Carol Tang

Protein phosphatase 2A regulates self-renewal of Drosophila neural stem cells.

Drosophila larval brain neural stem cells, also known as neuroblasts, divide asymmetrically to generate a self-renewing neuroblast and a ganglion mother cell (GMC) that divides terminally to produce two differentiated neurons or glia. Failure of asymmetric cell division can result in hyperproliferation of neuroblasts, a phenotype resembling brain tumors. Here we have identified Drosophila Protein phosphatase 2A (PP2A) as a brain tumor-suppressor that can inhibit self-renewal of neuroblasts. Supernumerary larval brain neuroblasts are generated at the expense of differentiated neurons in PP2A mutants. Neuroblast overgrowth was observed in both dorsomedial (DM)/posterior Asense-negative (PAN) neuroblast lineages and non-DM neuroblast lineages. The PP2A heterotrimeric complex, composed of the catalytic subunit (Mts), scaffold subunit (PP2A-29B) and a B-regulatory subunit (Tws), is required for the asymmetric cell division of neuroblasts. The PP2A complex regulates asymmetric localization of Numb, Pon and Atypical protein kinase C, as well as proper mitotic spindle orientation. Interestingly, PP2A and Polo kinase enhance Numb and Pon phosphorylation. PP2A, like Polo, acts to prevent excess neuroblast self-renewal primarily by regulating asymmetric localization and activation of Numb. Reduction of PP2A function in larval brains or S2 cells causes a marked decrease in Polo transcript and protein abundance. Overexpression of Polo or Numb significantly suppresses neuroblast overgrowth in PP2A mutants, suggesting that PP2A inhibits excess neuroblast self-renewal in the Polo/Numb pathway.

Parkin pathway activation mitigates glioma cell proliferation and predicts patient survival

Mutations in the parkin gene, which encodes a ubiquitin ligase, are a major genetic cause of parkinsonism. Interestingly, parkin also plays a role in cancer as a putative tumor suppressor, and the gene is frequently targeted by deletion and inactivation in human malignant tumors. Here, we investigated a potential tumor suppressor role for parkin in gliomas. We found that parkin expression was dramatically reduced in glioma cells. Restoration of parkin expression promoted G(1) phase cell-cycle arrest and mitigated the proliferation rate of glioma cells in vitro and in vivo. Notably, parkin-expressing glioma cells showed a reduction in levels of cyclin D1, but not cyclin E, and a selective downregulation of Akt serine-473 phosphorylation and VEGF receptor levels. In accordance, cells derived from a parkin-null mouse model exhibited increased levels of cyclin D1, VEGF receptor, and Akt phosphorylation, and divided significantly faster when compared with wild-type cells, with suppression of these changes following parkin reintroduction. Clinically, analysis of parkin pathway activation was predictive for the survival outcome of patients with glioma. Taken together, our study provides mechanistic insight into the tumor suppressor function of parkin in brain tumors and suggests that measurement of parkin pathway activation may be used clinically as a prognostic tool in patients with brain tumor.

Cryopreservation of neurospheres derived from human glioblastoma multiforme.

Cancer stem cells have been shown to initiate and sustain tumor growth. In many instances, clinical material is limited, compounded by a lack of methods to preserve such cells at convenient time points. Although brain tumor‐initiating cells grown in a spheroid manner have been shown to maintain their integrity through serial transplantation in immune‐compromised animals, practically, it is not always possible to have access to animals of suitable ages to continuously maintain these cells. We therefore explored vitrification as a cryopreservation technique for brain tumor‐initiating cells. Tumor neurospheres were derived from five patients with glioblastoma multiforme (GBM). Cryopreservation in 90% serum and 10% dimethyl sulfoxide yielded greatest viability and could be explored in future studies. Vitrification yielded cells that maintained self‐renewal and multipotentiality properties. Karyotypic analyses confirmed the presence of GBM hallmarks. Upon implantation into NOD/SCID mice, our vitrified cells reformed glioma masses that could be serially transplanted. Transcriptome analysis showed that the vitrified and nonvitrified samples in either the stem‐like or differentiated states clustered together, providing evidence that vitrification does not change the genotype of frozen cells. Upon induction of differentiation, the transcriptomes of vitrified cells associated with the original primary tumors, indicating that tumor stem‐like cells are a genetically distinct population from the differentiated mass, underscoring the importance of working with the relevant tumor‐initiating population. Our results demonstrate that vitrification of brain tumor‐initiating cells preserves the biological phenotype and genetic profiles of the cells. This should facilitate the establishment of a repository of tumor‐initiating cells for subsequent experimental designs. STEM CELLS 2009;27:29–39

Parkin enhances the expression of cyclin-dependent kinase 6 and negatively regulates the proliferation of breast cancer cells

