapeutic agents for gliomas. Metabolic reprogramming is one of the hallmarks of cancer cells, as well as one of the oldest fields in cancer biology research. Altered cancer cell metabolism not only provides energy and metabolites to support tumor growth, but also mediates the resistance of tumor cells to antitumor therapies. The interactions between cancer metabolism and DNA repair pathways, and the enhancement of radiotherapy sensitivity and assessment of radiation response by modulation of glioma metabolism are discussed herein.

Interactions between Radiation and One-Carbon Metabolism

Metabolic reprogramming is a hallmark of cancer. Cancer cells rewire one-carbon metabolism, a central metabolic pathway, to turn nutritional inputs into essential biomolecules required for cancer cell growth and maintenance. Radiation therapy, a common cancer therapy, also interacts and alters one-carbon metabolism. This review discusses the interactions between radiation therapy, one-carbon metabolism and its component metabolic pathways.

Metabolic response to radiation therapy in cancer

Tumor metabolism has emerged as a hallmark of cancer and is involved in carcinogenesis and tumor growth. Reprogramming of tumor metabolism is necessary for cancer cells to sustain high proliferation rates and enhanced demands for nutrients. Recent studies suggest that metabolic plasticity in cancer cells can decrease the efficacy of anticancer therapies by enhancing antioxidant defenses and DNA repair mechanisms. Studying radiation-induced metabolic changes will lead to a better understanding of radiation response mechanisms as well as the identification of new therapeutic targets, but there are few robust studies characterizing the metabolic changes induced by radiation therapy in cancer. In this review, we will highlight studies that provide information on the metabolic changes induced by radiation and oxidative stress in cancer cells and the associated underlying mechanisms.

Guanosine primes acute myeloid leukemia for differentiation via guanine nucleotide salvage synthesis

Differentiation arrest represents a distinct hallmark of acute myeloid leukemia (AML). Identification of differentiation-induction agents that are effective across various subtypes remains an unmet challenge. GTP biosynthesis is elevated in several types of cancers, considered to support uncontrolled tumor growth. Here we report that GTP overload by supplementation of guanosine, the nucleoside precursor of GTP, poises AML cells for differentiation and growth inhibition. Transcriptome profiling of guanosine-treated AML cells reveals a myeloid differentiation pattern. Importantly, the treatment compromises leukemia progression in AML xenograft models. Mechanistically, GTP overproduction requires sequential metabolic conversions executed by the purine salvage biosynthesis pathway including the involvement of purine nucleoside phosphorylase (PNP) and hypoxanthine phosphoribosyltransferase 1 (HPRT1). Taken together, our study offers novel metabolic insights tethering GTP homeostasis to myeloid differentiation and provides an experimental basis for further clinical investigations of guanosine or guanine nucleotides in the treatment of AML patients.

DDRugging glioblastoma: understanding and targeting the DNA damage response to improve future therapies

Glioblastoma is the most frequently diagnosed type of primary brain tumour in adults. These aggressive tumours are characterised by inherent treatment resistance and disease progression, contributing to ~ 190 000 brain tumour-related deaths globally each year. Current therapeutic interventions consist of surgical resection followed by radiotherapy and temozolomide chemotherapy, but average survival is typically around 1 year, with < 10% of patients surviving more than 5 years. Recently, a fourth treatment modality of intermediate-frequency low-intensity electric fields [called tumour-treating fields (TTFields)] was clinically approved for glioblastoma in some countries after it was found to increase median overall survival rates by ~ 5 months in a phase III randomised clinical trial. However, beyond these treatments, attempts to establish more effective therapies have yielded little improvement in survival for patients over the last 50 years. This is in contrast to many other types of cancer and highlights glioblastoma as a recognised tumour of unmet clinical need. Previous work has revealed that glioblastomas contain stem cell-like subpopulations that exhibit heightened expression of DNA damage response (DDR) factors, contributing to therapy resistance and disease relapse. Given that radiotherapy, chemotherapy and TTFields-based therapies all impact DDR mechanisms, this Review will focus on our current knowledge of the role of the DDR in glioblastoma biology and treatment. We also discuss the potential of effective multimodal targeting of the DDR combined with standard-of-care therapies, as well as emerging therapeutic targets, in providing much-needed improvements in survival rates for patients.

