Induction of oligodendrogenesis in glioblastoma-initiating cells by IFN-mediated activation of STAT3 signaling

Loss of RBM45 inhibits breast cancer progression by reducing the SUMOylation of IRF7 to promote IFNB1 transcription

Type I interferons exhibit anti-proliferative and anti-cancer activities, but their detailed regulatory mechanisms in cancer have not been fully elucidated yet. RNA binding proteins are master orchestrators of gene regulation, which are closely related to tumor progression. Here we show that the upregulated RNA binding protein RBM45 correlates with poor prognosis in breast cancer. Depletion of RBM45 suppresses breast cancer progression both in cultured cells and xenograft mouse models. Mechanistically, RBM45 ablation inhibits breast cancer progression through regulating type I interferon signaling, particularly by elevating IFN-β production. Importantly, RBM45 recruits TRIM28 to IRF7 and stimulates its SUMOylation, thereby repressing IFNB1 transcription. Loss of RBM45 reduced the SUMOylation of IRF7 by reducing the interaction between TRIM28 and IRF7 to promote IFNB1 transcription, leading to the inhibition of breast cancer progression. Taken together, our finding uncovers a vital role of RBM45 in modulating type I interferon signaling and cancer aggressive progression, implicating RBM45 as a potential therapeutic target in breast cancer.

A novel defined risk signature of interferon response genes predicts the prognosis and correlates with immune infiltration in glioblastoma

BACKGROUND

Interferons (IFNs) have been implemented as anti-tumor immunity agents in clinical trials of glioma, but only a subset of glioblastoma (GBM) patients profits from it. The predictive role of IFNs stimulated genes in GBM needs further exploration to investigate the clinical role of IFNs.

METHODS

This study screened 526 GBM patients from three independent cohorts. The transcriptome data with matching clinical information were analyzed using R. Immunohistochemical staining data from the Human Protein Atlas and DNA methylation data from MethSurv were used for validation in protein and methylation level respectively.

RESULTS

We checked the survival effect of all 491 IFNs response genes, and found 54 genes characterized with significant hazard ratio in overall survival (OS). By protein-protein interaction analysis, 10 hub genes were selected out for subsequent study. And based on the expression of these 10 genes, GBM patients could be divided into two subgroups with significant difference in OS. Furthermore, the least absolute shrinkage and selection operator cox regression model was utilized to construct a multigene risk signature, including STAT3, STAT2 and SOCS3, which could serve as an independent prognostic predictor for GBM. The risk model was validated in two independent GBM cohorts. The GBM patients with high risk scores mainly concentrated in the GBM Mesenchymal subtype. The higher risk group was enriched in hypoxia, angiogenesis, EMT, glycolysis and immune pathways, and had increased Macrophage M2 infiltration and high expression of immune checkpoint CD274 (namely PD-L1).

CONCLUSIONS

Our findings revealed the three-gene risk model could be an independent prognostic predictor for GBM, and they were crucial participants in immunosuppressive microenvironment of GBM.

The Yin and Yang of Type I IFNs in Cancer Promotion and Immune Activation

Type I Interferons (IFNs) are key regulators of natural and therapy-induced host defense against viral infection and cancer. Several years of remarkable progress in the field of oncoimmunology have revealed the dual nature of these cytokines. Hence, Type I IFNs may trigger anti-tumoral responses, while leading immune dysfunction and disease progression. This dichotomy relies on the duration and intensity of the transduced signaling, the nature of the unleashed IFN stimulated genes, and the subset of responding cells. Here, we discuss the role of Type I IFNs in the evolving relationship between the host immune system and cancer, as we offer a view of the therapeutic strategies that exploit and require an intact Type I IFN signaling, and the role of these cytokines in inducing adaptive resistance. A deep understanding of the complex, yet highly regulated, network of Type I IFN triggered molecular pathways will help find a timely and immune"logical" way to exploit these cytokines for anticancer therapy.

The Interactions Between Cancer Stem Cells and the Innate Interferon Signaling Pathway

Interferons (IFNs) form a family of cytokines with pleiotropic effects that modulate the immune response against multiple challenges like viral infections, autoimmune diseases, and cancer. While numerous anti-tumor activities have been described for IFNs, IFNs have also been associated with tumor growth and progression. The effect of IFNs on apoptosis, angiogenesis, tumor cell immunogenicity, and modulation of immune cells have been largely studied; however, less is known about their specific effects on cancer stem cells (CSCs). CSCs constitute a subpopulation of tumor cells endowed with stem-like properties including self-renewal, chemoresistance, tumorigenic capacity, and quiescence. This rare and unique subpopulation of cells is believed to be responsible for tumor maintenance, metastatic spread, and relapse. Thus, this review aims to summarize and discuss the current knowledge of the anti- and pro-CSCs effects of IFNs and also to highlight the need for further research on the interplay between IFNs and CSCs. Importantly, understanding this interplay will surely help to exploit the anti-tumor effects of IFNs, specifically those that target CSCs.