Although mutations in the parkin gene are frequently associated with familial Parkinsonism, emerging evidence suggests that parkin also plays a role in cancers as a putative tumor suppressor. Supporting this, we show here that parkin expression is dramatically reduced in several breast cancer-derived cell lines as well as in primary breast cancer tissues. Importantly, we found that ectopic parkin expression in parkin-deficient breast cancer cells mitigates their proliferation rate both in vitro and in vivo, as well as reduces the capacity of these cells to migrate. Cell cycle analysis revealed the arrestment of a significant percentage of parkin-expressing breast cancer cells at the G1-phase. However, we did not observe significant changes in the levels of the G1-associated cyclin D1 and E. On the other hand, the level of cyclin-dependent kinase 6 (CDK6) is dramatically and selectively elevated in parkin-expressing breast cancer cells, the extent of which correlates well with the expression of parkin. Interestingly, a recent study demonstrated that CDK6 restrains the proliferation of breast cancer cells. Taken together, our results support a negative role for parkin in tumorigenesis and provide a potential mechanism by which parkin exerts its suppressing effects on breast cancer cell proliferation.

Progenitor-like Traits Contribute to Patient Survival and Prognosis in Oligodendroglial Tumors

Purpose: Patient-derived glioma-propagating cells (GPC) contain karyotypic and gene expression profiles that are found in the primary tumor. However, their clinical relevance is unclear. We ask whether GPCs contribute to disease progression and survival outcome in patients with glioma by analyzing gene expression profiles. Experimental Design: We tapped into public sources of GPC gene expression data and derived a gene signature distinguishing oligodendroglial from glioblastoma multiforme (GBM) GPCs. By adapting a method in glioma biology, the Connectivity Map, we interrogated its strength of association in public clinical databases. We validated the top-ranking signaling pathways Wnt, Notch, and TGFβ, in GPCs and primary tumor specimens. Results: We observed that patients with better prognosis correlated with oligodendroglial GPC features and lower tumor grade, and this was independent of the current clinical indicator, 1p/19q status. Patients with better prognosis had proneural tumors whereas the poorly surviving cohort had mesenchymal tumors. In addition, oligodendroglial GPCs were more sensitive to Wnt and Notch inhibition whereas GBM GPCs responded to TGFβR1 inhibition. Conclusions: We provide evidence that GPCs are clinically relevant. In addition, the more favorable prognosis of oligodendroglial tumors over GBM could be recapitulated transcriptomically at the GPC level, underscoring the relevance of this cellular model. Our gene signature detects molecular heterogeneity in oligodendroglial tumors that cannot be accounted for by the 1p/19q status alone, indicating that stem-like traits contribute to clinical status. Collectively, these data highlight the limitation of morphology-based histologic analyses in tumor classification, consequently impacting on treatment decisions. Clin Cancer Res; 18(15); 4122–35. ©2012 AACR.

A Distinct Reactive Oxygen Species Profile Confers Chemoresistance in Glioma-Propagating Cells and Associates with Patient Survival Outcome

AIMS We explore the role of an elevated O2(-):H2O2 ratio as a prosurvival signal in glioma-propagating cells (GPCs). We hypothesize that depleting this ratio sensitizes GPCs to apoptotic triggers. RESULTS We observed that an elevated O2(-):H2O2 ratio conferred enhanced resistance in GPCs, and depletion of this ratio by pharmacological and genetic methods sensitized cells to apoptotic triggers. We established the reactive oxygen species (ROS) Index as a quantitative measure of a normalized O2(-):H2O2 ratio and determined its utility in predicting chemosensitivity. Importantly, mice implanted with GPCs of a reduced ROS Index demonstrated extended survival. Analysis of tumor sections revealed effective targeting of complementarity determinant 133 (CD133)- and nestin-expressing neural precursors. Further, we established the Connectivity Map to interrogate a gene signature derived from a varied ROS Index for the patterns of association with individual patient gene expression in four clinical databases. We showed that patients with a reduced ROS Index demonstrate better survival. These data provide clinical evidence for the viability of our O2(-):H2O2-mediated chemosensitivity profiles. INNOVATION AND CONCLUSION Gliomas are notoriously recurrent and highly infiltrative, and have been shown to arise from stem-like cells. We implicate an elevated O2(-):H2O2 ratio as a prosurvival signal in GPC self-renewal and proliferation. The ROS Index provides quantification of O2(-):H2O2-mediated chemosensitivity, an advancement in a previously qualitative field. Intriguingly, glioma patients with a reduced ROS Index correlate with longer survival and the Proneural molecular classification, a feature frequently associated with tumors of better prognosis. These data emphasize the feasibility of manipulating the O2(-):H2O2 ratio as a therapeutic strategy.

Collaboration of 3D context and extracellular matrix in the development of glioma stemness in a 3D model.

