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Background N 6 -methyladenosine (m6A) and adenosine-to-inosine (A-to-I) RNA editing are two of the most abundant RNA modification events affecting adenosines in mammals. Both these RNA modifications determine mRNA fate and play a pivotal role in tumor development and progression. Results Here, we show that METTL3, upregulated in glioblastoma, methylates ADAR1 mRNA and increases its protein level leading to a pro-tumorigenic mechanism connecting METTL3, YTHDF1, and ADAR1. We show that ADAR1 plays a cancer-promoting role independently of its deaminase activity by binding CDK2 mRNA, underlining the importance of ADARs as essential RNA-binding proteins for cell homeostasis as well as cancer progression. Additionally, we show that ADAR1 knockdown is sufficient to strongly inhibit glioblastoma growth in vivo. Conclusions Hence, our findings underscore METTL3/ADAR1 axis as a novel crucial pathway in cancer progression that connects m6A and A-to-I editing post-transcriptional events.

Glioblastoma (GBM) is the most aggressive primary brain tumor in adults. Despite progress in surgical and medical neuro-oncology, prognosis for GBM patients remains dismal, with a median survival of only 14–15 months. The modest benefit of conventional therapies is due to the presence of GBM stem cells (GSCs) that cause tumor relapse and chemoresistance and, therefore, that play a key role in GBM aggressiveness and recurrence. So far, strategies to identify and target GSCs have been unsuccessful. Thus, the development of an approach for GSC detection and targeting would be fundamental for improving the survival of GBM patients. Here, using the cell-systematic evolution of ligand by exponential (SELEX) methodology on human primary GSCs, we generated and characterized RNA aptamers that selectively bind GSCs versus undifferentiated GBM cells. We found that the shortened version of the aptamer 40L, which we have called A40s, costained with CD133-labeled cells in human GBM tissue, suggestive of an ability to specifically recognize GSCs in fixed human tissues. Of note, both 40L and A40s were rapidly internalized by cells, allowing for the delivery of the microRNA miR-34c and the anti-microRNA anti-miR-10b, demonstrating that these aptamers can serve as selective vehicles for therapeutics. In conclusion, the aptamers 40L and A40s can selectively target GSCs. Given the crucial role of GSCs in GBM recurrence and therapy resistance, these aptamers represent innovative drug delivery candidates with a great potential in the treatment of GBM. [Display omitted]

Zika virus (ZIKV) is a flavivirus with a marked effect on fetal nervous system development. ZIKV treatment has recently been found to also have a benefit against glioblastoma, a highly aggressive brain tumor with a poor prognosis. The reported data do not completely explain the mechanism beyond this effect. Nevertheless, in the majority of the cases no adverse effect has been found in healthy adult humans. In this study, we characterized the ZIKV infection mechanism on glioblastoma stem cells, which are considered responsible for the tumor progression and resistance to conventional therapies. Moreover, we explain why the action of this virus is directed to the stem cells in the nervous system counterpart. Our results confirm the effectiveness of ZIKV treatment against glioblastoma, indicating novel molecular targets that can be introduced for more powerful therapies.

Background Glioblastoma (GBM) is a highly migratory, invasive, and angiogenic brain tumor. Like vascular endothelial growth factor-A (VEGF-A), placental growth factor (PlGF) promotes GBM angiogenesis. VEGF-A is a ligand for both VEGF receptor-1 (VEGFR-1) and VEGFR-2, while PlGF interacts exclusively with VEGFR-1. We recently generated the novel anti-VEGFR-1 monoclonal antibody (mAb) D16F7 that diminishes VEGFR-1 homodimerization/activation without affecting VEGF-A and PlGF binding. Methods In the present study, we evaluated the expression of VEGFR-1 in human GBM tissue samples ( n  = 42) by immunohistochemistry, in cell lines ( n  = 6) and GBM stem cells (GSCs) ( n  = 18) by qRT-PCR and/or western blot analysis. In VEGFR-1 positive GBM or GSCs we also analyzed the ability of D16F7 to inhibit GBM invasiveness in response to VEGF-A and PlGF. Results Most of GBM specimens stained positively for VEGFR-1 and all but one GBM cell lines expressed VEGFR-1. On the other hand, in GSCs the expression of the receptor was heterogeneous. D16F7 reduced migration and invasion of VEGFR-1 positive GBM cell lines and patient-derived GSCs in response to VEGF-A and PlGF. Interestingly, this effect was also observed in VEGFR-1 positive GSCs transfected to over-express wild-type EGFR (EGFRwt + ) or mutant EGFR (ligand binding domain-deficient EGFRvIII + ). Furthermore, D16F7 suppressed intracellular signal transduction in VEGFR-1 over-expressing GBM cells by reducing receptor auto-phosphorylation at tyrosine 1213 and downstream Erk1/2 activation induced by receptor ligands. Conclusion The results from this study suggest that VEGFR-1 is a relevant target for GBM therapy and that D16F7-derived humanized mAbs warrant further investigation.

