A number of recent reports have demonstrated that only CD133-positive cancer cells of glioblastoma multiforme (GBM) have tumor-initiating potential. These findings raise an attractive hypothesis that GBMs can be cured by eradicating CD133-positive cancer stem cells (CSCs), which are a small portion of GBM cells. However, as GBMs are known to possess various genetic alterations, GBMs might harbor heterogeneous CSCs with different genetic alterations. Here, we compared the clinical characteristics of two GBM patient groups divided according to CD133-positive cell ratios. The CD133-low GBMs showed more invasive growth and gene expression profiles characteristic of mesenchymal or proliferative subtypes, whereas the CD133-high GBMs showed features of cortical and well-demarcated tumors and gene expressions typical of proneuronal subtype. Both CD133-positive and CD133-negative cells purified from four out of six GBM patients produced typical GBM tumor masses in NOD-SCID brains, whereas brain mass from CD133-negative cells showed more proliferative and angiogenic features compared to that from CD133-positive cells. Our results suggest, in contrast to previous reports that only CD133-positive cells of GBMs can initiate tumor formation in vivo CD133-negative cells also possess tumor-initiating potential, which is indicative of complexity in the identification of cancer cells for therapeutic targeting. A recent concept in brain tumor biology is that brain tumors arise from cancer stem cells (CSCs) that are CD133 positive (CD133 ( þ ) ). It has been reported that a small number of CD133 ( þ ) glioblastoma multiforme (GBM) cells are able to recapitulate the original tumor in vivo, whereas millions of CD133-negative (CD133 (À) ) cells could not produce brain tumor masses. 1-6 However, accumulating evidence suggests that CD133 (À) GBM cells can also regenerate heterogenous tumors in vivo, 7,8 and generation of the huge and rapidly growing tumors by only CD133 ( þ ) CSCs would be difficult because more than 50% of GBM patients have few CD133 ( þ ) cells. 9 As a majority of neurogenic astrocytes in the adult brain are not recognized by a CD133 antibody, 8 it is likely that CD133 might be newly expressed in GBM CSCs that are derived from CD133 (À) adult neural stem cells (NSCs) or terminally differentiated brain cells, such as astrocytes, neurons, and oligodendrocytes. Given that the gene expression profile is changed when GBM recurs after treatments, 10 it is plausible that new CD133 expression may occur if the characteristics of CSCs are changed or if some CSCs are selected by treatment. Furthermore, the wide-range variation in CD133 ( þ ) cell ratio (0.1-50% in GBM patients) 1-6 also suggests the existence of other GBM CSCs that do not express CD133.Therefore, we hypothesize that there are several kinds of CSCs in the tumor mass of GMB, and these diverse CSCs

Most of the cancers are still incurable human diseases. According to recent findings, especially targeting cancer stem cells (CSCs) is the most promising therapeutic strategy. CSCs take charge of a cancer hierarchy, harboring stem cell-like properties involving self-renewal and aberrant differentiation potential. Most of all, the presence of CSCs is closely associated with tumorigenesis and therapeutic resistance. Despite the numerous efforts to target CSCs, current anti-cancer therapies are still impeded by CSC-derived cancer malignancies; increased metastases, tumor recurrence, and even acquired resistance against the anti-CSC therapies developed in experimental models. One of the most forceful underlying reasons is a “cancer heterogeneity” due to “CSC plasticity” A comprehensive understanding of CSC-derived heterogeneity will provide novel insights into the establishment of efficient targeting strategies to eliminate CSCs. Here, we introduce findings on mechanisms of CSC reprogramming and CSC plasticity, which give rise to phenotypically varied CSCs. Also, we suggest concepts to improve CSC-targeted therapy in order to overcome therapeutic resistance caused by CSC plasticity and heterogeneity.

Glioblastoma (GBM) is an astrocytic brain tumor characterized by an aggressive clinical course and intense resistance to all therapeutic modalities. Here, we report the identification and functional characterization of Bcl2L12 (Bcl2-like-12) that is robustly expressed in nearly all human primary GBMs examined. Enforced Bcl2L12 expression confers marked apoptosis resistance in primary cortical astrocytes, and, conversely, its RNA interference (RNAi)-mediated knockdown sensitizes human glioma cell lines toward apoptosis in vitro and impairs tumor growth with increased intratumoral apoptosis in vivo. Mechanistically, Bcl2L12 expression does not affect cytochrome c release or apoptosome-driven caspase-9 activation, but instead inhibits post-mitochondrial apoptosis signaling at the level of effector caspase activation. One of Bcl2L12's mechanisms of action stems from its ability to interact with and neutralize caspase-7. Notably, while enforced Bcl2L12 expression inhibits apoptosis, it also engenders a pronecrotic state, which mirrors the cellular phenotype elicited by genetic or pharmacologic inhibition of post-mitochondrial apoptosis molecules. Thus, Bcl2L12 contributes to the classical tumor biological features of GBM such as intense apoptosis resistance and florid necrosis, and may provide a target for enhanced therapeutic responsiveness of this lethal cancer. Supplemental material is available at http://www.genesdev.org.