Indoleamine 2,3-dioxygenase 1 is highly expressed in glioma stem cells

Recent research has revealed that glioblastoma (GBM) avoids the immune system via strong expression of indoleamine 2,3-dioxygenase 1 (IDO1). IDO1, an enzyme involved in tryptophan metabolism, is now proposed as a new target in GBM treatment, since several reports have demonstrated that IDO1 expression is related to GBM malignancy. On the other hand, it is well known that glioma stem cells (GSCs) are strongly related to the malignancy of GBM. However, there is as yet no report evaluating the relationship between GSCs and IDO1. We therefore examined the expression levels of IDO1 in GSCs in order to identify a new therapeutic target for GBM based on the immune systems of GSCs. In the present study, we employed human GBM cell lines (U-138MG, U-251MG) and patient-derived GSC model cell lines (0125-GSC, 0222-GSC). GSC model cell lines Rev-U-138MG and Rev-U-251MG were established by culturing U-138MG and U-251MG in serum-free media, while differentiated GBM model cell lines 0125-DGC and 0222-DGC were established by culturing 0125-GSC and 0222-GSC in serum-containing media. The expression levels of stem cell markers (Nanog, Nestin, Oct4 and Sox2) and IDO1 protein and mRNA were determined. Rev-U-138MG and Rev-U-251MG formed spheres and their expression levels of stem cell markers were increased as compared to U-138MG and U-251MG. On the other hand, 0125-DGC and 0222-DGC suffered breakdown of sphere formation, despite the original 0125-GSC and 0222-GSC forming spheres, and their expression levels of the markers were decreased. IDO1 expressions were strongly recognized in Rev-U-138MG, Rev-U-251MG, 0125-GSC and 0222-GSC as compared to U-138MG, U-251MG, 0125-DGC and 0222-DGC. These findings demonstrate that GSCs exhibit treatment resistance with immunosuppression via high expression levels of IDO1, and could represent a novel target for GBM treatment.

Autocrine activation of the IFN signaling pathway may promote immune escape in glioblastoma

BACKGROUND

Interferons (IFNs) are cytokines typically induced upon viral infection but are constitutively expressed also in the absence of acute infection. The physiological role of autocrine and paracrine IFN signaling, however, remains poorly understood, and its function in glioblastoma has not been explored in depth.

METHODS

Using RNA interference-mediated gene silencing, we characterized constitutive type I IFN signaling and its role in human glioma cells.

RESULTS

We observed constitutive expression of phosphorylated signal transducer and activator of transcription 1 (pSTAT1) and myxovirus resistance protein A (MxA), a classical IFN-response marker, in the absence of exogenous IFN-β. In vivo, we found higher MxA expression in gliomas than in normal tissue, suggesting that IFN signaling is constitutively active in these tumors. To demonstrate the presence of an autocrine type I IFN signaling loop in glioma cells in vitro, we first confirmed the expression of the type I alpha/beta receptor (IFNAR)1/2, and its ligands, IFN-α and IFN-β. Small interfering RNA-mediated receptor gene silencing resulted in reduced expression of MxA at mRNA and protein levels, as did gene silencing of the ligands, corroborating the hypothesis of an autocrine signaling loop in which type I IFNs induce intracellular signaling through IFNAR1/2. On a functional level, following IFNAR1 or IFNAR2 gene silencing, we observed reduced programmed death ligand 1 (PD-L1) and major histocompatibility complex (MHC) class I and II expression as well as an enhanced susceptibility to natural killer immune cell lysis, suggesting that autocrine IFN signaling contributes to the immune evasion of glioma cells.

CONCLUSIONS

Our findings point to an important role of constitutive IFN signaling in glioma cells by modulating their interaction with the microenvironment.

Significance of low mTORC1 activity in defining the characteristics of brain tumor stem cells

Background

The significance of mammalian target of rapamycin complex 1 (mTORC1) activity in the maintenance of cancer stem cells (CSCs) remains controversial. Previous findings showed that mTORC1 activation depleted the population of leukemia stem cells in leukemia, while maintaining the stemness in pancreatic CSCs. The purpose of this study was to examine the currently unknown role and significance of mTORC1 activity in brain tumor stem cells (BTSCs).

Methods

Basal mTORC1 activity and its kinetics were investigated in BTSC clones isolated from patients with glioblastoma and their differentiated progenies (DIFFs). The effects of nutrient deprivation and the mTORC1 inhibitors on cell proliferation were compared between the BTSCs and DIFFs. Tissue sections from patients with brain gliomas were examined for expression of BTSC markers and mTORC1 activity by immunohistochemistry.