The role of autophagy in cancer onset and progression appears still controversial. On one hand, autophagy allows cancer cell to survive in unfavorable environmental conditions, on the other hand, once internal energy resources are exhausted, it leads to cell death. In addition, autophagy interpheres with cell cycle progression, de facto exerting a cytostatic activity. Hence, it represents an important target for anticancer therapy. For example, temozolomide (TMZ), of use for glioblastoma (GBM) treatment, appears as capable of inducing autophagy partially inhibiting cancer cell proliferation. However, GBM, a very aggressive brain tumor with poor prognosis even after surgery and radio-chemotherapy, invariably recurs and leads to patient death. Since cancer stem cells have been hypothesized to play a role in refractory/relapsing cancers, in the present work we investigated if autophagy could represent a constitutive cytoprotection mechanism for glioblastoma stem-like cells (GSCs) and if the modulation of autophagic process could affect GBM growth and survival. Thus, in the present study we first evaluated the relevance of autophagy in GBM tumor specimens, then its occurrence in GSCs and, finally, if modulation of autophagy could influence GSC response to TMZ. Our results suggested that, in vitro, the impairing autophagic process with quinacrine, a compound able to cross the blood-brain barrier, increased GSC susceptibility to TMZ. Death of GSCs was apparently due to the iron dependent form of programmed cell death characterized by the accumulation of lipid peroxides called ferroptosis. These results underscore the relevance of the modulation of autophagy in the GSC survival and death and suggest that triggering of ferroptosis in GSCs could represent a novel and important target for the management of glioblastoma.

Cancer stem cells Two papers in this issue report the identification of a specific type of colon cancer cell that initiates tumour growth in mice. This lends support to the theory that only a few cells within a tumour, the cancer stem cells, are responsible for tumour formation and maintenance. The finding highlights the importance of finding therapeutics to target cancer stem cells. Colon carcinoma is the second most common cause of death from cancer 1 . The isolation and characterization of tumorigenic colon cancer cells may help to devise novel diagnostic and therapeutic procedures. Although there is increasing evidence that a rare population of undifferentiated cells is responsible for tumour formation and maintenance 2 , 3 , 4 , this has not been explored for colorectal cancer. Here, we show that tumorigenic cells in colon cancer are included in the high-density CD133 + population, which accounts for about 2.5% of the tumour cells. Subcutaneous injection of colon cancer CD133 + cells readily reproduced the original tumour in immunodeficient mice, whereas CD133 - cells did not form tumours. Such tumours were serially transplanted for several generations, in each of which we observed progressively faster tumour growth without significant phenotypic alterations. Unlike CD133 - cells, CD133 + colon cancer cells grew exponentially for more than one year in vitro as undifferentiated tumour spheres in serum-free medium, maintaining the ability to engraft and reproduce the same morphological and antigenic pattern of the original tumour. We conclude that colorectal cancer is created and propagated by a small number of undifferentiated tumorigenic CD133 + cells, which should therefore be the target of future therapies.

Background Glioblastoma (GBM) is the most common and aggressive primary malignant brain tumor in adults, characterized by a poor prognosis mainly due to recurrence and therapeutic resistance. It has been widely demonstrated that glioblastoma stem-like cells (GSCs), a subpopulation of tumor cells endowed with stem-like properties is responsible for tumor maintenance and progression. Moreover, it has been demonstrated that GSCs contribute to GBM-associated neovascularization processes, through different mechanisms including the transdifferentiation into GSC-derived endothelial cells (GdECs). Methods In order to identify druggable cancer-related pathways in GBM, we assessed the effect of a selection of 349 compounds on both GSCs and GdECs and we selected elesclomol (STA-4783) as the most effective agent in inducing cell death on both GSC and GdEC lines tested. Results Elesclomol has been already described to be a potent oxidative stress inducer. In depth investigation of the molecular mechanisms underlying GSC and GdEC response to elesclomol, confirmed that this compound induces a strong increase in mitochondrial reactive oxygen species (ROS) in both GSCs and GdECs ultimately leading to a non-apoptotic copper-dependent cell death. Moreover, combined in vitro treatment with elesclomol and the alkylating agent temozolomide (TMZ) enhanced the cytotoxicity compared to TMZ alone. Finally, we used our experimental model of mouse brain xenografts to test the combination of elesclomol and TMZ and confirmed their efficacy in vivo. Conclusions Our results support further evaluation of therapeutics targeting oxidative stress such as elesclomol with the aim of satisfying the high unmet medical need in the management of GBM.