Adaptation of influenza A viruses to a new host species usually involves the mutation of one or more of the eight viral gene segments, and the molecular basis for host range restriction is still poorly understood. To investigate the molecular changes that occur during adaptation of a low-pathogenic avian influenza virus subtype commonly isolated from migratory birds to a mammalian host, we serially passaged the avirulent wild-bird H5N2 strain A/Aquatic bird/Korea/W81/05 (W81) in the lungs of mice. The resulting mouse-adapted strain (ma81) was highly virulent (50% mouse lethal dose ‫؍‬ 2.6 log 10 50% tissue culture infective dose) and highly lethal. Nonconserved mutations were observed in six viral genes (those for PB2, PB1, PA, HA, NA, and M). Reverse genetic experiments substituting viral genes and mutations demonstrated that the PA gene was a determinant of the enhanced virulence in mice and that a Thr-to-Iso substitution at position 97 of PA played a key role. In growth kinetics studies, ma81 showed enhanced replication in mammalian but not avian cell lines; the PA 97I mutation in strain W81 increased its replicative fitness in mice but not in chickens. The high virulence associated with the PA 97I mutation in mice corresponded to considerably enhanced polymerase activity in mammalian cells. Furthermore, this characteristic mutation is not conserved among avian influenza viruses but is prevalent among mouse-adapted strains, indicating a host-dependent mutation. To our knowledge, this is the first study that the isoleucine residue at position 97 in PA plays a key role in enhanced virulence in mice and is implicated in the adaptation of avian influenza viruses to mammalian hosts.

Glioblastomas multiforme (GBM) contain highly tumorigenic, self-renewing populations of stem/initiating cells [glioblastoma stem cells (GSC)] that contribute to tumor propagation and treatment resistance. However, our knowledge of the specific signaling pathways that regulate GSCs is limited. The MET tyrosine kinase is known to stimulate the survival, proliferation, and invasion of various cancers including GBM. Here, we identified a distinct fraction of cells expressing a high level of MET in human primary GBM specimens that were preferentially localized in perivascular regions of human GBM biopsy tissues and were found to be highly clonogenic, tumorigenic, and resistant to radiation. Inhibition of MET signaling in GSCs disrupted tumor growth and invasiveness both in vitro and in vivo, suggesting that MET activation is required for GSCs. Together, our findings indicate that MET activation in GBM is a functional requisite for the cancer stem cell phenotype and a promising therapeutic target. Cancer Res; 72(15); 3828-38. Ó2012 AACR.

Glioma stem cells (GSC) possess tumor-initiating potential and are relatively resistant to conventional chemotherapy and irradiation. Thus, they are considered to be major drivers for glioma initiation, progression, and recurrence. However, the precise mechanism governing acquisition of their drug resistance remains to be elucidated. Our previous study has shown that inhibitor of differentiation 4 (ID4) dedifferentiates Ink4a/Arf À/À mouse astrocytes and human glioma cells to glioma stem-like cells (induced GSCs or iGSCs). In this article, we report that ID4-driven iGSCs exhibit chemoresistant behavior to anticancer drugs through activation of ATPbinding cassette (ABC) transporters. We found that ID4 enhanced SOX2 protein expression by suppressing microRNA-9* (miR-9*), which can repress SOX2 by targeting its 3 0 -untranslated region. Consequently, ID4-mediated SOX2 induction enhanced ABCC3 and ABCC6 expression through direct transcriptional regulation, indicating that ID4 regulates the chemoresistance of iGSCs by promoting SOX2-mediated induction of ABC transporters. Furthermore, we found that short hairpin RNA-mediated knockdown of SOX2 in ID4-driven iGSCs resulted in loss of cancer stemness. Moreover, ectopic expression of SOX2 could dedifferentiate Ink4a/Arf À/À astrocytes and glioma cells to iGSCs, indicating a crucial role of SOX2 in genesis and maintenance of GSCs. Finally, we found that the significance of the ID4-miR-9*-SOX2-ABCC3/ABCC6 regulatory pathway is recapitulated in GSCs derived from patients with glioma. Together, our results reveal a novel regulatory mechanism by which ID4-driven suppression of miR-9* induces SOX2, which imparts stemness potential and chemoresistance to glioma cells and GSCs. Cancer Res; 71(9); 3410-21. Ó2011 AACR.