Results

BTSCs presented lower basal mTORC1 activity under each culture condition tested and a more rapid decline of mTORC1 activity after nutrient deprivation than observed in DIFFs. The self-renewal capacity of BTSCs was unaffected by mTORC1 inhibition, whereas it effectively suppressed DIFF proliferation. In agreement, immunohistochemical staining of glioma tissues revealed low mTORC1 activity in tumor cells positive for BTSC markers. In in vitro culture, BTSCs exhibited resistance to the antitumor agent temozolomide.

Conclusions

Our findings indicated the importance of low mTORC1 activity in maintaining the undifferentiated state of BTSCs, implicating the relevance of manipulating mTORC1 activity when developing future strategies that target BTSCs.

In Vitro and In Vivo Preclinical Effects of Type I IFNs on Gliomas

The interferons (IFNs) are a family of cytokines with diverse cellular actions such as control of cell proliferation and regulation of immune responses; therefore, they have been extensively studied as antitumor agents for a variety of malignancies, including gliomas. Type I IFNs exert their antitumor effects either directly, by targeting the tumor cells or the tumor stem cells, or indirectly, by regulating the anticancer activities of the immune system. More specifically, IFN-beta and IFN-alpha exhibit antiproliferative effects by p53 induction, CD8⁺ T-lymphocyte and macrophage activation, chemokine secretion, and miR-21 downregulation. In vitro and in vivo studies provide evidence that immunotherapy could have a role in glioma treatment, especially when first-line therapeutic interventions fail to produce durable responses. These effects are more obvious when combining IFN-beta with classical antitumor therapies such as temozolamide, an oral chemotherapeutic, for both newly diagnosed and recurrent gliomas. However, further clinical studies are needed to determine whether IFNs will have a definite place in the management of gliomas.

Antitumorigenic effect of interferon-β by inhibition of undifferentiated glioblastoma cells

Glioma stem-like cells (GSCs) are undifferentiated cells that are considered to be an origin of glioblastomas. Furthermore, they may contribute to treatment resistance and recurrence in glioblastomas. GSCs differentiate into differentiated glioma cells (non-glioma stem-like cells: non-GSCs), and interconversion might occur between GSCs and non-GSCs. We investigated whether interferon-beta (IFN-β) could exert any efficacy towards GSCs or such interconversion processes. The neural stem cell marker CD133 and pluripotency marker Nanog in GSCs were analyzed to evaluate their differentiation levels. GSCs were considered to undergo differentiation into non-GSCs upon serum exposure, since the expression of CD133 and Nanog in the GSCs was negatively affected. Furthermore, the cells regained their undifferentiated features upon removal of the serum. However, we verified that IFN-β reduced cell proliferation and tumor sphere formation in GSCs, and induced suppression of the restoration of such undifferentiated features. In addition, we also confirmed that IFN-β suppressed the acquisition process of undifferentiated features in human malignant glioma cell lines. Our data thus suggest that IFN-β could be an effective agent not only through its cell growth inhibitory effect on GSCs but also as a means of targeting the interconversion between GSCs and non-GSCs, indicating the possibility of IFN-β being used to prevent treatment resistance and recurrence in glioblastomas, via the inhibition of undifferentiated features.

Characterization of glioma stem-like cells from human glioblastomas

Glioma stem-like cells (GSCs) could have potential for tumorigenesis, treatment resistance, and tumor recurrence (GSC hypothesis). However, the mechanisms underlying such potential has remained elusive and few ultrastructural features of the cells have been reported in detail. We therefore undertook observations of the antigenic characteristics and ultrastructural features of GSCs isolated from human glioblastomas. Tumor spheres formed by variable numbers of cells, exhibiting a variable appearance in both their size and shape, were frequently seen in GSCs expressing the stem cell surface markers CD133 and CD15. Increased cell nucleus atypia, mitochondria, rough endoplasmic reticulum, coated vesicles, and microvilli, were noted in the GSCs. Furthermore, cells at division phases and different phases of the apoptotic process were occasionally observed. These findings could imply that GSCs have certain relations with human neural stem cells (NSCs) but are primitively different from undifferentiated NSCs. The data may provide support for the GSC hypothesis, and also facilitate the establishment of future glioblastoma treatments targeting GSCs.

Interferon-β and temozolomide combination therapy for temozolomide monotherapy-refractory malignant gliomas

Interferon-β (IFN-β) has been found to downregulate O⁶-methyl-guanine-DNA methyltransferase and sensitize glioma cells to chemoradiation therapy. The effectiveness of IFN-β and temozolomide (TMZ) combination therapy for newly diagnosed glioblastomas was previously reported. However, there is no clinical report of recurrent of malignant gliomas treated with the combination of IFN-β and TMZ. In the present study, we reported 7 cases of gliomas classified as uncontrollable with adjuvant TMZ monotherapy, who were then treated with IFN-β and TMZ combination therapy. The magnetic resonance imaging findings and clinical symptoms improved in the majority of the cases, with tolerable adverse events and minimal residual disability. The overall survival (OS) time from the date of the initial surgery exceeded 13 months, suggesting that this combination therapy was successful in improving the prognosis of malignant gliomas refractory to adjuvant TMZ monotherapy.

