New strategy for the analysis of phenotypic marker antigens in brain tumor-derived neurospheres in mice and humans

The development of a rapid patient-derived xenograft model to predict chemotherapeutic drug sensitivity/resistance in malignant glial tumors

BACKGROUND

High-grade gliomas (HGG) are aggressive brain tumors associated with short median patient survival and limited response to therapies, driving the need to develop tools to improve patient outcomes. Patient-derived xenograft (PDX) models, such as mouse PDX, have emerged as potential Avatar platforms for personalized oncology approaches, but the difficulty for some human grafts to grow successfully and the long time required for mice to develop tumors preclude their use for HGG.

METHODS

We used a rapid and efficient ex-ovo chicken embryo chorioallantoic membrane (CAM) culture system to evaluate the efficacy of oncologic drug options for HGG patients.

RESULTS

Implantation of fresh glioma tissue fragments from 59 of 60 patients, that include difficult-to-grow IDH-mutated samples, successfully established CAM tumor xenografts within 7 days, with a tumor take rate of 98.3%. These xenografts faithfully recapitulate the histological and molecular characteristics of the primary tumor, and the ability of individual fragments to form tumors was predictive of poor patient prognosis. Treatment of drug-sensitive or drug-resistant xenografts indicates that the CAM-glioma assay enables testing tumor sensitivity to temozolomide and carboplatin at doses consistent with those administered to patients. In a proof-of-concept study involving 14 HGG patients, we observed a correlation of 100% between the CAM xenograft response to temozolomide or carboplatin and the clinical response of patients.

CONCLUSION

The CAM-glioma model is a fast and reliable assay that has the potential to serve as a complementary model to drug discovery and a real-time Avatar platform to predict the best treatment for HGG patients.

KLF4 is a tumor suppressor in anaplastic meningioma stem-like cells and human meningiomas

Meningiomas are the most common primary tumors in central nervous system. While recent studies have revealed genetic clues to lower grade human meningiomas, the molecular determinants driving the progression and recurrence of anaplastic meningioma, the most malignant subtype with a low prevalence but high morbidity, are still poorly understood. It has been proposed that high recurrence rates of malignant meningiomas are linked to cancer stem cells. Indeed, tumor stem-like cells have been isolated from various meningioma subtypes, but never been obtained from anaplastic meningioma. In this study, we successfully isolated stem-like cells from human anaplastic meningioma. These cells are capable of forming spheres and initiating xenograft tumors that recapitulate anaplastic meningioma phenotypes, and thus could serve as an in vitro model for malignant meningiomas. KLF4, a transcription factor known for its role in stemness maintenance, was identified as one of the most frequently mutated genes in the benign secretory meningioma. Interestingly, we found that KLF4 is downregulated in anaplastic meningioma compared with low-grade meningioma subtypes. By manipulating KLF4 expression in anaplastic meningioma stem-like cells, we demonstrated that KLF4 acts as a tumor suppressor during malignant progression in meningioma, affecting apoptosis, proliferation, invasion, and cell cycle. These results suggest a potential therapeutic value of KLF4 for clinical intervention of anaplastic meningioma.

Immunophenotypic characterization of human glioblastoma stem cells: correlation with clinical outcome

Recently, glioma stem cells have been identified as the main cause of glioma propagation and recurrence and a number of several cell markers have been indicated as putative GSC markers. In the present work, a retrospective study to evaluate the prognostic potential of ability to generate GSCs in our series of 15 glioblastoma patients is described. β-tubulin III, nestin, CD133, GFAP, and SOX-2 marker expression, both in primary GBM cultures and in respective glioblastoma stem cells (GSCs), was evaluated by flow cytometric analysis. Our results demonstrated various expression levels of these markers in both cell cultures; of note, only those cells expressing SOX-2 at greater than 30% levels were able to produce in vitro neurospheres. Moreover, statistical analysis revealed that the GSCs generation negatively affected overall survival (OS) (P = 0.000) and progression-free survival (PFS) (P = 0.001). In addition, a very poor OS (P = 0.000) and PFS (P = 0.000) were observed among patients whose tumors expressed Ki67, evaluated by immunohistochemistry, and showed the ability to generate in vitro GSCs. Overall, the results suggest that in vitro GSCs generation associated to the expression of Ki67 and SOX-2 may be useful to identify patients at risk of disease progression.