Factors affecting the radiation response in glioblastoma

Glioblastoma (GBM) is a highly invasive primary brain tumor in adults with a 5-year survival rate of less than 10%. Conventional radiotherapy with photons, along with concurrent and adjuvant temozolomide, is the mainstay for treatment of GBM although no significant improvement in survival rates has been observed over the last 20 years. Inherent factors such as tumor hypoxia, radioresistant GBM stem cells, and upregulated DNA damage response mechanisms are well established as contributing to treatment resistance and tumor recurrence. While it is understandable that efforts have focused on targeting these factors to overcome this phenotype, there have also been striking advances in precision radiotherapy techniques, including proton beam therapy and carbon ion radiotherapy (CIRT). These enable higher doses of radiation to be delivered precisely to the tumor, while minimizing doses to surrounding normal tissues and organs at risk. These alternative radiotherapy techniques also benefit from increased biological effectiveness, particularly in the case of CIRT. Although not researched extensively to date, combining these new radiation modalities with radio-enhancing agents may be particularly effective in improving outcomes for patients with GBM.

DNA damage response proteins synergistically affect the cancer prognosis and resistance

Amplification of oxidative stress can be utilized as a strategy to attenuate cancer progression by instigating apoptosis. However, the duration of positive response to such therapies is limited, as cancer cells eventually develop resistance. The underlying molecular mechanisms of cancer cells to escape apoptosis under oxidative stress is unknown. Employing big data, and its integration with transcriptome, proteome and network analysis in six cancer types revealed system-level interactions between DNA damage response (DDR) proteins, including; DNA damage repair, cell cycle checkpoints and anti-apoptotic proteins. Cancer system biology is used to elucidate mechanisms for cancer progression, but networks defining mechanisms causing resistance is less explored. Using system biology, we identified DDR hubs between G1-S and M phases that were associated with bad prognosis. The increased expression of DDR network was involved in resistance under high oxidative stress. We validated our findings by combining H₂O₂ induced oxidative stress and DDR inhibitors in human lung cancer cells to conclude the necessity of targeting a 'disease-causing network'. Collectively, our work provides insights toward designing strategies for network pharmacology to combat resistance in cancer research.

GammaTile® brachytherapy in the treatment of recurrent glioblastomas

Background

GammaTile® (GT) is a recent U.S. Food and Drug Administration (FDA) cleared brachytherapy platform. Here, we report clinical outcomes for recurrent glioblastoma patients after GT treatment following maximal safe resection.

Methods

We prospectively followed twenty-two consecutive Isocitrate Dehydrogenase (IDH) wild-type glioblastoma patients (6 O⁶-Methylguanine-DNA methyltransferase methylated (MGMTm); sixteen MGMT unmethylated (MGMTu)) who underwent maximal safe resection of recurrent tumor followed by GT placement.

Results

The cohort consisted of 14 second and eight third recurrences. In terms of procedural safety, there was one 30-day re-admission (4.5%) for an incisional cerebrospinal fluid leak, which resolved with lumbar drainage. No other wound complications were observed. Six patients (27.2%) declined in Karnofsky Performance Score (KPS) after surgery due to worsening existing deficits. One patient suffered a new-onset seizure postsurgery (4.5%). There was one (4.5%) 30-day mortality from intracranial hemorrhage secondary to heparinization for an ischemic limb. The mean follow-up was 733 days (range 279–1775) from the time of initial diagnosis. Six-month local control (LC₆) and twelve-month local control (LC₁₂) were 86 and 81%, respectively. Median progression-free survival (PFS) was comparable for MGMTu and MGMTm patients (~8.0 months). Median overall survival (OS) was 20.0 months for the MGMTu patients and 37.4 months for MGMTm patients. These outcomes compared favorably to data in the published literature and an independent glioblastoma cohort of comparable patients without GT treatment.

Conclusions

This clinical experience supports GT brachytherapy as a treatment option in a multi-modality treatment strategy for recurrent glioblastomas.