Effect of the STAT3 inhibitor STX-0119 on the proliferation of a temozolomide-resistant glioblastoma cell line

Glioblastoma multiforme (GBM) is one of the most malignant and aggressive tumors and has a very poor prognosis, with a median survival time of less than 2 years. Once recurrence develops, there are few therapeutic approaches to control the growth of glioblastoma. In particular, temozolomide (TMZ)-resistant (TMZ-R) GBM is very difficult to treat, and a novel approach to overcome resistance is eagerly awaited. Previously, we reported a novel small molecule inhibitor of STAT3 dimerization, STX-0119, as a cancer therapeutic. In the current study, the efficacy of STX-0119 was evaluated against our established TMZ-resistant U87 cell line using quantitative PCR-based gene expression analysis, in vitro assay and animal experiments. The growth inhibitory effect of STX-0119 on U87 and TMZ-R U87 cells was moderate (IC₅₀, 34 and 45 µM, respectively). In particular, STX-0119 did not show significant inhibition of U87 tumor growth; however, it suppressed the growth of the TMZ-R U87 tumor in nude mice by more than 50%, and prolonged the median survival time compared to the control group. Quantitative PCR revealed that YKL-40, MAGEC1, MGMT, several EMT genes, mesenchymal genes and STAT3 target genes were upregulated, but most of those genes were downregulated by STX-0119 treatment. Furthermore, the invasive activity of TMZ-R U87 cells was significantly inhibited by STX-0119. YKL-40 levels in TMZ-R U87 cells and their supernatants were significantly decreased by STX-0119 administration. These results suggest that STX-0119 is an efficient therapeutic to overcome TMZ resistance in recurrent GBM tumors, and could be the next promising compound leading to survival prolongation, and YKL-40 may be a possible surrogate marker for STAT3 targeting.

Smac mimetic promotes glioblastoma cancer stem-like cell differentiation by activating NF-κB

Recently, a broader role of inhibitor of apoptosis (IAP) proteins besides their antiapoptotic functions has been described. Therefore, we investigated the effect of non-toxic concentrations of the small-molecule Smac mimetic BV6, which antagonizes IAP proteins, on differentiation of cancer stem-like cells (CSLCs) derived from primary glioblastoma (GBM) specimens. Here, we identify a novel function of BV6 in regulating differentiation of GBM CSLCs by activating NF-κB. BV6 at non-lethal doses stimulates morphological changes associated with the differentiation of GBM CSLCs. BV6 increases transcriptional activity, mRNA and protein levels of the astrocytic marker GFAP without altering expression of the neuronal marker β-III-tubulin, indicating that BV6 induces astrocytic differentiation of GBM CSLCs. Molecular studies reveal that BV6 triggers processing of the NF-κB subunit p100 to p52, nuclear translocation of p52 and p50 and increased NF-κB DNA-binding. Intriguingly, inhibition of NF-κB by overexpression of dominant-negative IκBα super-repressor (IκBα-SR) blocks the BV6-stimulated increase in GFAP and differentiation. Interestingly, this BV6-stimulated differentiation is associated with reduced expression of stemness markers such as CD133, Nanog and Sox2 in GBM CSLCs. In contrast, BV6 does not alter cell morphology, differentiation and expression of stemness markers in non-malignant neural stem cells. Importantly, BV6 treatment reduces clonogenicity of GBM CSLCs in vitro and in vivo, suppresses their tumorigenicity in orthotopic and subcutaneous mouse models and significantly increases the survival of mice. By identifying a novel role of BV6 in promoting differentiation of GBM CSLCs, these findings provide new insights into Smac mimetic-regulated non-apoptotic functions with important implications for targeting GBM CSLCs.

Brain tumor stem cells: Molecular characteristics and their impact on therapy

Glioblastoma (GBM) is the most prevalent primary brain tumor and ranks among the most lethal of human cancers with conventional therapy offering only palliation. Great strides have been made in understanding brain cancer genetics and modeling these tumors with new targeted therapies being tested, but these advances have not translated into substantially improved patient outcomes. Multiple chemotherapeutic agents, including temozolomide, the first-line treatment for glioblastoma, have been developed to kill cancer cells. However, the response to temozolomide in GBM is modest. Radiation is also moderately effective but this approach is plagued by limitations due to collateral radiation damage to healthy brain tissue and development of radioresistance. Therapeutic resistance is attributed at least in part to a cell population within the tumor that possesses stem-like characteristics and tumor propagating capabilities, referred to as cancer stem cells. Within GBM, the intratumoral heterogeneity is derived from a combination of regional genetic variance and a cellular hierarchy often regulated by distinct cancer stem cell niches, most notably perivascular and hypoxic regions. With the recent emergence as a key player in tumor biology, cancer stem cells have symbiotic relationships with the tumor microenvironment, oncogenic signaling pathways, and epigenetic modifications. The origins of cancer stem cells and their contributions to brain tumor growth and therapeutic resistance are under active investigation with novel anti-cancer stem cell therapies offering potential new hope for this lethal disease.