Identification of Global DNA Methylation Signatures in Glioblastoma-Derived Cancer Stem Cells

Glioblastoma (GBM) is the most common and most aggressive primary brain tumor in adults. The existence of a small population of stem-like tumor cells that efficiently propagate tumors and resist cytotoxic therapy is one proposed mechanism leading to the resilient behavior of tumor cells and poor prognosis. In this study, we performed an in-depth analysis of the DNA methylation landscape in GBM-derived cancer stem cells (GSCs). Parallel comparisons of primary tumors and GSC lines derived from these tumors with normal controls (a neural stem cell (NSC) line and normal brain tissue) identified groups of hyper- and hypomethylated genes that display a trend of either increasing or decreasing methylation levels in the order of controls, primary GBMs, and their counterpart GSC lines, respectively. Interestingly, concurrent promoter hypermethylation and gene body hypomethylation were observed in a subset of genes including MGMT, AJAP1 and PTPRN2. These unique DNA methylation signatures were also found in primary GBM-derived xenograft tumors indicating that they are not tissue culture-related epigenetic changes. Integration of GSC-specific epigenetic signatures with gene expression analysis further identified candidate tumor suppressor genes that are frequently down-regulated in GBMs such as SPINT2, NEFM and PENK. Forced re-expression of SPINT2 reduced glioma cell proliferative capacity, anchorage independent growth, cell motility, and tumor sphere formation in vitro. The results from this study demonstrate that GSCs possess unique epigenetic signatures that may play important roles in the pathogenesis of GBM.

Tight regulation between cell survival and programmed cell death in GBM stem-like cells by EGFR/GSK3b/PP2A signaling

Malignant gliomas represent one of the most aggressive forms of cancer, displaying high mortality rates and limited treatment options. Specific subpopulations of cells residing in the tumor niche with stem-like characteristics have been postulated to initiate and maintain neoplasticity while resisting conventional therapies. The study presented here aims to define the role of glycogen synthase kinase 3 beta (GSK3b) in patient-derived glioblastoma (GBM) stem-like cell (GSC) proliferation, apoptosis and invasion. To evaluate the potential role of GSK3b in GBM, protein profiles from 68 GBM patients and 20 normal brain samples were analyzed for EGFR-mediated PI3kinase/Akt and GSK3b signaling molecules including protein phosphatase 2A (PP2A). To better understand the function of GSK3b in GBM, GSCs were isolated from GBM patient samples. Blocking GSK3b phosphorylation at Serine 9 attenuated cell proliferation while concomitantly stimulating apoptosis through activation of Caspase-3 in patient-derived GSCs. Increasing GSK3b protein content resulted in the inhibition of cell proliferation, colony formation and stimulated programmed cell death. Depleting GSK3b in GSCs down regulated PP2A. Furthermore, knocking down PP2A or blocking its activity by okadaic acid inactivated GSK3b by increasing GSK3b phosphorylation at Serine 9. Our data suggests that GSK3b may function as a regulator of apoptosis and tumorigenesis in GSCs. Therapeutic approaches targeting GSK3b in glioblastoma stem-like cells may be a useful addition to our current therapeutic armamentarium.

Isolation of cancer stem cells from three human glioblastoma cell lines: characterization of two selected clones

