Eyes wide open: a critical review of sphere-formation as an assay for stem cells

Cancer Stem Cells: A Review of Potential Clinical Applications

Context.—Cancer stem cells (CSCs) comprise a minor cell population in a tumor; however, they possess self-renewal capacity and are responsible for tumor recurrence and the emerging issue of tumor resistance. Despite recent advances in the study of pathogenesis and mechanisms of CSC-mediated disease recurrence and multidrug resistance, many questions remain unanswered.

Objectives.—To provide an overview of CSC theory and to describe major methods of CSC detection and isolation, with the emphasis on those techniques that are potentially relevant in clinical laboratory practice. Particular attention is given to CSC markers, such as cancer testis antigens, which could become promising targets in the development of immunotherapy in settings of minimal residual disease.

Data Sources.—The review is based on analysis of peer-reviewed literature cited in PubMed, as well as preliminary results of studies conducted in our laboratory.

Conclusions.—Despite a lack of consensus in the scientific community on research methodology, CSCs have demonstrated significant potential as therapeutic targets in the treatment of cancer. Further research of CSC biology and markers will eventually lead to the development of novel therapeutic approaches for targeting these cells to treat resistant and recurrent tumors and minimal residual disease.

The role of cancer stem cells in the anti-carcinogenicity of curcumin

Many cancers contain cell subpopulations that display characteristics of stem cells. These cells are characterised by their ability to self-renew, form differentiated progeny and develop resistance to chemotherapeutic strategies. Cancer stem cells may utilise many of the same signalling pathways as normal stem cells including Wnt, Notch and Hedgehog. The dietary agent curcumin exerts a plethora of anti-carcinogenic effects both in vitro and in vivo, and can also inhibit many of the signalling pathways associated with stem cell biology. Emerging evidence suggests that curcumin can exert its anti-carcinogenic activity via targeting cancer stem cells through the disruption of stem cell signalling pathways. In this review we summarise the ability of curcumin to interfere with signalling pathways Wnt, Hedgehog, Notch, Signal Transducers and Activator (STAT) and interleukin-8, and report curcumin-induced changes in function and properties of cancer stem cells. We present evidence that the effects of curcumin on cancer stem cells mediate, or contribute to, its anti-carcinogenic activity.

FOXO3 shares common targets with ASCL1 genome-wide and inhibits ASCL1-dependent neurogenesis

FOXO transcription factors are central regulators of longevity from worms to humans. FOXO3, the FOXO isoform associated with exceptional human longevity, preserves adult neural stem cell pools. Here, we identify FOXO3 direct targets genome-wide in primary cultures of adult neural progenitor cells (NPCs). Interestingly, FOXO3-bound sites are enriched for motifs for bHLH transcription factors, and FOXO3 shares common targets with the proneuronal bHLH transcription factor ASCL1/MASH1 in NPCs. Analysis of the chromatin landscape reveals that FOXO3 and ASCL1 are particularly enriched at the enhancers of genes involved in neurogenic pathways. Intriguingly, FOXO3 inhibits ASCL1-dependent neurogenesis in NPCs and direct neuronal conversion in fibroblasts. FOXO3 also restrains neurogenesis in vivo. Our study identifies a genome-wide interaction between the prolongevity transcription factor FOXO3 and the cell-fate determinant ASCL1 and raises the possibility that FOXO3's ability to restrain ASCL1-dependent neurogenesis may help preserve the neural stem cell pool.

The late response of rat subependymal zone stem and progenitor cells to stroke is restricted to directly affected areas of their niche

Ischaemia leads to increased proliferation of progenitors in the subependymal zone (SEZ) neurogenic niche of the adult brain and to generation and migration of newborn neurons. Here we investigated the spatiotemporal characteristics of the mitotic activity of adult neural stem and progenitor cells in the SEZ during the sub-acute and chronic post-ischaemic phases. Ischaemia was induced by performing a 1h unilateral middle cerebral artery occlusion (MCAO) and tissue was collected 4/5 weeks and 1 year after the insult. Neural stem cells (NSCs) responded differently from their downstream progenitors to MCAO, with NSCs being activated only transiently whilst progenitors remain activated even at 1 year post-injury. Importantly, mitotic activation was observed only in the affected areas of the niche and specifically in the dorsal half of the SEZ. Analysis of the topography of mitoses, in relation to the anatomy of the lesion and to the position of ependymal cells and blood vessels, suggested an interplay between lesion-derived recruiting signals and the local signals that normally control proliferation in the chronic post-ischaemic phase.

Cancer stem cells in lung cancer: Evidence and controversies

The cancer stem cell (CSC) model is based on a myriad of experimental and clinical observations suggesting that the malignant phenotype is sustained by a subset of cells characterized by the capacity for self-renewal, differentiation and innate resistance to chemotherapy and radiation. CSC may be responsible for disease recurrence after definitive therapy and may therefore be functionally synonymous with minimal residual disease. Similar to other solid tumours, several putative surface markers for lung CSC have been identified, including CD133 and CD44. In addition, expression and/or activity of the cytoplasmic enzyme aldehyde dehydrogenase ALDH and capacity of cells to exclude membrane permeable dyes (known as the 'side population') correlate with stem-like function in vitro and in vivo. Embryonic stem cell pathways such as Hedgehog, Notch and WNT may also be active in lung cancers stem cells and therefore may be therapeutically targetable for maintenance therapy in patients achieving a complete response to surgery, radiotherapy or chemotherapy. This paper will review the evidence regarding the existence and function of lung CSC in the context of the experimental and clinical evidence and discuss some ongoing controversies regarding this model.

Cancer stem cells in pediatric sarcomas

Sarcomas represent a clinically and biologically diverse group of malignant connective tissue tumors. Despite aggressive conventional therapy, a large proportion of sarcoma patients experience disease recurrence which will ultimately result in mortality. The presence of a unique population of cells, referred to as cancer stem cells (CSCs), have been proposed to be responsible for refractory responses to current chemotherapies as well underlying the basis for metastasis and relapse of disease – clinical corollaries to what has been termed the CSC hypothesis. The presence of CSCs have been suggested in a variety of hematologic and solid malignancies, and only more recently in sarcomas. Based on our current understanding of normal stem cell biology and evidence obtained from the study of malignant hematopoietic and solid tumors, researchers have identified candidate cell surface markers (CD133, CD117, Stro-1), biochemical markers (aldehyde dehydrogenase activity), and cytological characteristics (side population and spherical colony formation) that may identify putative sarcoma CSCs. In this review, we explore the current state of evidence that may suggest the existence of sarcoma CSCs. We present research in osteosarcoma, the Ewing’s sarcoma family of tumors, rhabdomyosarcoma, as well as other sarcoma subtypes to describe commonly used molecular and biochemical markers, as well as techniques, used in the identification, isolation, and characterization of candidate sarcoma CSCs. We will also discuss the current controversies and challenges that face research in sarcoma CSC.