Resistance to oncolytic myxoma virus therapy in nf1(-/-)/trp53(-/-) syngeneic mouse glioma models is independent of anti-viral type-I interferon

Despite promising preclinical studies, oncolytic viral therapy for malignant gliomas has resulted in variable, but underwhelming results in clinical evaluations. Of concern are the low levels of tumour infection and viral replication within the tumour. This discrepancy between the laboratory and the clinic could result from the disparity of xenograft versus syngeneic models in determining in vivo viral infection, replication and treatment efficacy. Here we describe a panel of primary mouse glioma lines derived from Nf1 (+/-) Trp53 (+/-) mice in the C57Bl/6J background for use in the preclinical testing of the oncolytic virus Myxoma (MYXV). These lines show a range of susceptibility to MYXV replication in vitro, but all succumb to viral-mediated cell death. Two of these lines orthotopically grafted produced aggressive gliomas. Intracranial injection of MYXV failed to result in sustained viral replication or treatment efficacy, with minimal tumour infection that was completely resolved by 7 days post-infection. We hypothesized that the stromal production of Type-I interferons (IFNα/β) could explain the resistance seen in these models; however, we found that neither the cell lines in vitro nor the tumours in vivo produce any IFNα/β in response to MYXV infection. To confirm IFNα/β did not play a role in this resistance, we ablated the ability of tumours to respond to IFNα/β via IRF9 knockdown, and generated identical results. Our studies demonstrate that these syngeneic cell lines are relevant preclinical models for testing experimental glioma treatments, and show that IFNα/β is not responsible for the MYXV treatment resistance seen in syngeneic glioma models.

Chromatin regulator PRC2 is a key regulator of epigenetic plasticity in glioblastoma

Tumor cell plasticity contributes to functional and morphologic heterogeneity. To uncover the underlying mechanisms of this plasticity, we examined glioma stem-like cells (GSC) where we found that the biologic interconversion between GSCs and differentiated non-GSCs is functionally plastic and accompanied by gain or loss of polycomb repressive complex 2 (PRC2), a complex that modifies chromatin structure. PRC2 mediates lysine 27 trimethylation on histone H3 and in GSC it affected pluripotency or development-associated genes (e.g., Nanog, Wnt1, and BMP5) together with alterations in the subcellular localization of EZH2, a catalytic component of PRC2. Intriguingly, exogenous expression of EZH2-dNLS, which lacks nuclear localization sequence, impaired the repression of Nanog expression under differentiation conditions. RNA interference (RNAi)-mediated attenuation or pharmacologic inhibition of EZH2 had little to no effect on apoptosis or bromodeoxyuridine incorporation in GSCs, but it disrupted morphologic interconversion and impaired GSC integration into the brain tissue, thereby improving survival of GSC-bearing mice. Pathologic analysis of human glioma specimens revealed that the number of tumor cells with nuclear EZH2 is larger around tumor vessels and the invasive front, suggesting that nuclear EZH2 may help reprogram tumor cells in close proximity to this microenvironment. Our results indicate that epigenetic regulation by PRC2 is a key mediator of tumor cell plasticity, which is required for the adaptation of glioblastoma cells to their microenvironment. Thus, PRC2-targeted therapy may reduce tumor cell plasticity and tumor heterogeneity, offering a new paradigm for glioma treatment.

Delineating the cytogenomic and epigenomic landscapes of glioma stem cell lines

Glioblastoma multiforme (GBM), the most common and malignant type of glioma, is characterized by a poor prognosis and the lack of an effective treatment, which are due to a small sub-population of cells with stem-like properties, termed glioma stem cells (GSCs). The term "multiforme" describes the histological features of this tumor, that is, the cellular and morphological heterogeneity. At the molecular level multiple layers of alterations may reflect this heterogeneity providing together the driving force for tumor initiation and development. In order to decipher the common "signature" of the ancestral GSC population, we examined six already characterized GSC lines evaluating their cytogenomic and epigenomic profiles through a multilevel approach (conventional cytogenetic, FISH, aCGH, MeDIP-Chip and functional bioinformatic analysis). We found several canonical cytogenetic alterations associated with GBM and a common minimal deleted region (MDR) at 1p36.31, including CAMTA1 gene, a putative tumor suppressor gene, specific for the GSC population. Therefore, on one hand our data confirm a role of driver mutations for copy number alterations (CNAs) included in the GBM genomic-signature (gain of chromosome 7- EGFR gene, loss of chromosome 13- RB1 gene, loss of chromosome 10-PTEN gene); on the other, it is not obvious that the new identified CNAs are passenger mutations, as they may be necessary for tumor progression specific for the individual patient. Through our approach, we were able to demonstrate that not only individual genes into a pathway can be perturbed through multiple mechanisms and at different levels, but also that different combinations of perturbed genes can incapacitate functional modules within a cellular networks. Therefore, beyond the differences that can create apparent heterogeneity of alterations among GSC lines, there's a sort of selective force acting on them in order to converge towards the impairment of cell development and differentiation processes. This new overview could have a huge importance in therapy.