Translation of DNA Damage Response Inhibitors as Chemoradiation Sensitizers From the Laboratory to the Clinic

Combination therapies with agents targeting the DNA damage response (DDR) offer an opportunity to selectively enhance the therapeutic index of chemoradiation or eliminate use of chemotherapy altogether. The successful translation of DDR inhibitors to clinical use requires investigating both their direct actions as (chemo)radiosensitizers and their potential to stimulate tumor immunogenicity. Beginning with high-throughput screening using both viability and DNA damage-reporter assays, followed by validation in gold-standard radiation colony-forming assays and in vitro assessment of mechanistic effects on the DDR, we describe proven strategies and methods leading to the clinical development of DDR inhibitors both with radiation alone and in combination with chemoradiation. Beyond these in vitro studies, we discuss the impact of key features of human xenograft and syngeneic mouse models on the relevance of in vivo tumor efficacy studies, particularly with regard to the immunogenic effects of combined therapy with radiation and DDR inhibitors. Finally, we describe recent technological advances in radiation delivery (using the small animal radiation research platform) that allow for conformal, clinically relevant radiation therapy in mouse models. This overall approach is critical to the successful clinical development and ultimate Food and Drug Administration approval of DDR inhibitors as (chemo)radiation sensitizers.

P2Y12 Purinergic Receptor and Brain Tumors: Implications on Glioma Microenvironment

Gliomas are the most common malignant brain tumors in adults, characterized by a high proliferation and invasion. The tumor microenvironment is rich in growth-promoting signals and immunomodulatory pathways, which increase the tumor's aggressiveness. In response to hypoxia and glioma therapy, the amounts of adenosine triphosphate (ATP) and adenosine diphosphate (ADP) strongly increase in the extracellular space, and the purinergic signaling is triggered by nucleotides' interaction in P2 receptors. Several cell types are present in the tumor microenvironment and can facilitate tumor growth. In fact, tumor cells can activate platelets by the ADP-P2Y₁₂ engagement, which plays an essential role in the cancer context, protecting tumors from the immune attack and providing molecules that contribute to the growth and maintenance of a rich environment to sustain the protumor cycle. Besides platelets, the P2Y₁₂ receptor is expressed by some tumors, such as renal carcinoma, colon carcinoma, and gliomas, being related to tumor progression. In this context, this review aims to depict the glioma microenvironment, focusing on the relationship between platelets and tumor malignancy.

Clinical Targeting of Altered Metabolism in High-Grade Glioma

ABSTRACT

High-grade gliomas are among the deadliest of all cancers despite standard treatments, and new therapeutic strategies are needed to improve patient outcome. Targeting the altered metabolic state of tumors with traditional chemotherapeutic agents has a history of success, and our increased understanding of cellular metabolism in the past 2 decades has reinvigorated the concept of novel metabolic therapies in brain tumors. Here we highlight metabolic alterations in advanced gliomas and their translation into clinical trials using both novel agents and already established drugs repurposed for cancer treatment in an effort to improve outcome for these deadly diseases.

Uncovering Spatiotemporal Heterogeneity of High-Grade Gliomas: From Disease Biology to Therapeutic Implications

Glioblastomas (GBM) are the most common and aggressive tumors of the central nervous system. Rapid tumor growth and diffuse infiltration into healthy brain tissue, along with high intratumoral heterogeneity, challenge therapeutic efficacy and prognosis. A better understanding of spatiotemporal tumor heterogeneity at the histological, cellular, molecular, and dynamic levels would accelerate the development of novel treatments for this devastating brain cancer. Histologically, GBM is characterized by nuclear atypia, cellular pleomorphism, necrosis, microvascular proliferation, and pseudopalisades. At the cellular level, the glioma microenvironment comprises a heterogeneous landscape of cell populations, including tumor cells, non-transformed/reactive glial and neural cells, immune cells, mesenchymal cells, and stem cells, which support tumor growth and invasion through complex network crosstalk. Genomic and transcriptomic analyses of gliomas have revealed significant inter and intratumoral heterogeneity and insights into their molecular pathogenesis. Moreover, recent evidence suggests that diverse dynamics of collective motion patterns exist in glioma tumors, which correlate with histological features. We hypothesize that glioma heterogeneity is not stochastic, but rather arises from organized and dynamic attributes, which favor glioma malignancy and influences treatment regimens. This review highlights the importance of an integrative approach of glioma histopathological features, single-cell and spatially resolved transcriptomic and cellular dynamics to understand tumor heterogeneity and maximize therapeutic effects.

