Arsenic trioxide sensitizes glioblastoma to a myc inhibitor

Glioblastoma multiforme (GBM) is associated with high mortality due to infiltrative growth and recurrence. Median survival of the patients is less than 15 months, increasing requirements for new therapies. We found that both arsenic trioxide and 10058F4, an inhibitor of Myc, induced differentiation of cancer stem-like cells (CSC) of GBM and that arsenic trioxide drastically enhanced the anti-proliferative effect of 10058F4 but not apoptotic effects. EGFR-driven genetically engineered GBM mouse model showed that this cooperative effect is higher in EGFRvIII-expressing INK4a/Arf^-/- neural stem cells (NSCs) than in control wild type NSCs. In addition, treatment of GBM CSC xenografts with arsenic trioxide and 10058F4 resulted in significant decrease in tumor growth and increased differentiation with concomitant decrease of proneural and mesenchymal GBM CSCs in vivo. Our study was the first to evaluate arsenic trioxide and 10058F4 interaction in GBM CSC differentiation and to assess new opportunities for arsenic trioxide and 10058F4 combination as a promising approach for future differentiation therapy of GBM.

Synergistic antiproliferative effect of imatinib and adriamycin in platelet-derived growth factor receptor-expressing osteosarcoma cells

Osteosarcoma (OS) is the most frequent primary solid malignant tumor of bone. Its prognosis remains poor in the substantial proportion of patients who do not respond to chemotherapy and novel therapeutic options are therefore needed. We previously established a mouse model that mimics the aggressive behavior of human OS. Enzyme-linked immunosorbent assay-based screening of such mouse tumor lysates identified platelet-derived growth factor–BB (PDGF-BB) as an abundant soluble factor, the gene for which was expressed dominantly in surrounding non-malignant cells of the tumor, whereas that for the cognate receptor (PDGF receptor β) was highly expressed in OS cells. Platelet-derived growth factor-BB induced activation of both MEK–ERK and phosphatidylinositol 3-kinase–protein kinase B signaling pathways and promoted survival in OS cells deprived of serum, and these effects were blocked by the PDGF receptor inhibitor imatinib. However, these actions of PDGF-BB and imatinib were mostly masked in the presence of serum. Whereas imatinib alone did not manifest an antitumor effect in mice harboring OS tumors, combined treatment with imatinib and adriamycin exerted a synergistic antiproliferative effect on OS cells in vivo. These results suggest that treatment of OS with imatinib is effective only when cell survival is dependent on PDGF signaling or when imatinib is combined with another therapeutic intervention that renders the tumor cells susceptible to imatinib action, such as by inducing cellular stress.

Abstract 4342: Crosstalk between initiating cells with different metabolism in a murine model of malignant glioma

Background

The metabolic preference of malignant glioma for glycolysis as an energy source is a potential therapeutic target. As a result of the cellular heterogeneity of these tumors, however, the relation between glycolytic preference, tumor formation, and tumor cell clonogenicity has remained unknown. To address this issue, we analyzed the metabolic profiles of isogenic glioma-initiating cells (GICs) in a mouse model.

Methods

GICs were established by overexpression of H-RasV12 in Ink4a/Arf-null neural stem cells. Subpopulations of these cells were obtained by single-cell cloning, and clones differing in extracellular acidification potential were assessed for metabolic characteristics by quantification of intra- and extracellular metabolites. Tumorigenicity was assessed by implantation of 100 cells of each subpopulation into the forebrain of wild-type mice. Tumors were examined for pathological features of glioma and expression of glycolytic enzymes.

Results

Malignant transformation of neural stem cells resulted in a shift in metabolism characterized by a significant increase in glucose uptake and lactic acid production. Clonal populations of GICs also manifested pronounced differences in their metabolic profiles. Certain GICs consumed more glucose and produced more lactate, while others had higher oxygen consumption. These differences were reflected in the levels of intracellular metabolites and they were paralleled by a differential expression of glycolytic enzymes such as hexokinase 2 and pyruvate kinase M2. GIC clones with different metabolic profiles had the same level of tumorigenic ability and the tumors formed by all types of GICs displayed the histopathological features of glioblastoma. However, the differential expression of the glycolytic enzymes was also evident in the tumors formed by each of the clones. Implantation of a mix of GICs with different metabolic profiles showed that clones with higher glycolytic ability were the major component of the tumor mass and that they provided a scaffold for the less glycolytic clones, supporting their expansion.