The future of glioma treatment: stem cells, nanotechnology and personalized medicine

The development of novel therapies, imaging techniques and insights into the processes that drive growth of CNS tumors have allowed growing enthusiasm for the treatment of CNS malignancies. Despite this energized effort to investigate and treat brain cancer, clinical outcomes for most patients continue to be dismal. Recognition of diverse tumor subtypes, behaviors and outcomes has led to an interest in personalized medicine for the treatment of brain tumors. This new paradigm requires evaluation of the tumor phenotype at the time of diagnosis so that therapy can be specifically tailored to each individual patient. Investigating novel therapies involving stem cells, nanotechnology and molecular medicine will allow diversity of therapeutic options for patients with brain cancer. These exciting new therapeutic strategies for brain tumors are reviewed in this article.

Therapeutic strategies targeting cancer stem cells

Increasing studies have demonstrated a small proportion of cancer stem cells (CSCs) exist in the cancer cell population. CSCs have powerful self-renewal capacity and tumor-initiating ability and are resistant to chemotherapy and radiation. Conventional anticancer therapies kill the rapidly proliferating bulk cancer cells but spare the relatively quiescent CSCs, which cause cancer recurrence. So it is necessary to develop therapeutic strategies acting specifically on CSCs. In recent years, studies have shown that therapeutic agents such as metformin, salinomycin, DECA-14, rapamycin, oncostatin M (OSM), some natural compounds, oncolytic viruses, microRNAs, cell signaling pathway inhibitors, TNF-related apoptosis inducing ligand (TRAIL), interferon (IFN), telomerase inhibitors, all-trans retinoic acid (ATRA) and monoclonal antibodies can suppress the self-renewal of CSCs in vitro and in vivo. A combination of these agents and conventional chemotherapy drugs can significantly inhibit tumor growth, metastasis and recurrence. These strategies targeting CSCs may bring new hopes to cancer therapy.

Biological and clinical implications of cancer stem cells in primary brain tumors

Despite therapeutic advances, glioblastoma multiforme (GBM) remains a lethal disease. The infiltrative nature of this disease and the presence of a cellular population resistant to current medical treatments account for the poor prognosis of these patients. Growing evidence indicates the existence of a fraction of cancer cells sharing the functional properties of adult stem cells, including self-renewal and a greater ability to escape chemo-radiotherapy-induced death stimuli. Therefore, these cells are commonly defined as cancer stem cells (GBM-SCs). The initial GBM-SC concept has been challenged, and refined according to the emerging molecular taxonomy of GBM. This allowed to postulate the existence of multiple CSC types, each one driving a given molecular entity. Furthermore, it is becoming increasingly clear that GBM-SCs thrive through a dynamic and bidirectional interaction with the surrounding microenvironment. In this article, we discuss recent advances in GBM-SC biology, mechanisms through which these cells adapt to hostile conditions, pharmacological strategies for selectively killing GBM-SCs, and how novel CSC-associated endpoints have been investigated in the clinical setting.

NGF/γ-IFN inhibits androgen-independent prostate cancer and reverses androgen receptor function through downregulation of FGFR2 and decrease in cancer stem cells

Androgen-independent prostate cancer (AIPC) is difficult to treat. Present study is to explore the inhibitory effect of a cytokine environment on AIPC and its mechanism. We utilized nerve growth factor (NGF)/γ-interferon (γ-IFN) to change the cytokine environment. Animal models and 2 androgen receptor (AR)-negative prostate cancer cell lines were used to evaluate the effect of NGF/γ-IFN. Flow cytometry, immunocytochemistry, western blotting, Tunel assay, colony formation efficiency, gene microarray, and in vivo bioluminescence were used to discern the mechanisms within NGF/γ-IFN that effect the environment. In vitro, NGF/γ-IFN effectively inhibited the proliferation of AIPC cell lines and promoted the apoptosis of the cancer cells. In vivo, NGF/γ-IFN suppressed the growth and metastasis of a tumor mass that arose from the AIPC cell line. After NGF/γ-IFN treatment, the AR-negative cell lines re-expressed AR and were then able to respond to the androgen. Contrary to expectations, the proliferation of cells was inhibited after dihydrotestosterone was added, and the results indicated that NGF/γ-IFN decreased the proportion of cancer stem cells. NGF/γ-IFN worked mainly through the downregulation of fibroblast growth factor receptor 2.