A hierarchy of cellular stemness exists in certain cancers, and any successful strategy to treat such cancers would have to eliminate the self-renewing tumor-initiating cells at the apex of the hierarchy. The cellular microenvironment, in particular the extracellular matrix (ECM), is believed to have a role in regulating stemness. In this work, U251 glioblastoma cells are cultured on electrospun polystyrene (ESPS) scaffolds coated with an array of 7 laminin isoforms to provide a 3D model for stem cell-related genes and proteins expression studies. We observed collaboration between 3D context and laminins in promoting glioma stemness. Depending on the laminin isoform presented, U251 cells cultured on ESPS scaffolds (3D) exhibited increased expression of stemness markers compared to those cultured on tissue culture polystyrene (2D). Our results indicate the influence of 3D (versus 2D) context on integrin expression, specifically, the upregulation of the laminin-binding integrins alpha 6 and beta 4. By a colony forming assay, we showed enhanced clonogenicity of cells grown on ESPS scaffolds in collaboration with laminins 411, 421, 511 and 521. Evaluation of patient glioma databases demonstrated significant enrichment of integrin and ECM pathway networks in tumors of worse prognosis, consistent with our observations. The present results demonstrate how 3D versus 2D context profoundly affects ECM signaling, leading to stemness.

The SCFSlimb E3 ligase complex regulates asymmetric division to inhibit neuroblast overgrowth

Drosophila larval brain neuroblasts divide asymmetrically to balance between self‐renewal and differentiation. Here, we demonstrate that the SCFSlimb E3 ubiquitin ligase complex, which is composed of Cul1, SkpA, Roc1a and the F‐box protein Supernumerary limbs (Slimb), inhibits ectopic neuroblast formation and regulates asymmetric division of neuroblasts. Hyperactivation of Akt leads to similar neuroblast overgrowth and defects in asymmetric division. Slimb associates with Akt in a protein complex, and SCFSlimb acts through SAK and Akt to inhibit neuroblast overgrowth. Moreover, Beta‐transducin repeat containing, the human ortholog of Slimb, is frequently deleted in highly aggressive gliomas, suggesting a conserved tumor suppressor‐like function.

Mechanical confinement triggers glioma linear migration dependent on formin FHOD3

Glioma linear migration on brain blood vessels is recapitulated in vitro. This migration is characterized by a two-phase process activated by laminin, confinement, and linear topology. It requires a unique balance of actin polymerization systems emphasizing formins and not Arp2/3 and is a powerful tool for identifying new targets such as the formin FHOD3.

ST3GAL1-Associated Transcriptomic Program in Glioblastoma Tumor Growth, Invasion, and Prognosis

Background: Cell surface sialylation is associated with tumor cell invasiveness in many cancers. Glioblastoma is the most malignant primary brain tumor and is highly infiltrative. ST3GAL1 sialyltransferase gene is amplified in a subclass of glioblastomas, and its role in tumor cell self-renewal remains unexplored. Methods: Self-renewal of patient glioma cells was evaluated using clonogenic, viability, and invasiveness assays. ST3GAL1 was identified from differentially expressed genes in Peanut Agglutinin–stained cells and validated in REMBRANDT (n = 390) and Gravendeel (n = 276) clinical databases. Gene set enrichment analysis revealed upstream processes. TGFβ signaling on ST3GAL1 transcription was assessed using chromatin immunoprecipitation. Transcriptome analysis of ST3GAL1 knockdown cells was done to identify downstream pathways. A constitutively active FoxM1 mutant lacking critical anaphase-promoting complex/cyclosome ([APC/C]-Cdh1) binding sites was used to evaluate ST3Gal1-mediated regulation of FoxM1 protein. Finally, the prognostic role of ST3Gal1 was determined using an orthotopic xenograft model (3 mice groups comprising nontargeting and 2 clones of ST3GAL1 knockdown in NNI-11 [8 per group] and NNI-21 [6 per group]), and the correlation with patient clinical information. All statistical tests on patients’ data were two-sided; other P values below are one-sided. Results: High ST3GAL1 expression defines an invasive subfraction with self-renewal capacity; its loss of function prolongs survival in a mouse model established from mesenchymal NNI-11 (P < .001; groups of 8 in 3 arms: nontargeting, C1, and C2 clones of ST3GAL1 knockdown). ST3GAL1 transcriptomic program stratifies patient survival (hazard ratio [HR] = 2.47, 95% confidence interval [CI] = 1.72 to 3.55, REMBRANDT P = 1.92x10-8; HR = 2.89, 95% CI = 1.94 to 4.30, Gravendeel P = 1.05x10-11), independent of age and histology, and associates with higher tumor grade and T2 volume (P = 1.46x10-4). TGFβ signaling, elevated in mesenchymal patients, correlates with high ST3GAL1 (REMBRANDT gliomacor = 0.31, P = 2.29x10-10; Gravendeel gliomacor = 0.50, P = 3.63x10-20). The transcriptomic program upon ST3GAL1 knockdown enriches for mitotic cell cycle processes. FoxM1 was identified as a statistically significantly modulated gene (P = 2.25x10-5) and mediates ST3Gal1 signaling via the (APC/C)-Cdh1 complex. Conclusions: The ST3GAL1-associated transcriptomic program portends poor prognosis in glioma patients and enriches for higher tumor grades of the mesenchymal molecular classification. We show that ST3Gal1-regulated self-renewal traits are crucial to the sustenance of glioblastoma multiforme growth.