Genetic reconstitution of tumorigenesis in primary intestinal cells

Animal models for human colorectal cancer recapitulate multistep carcinogenesis that is typically initiated by activation of the Wnt pathway. Although potential roles of both genetic and environmental modifiers have been extensively investigated in vivo, it remains elusive whether epithelial cells definitely require interaction with stromal cells or microflora for tumor development. Here we show that tumor development could be simply induced independently of intestinal microenvironment, even with WT murine primary intestinal cells alone. We developed an efficient method for lentiviral transduction of intestinal organoids in 3D culture. Despite seemingly antiproliferative effects by knockdown of adenomatous polyposis coli (APC), we managed to reproducibly induce APC-inactivated intestinal organoids. As predicted, these organoids were constitutively active in the Wnt signaling pathway and proved tumorigenic when injected into nude mice, yielding highly proliferative tubular epithelial glands accompanied by prominent stromal tissue. Consistent with cellular transformation, tumor-derived epithelial cells acquired sphere formation potential, gave rise to secondary tumors on retransplantation, and highly expressed cancer stem cell markers. Inactivation of p53 or phosphatase and tensin homolog deleted from chromosome 10, or activation of Kras, promoted tumor development only in the context of APC suppression, consistent with earlier genetic studies. These findings clearly indicated that genetic cooperation for intestinal tumorigenesis could be essentially recapitulated in intestinal organoids without generating gene-modified mice. Taken together, this in vitro model for colon cancer described herein could potentially provide unique opportunities for carcinogenesis studies by serving as a substitute or complement to the currently standard approaches.

Proliferation of murine midbrain neural stem cells depends upon an endogenous sonic hedgehog (Shh) source

The Sonic Hedgehog (Shh) pathway is responsible for critical patterning events early in development and for regulating the delicate balance between proliferation and differentiation in the developing and adult vertebrate brain. Currently, our knowledge of the potential role of Shh in regulating neural stem cells (NSC) is largely derived from analyses of the mammalian forebrain, but for dorsal midbrain development it is mostly unknown. For a detailed understanding of the role of Shh pathway for midbrain development in vivo, we took advantage of mouse embryos with cell autonomously activated Hedgehog (Hh) signaling in a conditional Patched 1 (Ptc1) mutant mouse model. This animal model shows an extensive embryonic tectal hypertrophy as a result of Hh pathway activation. In order to reveal the cellular and molecular origin of this in vivo phenotype, we established a novel culture system to evaluate neurospheres (nsps) viability, proliferation and differentiation. By recreating the three-dimensional (3-D) microenvironment we highlight the pivotal role of endogenous Shh in maintaining the stem cell potential of tectal radial glial cells (RGC) and progenitors by modulating their Ptc1 expression. We demonstrate that during late embryogenesis Shh enhances proliferation of NSC, whereas blockage of endogenous Shh signaling using cyclopamine, a potent Hh pathway inhibitor, produces the opposite effect. We propose that canonical Shh signaling plays a central role in the control of NSC behavior in the developing dorsal midbrain by acting as a niche factor by partially mediating the response of NSC to epidermal growth factor (EGF) and fibroblast growth factor (FGF) signaling. We conclude that endogenous Shh signaling is a critical mechanism regulating the proliferation of stem cell lineages in the embryonic dorsal tissue.

Phosphorylation of EZH2 activates STAT3 signaling via STAT3 methylation and promotes tumorigenicity of glioblastoma stem-like cells

Glioblastoma multiforme (GBM) displays cellular hierarchies harboring a subpopulation of stem-like cells (GSCs). Enhancer of Zeste Homolog 2 (EZH2), the lysine methyltransferase of Polycomb repressive complex 2, mediates transcriptional repression of prodifferentiation genes in both normal and neoplastic stem cells. An oncogenic role of EZH2 as a transcriptional silencer is well established; however, additional functions of EZH2 are incompletely understood. Here, we show that EZH2 binds to and methylates STAT3, leading to enhanced STAT3 activity by increased tyrosine phosphorylation of STAT3. The EZH2-STAT3 interaction preferentially occurs in GSCs relative to non-stem bulk tumor cells, and it requires a specific phosphorylation of EZH2. Inhibition of EZH2 reverses the silencing of Polycomb target genes and diminishes STAT3 activity, suggesting therapeutic strategies.

The isolation, differentiation, and survival in vivo of multipotent cells from the postnatal rat filum terminale

Neural stem cells (NSCs) are undifferentiated cells in the central nervous system (CNS) that are capable of self-renewal and can be induced to differentiate into neurons and glia. Current sources of mammalian NSCs are confined to regions of the CNS that are critical to normal function and surgically difficult to access, which limits their therapeutic potential in human disease. We have found that the filum terminale (FT), a previously unexplored, expendable, and easily accessible tissue at the caudal end of the spinal cord, is a source of multipotent cells in postnatal rats and humans. In this study, we used a rat model to isolate and characterize the potential of these cells. Neurospheres derived from the rat FT are amenable to in vitro expansion in the presence of a combination of growth factors. These proliferating, FT-derived cells formed neurospheres that could be induced to differentiate into neural progenitor cells, neurons, astrocytes, and oligodendrocytes by exposure to serum and/or adhesive substrates. Through directed differentiation using sonic hedgehog and retinoic acid in combination with various neurotrophic factors, FT-derived neurospheres generated motor neurons that were capable of forming neuromuscular junctions in vitro. In addition, FT-derived progenitors that were injected into chick embryos survived and could differentiate into both neurons and glia in vivo.