Contribution of microRNA-1275 to Claudin11 protein suppression via a polycomb-mediated silencing mechanism in human glioma stem-like cells

Glioblastomas show heterogeneous histological features, and tumor cells show distinct phenotypic states that confer different functional attributes and an aggressive character. However, the molecular mechanisms underlying the heterogeneity in this disease are poorly understood. Glioma stem-like cells (GSCs) are considered able to aberrantly differentiate into diverse cell types and may contribute to the establishment of tumor heterogeneity. Using a GSC model, we investigated differentially expressed microRNAs (miRNAs) and associated epigenetic mechanisms that regulate the differentiation of GSCs. miRNA profiling using microarray technology showed that 13 and 34 miRNAs were commonly up-regulated and down-regulated in two independent GSC lines during differentiation, respectively. Among this set of miRNAs, quantitative PCR analysis showed that miRNA-1275 (miR-1275) was consistently down-regulated during GSC differentiation, along with the up-regulation of its target, CLDN11, an important protein during oligodendroglial lineage differentiation. Inhibition of miR-1275 with a specific antisense oligonucleotide (anti-miR-1275) in GSCs increased the expression of CLDN11, together with significant growth suppression. Epigenetic analysis revealed that gain of histone H3 lysine 27 trimethylation (H3K27me3) in the primary microRNA-1275 promoter was closely associated with miR-1275 expression. Treatment with 3-deazaneplanocin A, an inhibitor of H3K27 methyltransferase, attenuated CLDN11 induction by serum stimulation in parallel with sustained miR-1275 expression. Our results have illuminated the epigenetic regulatory pathways of miR-1275 that are closely associated with oligodendroglial differentiation, which may contribute to the tissue heterogeneity seen in the formation of glioblastomas. Given that inhibition of miR-1275 induces expression of oligodendroglial lineage proteins and suppresses tumor cell proliferation, this may be a potential therapeutic target for glioblastomas.

Signal transducer and activator of transcription 1 (Stat1) maintains basal mRNA expression of pro-survival stat3-target genes in glioma C6 cells

The STAT proteins (signal transducers and activators of transcription) are transcription factors mediating cytokine/growth factor signaling, which play important role in controlling cell cycle progression and apoptosis. In many cancer cell lines and tumors (including gliomas) the STAT proteins (in particular Stats 1, 3, and 5) are persistently activated. In this study, we employed DNA decoys, siRNAs, and protein overexpression, to elucidate the role of Stat1 and Stat3 in regulation of expression of endogenous Stat3-target genes (Bcl2l1, Myc, Ccnd1) and a Stat-driven reporter plasmid, in rat C6 glioma cells. The results obtained with the decoys and siRNA suggest that in proliferating C6 cells, Stat1 supports the basal expression of Bcl2l1, while the decoy and chromatin immunoprecipitation results suggest it also plays a similar role for Myc. In the Stat-driven reporter system, overexpression of Stat1 stimulated, while overexpression of Stat3 inhibited the reporter gene expression. The level of Stat1 phosphorylation observed under basal conditions in proliferating glioma C6 cells is very low. Therefore, we speculate that it is the activity of the unphosphorylated Stat1 that is inhibited by Stat1 decoy or Stat1 siRNA. Taken together, our results demonstrate that it is Stat1 not Stat3 that maintains the expression of Bcl2l1 and possibly Myc in proliferating glioma C6 cells. An established paradigm is that Stat3 exerts a pro-survival and potentially oncogenic effects, while Stat1 is mainly associated with the immune response. Our results add to a number of reports that challenge this paradigm.

Girdin maintains the stemness of glioblastoma stem cells

Glioblastomas (GBMs) are the most common and aggressive type of brain tumor. GBMs usually show hyperactivation of the PI3K-Akt pathway, a pro-tumorigenic signaling cascade that contributes to pathogenesis. Girdin, an actin-binding protein identified as a novel substrate of Akt, regulates the sprouting of axons and the migration of neural progenitor cells during early postnatal-stage neurogenesis in the hippocampus. Here, we show that Girdin is highly expressed in human glioblastoma (GBM). Stable Girdin knockdown in isolated GBM stem cells resulted in decreased expression of stem cell markers, including CD133, induced multilineage neural differentiation, and inhibited in vitro cell motility, ex vivo invasion, sphere-forming capacity and in vivo tumor formation. Furthermore, exogenous expression of the Akt-binding domain of Girdin, which competitively inhibits its Akt-mediated phosphorylation, diminished the expression of stem cell markers, SOX2 and nestin, and migration on the brain slice and induced the expression of neural differentiation markers glial fibrillary acidic protein/βIII Tubulin. Our results reveal that Girdin is required for GBM-initiating stem cells to sustain the stemness and invasive properties.