Cancer stem cells (CSC) were isolated via a non-adherent neurosphere assay from three glioma cell lines: LI, U87, and U373. Using a clonal assay, two clones (D2 and F11) were selected from spheres derived from LI cells and were characterized for the: expression of stem cell markers (CD133, Nestin, Musashi-1 and Sox2); proliferation; differentiation capability (determined by the expression of GalC, βIII-Tubulin and GFAP); Ca²⁺ signaling and tumorigenicity in nude mice. Both D2 and F11 clones expressed higher levels of all stem cell markers with respect to the parental cell line. Clones grew more slowly than LI cells with a two-fold increase in duplication time. Markers of differentiation (βIII-Tubulin and GFAP) were expressed at high levels in both LI cells and in neurospheres. The expression of Nestin, Sox2, and βIII-Tubulin was down-regulated in D2 and F11 when cultured in serum-containing medium, whereas Musashi-1 was increased. In this condition, duplication time of D2 and F11 increased without reaching that of LI cells. D2, F11 and parental cells did not express voltage-dependent Ca²⁺-channels but they exhibited increased intracellular Ca²⁺ levels in response to ATP. These Ca²⁺ signals were larger in LI cells and in spheres cultured in serum-containing medium, while they were smaller in serum-free medium. The ATP treatment did not affect cell proliferation. Both D2 and F11 induced the appearance of tumors when ortotopically injected in athymic nude mice at a density 50-fold lower than that of LI cells. All these data indicate that both clones have characteristics of CSC and share the same stemness properties. The findings regarding the expression of differentiation markers and Ca²⁺-channels show that both clones are unable to reach the terminal differentiation. Both D2 and F11 might represent a good model to improve the knowledge on CSC in glioblastoma and to identify new therapeutic approaches.

Inhibition of Notch signaling alters the phenotype of orthotopic tumors formed from glioblastoma multiforme neurosphere cells but does not hamper intracranial tumor growth regardless of endogene Notch pathway signature

BACKGROUND

Brain cancer stem-like cells (bCSC) are cancer cells with neural stem cell (NSC)-like properties found in the devastating brain tumor glioblastoma multiforme (GBM). bCSC are proposed a central role in tumor initiation, progression, treatment resistance and relapse and as such present a promising target in GBM research. The Notch signaling pathway is often deregulated in GBM and we have previously characterized GBM-derived bCSC cultures based on their expression of the Notch-1 receptor and found that it could be used as predictive marker for the effect of Notch inhibition. The aim of the present project was therefore to further elucidate the significance of Notch pathway activity for the tumorigenic properties of GBM-derived bCSC.

METHODS

Human-derived GBM xenograft cells previously established as NSC-like neurosphere cultures were used. Notch inhibition was accomplished by exposing the cells to the gamma-secretase inhibitor DAPT prior to gene expression analysis and intracranial injection into immunocompromised mice.

RESULTS

By analyzing the expression of several Notch pathway components, we found that the cultures indeed displayed different Notch pathway signatures. However, when DAPT-treated neurosphere cells were injected into the brain of immunocompromised mice, no increase in survival was obtained regardless of Notch pathway signature and Notch inhibition. We did however observe a decrease in the expression of the stem cell marker Nestin, an increase in the proliferative marker Ki-67 and an increased number of abnormal vessels in tumors formed from DAPT-treated, high Notch-1 expressing cultures, when compared with the control.

CONCLUSION

Based on the presented results we propose that Notch inhibition partly induces differentiation of bCSC, and selects for a cell type that more strongly induces angiogenesis if the treatment is not sustained. However, this more differentiated cell type might prove to be more sensitive to conventional therapies.

Glioblastoma stem-like cells-biology and therapeutic implications

The cancer stem-cell hypothesis proposes that malignant tumors are likely to encompass a cellular hierarchy that parallels normal tissue and may be responsible for the maintenance and recurrence of glioblastoma multiforme (GBM) in patients. The purpose of this manuscript is to review methods for optimizing the derivation and culturing of stem-like cells also known as tumor stem cells (TSCs) from patient-derived GBM tissue samples. The hallmarks of TSCs are that they must be able to self-renew and retain tumorigenicity. The isolation, optimization and derivation of TSCs as outlined in this review, will be important in understanding biology and therapeutic applications related to these cells.