Conclusions

The metabolic characteristics of glioma cells appear early during malignant transformation and persist until the late stages of tumor formation. Even isogenic clones may be heterogeneous in terms of metabolic features, however, and this heterogeneity may play a role in tumor cell proliferation and survival. Our results suggest that a more detailed understanding of the metabolic profile of malignant gliomas is imperative for their effective therapeutic targeting.

Citation Format: Oltea Sampetrean, Isako Saga, Shunsuke Shibao, Jun Okubo, Satoru Osuka, Nobuyuki Onishi, Hideyuki Saya. Crosstalk between initiating cells with different metabolism in a murine model of malignant glioma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4342. doi:10.1158/1538-7445.AM2014-4342

Abstract 2041: Development and analysis of mouse brain tumor models derived from neural stem cells expressing activated ALK

Neural Stem Cells (NSCs), having self-renewal and multipotent ability, are considered one of the cells-of-origin of glioblastoma maltifome (GBM). Although NSCs can be enriched in neurosphere floating culture with serum-free media containing EGF/FGF as a classical method, neurosphere is composed of not only NSCs but also differentiated and apoptotic cells. Therefore, we utilize the method for efficient derivation of NSCs with long-term self-renewal and differentiate capacity in adherent culture. Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase first identified in a chromosomal translocation associated with anaplastic large cell lymphomas. Subsequently, new ALK translocations were found in a fraction of non-small-cell lung cancers and in other solid tumors. The function of full-length ALK is involved in neuronal cell differentiation, regeneration and synapse formation. Recently, gene amplification and mutations of full-length ALK were identified in neuroblastoma. Furthermore, it is reported that ALK and PTN, ligand of ALK, are required for maintenance of the stem cell population in GBM. Although constitutive activation of ALK signaling results in cell transformation, little is known about the tumorigenic mechanisms induced by activated ALK. We reasoned that ALK activity has a crucial role in GBM. To verify this hypothesis, human GBM stem cell lines were treated with crizotinib, ALK inhibitor. Crizotinib could suppress the proliferation of human GBM stem cell lines in dose-dependent-manner. Next, we have established a stable mouse model of brain tumor transplanting the genetically modified NSCs. Active mutant of H-RAS could transform the Ink4a/Arf KO NSCs but not WT NSCs. However, transplanting of WT NSCs transduced activated ALK could rapidly formed highly proliferative and invasive brain tumors. Histological characteristics of these tumors resembled human GBM phenotype demonstrating necrosis, perivascular cuffing and giant cell formation. Although the activated H-RAS increased expression of Ink4a in WT NSCs, but the activated ALK never changed. On the basis of these findings, we propose a specific regulation against tumor suppressor genes by activated ALK.

Citation Format: Nobuyuki Onishi, Oltea Sampetrean, Eiji Sugihara, Hideyuki Saya. Development and analysis of mouse brain tumor models derived from neural stem cells expressing activated ALK. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2041. doi:10.1158/1538-7445.AM2014-2041

Regulation of MKL1 via actin cytoskeleton dynamics drives adipocyte differentiation

Cellular differentiation is regulated through activation and repression of defined transcription factors. A hallmark of differentiation is a pronounced change in cell shape, which is determined by dynamics of the actin cytoskeleton. Here we show that regulation of the transcriptional coactivator MKL1 (megakaryoblastic leukemia 1) by actin cytoskeleton dynamics drives adipocyte differentiation mediated by peroxisome proliferator-activated receptor γ (PPARγ), a master transcriptional regulator of adipogenesis. Induction of adipocyte differentiation results in disruption of actin stress fibres through downregulation of RhoA-ROCK signalling. The consequent rapid increase in monomeric G-actin leads to the interaction of G-actin with MKL1, which prevents nuclear translocation of MKL1 and allows expression of PPARγ followed by adipogenic differentiation. Moreover, we found that MKL1 and PPARγ act in a mutually antagonistic manner in the adipocytic differentiation programme. Our findings thus provide new mechanistic insight into the relation between the dynamics of cell shape and transcriptional regulation during cellular differentiation.