Aberrant activation of receptor tyrosine kinases (RTK) is causally linked to the pathobiological traits of glioblastoma and genesis of glioma stem-like cells (GSC), but the underlying mechanism is still unknown. Here, we show that epidermal growth factor receptor (EGFR) signaling regulates the proliferation, angiogenesis, and acquisition of GSC characteristics by inducing inhibitor of differentiation 3 (ID3) and ID3-regulated cytokines [GRO1 and interleukins (IL)-6 and 8] induction. We found that EGFR-mediated ID3 expression was regulated by Smad5, which was directly phosphorylated by AKT. Furthermore, ID3 alone imparted GSC features to primary astrocytes derived from Ink4a/Arf-deficient mouse, and EGFR-ID3-IL-6 signaling axis gave rise to tumor cell heterogeneity. Conversely, EGFR inhibitors suppressed EGFR-AKT-Smad5-driven induction of ID3, which led to a decrease in the tumorsphere forming ability of GSCs and U87MG cells that possess an active mutant EGFR, EGFRvIII, without obvious cytotoxic effects. However, these cells seemed to regain colonogenic ability after removal of the EGFR inhibitors. Together, the results delineate a novel integrative molecular mechanism in which the RTK-ID signaling pathway governs genesis and maintenance of GBM histopathologic features, such as GSCsbased tumor initiation, progression, and angiogenesis. Cancer Res; 71(22); 7125-34. Ó2011 AACR.

Cellular origins and genetic factors governing the genesis and maintenance of glioblastomas (GBM) are not well understood. Here, we report a pathogenetic role of the developmental regulator Id4 (inhibitor of differentiation 4) in GBM. In primary murine Ink4a/Arf −/− astrocytes, and human glioma cells, we provide evidence that enforced Id4 can drive malignant transformation by stimulating increased cyclin E to produce a hyperproliferative profile and by increased Jagged1 expression with Notch1 activation to drive astrocytes into a neural stem-like cell state. Thus, Id4 plays an integral role in the transformation of astrocytes via its combined actions on two-key cell cycle and differentiation regulatory molecules.Supplemental material is available at http://www.genesdev.org. The self-renewal, differentiative, and proliferative states of tissue stem and progenitor cells are maintained by a variety of well-defined cell fate determinant factors (Keller 2005). Disregulation of such key developmental regulators and acquisition and maintenance of an immature differentiation phenotype are common themes across a broad spectrum of cancer types, including primary brain cancers (Garraway and Sellers 2006). Indeed, a growing body of evidence supports the view that developmental regulators play direct roles in driving aspiring cancer cells toward a malignant phenotype, one that is endowed with stem-like cell properties including robust renewal potential (Beachy et al. 2004). This cellular phenotype, embodied in the cancer stem cell paradigm (Pardal et al. 2003), has been well documented in the highly malignant brain cancer, primary glioblastoma multiforme (GBM) (Galli et al. 2004;Singh et al. 2004). The cellular origins and specific genetic elements involved in the genesis and maintenance of these so-called brain cancer stem cells (BCSCs) remain areas of intensive investigation. Results and Discussion Id4 is overexpressed human GBM, and can drive transformation of murine Ink4a/Arf −/− astrocytesGiven the biological significance of stem cell fate determinants in the regulation of normal self-renewal and differentiation, we hypothesized that the genetic or epigenetic alterations in classical cell fate determinants might cooperate with prototypical oncogenes and tumor suppressor genes to both effect malignant transformation and endow stem cell-like renewal activity in these tumors (Leung et al. 2004;Ligon et al. 2007). We were particularly focused on the expression and functional activity of inhibitor of differentiation 4 (Id4) in GBM given its prominent role in governing neural stem cell (NSC) fate decisions (Yun et al. 2004). Id4 mRNA levels were found to be increased in nine of 15 GBM samples as compared with human normal brain tissue (Supplemental Fig. S1A) and abundant Id4 mRNA and protein levels were detected in five of seven human glioma cell lines (A172, LN229, LN18, U87MG, and T98G) and an immortalized human NSC line (HB1.F3) (Cho et al. 2002), but not in the normal human astrocytes (NHA) (Supplemental Fig. S1B). Tumor ...