Genomic basis of deep-water adaptation in Arctic Charr (Salvelinus alpinus) morphs

The post-glacial colonization of Gander Lake in Newfoundland, Canada, by Arctic Charr (Salvelinus alpinus) provides the opportunity to study the genomic basis of adaptation to extreme deep-water environments. Colonization of deep-water (>50 m) habitats often requires extensive adaptation to cope with novel environmental challenges from high hydrostatic pressure, low temperature, and low light, but the genomic mechanisms underlying evolution in these environments are rarely known. Here, we compare genomic divergence between a deep-water morph adapted to depths of up to 288 m and a larger, piscivorous pelagic morph occupying shallower depths. Using both a SNP array and resequencing of whole nuclear and mitochondrial genomes, we find clear genetic divergence (F_ST  = 0.11-0.15) between deep and shallow water morphs, despite an absence of morph divergence across the mitochondrial genome. Outlier analyses identified many diverged genomic regions containing genes enriched for processes such as gene expression and DNA repair, cardiac function, and membrane transport. Detection of putative copy number variants (CNVs) uncovered 385 genes with CNVs distinct to piscivorous morphs, and 275 genes with CNVs distinct to deep-water morphs, enriched for processes associated with synapse assembly. Demographic analyses identified evidence for recent and local morph divergence, and ongoing reductions in diversity consistent with postglacial colonization. Together, these results show that Arctic Charr morph divergence has occurred through genome-wide differentiation and elevated divergence of genes underlying multiple cellular and physiological processes, providing insight into the genomic basis of adaptation in a deep-water habitat following postglacial recolonization.

One-Carbon Metabolism Associated Vulnerabilities in Glioblastoma: A Review

Simple Summary

Glioblastoma tumours are the most malignant and common type of central nervous system tumours. Despite aggressive treatment measures, disease recurrence in patients with glioblastoma is inevitable and survival rates remain low. Glioblastoma cells, like other cancer cells, can leverage metabolic pathways to increase their rate of proliferation, maintain self-renewal, and develop treatment resistance. Furthermore, many of the metabolic strategies employed by cancer cells are similar to those employed by stem cells in order to maintain self-renewal and proliferation. One-carbon metabolism and de novo purine synthesis are metabolic pathways that are essential for biosynthesis of macromolecules and have been found to be essential for tumourigenesis. In this review, we summarize the evidence showing the significance of 1-C-mediated de novo purine synthesis in glioblastoma cell proliferation and tumourigenesis, as well as evidence suggesting the effectiveness of targeting this metabolic pathway as a therapeutic modality.

Abstract

Altered cell metabolism is a hallmark of cancer cell biology, and the adaptive metabolic strategies of cancer cells have been of recent interest to many groups. Metabolic reprogramming has been identified as a critical step in glial cell transformation, and the use of antimetabolites against glioblastoma has been investigated. One-carbon (1-C) metabolism and its associated biosynthetic pathways, particularly purine nucleotide synthesis, are critical for rapid proliferation and are altered in many cancers. Purine metabolism has also been identified as essential for glioma tumourigenesis. Additionally, alterations of 1-C-mediated purine synthesis have been identified as commonly present in brain tumour initiating cells (BTICs) and could serve as a phenotypic marker of cells responsible for tumour recurrence. Further research is required to elucidate mechanisms through which metabolic vulnerabilities may arise in BTICs and potential ways to therapeutically target these metabolic processes. This review aims to summarize the role of 1-C metabolism-associated vulnerabilities in glioblastoma tumourigenesis and progression and investigate the therapeutic potential of targeting this pathway in conjunction with other treatment strategies.