ABSTRACT Objective Axitinib is a tyrosine kinase inhibitor characterized by a strong affinity for Vascular Endothelial Growth Factor Receptors (VEGFRs). It was approved in 2012 by Food and Drug Administration and European Medicines Agency as a second line treatment for advanced renal cell carcinoma and is currently under evaluation in clinical trial for the treatment of other cancers. Glioblastoma IDH‐wild type (GBM) is a highly malignant brain tumor characterized by diffusely infiltrative growth pattern and by a prominent neo‐angiogenesis. In GBM, axitinib has demonstrated a limited effectiveness as a monotherapy, while it was recently shown to significantly improve its efficacy in combination treatments. In preclinical models, axitinib has been reported to trigger cellular senescence both in tumor as well as in normal cells, through a mechanism involving intracellular reactive oxygen species (ROS) accumulation and activation of Ataxia Telangiectasia Mutated kinase (ATM). Limiting axitinib‐dependent ROS increase by antioxidants prevents senescence specifically in normal cells, without affecting tumor cells. Methods We used brain tumor xenografts obtained by engrafting Glioma Stem Cells (GSCs) into the brain of immunocompromised mice, to investigate the hypothesis that the antioxidant molecule N‐Acetyl‐L‐Cysteine (NAC) might be used to reduce senescence‐associated adverse effects of axitinib treatment without altering its anti‐tumor activity. Results We demonstrate that the use of the antioxidant molecule N‐Acetyl‐Cysteine (NAC) in combination with axitinib stabilizes tumor microvessels in GBM tumor orthotopic xenografts, eventually resulting in vessel normalization, and protects liver vasculature from axitinib‐dependent toxicity. Conclusion Overall, we found that NAC co‐treatment allows vessel normalization in brain tumor vessels and exerts a protective effect on liver vasculature, therefore minimizing axitinib‐dependent toxicity. Schematic representation of the experimental approach. In GBM preclinical models, we observed the efficacy of NAC cotreatment in protecting vessels from axitinib‐induced toxicity.

Glioblastoma multiforme (GBM) is the most common and malignant primary brain tumor in adults, characterized by aggressive growth, limited response to therapy, and inexorable recurrence. Because of the extremely unfavorable prognosis of GBM, it is important to develop more effective diagnostic and therapeutic strategies based on biologically and clinically relevant patient stratification systems. Analyzing a collection of patient‐derived GBM stem‐like cells (GSCs) by gene expression profiling, nuclear magnetic resonance spectroscopy, and signal transduction pathway activation, we identified two GSC clusters characterized by different clinical features. Due to the widely documented role played by microRNAs (miRNAs) in the tumorigenesis process, in this study we explored whether these two GBM patient subtypes could also be discriminated by different miRNA signatures. Global miRNA expression pattern was analyzed by oblique principal component analysis and principal component analysis. By a combined inferential strategy on PCA results, we identified a reduced set of three miRNAs – miR‐23a, miR‐27a, and miR‐9* (miR‐9‐3p) – able to discriminate the proneural‐ and mesenchymal‐like GSC phenotypes as well as mesenchymal and proneural subtypes of primary GBM included in The Cancer Genome Atlas (TCGA) data set. Kaplan–Meier analysis showed a significant correlation between the selected miRNAs and overall survival in 429 GBM specimens from TCGA‐identifying patients who had an unfavorable outcome. The survival prognostic capability of the three‐miRNA signatures could have important implications for the understanding of the biology of GBM subtypes and could be useful in patient stratification to facilitate interpretation of results from clinical trials. The global miRNA expression pattern in a collection of glioblastoma stem‐like cell lines (GSCs) was analyzed. We identified a reduced set of three miRNAs – miR‐23a, miR‐27a, and miR‐9* (miR‐9‐3p) – able to discriminate the proneural‐ and mesenchymal‐like GSC phenotypes as well as mesenchymal and proneural subtypes of primary GBM included in The Cancer Genome Atlas (TCGA) data set. This three‐miRNA signature showed a positive correlation with overall survival in 429 specimens from TCGA.

Tumor hypoxic microenvironment causes hypoxia inducible factor 1 alpha (HIF-1α) activation and necrosis with alarmins release. Importantly, HIF-1α also controls the expression of alarmin receptors in tumor cells that can bind to and be activated by alarmins. Human tumor tissues possess 1–2% of cancer stem cells (CSCs) residing in hypoxic niches and responsible for the metastatic potential of tumors. Our hypothesis is that hypoxic CSCs express alarmin receptors that can bind alarmins released during necrosis, an event favoring CSCs migration. To investigate this aspect, glioblastoma stem-like cell (GSC) lines were kept under hypoxia to determine the expression of hypoxic markers as well as receptor for advanced glycation end products (RAGE). The presence of necrotic extracts increased migration, invasion and cellular adhesion. Importantly, HIF-1α inhibition by digoxin or acriflavine prevented the response of GSCs to hypoxia alone or plus necrotic extracts. In vivo, GSCs injected in one brain hemisphere of NOD/SCID mice were induced to migrate to the other one in which a necrotic extract was previously injected. In conclusion, our results show that hypoxia is important not only for GSCs maintenance but also for guiding their response to external necrosis. Inhibition of hypoxic pathway may therefore represent a target for preventing brain invasion by glioblastoma stem cells (GSCs).