In Vivo c-Met Pathway Inhibition Depletes Human Glioma Xenografts of Tumor-Propagating Stem-Like Cells

Solid malignancies contain sphere-forming stem-like cells that are particularly efficient in propagating tumors. Identifying agents that target these cells will advance the development of more effective therapies. Recent converging evidence shows that c-Met expression marks tumor-initiating stem-like cells and that c-Met signaling drives human glioblastoma multiforme (GBM) cell stemness in vitro. However, the degree to which tumor-propagating stem-like cells depend on c-Met signaling in histologically complex cancers remains unknown. We examined the effects of in vivo c-Met pathway inhibitor therapy on tumor-propagating stem-like cells in human GBM xenografts. Animals bearing pre-established tumor xenografts expressing activated c-Met were treated with either neutralizing anti- hepatocyte growth factor (HGF) monoclonal antibody L2G7 or with the c-Met kinase inhibitor PF2341066 (Crizotinib). c-Met pathway inhibition inhibited tumor growth, depleted tumors of sphere-forming cells, and inhibited tumor expression of stem cell markers CD133, Sox2, Nanog, and Musashi. Withdrawing c-Met pathway inhibitor therapy resulted in a substantial rebound in stem cell marker expression concurrent with tumor recurrence. Cells derived from xenografts treated with anti-HGF in vivo were depleted of tumor-propagating potential as determined by in vivo serial dilution tumor-propagating assay. Furthermore, daughter xenografts that did form were 12-fold smaller than controls. These findings show that stem-like tumor-initiating cells are dynamically regulated by c-Met signaling in vivo and that c-Met pathway inhibitors can deplete tumors of their tumor-propagating stem-like cells.

B7-H4 expression is elevated in human U251 glioma stem-like cells and is inducible in monocytes cultured with U251 stem-like cell conditioned medium

Previous studies indicated that B7-H4, the youngest B7 family, negatively regulates T cell-mediated immunity and is significantly overexpressed in many human tumors. Tumor stem cells are purported to play a role in tumor renewal and resistance to radiation and chemotherapy. However, the link between B7-H4 and tumor stem cells is unclear. In this study, we investigated B7-H4 expression in the medium of human glioma U251 cell cultures. Immunofluorescence results showed that U251 cells cultured in serum-free medium (supplemented with 2% B27, 20 ng/mL epidermal growth factor, 20 ng/mL basic fibroblast growth factor) maintained stem-like cell characteristics, including expression of stem cell marker CD133 and the neural progenitor cell markers nestin and SOX2. In contrast, U251 cells cultured in serum-containing medium highly expressed differentiation marker glial fibrillary acidic protein. Flow cytometry analysis showed serum-free medium-cultured U251 cells expressed higher intracellular B7-H4 than serum-containing medium-cultured U251 cells (24%–35% vs. 8%–11%, P < 0.001). Immunofluorescence in purified monocytes from normal human peripheral blood mononuclear cells revealed moderate expression of B7-H4 after stimulation with conditioned medium from U251 cells cultured in serum-containing medium. Moreover, conditioned medium from U251 stem-like cells had a significant stimulation effect on B7-H4 expression compared with serum-containing conditioned medium (P < 0.01). Negative costimulatory molecule B7-H4 was preferentially expressed in U251 stem-like cells, and conditioned medium from these cells more effectively induced monocytes to express B7-H4 than conditioned medium from U251 cells cultured in the presence of serum. Our results show that U251 stem-like cells may play a more crucial role in tumor immunoloregulation with high expression of B7-H4.

Candidate pathways for promoting differentiation or quiescence of oligodendrocyte progenitor-like cells in glioma

Platelet-derived growth factor receptor alpha-positive oligodendrocyte progenitor cells (OPC) located within the mature central nervous system may remain quiescent, proliferate, or differentiate into oligodendrocytes. Human glioblastoma multiforme tumors often contain rapidly proliferating oligodendrocyte lineage transcription factor 2 (Olig2)-positive cells that resemble OPCs. In this study, we sought to identify candidate pathways that promote OPC differentiation or quiescence rather than proliferation. Gene expression profiling conducted in both normal murine OPCs and highly proliferative Olig2-positive glioma cells identified all the transcripts associated with the highly proliferative state of these cells and showed that among the various cell types found within the brain, Olig2-positive tumor cells are most similar to OPCs. We then subtracted OPC transcripts found in tumor samples from those found in normal brain samples and identified 28 OPC transcripts as candidates for promoting differentiation or quiescence. Systematic analysis of human glioma data revealed that these genes have similar expression profiles in human tumors and were significantly enriched in genomic deletions, suggesting an antiproliferative role. Treatment of primary murine glioblastoma cells with agonists of one candidate gene, Gpr17, resulted in a decreased number of neurospheres. Together, our findings show that comparison of the molecular phenotype of progenitor cells in tumors to the equivalent cells in the normal brain represents a novel approach for the identification of targeted therapies.