STING enhances cell death through regulation of reactive oxygen species and DNA damage

Resistance to DNA-damaging agents is a significant cause of treatment failure and poor outcomes in oncology. To identify unrecognized regulators of cell survival we performed a whole-genome CRISPR-Cas9 screen using treatment with ionizing radiation as a selective pressure, and identified STING (stimulator of interferon genes) as an intrinsic regulator of tumor cell survival. We show that STING regulates a transcriptional program that controls the generation of reactive oxygen species (ROS), and that STING loss alters ROS homeostasis to reduce DNA damage and to cause therapeutic resistance. In agreement with these data, analysis of tumors from head and neck squamous cell carcinoma patient specimens show that low STING expression is associated with worse outcomes. We also demonstrate that pharmacologic activation of STING enhances the effects of ionizing radiation in vivo, providing a rationale for therapeutic combinations of STING agonists and DNA-damaging agents. These results highlight a role for STING that is beyond its canonical function in cyclic dinucleotide and DNA damage sensing, and identify STING as a regulator of cellular ROS homeostasis and tumor cell susceptibility to reactive oxygen dependent, DNA damaging agents.

Metabolic Vulnerabilities in Brain Cancer

Glioblastomas (GBMs) exhibit altered metabolism to support a variety of bioenergetic and biosynthetic demands for tumor growth, invasion, and drug resistance. Changes in glycolytic flux, oxidative phosphorylation, the pentose phosphate pathway, fatty acid biosynthesis and oxidation, and nucleic acid biosynthesis are observed in GBMs to help drive tumorigenesis. Both the genetic landscape of GBMs and the unique brain tumor microenvironment shape metabolism; therefore, an understanding of how both intrinsic and extrinsic factors modulate metabolism is becoming increasingly important for finding effect targets and therapeutics for GBM.

Rewiring of purine metabolism in response to acidosis stress in glioma stem cells

Glioma stem cells (GSCs) contribute to therapy resistance and poor outcomes for glioma patients. A significant feature of GSCs is their ability to grow in an acidic microenvironment. However, the mechanism underlying the rewiring of their metabolism in low pH remains elusive. Here, using metabolomics and metabolic flux approaches, we cultured GSCs at pH 6.8 and pH 7.4 and found that cells cultured in low pH exhibited increased de novo purine nucleotide biosynthesis activity. The overexpression of glucose-6-phosphate dehydrogenase, encoded by G6PD or H6PD, supports the metabolic dependency of GSCs on nucleotides when cultured under acidic conditions, by enhancing the pentose phosphate pathway (PPP). The high level of reduced glutathione (GSH) under acidic conditions also causes demand for the PPP to provide NADPH. Taken together, upregulation of G6PD/H6PD in the PPP plays an important role in acidic-driven purine metabolic reprogramming and confers a predilection toward glioma progression. Our findings indicate that targeting G6PD/H6PD, which are closely related to glioma patient survival, may serve as a promising therapeutic target for improved glioblastoma therapeutics. An integrated metabolomics and metabolic flux analysis, as well as considering microenvironment and cancer stem cells, provide a precise insight into understanding cancer metabolic reprogramming.

Metabolic Reprogramming in Anticancer Drug Resistance: A Focus on Amino Acids

Overcoming anticancer drug resistance is a major challenge in cancer therapy, requiring innovative strategies that consider the extensive tumor heterogeneity and adaptability. We provide recent evidence highlighting the key role of amino acid (AA) metabolic reprogramming in cancer cells and the supportive microenvironment in driving resistance to anticancer therapies. AAs sustain the acquisition of anticancer resistance by providing essential building blocks for biosynthetic pathways and for maintaining a balanced redox status, and modulating the epigenetic profile of both malignant and non-malignant cells. In addition, AAs support the reduced intrinsic susceptibility of cancer stem cells to antineoplastic therapies. These findings shed new light on the possibility of targeting nonresponding tumors by modulating AA availability through pharmacological or dietary interventions.

The Many Facets of Therapy Resistance and Tumor Recurrence in Glioblastoma

Glioblastoma (GBM) is the most lethal type of primary brain cancer. Standard care using chemo- and radio-therapy modestly increases the overall survival of patients; however, recurrence is inevitable, due to treatment resistance and lack of response to targeted therapies. GBM therapy resistance has been attributed to several extrinsic and intrinsic factors which affect the dynamics of tumor evolution and physiology thus creating clinical challenges. Tumor-intrinsic factors such as tumor heterogeneity, hypermutation, altered metabolomics and oncologically activated alternative splicing pathways change the tumor landscape to facilitate therapy failure and tumor progression. Moreover, tumor-extrinsic factors such as hypoxia and an immune-suppressive tumor microenvironment (TME) are the chief causes of immunotherapy failure in GBM. Amid the success of immunotherapy in other cancers, GBM has occurred as a model of resistance, thus focusing current efforts on not only alleviating the immunotolerance but also evading the escape mechanisms of tumor cells to therapy, caused by inter- and intra-tumoral heterogeneity. Here we review the various mechanisms of therapy resistance in GBM, caused by the continuously evolving tumor dynamics as well as the complex TME, which cumulatively contribute to GBM malignancy and therapy failure; in an attempt to understand and identify effective therapies for recurrent GBM.