Targeting glioma stem cells: a novel framework for brain tumors

The past decade has seen a dramatic increase in stem cell research that focuses on glioma stem cells (GSC) and their mechanisms of action, revealing multiple potential targets for primary malignant brain tumors. Herein, we present a novel framework for considering GSC targets based on direct and indirect strategies. Direct strategies target GSC molecular pathways to overcome their resistance to radiation and chemotherapy, block their function or induce their differentiation. Indirect strategies target the microenvironment of the GSC, namely the perivascular, hypoxic and immune niches. Progress made on GSC targets is reviewed in detail and specific pathways are identified in context of the proposed framework. The potential barriers for translation to the clinical setting are also discussed. Overall, targeting GSC provides an unprecedented opportunity for revolutionary approaches to treat high-grade gliomas that continue to have a poor patient prognosis.

MicroRNAs involved in the EGFR/PTEN/AKT pathway in gliomas

Gliomas are the most common type of malignant primary brain tumor. Despite advances in surgery, radiation therapy, and chemotherapy, the prognosis of patients with gliomas has not significantly improved. MicroRNAs (miRNAs), a class of non-coding RNAs, 21-25 nucleotides long, negatively regulate the expression of target genes by interacting with specific sites in mRNAs, and play a critical role in the development of gliomas. The EGFR/PTEN/AKT pathway is a promising target for anti-glioma therapy. Recent studies have showed that regulation of the EGFR/PTEN/AKT pathway by miRNAs plays a major role in glioma progression, indicating a novel way to investigate the tumorigenesis, diagnosis, and therapy of gliomas. Here, we focus on recent findings of miRNAs with respect to the EGFR/PTEN/AKT pathway in gliomas.

DNA repair: the culprit for tumor-initiating cell survival?

The existence of "tumor-initiating cells" (TICs) has been a topic of heated debate for the last few years within the field of cancer biology. Their continuous characterization in a variety of solid tumors has led to an abundance of evidence supporting their existence. TICs are believed to be responsible for resistance against conventional treatment regimes of chemotherapy and radiation, ultimately leading to metastasis and patient demise. This review summarizes DNA repair mechanism(s) and their role in the maintenance and regulation of stem cells. There is evidence supporting the hypothesis that TICs, similar to embryonic stem (ES) cells and hematopoietic stem cells (HSCs), display an increase in their ability to survive genotoxic stress and injury. Mechanistically, the ability of ES cells, HSCs and TICs to survive under stressful conditions can be attributed to an increase in the efficiency at which these cells undergo DNA repair. Furthermore, the data presented in this review summarize the results found by our lab and others demonstrating that TICs have an increase in their genomic stability, which can allow for TIC survival under conditions such as anticancer treatments, while the bulk population of tumor cells dies. We believe that these data will greatly impact the development and design of future therapies being engineered to target and eradicate this highly aggressive cancer cell population.

Frontiers in targeting glioma stem cells

Patients with glioblastoma multiforme (GBM - WHO grade IV) seldom recover. This is due to the infiltrative nature of these tumours and the presence of cellular populations with ability to escape therapies and drive tumour recurrence and progression. In some cases, these resistant cells exhibit stem properties [glioma stem cells (GSC)]. This article aims at discussing relevant issues on GSC resistance to current therapies and outlines possible and promising avenues in regard to novel therapeutic strategies, such as pharmacological, immunological and viral interventions.

The modulation of microRNAs by type I IFN through the activation of signal transducers and activators of transcription 3 in human glioma

Type I IFNs are involved in double-stranded RNA responses. Here, we investigated the possibility that IFN-beta may induce or downregulate cellular microRNAs (miRNA) in human neoplasms and thereby use the RNA interference system to show antitumor effects. Because of its known connection to glioma biology, we focused on miR-21 among seven miRNAs influenced by IFN-beta. We analyzed the effect of IFN-beta treatment on miR-21 expression in glioma cells and intracranial glioma xenografts. IFN-beta treatment reduced miR-21 expression in glioma cells markedly, and IFN-beta administration suppressed the growth of glioma-initiating cell-derived intracranial tumors. The levels of primary miR-21 gene transcripts, precursor miR-21, and mature miR-21 decreased 6 hours after the addition of IFN-beta, indicating that the reduction in miR-21 levels was due to transcriptional suppression. We did reporter assays to elucidate the IFN-beta-mediated suppression of miR-21; the addition of signal transducers and activators of transcription 3 (STAT3)-expressing vectors induced the IFN-beta-mediated suppression of miR-21, whereas STAT3-inhibiting agents inhibited the miR-21 suppression. Thus, the results of our study show that the downregulation of miR-21 contributes to the antitumor effects of IFN-beta and that miR-21 expression is negatively regulated by STAT3 activation. These results highlight the importance of understanding the transcriptional regulation of the miRNAs involved in oncogenesis.