IGF1 receptor signaling regulates adaptive radioprotection in glioma stem cells

Cancer stem cells (CSCs) play an important role in disease recurrence after radiation treatment as a result of intrinsic properties such as high DNA repair capability and antioxidative capacity. It is unclear, however, how CSCs further adapt to escape the toxicity of the repeated irradiation regimens used in clinical practice. Here, we have exposed a population of murine glioma stem cells (GSCs) to fractionated radiation in order to investigate the associated adaptive changes, with the ultimate goal of identifying a targetable factor that regulates acquired radioresistance. We have shown that fractionated radiation induces an increase in IGF1 secretion and a gradual upregulation of the IGF type 1 receptor (IGF1R) in GSCs. Interestingly, IGF1R upregulation exerts a dual radioprotective effect. In the resting state, continuous IGF1 stimulation ultimately induces downregulation of Akt/extracellular-signal-regulated kinases (ERK) and FoxO3a activation, which results in slower proliferation and enhanced self-renewal. In contrast, after acute radiation, the abundance of IGF1R and increased secretion of IGF1 promote a rapid shift from a latent state toward activation of Akt survival signaling, protecting GSCs from radiation toxicity. Treatment of tumors formed by the radioresistant GSCs with an IGF1R inhibitor resulted in a marked increase in radiosensitivity, suggesting that blockade of IGF1R signaling is an effective strategy to reverse radioresistance. Together, our results show that GSCs evade the damage of repeated radiation not only through innate properties but also through gradual inducement of resistance pathways and identify the dynamic regulation of GSCs by IGF1R signaling as a novel mechanism of adaptive radioprotection.

Acquired expression of NFATc1 downregulates E-cadherin and promotes cancer cell invasion

NFATc1 is a transcription factor that regulates T-cell development, osteoclastogenesis, and macrophage function. Given that T cells, osteoclasts, and macrophages in the tumor microenvironment are thought to modulate tumor progression, tumor cells may acquire NFATc1 expression through fusion with these NFATc1-expressing normal cells. We here revealed that a small proportion of tumor cells in human carcinoma specimens expressed NFATc1. To investigate the consequences of NFATc1 acquisition by tumor cells, we established A549 and MCF7 cell lines expressing a constitutively active form of NFATc1 (NFATc1CA) in an inducible manner. The expression of NFATc1CA promoted cancer cell invasion in association with changes in cell morphology. Analysis of gene expression and RNA interference experiments revealed that NFATc1CA suppressed E-cadherin expression by upregulating the transcriptional repressors Snail and Zeb1 in a manner independent of TGF-β signaling. Induced expression of NFATc1CA also downregulated E-cadherin expression and increased invasive activity in tumor xenografts in vivo. Our results thus suggest that the acquisition of NFATc1 expression contributes to tumor progression.

Targeting Aurora B to the equatorial cortex by MKlp2 is required for cytokinesis

Although Aurora B is important in cleavage furrow ingression and completion during cytokinesis, the mechanism by which kinase activity is targeted to the cleavage furrow and the molecule(s) responsible for this process have remained elusive. Here, we demonstrate that an essential mitotic kinesin MKlp2 requires myosin-II for its localization to the equatorial cortex, and this event is required to recruit Aurora B to the equatorial cortex in mammalian cells. This recruitment event is also required to promote the highly focused accumulation of active RhoA at the equatorial cortex and stable ingression of the cleavage furrow in bipolar cytokinesis. Specifically, in drug-induced monopolar cytokinesis, targeting Aurora B to the cell cortex by MKlp2 is essential for cell polarization and furrow formation. Once the furrow has formed, MKlp2 further recruits Aurora B to the growing furrow. This process together with continuous Aurora B kinase activity at the growing furrow is essential for stable furrow propagation and completion. In contrast, a MKlp2 mutant defective in binding myosin-II does not recruit Aurora B to the cell cortex and does not promote furrow formation during monopolar cytokinesis. This mutant is also defective in maintaining the ingressing furrow during bipolar cytokinesis. Together, these findings reveal that targeting Aurora B to the cell cortex (or the equatorial cortex) by MKlp2 is essential for the maintenance of the ingressing furrow for successful cytokinesis.