[Animal models of human glioma: the progress of application and investigation]

The glioma accounts for half of the central nervous tumors, among which the glioblastoma multiforme (GBM) is one of the most aggressive and lethal brain tumors. The difficulties in glioma therapy indicate the need of appropriate animal models for preclinical studies. Benefiting from the development of molecular biology, genetics, and transgenic technology, variable animal models of glioma have been established. These animal models of human glioma are reviewed in this paper.

Co-localization of PCNA, VCAM-1 and caspase-3 with nestin in xenografts derived from human anaplastic astrocytoma and glioblastoma multiforme tumor spheres

The cancer stem cell hypothesis proposes that tumors contain a small subset of cancer cells, the cancer stem cells, which constitute a reservoir of self-sustaining cells with the exclusive ability to self-renew and maintain the tumor. Markers that define cancer stem cells that are capable of recapitulating brain tumors as xenografts in mice has not been described. We investigated the relationship between expression of nestin and that of PCNA, VCAM-1 and caspase-3 in the xenografts developed from human anaplastic astrocytoma and glioblastoma tumor-derived spheres in the brain of nude mouse. Xenografts obtained from astrocytoma tumor stem cells (ATSC) and glioblastoma tumor stem cells (GTSC) have showed a large number of cells positive for both PCNA and the nestin, demonstrating that nestin expressing cells have a high rate of proliferation. Xenografts from GTSC showed heterogeneous staining pattern with cells that express both nestin and VCAM-1, whereas others cells remained complete negative. In this case it was noticed that most tumor cells with large or multinucleated nuclei coexpress nestin and VCAM-1. In xenografts from ATSC most cells positive for nestin express VCAM-1 and in this case the two proteins appear to occupy the same cytoplasmic region. Both GTSC and ATSC derived xenografts showed cells positive for both caspase-3 and for nestin detected mainly as single cells and as cell clusters located near or around a blood vessel.

Isolation and characterization of a population of stem-like progenitor cells from an atypical meningioma

The majority of meningiomas are benign tumors associated with favorable outcomes; however, the less common aggressive variants with unfavorable outcomes often recur and may be due to subpopulations of less-differentiated cells residing within the tumor. These subpopulations of tumor cells have tumor-initiating properties and may be isolated from heterogeneous tumors when sorted or cultured in defined medium. We report the isolation and characterization of a population of tumor-initiating cells derived from an atypical meningioma. We identify a tumor-initiating population from an atypical meningioma, termed meningioma-initiating cells (MICs). These MICs self-renew, differentiate, and can recapitulate the histological characteristics of the parental tumor when transplanted at 1000 cells into the flank regions of athymic nude mice. Immunohistochemistry reveals stem-like protein expression patterns similar to neural stem and progenitor cells (NSPCs) while genomic profiling verified the isolation of cancer cells (with defined meningioma chromosomal aberrations) from the bulk tumor. Microarray and pathway analysis identifies biochemical processes and gene networks related to aberrant cell cycle progression, particularly the loss of heterozygosity of tumor suppressor genes CDKN2A (p16(INK4A)), p14(ARF), and CDKN2B (p15(INK4B)). Flow cytometric analysis revealed the expression of CD44 and activated leukocyte adhesion molecule (ALCAM/CD166); these may prove to be markers able to identify this cell type. The isolation and identification of a tumor-initiating cell population capable of forming meningiomas demonstrates a useful model for understanding meningioma development. This meningioma model may be used to study the cell hierarchy of meningioma tumorogenesis and provide increased understanding of malignant progression.