In vitro analysis of breast cancer cell line tumourspheres and primary human breast epithelia mammospheres demonstrates inter- and intrasphere heterogeneity

Mammosphere and breast tumoursphere culture have gained popularity as in vitro assays for propagating and analysing normal and cancer stem cells. Whether the spheres derived from different sources or parent cultures themselves are indeed single entities enriched in stem/progenitor cells compared to other culture formats has not been fully determined. We surveyed sphere-forming capacity across 26 breast cell lines, immunophenotyped spheres from six luminal- and basal-like lines by immunohistochemistry and flow cytometry and compared clonogenicity between sphere, adherent and matrigel culture formats using in vitro functional assays. Analyses revealed morphological and molecular intra- and inter-sphere heterogeneity, consistent with adherent parental cell line phenotypes. Flow cytometry showed sphere culture does not universally enrich for markers previously associated with stem cell phenotypes, although we found some cell-line specific changes between sphere and adherent formats. Sphere-forming efficiency was significantly lower than adherent or matrigel clonogenicity and constant over serial passage. Surprisingly, self-renewal capacity of sphere-derived cells was similar/lower than other culture formats. We observed significant correlation between long-term-proliferating-cell symmetric division rates in sphere and adherent cultures, suggesting functional overlap between the compartments sustaining them. Experiments with normal primary human mammary epithelia, including sorted luminal (MUC1⁺) and basal/myoepithelial (CD10⁺) cells revealed distinct luminal-like, basal-like and mesenchymal entities amongst primary mammospheres. Morphological and colony-forming-cell assay data suggested mammosphere culture may enrich for a luminal progenitor phenotype, or induce reversion/relaxation of the basal/mesenchymal in vitro selection occurring with adherent culture. Overall, cell line tumourspheres and primary mammospheres are not homogenous entities enriched for stem cells, suggesting a more cautious approach to interpreting data from these assays and careful consideration of its limitations. Sphere culture may represent an alternative 3-dimensional culture system which rather than universally ‘enriching’ for stem cells, has utility as one of a suite of functional assays that provide a read-out of progenitor activity.

Sphere-forming cells from peripheral cornea demonstrate polarity and directed cell migration

Sphere-forming cells from peripheral cornea represent a potential source of progenitor cells for treatment of corneal degenerative diseases. Control of cellular repopulation on transplantable substrates is important to prevent uncontrolled growth in unfavourable directions. The coordination of cellular outgrowth may be in response to environmental cues and/or cellular signals from other spheres. To investigate this, cell migration patterns were observed following placement of spheres on an adhesive surface. Human peripheral corneal cells were maintained using a sphere-forming assay and their behaviour on collagen substrate recorded by time-lapse imaging. Immunocytochemistry and proliferation assays were used to detect protein expression and cell division. Proliferation assays showed that spheres formed by a combination of cell division and aggregation. Cell division continued within spheres for up to 4 months and was up-regulated when exposed to differentiation medium and collagen substrate. The spheres expressed both epithelial and stromal cell markers. When exposed to collagen; (1) 25% of the spheres showed spontaneous polarised outgrowth. (2) One sphere initially showed polarised outgrowth followed by collective migration with discrete morphological changes to form leading and trailing compartments. (3) A sphere which did not show polarised outgrowth was also capable of collective migration using cell protrusion and retraction. (4) Active recruitment of cells into spheres was observed. (5) Placement of spheres in close proximity led to production of a cell exclusion area adjacent to spheres. Thus peripheral corneal cell spheres are dynamic entities capable of developing polarity and modifying migration in response to their environment.

Phenotypic characterization of metastatic anaplastic thyroid cancer stem cells

Emerging evidence suggests cancer stem cells (CSCs) may initiate new tumors in anaplastic thyroid carcinoma (ATC), one of the most aggressive solid tumors in humans. However, the involvement of CSCs in human tumorigenesis has not been previously studied in authenticated ATC cell lines. Here we demonstrate a functional role of CSCs in four new validated human ATC cell lines (THJ-11T, THJ-16T, THJ-21T and THJ-29T). We identified and enriched CSCs using a spheroid-forming assay. About 3 to 9% of cells from four ATC cell lines formed thyrospheres. The thyrospheres expressed the stem cell markers NANOG and Oct4 and possessed the ability to self-renew. Injection of these thyrospheres into the thyroids of NOD/SCID Il2rg-/- mice resulted in the formation of metastatic tumors that recapitulated the clinical features of human ATC. To our knowledge, this is the first in vivo characterization of thyroid CSCs using validated human ATC cell lines. The availability of disease-specific thyrospheres and our orthotopic tumor models will enable the elucidation of disease mechanisms and the environmental niche of CSCs. They may also be useful for preclinical therapeutic screening and for monitoring the effects of biological therapies on ATC.

Medullospheres from DAOY, UW228 and ONS-76 cells: increased stem cell population and proteomic modifications

Background

Medulloblastoma (MB) is an aggressive pediatric tumor of the Central Nervous System (CNS) usually treated according to a refined risk stratification. The study of cancer stem cells (CSC) in MB is a promising approach aimed at finding new treatment strategies.

Methodology/Principal Findings

The CSC compartment was studied in three characterized MB cell lines (DAOY, UW228 and ONS-76) grown in standard adhesion as well as being grown as spheres, which enables expansion of the CSC population. MB cell lines, grown in adherence and as spheres, were subjected to morphologic analysis at the light and electron microscopic level, as well as cytofluorimetric determinations. Medullospheres (MBS) were shown to express increasingly immature features, along with the stem cells markers: CD133, Nestin and β-catenin. Proteomic analysis highlighted the differences between MB cell lines, demonstrating a unique protein profile for each cell line, and minor differences when grown as spheres. In MBS, MALDI-TOF also identified some proteins, that have been linked to tumor progression and resistance, such as Nucleophosmin (NPM). In addition, immunocytochemistry detected Sox-2 as a stemness marker of MBS, as well as confirming high NPM expression.

Conclusions/Significance

Culture conditioning based on low attachment flasks and specialized medium may provide new data on the staminal compartment of CNS tumors, although a proteomic profile of CSC is still elusive for MB.