Twist2 functions as a tumor suppressor in murine osteosarcoma cells

The epithelial-mesenchymal transition (EMT) contributes to the malignant progression of cancer cells including acquisition of the ability to undergo metastasis. However, whereas EMT-related transcription factors (EMT-TF) are known to play an important role in the malignant progression of epithelial tumors, their role in mesenchymal tumors remains largely unknown. We show that expression of the gene for Twist2 is downregulated in human osteosarcoma and correlates inversely with tumorigenic potential in mouse osteosarcoma. Forced expression of Twist2 in highly tumorigenic murine osteosarcoma cells induced a slight inhibition of cell growth in vitro but markedly suppressed tumor formation in vivo. Conversely, knockdown of Twist2 in osteosarcoma cells with a low tumorigenic potential promoted tumor formation in vivo, suggesting that Twist2 functions as a tumor suppressor in osteosarcoma cells. Furthermore, Twist2 induced expression of fibulin-5, which has been reported as a tumor suppressor. Medium conditioned by mouse osteosarcoma cells overexpressing Twist2 inhibited expression of the MMP9 gene as well as invasion in mouse embryonic fibroblasts, and forced expression of Twist2 in osteosarcoma cells suppressed MMP9 gene expression in tumor tissue. Data from the present study suggest that Twist2 inhibits formation of a microenvironment conducive to tumor growth and thereby attenuates tumorigenesis in osteosarcoma.

Abstract 238: IGF1 receptor signaling regulates adaptive radioprotection in glioma stem cells

Background: Cancer stem cells play an important role in disease recurrence after radiation treatment as a result of intrinsic properties such as quiescence and high DNA repair capability. It is unclear, however, how cancer stem cells further adapt to escape the toxicity of the repeated irradiation regimens used in clinical practice. Here, we have exposed a population of murine glioma stem cells (GSCs) to fractionated radiation in order to investigate the associated adaptive changes, with the ultimate goal of identifying a targetable factor that regulates acquired radioresistance.

Methods: Initial tumors were formed by implantation of Ink4a/Arf -/- neural stem cells overexpressing H-RASV12 into the forebrain of wild-type mice. GSCs purified from the tumors were then grown as tumorspheres (TS), with a subgroup, TS-RR, surviving repeated radiation (12x 5Gy). The two types of cells, their subclones and allografts were compared to identify differentially expressed factors that underlie acquired radioresistance.

Results: TS-RR were more resistant than TS to further radiation, both in vitro and in vivo. Analysis of the subclones showed that even the most resistant TS subclones did not reach the radioresistance level of TS-RR, suggesting that TS-RR may have acquired radioresistance de novo during the repeated irradiation. Analysis of the molecular changes induced in TS during fractionated radiation revealed an increase in IGF1 secretion and a gradual up-regulation of the IGF type 1 receptor (IGF1R). Interestingly, IGF1R up-regulation exerted a dual radioprotective effect: in the resting state, continuous IGF1 stimulation ultimately induced down-regulation of Akt/ERK and FoxO3a activation, which resulted in slower proliferation and enhanced self-renewal. In contrast, after acute radiation, the abundance of IGF1R and increased secretion of IGF1 promoted a rapid shift from a latent state towards activation of Akt survival signaling, protecting GSCs from radiation toxicity. Treatment of tumors formed by the radioresistant GSCs with an IGF1R inhibitor resulted in a marked increase in radiosensitivity.

Conclusion: Our results show that GSCs can evade the damage of repeated radiation not only through innate properties, but also by establishing an IGF1-IGF1R autocrine trophic loop, which results in acquired resistance to radiation. Elucidation of stem-cell-specific adaptive radioprotection mechanisms and identification of targetable key factors are crucial to the refinement of radiosensitizing strategies and prevention of tumor relapse.