The Role of Cancer Stem Cells (CD133+) in Malignant Gliomas

Malignant gliomas, particularly glioblastoma multiforme (GBM) tumors, are very difficult to treat by conventional approaches. Although most of the tumor mass can be removed by surgical resection, radiotherapy, and chemotherapy, it eventually recurs. There is growing evidence that cancer stem cells (CSCs) play an important role in tumor recurrence. These stem cells are radioresistant and chemoresistant. The most commonly used tumor marker for CSCs is CD133. The amount of CSC component is closely correlated with tumor malignancy grading. Isolating, identifying, and treating CSCs as the target is crucial for treating malignant gliomas. CSC-associated vascular endothelial growth factor (VEGF) promotes tumor angiogenesis, tumor hemorrhage, and tumor infiltration. Micro-RNA (miRNA) plays a role in CSC gene expression, which may regulate oncogenesis or suppression to influence tumor development or progression. The antigenesis of CSCs and normal stem cells may be different. The CSCs may escape the T-cell immune response. Identifying a new specific antigen from CSCs for vaccine treatment is a key point for immunotherapy. On the other hand, augmented treatment with radiosensitizer or chemosensitizer may lead to reduction of CSCs and lead to CSCs being vulnerable to radiotherapy and chemotherapy. The control of signaling pathway and cell differentiation to CSC growth is another new hope for treatment of malignant gliomas. Although the many physiological behavioral differences between CSCs and normal stem cells are unclear, the more we know about these differences the better we will be able to treat CSCs effectively.

Loss of ATM/Chk2/p53 pathway components accelerates tumor development and contributes to radiation resistance in gliomas

Maintenance of genomic integrity is essential for adult tissue homeostasis and defects in the DNA-damage response (DDR) machinery are linked to numerous pathologies including cancer. Here, we present evidence that the DDR exerts tumor suppressor activity in gliomas. We show that genes encoding components of the DDR pathway are frequently altered in human gliomas and that loss of elements of the ATM/Chk2/p53 cascade accelerates tumor formation in a glioma mouse model. We demonstrate that Chk2 is required for glioma response to ionizing radiation in vivo and is necessary for DNA-damage checkpoints in the neuronal stem cell compartment. Finally, we observed that the DDR is constitutively activated in a subset of human GBMs, and such activation correlates with regions of hypoxia.

CD117 and Stro-1 identify osteosarcoma tumor-initiating cells associated with metastasis and drug resistance

Emerging evidence indicates the presence of tumor-initiating cells (TIC) or cancer stem cells in osteosarcoma. However, no study has shown specific markers to identify osteosarcoma TICs with in vivo tumor formation ability. Additionally, there has been a lack of investigations gauging the contribution of osteosarcoma TICs to metastatic and drug-resistant properties. In this study, we have identified mouse and human osteosarcoma TICs using mesenchymal stem cell markers CD117 and Stro-1. These markers were preferentially expressed in spheres and doxorubicin-resistant cells. Both mouse and human cells expressing these markers were sorted and analyzed for their abilities of tumor formation with as few as 200 cells, self-renewability, multipotency, drug resistance, metastatic potential, and enrichment of a metastasis-associated marker (CXCR4) and a drug resistance marker (ABCG2). CD117(+)Stro-1(+) cells efficiently formed serially transplantable tumors, whereas CD117(-)Stro-1(-) cells rarely initiated tumors. On orthotopic injections, CD117(+)Stro-1(+ )cell-derived tumors metastasized at a high frequency. Further, CD117(+)Stro-1(+) cells showed high invasive and drug-resistant properties and were efficiently enriched for CXCR4 (20-90%) and ABCG2 (60-90%). These results suggest possible mechanisms for the high metastatic and drug-resistant properties of osteosarcoma TICs. In summary, CD117 and Stro-1 identify osteosarcoma TICs associated with the most lethal characteristics of the disease-metastasis and drug resistance-and these markers offer candidates for TIC-targeted drug delivery aimed at eradicating osteosarcoma.