Activation of type 1 cannabinoid receptor (CB1R) promotes neurogenesis in murine subventricular zone cell cultures

The endocannabinoid system has been implicated in the modulation of adult neurogenesis. Here, we describe the effect of type 1 cannabinoid receptor (CB1R) activation on self-renewal, proliferation and neuronal differentiation in mouse neonatal subventricular zone (SVZ) stem/progenitor cell cultures. Expression of CB1R was detected in SVZ-derived immature cells (Nestin-positive), neurons and astrocytes. Stimulation of the CB1R by (R)-(+)-Methanandamide (R-m-AEA) increased self-renewal of SVZ cells, as assessed by counting the number of secondary neurospheres and the number of Sox2+/+ cell pairs, an effect blocked by Notch pathway inhibition. Moreover, R-m-AEA treatment for 48 h, increased proliferation as assessed by BrdU incorporation assay, an effect mediated by activation of MAPK-ERK and AKT pathways. Surprisingly, stimulation of CB1R by R-m-AEA also promoted neuronal differentiation (without affecting glial differentiation), at 7 days, as shown by counting the number of NeuN-positive neurons in the cultures. Moreover, by monitoring intracellular calcium concentrations ([Ca²⁺]_i) in single cells following KCl and histamine stimuli, a method that allows the functional evaluation of neuronal differentiation, we observed an increase in neuronal-like cells. This proneurogenic effect was blocked when SVZ cells were co-incubated with R-m-AEA and the CB1R antagonist AM 251, for 7 days, thus indicating that this effect involves CB1R activation. In accordance with an effect on neuronal differentiation and maturation, R-m-AEA also increased neurite growth, as evaluated by quantifying and measuring the number of MAP2-positive processes. Taken together, these results demonstrate that CB1R activation induces proliferation, self-renewal and neuronal differentiation from mouse neonatal SVZ cell cultures.

Glioma stem cells and immunotherapy for the treatment of malignant gliomas

Stem cell research has led to the discovery of glioma stem cells (GSCs), and because these cells are resistant to chemotherapy and radiotherapy, analysis of their properties has been rapidly pursued for targeted treatment of malignant glioma. Recent studies have also revealed complex crosstalk between GSCs and their specialized environment (niche). Therefore, targeting not only GSCs but also their niche may be a principle for novel therapies of malignant glioma. One possible novel strategy for targeting GSCs and their niches is immunotherapy with different antitumor mechanism(s) from those of conventional therapy. Recent clinical studies of immunotherapy using peptide vaccines and antibodies have shown promising results. This review describes the recent findings related to GSCs and their niches, as well as immunotherapies for glioma, followed by discussion of immunotherapies that target GSCs for the treatment of malignant glioma.

Human cardiospheres as a source of multipotent stem and progenitor cells

Cardiospheres (CSs) are self-assembling multicellular clusters from the cellular outgrowth from cardiac explants cultured in nonadhesive substrates. They contain a core of primitive, proliferating cells, and an outer layer of mesenchymal/stromal cells and differentiating cells that express cardiomyocyte proteins and connexin 43. Because CSs contain both primitive cells and committed progenitors for the three major cell types present in the heart, that is, cardiomyocytes, endothelial cells, and smooth muscle cells, and because they are derived from percutaneous endomyocardial biopsies, they represent an attractive cell source for cardiac regeneration. In preclinical studies, CS-derived cells (CDCs) delivered to infarcted hearts resulted in improved cardiac function. CDCs have been tested safely in an initial phase-1 clinical trial in patients after myocardial infarction. Whether or not CDCs are superior to purified populations, for example, c-kit(+) cardiac stem cells, or to gene therapy approaches for cardiac regeneration remains to be evaluated.

Strategies for isolating and enriching cancer stem cells: well begun is half done

Cancer stem cells (CSCs) constitute a subpopulation of cancer cells that have the potential for self-renewal, multipotent differentiation, and tumorigenicity. Studies on CSC biology and CSC-targeted therapies depend on CSC isolation and/or enrichment methodologies. Scientists have conducted extensive research in this field since John Dick's group successfully isolated CSCs based on the expression of the CD34 and CD38 surface markers. Progress in CSC research has been greatly facilitated by the enrichment and isolation of these cells. In this review, we summarize the current strategies used in our and other laboratories for CSC isolation and enrichment, including methods based on stem cell surface markers, intracellular enzyme activity, the concentration of reactive oxygen species, the mitochondrial membrane potential, promoter-driven fluorescent protein expression, autofluorescence, suspension/adherent culture, cell division, the identification of side population cells, resistance to cytotoxic compounds or hypoxia, invasiveness/adhesion, immunoselection, and physical property. Although many challenges remain to be overcome, it is reasonable to believe that more reliable, efficient, and convenient methods will be developed in the near future.

The histone deacetylase inhibitor sodium butyrate promotes cell death and differentiation and reduces neurosphere formation in human medulloblastoma cells

Increasing evidence suggests that alterations in epigenetic mechanisms regulating chromatin state play a role in the pathogenesis of medulloblastoma (MB), the most common malignant brain tumor of childhood. Histone deacetylase (HDAC) inhibitors, which increase chromatin relaxation, have been shown to display anticancer activities. Here we show that the HDAC inhibitor sodium butyrate (NaB) markedly increases cell death and reduces colony formation in human MB cell lines. In addition, NaB increased the mRNA expression of Gria2, a neuronal differentiation marker, in D283 and DAOY cells and reduced the number of neurospheres in D283 cell cultures. Finally, NaB reduced the viability of D283 cells when combined with etoposide. These data show that NaB displays pronounced inhibitory effects on the survival of human MB cells and suggest that NaB might potentiate the effects of etoposide. In addition, our study suggests that HDAC inhibition might promote the neuronal differentiation of MB cells and provides the first evidence that an HDAC inhibitor might suppress the expansion or survival of MB cancer stem cells.

Left atrial appendages from adult hearts contain a reservoir of diverse cardiac progenitor cells

AIMS

There is strong evidence supporting the claim that endogenous cardiac progenitor cells (CPCs) are key players in cardiac regeneration, but the anatomic source and phenotype of the master cardiac progenitors remains uncertain. Our aim was to investigate the different cardiac stem cell populations in the left atrial appendage (LAA) and their fates.

METHODS AND RESULTS

We investigated the CPC content and profile of adult murine LAAs using immunohistochemistry and flow cytometry. We demonstrate that the LAA contains a large number of CPCs relative to other areas of the heart, representing over 20% of the total cell number. We grew two distinct CPC populations from the LAA by varying the degree of proteolysis. These differed by their histological location, surface marker profiles and growth dynamics. Specifically, CD45(pos) cells grew with milder proteolysis, while CD45(neg) cells grew mainly with more intense proteolysis. Both cell types could be induced to differentiate into cells with cardiomyocyte markers and organelles, albeit by different protocols. Many CD45(pos) cells expressed CD45 initially and rapidly lost its expression while differentiating.