Reuse of Molecules for Glioblastoma Therapy

Glioblastoma multiforme (GBM) is a highly malignant primary brain tumor. The current standard of care for GBM is the Stupp protocol which includes surgical resection, followed by radiotherapy concomitant with the DNA alkylator temozolomide; however, survival under this treatment regimen is an abysmal 12-18 months. New and emerging treatments include the application of a physical device, non-invasive 'tumor treating fields' (TTFs), including its concomitant use with standard of care; and varied vaccines and immunotherapeutics being trialed. Some of these approaches have extended life by a few months over standard of care, but in some cases are only available for a minority of GBM patients. Extensive activity is also underway to repurpose and reposition therapeutics for GBM, either alone or in combination with the standard of care. In this review, we present select molecules that target different pathways and are at various stages of clinical translation as case studies to illustrate the rationale for their repurposing-repositioning and potential clinical use.

Inhibition of the de novo pyrimidine biosynthesis pathway limits ribosomal RNA transcription causing nucleolar stress in glioblastoma cells

Glioblastoma is the most common and aggressive type of cancer in the brain; its poor prognosis is often marked by reoccurrence due to resistance to the chemotherapeutic agent temozolomide, which is triggered by an increase in the expression of DNA repair enzymes such as MGMT. The poor prognosis and limited therapeutic options led to studies targeted at understanding specific vulnerabilities of glioblastoma cells. Metabolic adaptations leading to increased synthesis of nucleotides by de novo biosynthesis pathways are emerging as key alterations driving glioblastoma growth. In this study, we show that enzymes necessary for the de novo biosynthesis of pyrimidines, DHODH and UMPS, are elevated in high grade gliomas and in glioblastoma cell lines. We demonstrate that DHODH's activity is necessary to maintain ribosomal DNA transcription (rDNA). Pharmacological inhibition of DHODH with the specific inhibitors brequinar or ML390 effectively depleted the pool of pyrimidines in glioblastoma cells grown in vitro and in vivo and impaired rDNA transcription, leading to nucleolar stress. Nucleolar stress was visualized by the aberrant redistribution of the transcription factor UBF and the nucleolar organizer nucleophosmin 1 (NPM1), as well as the stabilization of the transcription factor p53. Moreover, DHODH inhibition decreased the proliferation of glioblastoma cells, including temozolomide-resistant cells. Importantly, the addition of exogenous uridine, which reconstitutes the cellular pool of pyrimidine by the salvage pathway, to the culture media recovered the impaired rDNA transcription, nucleolar morphology, p53 levels, and proliferation of glioblastoma cells caused by the DHODH inhibitors. Our in vivo data indicate that while inhibition of DHODH caused a dramatic reduction in pyrimidines in tumor cells, it did not affect the overall pyrimidine levels in normal brain and liver tissues, suggesting that pyrimidine production by the salvage pathway may play an important role in maintaining these nucleotides in normal cells. Our study demonstrates that glioblastoma cells heavily rely on the de novo pyrimidine biosynthesis pathway to generate ribosomal RNA (rRNA) and thus, we identified an approach to inhibit ribosome production and consequently the proliferation of glioblastoma cells through the specific inhibition of the de novo pyrimidine biosynthesis pathway.

Purine metabolism promotes radioresistance and is a therapeutic target in glioblastoma

Profound intratumoral genomic heterogeneity has limited the ability of targeted therapies to overcome therapy resistance in glioblastoma. We have defined purine metabolism as a key mediator of DNA repair and radiation resistance in glioblastoma. Because many glioblastoma oncogenic drivers activate purine metabolism, its inhibition may overcome therapy resistance despite intratumoral genomic heterogeneity.