CONCLUSIONS

Our results demonstrate that the left atrial appendage plays a role as a reservoir of multiple types of progenitor cells in murine adult hearts. Two different types of CPCs were isolated, differing in their epicardial-myocardial localization. Considering studies demonstrating layer-specific origins of different cardiac progenitor cells, our findings may shed light on possible pathways to study and utilize the diversity of endogenous progenitor cells in the adult heart.

Neural stem/progenitor cells display a low requirement for oxidative metabolism independent of hypoxia inducible factor-1alpha expression

Neural stem/progenitor cells (NSPCs) are multipotent cells within the embryonic and adult brain that give rise to both neuronal and glial cell lineages. Maintenance of NSPC multipotency is promoted by low oxygen tension, although the metabolic underpinnings of this trait have not been described. In this study, we investigated the metabolic state of undifferentiated NSPCs in culture, and tested their relative reliance on oxidative versus glycolytic metabolism for survival, as well as their dependence on hypoxia inducible factor-1alpha (HIF-1α) expression for maintenance of metabolic phenotype. Unlike primary neurons, NSPCs from embryonic and adult mice survived prolonged hypoxia in culture. In addition, NSPCs displayed greater susceptibility to glycolytic inhibition compared with primary neurons, even in the presence of alternative mitochondrial TCA substrates. NSPCs were also more resistant than neurons to mitochondrial cyanide toxicity, less capable of utilizing galactose as an alternative substrate to glucose, and more susceptible to pharmacological inhibition of the pentose phosphate pathway by 6-aminonicotinamide. Inducible deletion of exon 1 of the Hif1a gene improved the ability of NSPCs to utilize pyruvate during glycolytic inhibition, but did not alter other parameters of metabolism, including their ability to withstand prolonged hypoxia. Taken together, these data indicate that NSPCs have a relatively low requirement for oxidative metabolism for their survival and that hypoxic resistance is not dependent upon HIF-1α signaling.

Canonical and non-canonical NF-κB signaling promotes breast cancer tumor-initiating cells

Tumor-initiating cells (TICs) are a sub-population of cells that exhibit a robust ability to self-renew and contribute to the formation of primary tumors, the relapse of previously treated tumors, and the development of metastases. TICs have been identified in various tumors, including those of the breast, and are particularly enriched in the basal-like and claudin-low subtypes of breast cancer. The signaling pathways that contribute to the function and maintenance of TICs are under intense study. We explored the potential involvement of the NF-κB family of transcription factors in TICs in cell lines that are representative of basal-like and claudin-low breast cancer. NF-κB was found to be activated in breast cancer cells that form tumorspheres efficiently. Moreover, both canonical and non-canonical NF-κB signaling is required for these cells to self-renew in vitro and to form xenograft tumors efficiently in vivo using limiting dilutions of cells. Consistent with this, canonical and non-canonical NF-κB signaling is activated in TICs isolated from breast cancer cell lines. Experimental results indicate that NF-κB promotes the function of TICs by stimulating epithelial-to-mesenchymal transition (EMT) and by upregulating the expression of the inflammatory cytokines IL-1β and IL-6. The results suggest the use of NF-κB inhibitors for clinical therapy of certain breast cancers.

Cancer stem cells: a shifting subpopulation of cells with stemness?

Carcinogenesis and tumor cells had long been thought to be produced by genetic mutations, until the emergence of the cancer stem cell (CSC) model. This relatively new model reveals another layer of complexity in malignancy formation and maintenance. However, these hard-to-track subpopulations have not only been challenging to understand at the molecular level but have also prevented the development of novel CSC-based cancer therapies. Accumulating evidence from previous studies has suggested that CSCs might be derived from either somatic stem cells (SCCs) or differentiated progenitor cells and that they constantly reestablish their niches; these factors underlie the variability in their frequencies and biological marker expression. Therefore, we present the hypothesis that CSCs possess constantly evolving features. During tumorigenesis, the characteristics of the CSCs may change, thus presenting a "moving target". We also suggest that a multifaceted, integrated strategy combining traditional cell surface markers and transcriptional factor targeting methods to identify "stemness" should lead to the better isolation and characterization of CSCs. Corresponding laboratory approaches to test this hypothesis are also provided.

VCAM1 is essential to maintain the structure of the SVZ niche and acts as an environmental sensor to regulate SVZ lineage progression

Neurons arise in the adult forebrain subventricular zone (SVZ) from Type B neural stem cells (NSCs), raising considerable interest in the molecules that maintain this life-long neurogenic niche. Type B cells are anchored by specialized apical endfeet in the center of a pinwheel of ependymal cells. Here we show that the apical endfeet express high levels of the adhesion and signaling molecule vascular cell adhesion molecule-1 (VCAM1). Disruption of VCAM1 in vivo causes loss of the pinwheels, disrupted SVZ cytoarchitecture, proliferation and depletion of the normally quiescent apical Type B cells, and increased neurogenesis in the olfactory bulb, demonstrating a key role in niche structure and function. We show that VCAM1 signals via NOX2 production of reactive oxygen species (ROS) to maintain NSCs. VCAM1 on Type B cells is increased by IL-1β, demonstrating that it can act as an environmental sensor, responding to chemokines involved in tissue repair.

A small molecule (pluripotin) as a tool for studying cancer stem cell biology: proof of concept

BACKGROUND

Cancer stem cells (CSC) are thought to be responsible for tumor maintenance and heterogeneity. Bona fide CSC purified from tumor biopsies are limited in supply and this hampers study of CSC biology. Furthermore, purified stem-like CSC subpopulations from existing tumor lines are unstable in culture. Finding a means to overcome these technical challenges would be a useful goal. In a first effort towards this, we examined whether a chemical probe that promotes survival of murine embryonic stem cells without added exogenous factors can alter functional characteristics in extant tumor lines in a fashion consistent with a CSC phenotype.

METHODOLOGY/PRINCIPAL FINDINGS

The seven tumor lines of the NCI60 colon subpanel were exposed to SC-1 (pluripotin), a dual kinase and GTPase inhibitor that promotes self-renewal, and then examined for tumorigenicity under limiting dilution conditions and clonogenic activity in soft agar. A statistically significant increase in tumor formation following SC-1 treatment was observed (p<0.04). Cloning efficiencies and expression of putative CSC surface antigens (CD133 and CD44) were also increased. SC-1 treatment led to sphere formation in some colon tumor lines. Finally, SC-1 inhibited in vitro kinase activity of RSK2, and another RSK2 inhibitor increased colony formation implicating a role for this kinase in eliciting a CSC phenotype.

CONCLUSIONS/SIGNIFICANCE

These findings validate a proof of concept study exposure of extant tumor lines to a small molecule may provide a tractable in vitro model for understanding CSC biology.

Gpr177 deficiency impairs mammary development and prohibits Wnt-induced tumorigenesis

Aberrant regulation of the Wnt pathway, essential for various developmental processes, is tightly linked to human breast cancers. By hijacking this evolutionary conserved signaling pathway, cancer cells acquire sustaining proliferation ability, leading to modification of physiologic properties necessary for tumor initiation and progression. An enormous wealth of knowledge on the importance of Wnt signaling in breast development and cancer has been obtained, but the cell types responsible for production of this proliferative signal operating within normal and malignant tissues remains poorly understood. Here we report that Wnt production mediated by Gpr177 is essential for mammary morphogenesis. The loss of Gpr177 interferes with mammary stem cells, leading to deficiencies in cell proliferation and differentiation. Genetic analysis further demonstrates an indispensable role of Gpr177 in Wnt-induced tumorigenesis. The Gpr177-deficiency mice are resistant to malignant transformation. This study not only demonstrates the necessity of Wnt in mammary organogenesis but also provides a proof-of-principle for targeting of Gpr177 as a potential new treatment for human diseases with aberrant Wnt stimulation.

βArrestin-1 and Mcl-1 modulate self-renewal growth of cancer stem-like side-population cells in non-small cell lung cancer

Side population (SP) cells have been reported to have properties of cancer stem-like cells (CSCs) in non-small cell lung carcinoma (NSCLC), yet their molecular features have not been fully elucidated. Here we show that, NSCLC-SP cells were enriched in G₀/G­₁ phase of cell cycle, had higher aldehyde dehydrogenase activity as well as higher clonogenic and self-renewing ability compared to main population (MP) cells. Interestingly, SP cells were also able to trans-differentiate into angiogenic tubules in vitro and were highly tumorigenic as compared to MP cells. SP-derived tumors demonstrated the intratumoral heterogeneity comprising of both SP and MP cells, suggesting the self-renewal and differentiation ability of SP cells are manifested in vivo as well. βArrestin-1 (βArr1) is involved in the progression of various cancers including NSCLCs and we find that depletion of βArr1 significantly blocked the SP phenotype; whereas depletion of βArr2 had relatively minor effects. Ectopic expression of βArr1 resulted in increased SP frequency and ABCG2 expression while abrogation of βArr1 expression suppressed the self-renewal growth and expansion of A549 cells. Anti-apoptotic protein Mcl-1 is known to be one of the key regulators of self-renewal of tissue stem cells and is thought to contribute to survival of NSCLC cells. Our experiments show that higher levels of Mcl-1 were expressed in SP cells compared to MP cells at both transcriptional and translational levels. In addition, Obatoclax, a pharmacological inhibitor of Mcl-1, could effectively prevent the self-renewal of both EGFR-inhibitor sensitive and resistant NSCLC cells. In conclusion, our findings suggest that βArr1 and Mcl-1 are involved in the self-renewal and expansion of NSCLC-CSCs and are potential targets for anti-cancer therapy.

Phosphosulindac (OXT-328) selectively targets breast cancer stem cells in vitro and in human breast cancer xenografts

Pharmacological targeting of breast cancer stem cells (CSCs) is highly promising for the treatment of breast cancer, as the small population of CSCs appears responsible for tumor initiation and progression and also for resistance to conventional treatment. Here we report that the novel phosphosulindac (OXT-328, PS) selectively and effectively eliminates breast CSCs both in vitro and in vivo. PS reduced cell proliferation and induced apoptosis in various breast CSCs. Breast CSCs are resistant to conventional cancer drugs but are sensitive to PS. Long-term treatment of mixtures of cultured breast CSCs and breast cancer cells with PS preferentially eliminated the CSCs. PS impaired the ability of CSCs to form mammospheres and markedly suppressed the expression of CSC-related genes. More importantly, PS prevented by half (p = .06) the formation of tumors initiated by CSCs in immunodeficient mice, and inhibited by 83% (p < .05) the growth of already formed breast cancer xenografts, reducing the proportion of CSCs in them. PS suppressed the Wnt/β-catenin pathway by stimulating the degradation of β-catenin and its relocalization to the cell membrane and also blocked the epithelial-mesenchymal transition and the generation of breast CSCs. These results indicate that PS has a strong inhibitory effect against breast cancer, acting, at least in part, by targeting CSCs through a signaling mechanism involving Wnt signaling.

Regulation of progenitor cell proliferation and neuronal differentiation in enteric nervous system neurospheres

Enteric nervous system (ENS) progenitor cells isolated from mouse and human bowel can be cultured in vitro as neurospheres which are aggregates of the proliferating progenitor cells, together with neurons and glial cells derived from them. To investigate the factors regulating progenitor cell proliferation and differentiation, we first characterised cell proliferation in mouse ENS neurospheres by pulse chase experiments using thymidine analogs. We demonstrate rapid and continuous cell proliferation near the neurosphere periphery, after which postmitotic cells move away from the periphery to become distributed throughout the neurosphere. While many proliferating cells expressed glial markers, expression of the neuronal markers β-tubulin III (Tuj1) and nitric oxide synthase was detected in increasing numbers of post-mitotic cells after a delay of several days. Treatment of both mouse and human neurospheres with the γ-secretase inhibitor N-[N-(3,5-Difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester (DAPT) reduced expression of the transcription factors Hes1 and Hes5, demonstrating inhibition of Notch signaling. DAPT treatment also inhibited progenitor cell proliferation and increased the numbers of differentiating neurons expressing Tuj1 and nitric oxide synthase. To confirm that the cellular effects of DAPT treatment were due to inhibition of Notch signaling, siRNA knockdown of RBPjκ, a key component of the canonical Notch signaling pathway, was demonstrated both to reduce proliferation and to increase neuronal differentiation in neurosphere cells. These observations indicate that Notch signaling promotes progenitor cell proliferation and inhibits neuronal differentiation in ENS neurospheres.

A global transcriptome analysis reveals molecular hallmarks of neural stem cell death, survival, and differentiation in response to partial FGF-2 and EGF deprivation

Neurosphere cell culture is a commonly used model to study the properties and potential applications of neural stem cells (NSCs). However, standard protocols to culture NSCs have yet to be established, and the mechanisms underlying NSC survival and maintenance of their undifferentiated state, in response to the growth factors FGF-2 and EGF are not fully understood. Using cultures of embryonic and adult olfactory bulb stem cells (eOBSCs and aOBSCs), we analyzed the consequences of FGF-2 and EGF addition at different intervals on proliferation, cell cycle progression, cell death and differentiation, as well as on global gene expression. As opposed to cultures supplemented daily, addition of FGF-2 and EGF every 4 days significantly reduced the neurosphere volume and the total number of cells in the spheres, mainly due to increased cell death. Moreover, partial FGF-2 and EGF deprivation produced an increase in OBSC differentiation during the proliferative phase. These changes were more evident in aOBSC than eOBSC cultures. Remarkably, these effects were accompanied by a significant upregulation in the expression of endogenous Fgf-2 and genes involved in cell death and survival (Cryab), lipid catabolic processes (Pla2g7), cell adhesion (Dscaml1), cell differentiation (Dscaml1, Gpr17, S100b, Ndrg2) and signal transduction (Gpr17, Ndrg2). These findings support that a daily supply of FGF-2 and EGF is critical to maintain the viability and the undifferentiated state of NSCs in culture, and they reveal novel molecular hallmarks of NSC death, survival and the initiation of differentiation.

Interleukin-10 regulates progenitor differentiation and modulates neurogenesis on adult brain

The adult subventricular zone (SVZ) is the main neurogenic niche in normal adult brain of mice and rats. The adult SVZ contains neural stem cells (NSCs) that mainly differentiate into committed neuroblasts. The new generated neuroblasts accumulate in dorsal SVZ where they further differentiate and initiate a long migration pathway to their final destination the olfactory bulb (OB).

In here we report a new role for Interleukin 10 (IL-10) different from its well known anti-inflammatory properties. We reveal that IL-10 receptor is expressed in Nestin+ progenitors restricted to the dorsal SVZ in adult brain. Through IL-10 gain models we observed that IL-10 maintains neural progenitors in an undifferentiated stage by keeping progenitors in active cycle and up-regulating the presence of pro-neural genes markers (Nestin, Sox genes, Musashi, Mash1) in detriment of neuronal gene expression (Numb, DCX, TUBB3). On top, IL-10 reduces neuronal differentiation and finally impairs endogenous neurogenesis. Consistently, in the absence of IL-10 in vivo neuronal differentiation among SVZ progenitors is enhanced and the incorporation of new neurons in the adult OB is increased.

Thus, our results provide the first evidence that IL-10 acts as a growth factor on SVZ progenitors and regulates adult neurogenesis in adult normal brain.

Sphere-forming assays for assessment of benign and malignant pancreatic stem cells

Sphere-forming assays are an in vitro technique to assay both normal and neoplastic cells for clonogenic growth potential. Currently, the identification of adult progenitors in the pancreas remains an area of intense investigation. The use of sphere-forming assays provides a critical step to identify new cell types in the pancreas that are capable of clonogenic growth and differentiation. In the field of cancer biology, cancer stem cells have been defined functionally by two major criteria: their ability to undergo self-renewal and their ability to produce differentiated progeny, two conditions which satisfy the criteria of stem cells. Here we briefly review both the capabilities of pancreatosphere and pancreatic tumorsphere assays, discuss important caveats regarding their use, and provide detailed protocols for the assay of both normal and neoplastic cells.

CANCER STEM CELLS IN OSTEOSARCOMA

Osteosarcoma is the most common type of bone cancer and the second leading cause of cancer-related deaths in pediatric patients. Despite conventional treatments such as surgery and chemotherapy, long-term survival rates for patients diagnosed with osteosarcoma have not improved over the last 30 years, likely due to drug-resistant metastasis and disease recurrence. An emerging concept in cancer research is that within a heterogeneous tumor there is a small subset of cells called "cancer stem cells" that are responsible for drug resistance, tumor recurrence and metastasis. This brief review summarizes our current knowledge about cancer stem cells in osteosarcoma, including their potential as a new target for osteosarcoma treatment.

CD44+ cancer stem-like cells in EBV-associated nasopharyngeal carcinoma

Nasopharyngeal carcinoma (NPC) is a unique EBV-associated epithelial malignancy, showing highly invasive and metastatic phenotype. Despite increasing evidence demonstrating the critical role of cancer stem-like cells (CSCs) in the maintenance and progression of tumors in a variety of malignancies, the existence and properties of CSC in EBV-associated NPC are largely unknown. Our study aims to elucidate the presence and role of CSCs in the pathogenesis of this malignant disease. Sphere-forming cells were isolated from an EBV-positive NPC cell line C666-1 and its tumor-initiating properties were confirmed by in vitro and in vivo assays. In these spheroids, up-regulation of multiple stem cell markers were found. By flow cytometry, we demonstrated that both CD44 and SOX2 were overexpressed in a majority of sphere-forming C666-1 cells. The CD44+SOX2+ cells was detected in a minor population in EBV-positive xenografts and primary tumors and considered as potential CSC in NPC. Notably, the isolated CD44+ NPC cells were resistant to chemotherapeutic agents and with higher spheroid formation efficiency, showing CSC properties. On the other hand, microarray analysis has revealed a number of differentially expressed genes involved in transcription regulation (e.g. FOXN4, GLI1), immune response (CCR7, IL8) and transmembrane transport (e.g. ABCC3, ABCC11) in the spheroids. Among these genes, increased expression of CCR7 in CD44+ CSCs was confirmed in NPC xenografts and primary tumors. Importantly, blocking of CCR7 abolished the sphere-forming ability of C666-1 in vitro. Expression of CCR7 was associated with recurrent disease and distant metastasis. The current study defined the specific properties of a CSC subpopulation in EBV-associated NPC. Our findings provided new insights into developing effective therapies targeting on CSCs, thereby potentiating treatment efficacy for NPC patients.

A gold(III) porphyrin complex as an anti-cancer candidate to inhibit growth of cancer-stem cells

A cytotoxic gold(III) complex of meso-tetraphenylporphyrin ([Au(TPP)]Cl, denoted gold-1a) blocks the self-renewal ability of cancer stem-like cells and shows appealing safety pharmacological profiles in rodents.

MEF promotes stemness in the pathogenesis of gliomas

High-grade gliomas are aggressive and uniformly fatal tumors, composed of a heterogeneous population of cells that include many with stem-cell-like properties. The acquisition of stem-like traits might contribute to glioma initiation, growth, and recurrence. Here we investigated the role of the transcription factor myeloid Elf-1 like factor (MEF, also known as ELF4) in gliomas. We found that MEF is highly expressed in both human and mouse glioblastomas and its absence impairs gliomagenesis in a PDGF-driven glioma mouse model. We show that modulation of MEF levels in both mouse neural stem cells and human glioblastoma cells has a significant impact on neurosphere formation. Moreover, we identify Sox2 as a direct downstream target of MEF. Taken together, our studies implicate MEF as a previously unrecognized gatekeeper gene in gliomagenesis that promotes stem cell characteristics through Sox2 activation.

Predicting stem cell fate changes by differential cell cycle progression patterns

Stem cell self-renewal, commitment and reprogramming rely on a poorly understood coordination of cell cycle progression and execution of cell fate choices. Using existing experimental paradigms, it has not been possible to probe this relationship systematically in live stem cells in vitro or in vivo. Alterations in stem cell cycle kinetics probably occur long before changes in phenotypic markers are apparent and could be used as predictive parameters to reveal changes in stem cell fate. To explore this intriguing concept, we developed a single-cell tracking approach that enables automatic detection of cell cycle phases in live (stem) cells expressing fluorescent ubiquitylation-based cell-cycle indicator (FUCCI) probes. Using this tool, we have identified distinctive changes in lengths and fluorescence intensities of G1 (red fluorescence) and S/G2-M (green) that are associated with self-renewal and differentiation of single murine neural stem/progenitor cells (NSCs) and embryonic stem cells (ESCs). We further exploited these distinctive features using fluorescence-activated cell sorting to select for desired stem cell fates in two challenging cell culture settings. First, as G1 length was found to nearly double during NSC differentiation, resulting in progressively increasing red fluorescence intensity, we successfully purified stem cells from heterogeneous cell populations by their lower fluorescence. Second, as ESCs are almost exclusively marked by the green (S/G2-M) FUCCI probe due to their very short G1, we substantially augmented the proportion of reprogramming cells by sorting green cells early on during reprogramming from a NSC to an induced pluripotent stem cell state. Taken together, our studies begin to shed light on the crucial relationship between cell cycle progression and fate choice, and we are convinced that the presented approach can be exploited to predict and manipulate cell fate in a wealth of other mammalian cell systems.

Sorting live stem cells based on Sox2 mRNA expression

While cell sorting usually relies on cell-surface protein markers, molecular beacons (MBs) offer the potential to sort cells based on the presence of any expressed mRNA and in principle could be extremely useful to sort rare cell populations from primary isolates. We show here how stem cells can be purified from mixed cell populations by sorting based on MBs. Specifically, we designed molecular beacons targeting Sox2, a well-known stem cell marker for murine embryonic (mES) and neural stem cells (NSC). One of our designed molecular beacons displayed an increase in fluorescence compared to a nonspecific molecular beacon both in vitro and in vivo when tested in mES and NSCs. We sorted Sox2-MB⁺SSEA1⁺ cells from a mixed population of 4-day retinoic acid-treated mES cells and effectively isolated live undifferentiated stem cells. Additionally, Sox2-MB⁺ cells isolated from primary mouse brains were sorted and generated neurospheres with higher efficiency than Sox2-MB⁻ cells. These results demonstrate the utility of MBs for stem cell sorting in an mRNA-specific manner.

Ultrastructural evidence of exosome secretion by progenitor cells in adult mouse myocardium and adult human cardiospheres

The demonstration of beneficial effects of cell therapy despite the persistence of only few transplanted cells in vivo suggests secreted factors may be the active component of this treatment. This so-called paracrine hypothesis is supported by observations that culture media conditioned by progenitor cells contain growth factors that mediate proangiogenic and cytoprotective effects. Cardiac progenitor cells in semi-suspension culture form spherical clusters (cardiospheres) that deliver paracrine signals to neighboring cells. A key component of paracrine secretion is exosomes, membrane vesicles that are stored intracellularly in endosomal compartments and are secreted when these structures fuse with the cell plasma membrane. Exosomes have been identified as the active component of proangiogenic effects of bone marrow CD34⁺ stem cells in mice and the regenerative effects of embryonic mesenchymal stem cells in infarcted hearts in pigs and mice. Here, we provide electron microscopic evidence of exosome secretion by progenitor cells in mouse myocardium and human cardiospheres. Exosomes are emerging as an attractive vector of paracrine signals delivered by progenitor cells. They can be stored as an “off-the-shelf” product. As such, exosomes have the potential for circumventing many of the limitations of viable cells for therapeutic applications in regenerative medicine.

Cyclopamine cooperates with EGFR inhibition to deplete stem-like cancer cells in glioblastoma-derived spheroid cultures

Putative cancer stem cells have been identified in glioblastoma (GBM), associated with resistance to conventional therapies. Overcoming this resistance is a major challenge to manage this deadly brain tumor. Epidermal growth factor receptor (EGFR) is commonly amplified, over-expressed, and/or mutated in GBM, making it a compelling target for therapy. This study investigates the behavior of 3 primary neurosphere (NS) cell lines and their adherent counterparts originated from human GBM resections, when treated with EGFR-tyrosine kinase inhibitor erlotinib, associated or not with cyclopamine, a hedgehog pathway inhibitor. Adherent cells cultured in the presence of serum expressed the glial fibrillary acidic protein, whereas NS-forming cells cultured in serum-free medium expressed CD133, nestin, and Oct-4, markers of neural stem and progenitor cells. For the 3 adherent cell lines, erlotinib has a moderate effect (50% inhibitory concentration [IC50], >10 µM). Conversely, erlotinib induced a strong cell growth inhibition (IC50, <1 µM) on NS-forming cells, related to the EGFR gene amplification and EGFR protein expression. A short exposure to erlotinib reduced nestin-positive cell proliferation, but NS-initiating activity and self-renewal were not altered. EGFR pathway seems essential for GBM progenitor cell proliferation but dispensable for cancer stem-like cell self-renewal. Inhibition of hedgehog pathway with cyclopamine was evaluated in association with erlotinib on NS growth. Although each drug separately had no effect on sphere initiation, their combination significantly decreased the sphere number (P < .001). Our findings show synergic efficiency for erlotinib-cyclopamine association and provide a suitable in vitro model to explore drug combinations on GBM cells.