Citation Format: Satoru Osuka, Oltea Sampetrean, Takatsune Shimizu, Isako Saga, Nobuyuki Onishi, Eiji Sugihara, Jun Okubo, Satoshi Fujita, Shingo Takano, Akira Matsumura, Hideyuki Saya. IGF1 receptor signaling regulates adaptive radioprotection in glioma stem cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 238. doi:10.1158/1538-7445.AM2013-238

xCT inhibition depletes CD44v-expressing tumor cells that are resistant to EGFR-targeted therapy in head and neck squamous cell carcinoma

The targeting of antioxidant systems that allow stem-like cancer cells to avoid the adverse consequences of oxidative stress might be expected to improve the efficacy of cancer treatment. Here, we show that head and neck squamous cell carcinoma (HNSCC) cells that express variant isoforms of CD44 (CD44v) rely on the activity of the cystine transporter subunit xCT for control of their redox status. xCT inhibition selectively induces apoptosis in CD44v-expressing tumor cells without affecting CD44v-negative differentiated cells in the same tumor. In contrast to CD44v-expressing undifferentiated cells, CD44v-negative differentiated cells manifest EGF receptor (EGFR) activation and rely on EGFR activity for their survival. Combined treatment with inhibitors of xCT-dependent cystine transport and of EGFR resulted in a synergistic reduction of EGFR-expressing HNSCC tumor growth. Thus, xCT-targeted therapy may deplete CD44v-expressing undifferentiated HNSCC cells and concurrently sensitize the remaining differentiating cells to available treatments including EGFR-targeted therapy.

Abstract P2-04-01: Development of mouse breast cancer models based on induced cancer stem cells (iCSC)

Breast cancer is one of the leading causes of death in women worldwide. To develop novel therapeutic approaches for the refractory cases, the mouse models which recapitulate the tumor tissues biologically and pathologically similar to human breast cancer are required. Although xenograft models of established cell lines in immune-deficient mice are frequently used for preclinical experiments, such xenograft models are not sufficient because the heterogenous structure based on the microenvironment and the intrinsic characteristics of cancer cells is not correctly formed.

In this study, we have established induced cancer stem cells (iCSC) from normal mouse mammary stem/progenitor cells through minimal required genetic manipulations and generated mouse breast cancer models by inoculating the iCSCs in the mammary fat pads. Initially, we established iCSC by introducing the H-RasV12 into Ink4a/Arf-knockout mammary stem/progenitor cells and this iCSC formed tumor similar to human triple negative breast cancer in mouse. This finding suggested that two genetic events, an activation of oncogenic signal and a tumor suppressor inactivation, are required for generating the breast cancer iCSC.

Anaplastic Lymphoma Kinase (ALK) gene, which encodes a receptor tyrosine kinase, was reported to be amplified and/or overexpressed up to 86% of patients in inflammatory breast cancer, and pleiotrophin (PTN), which is a physiological ligand for ALK, was also shown to be highly expressed in about 60% of human breast cancers. Therefore, we hypothesized that ALK pathway is involved in tumorigenesis of breast cancers and, then, attempted to generate iCSC by using ALK gene. Interestingly, we found that one of the naturally occurring mutations of ALK is sufficient for generating iCSC and tumor formation in vivo without any prior tumor suppressor inactivation. The ALK-induced iCSCs developed highly aggressive breast cancers in mice. Furthermore, the tumor formation was significantly suppressed when the ALK-induced iCSCs were generated by using mammary stem/progenitor cells derived from mouse deficient in CD44 which is a CSC marker. We have recently revealed a role of CD44, in particular that of a variant isoform (CD44v), in the protection of CSCs from high levels of oxidative stress derived from both tumor cells and their microenvironment (Cancer Cell 19: 387–400, 2011; Cancer Res 72: 1438–1448, 2012; Nat Commun 3: 883, 2012). We will discuss the underlying mechanism of ALK-induced tumorigenesis and a role of CD44 in the CSC functions.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P2-04-01.

Up-regulation of Imp3 confers in vivo tumorigenicity on murine osteosarcoma cells

Osteosarcoma is a high-grade malignant bone tumor that manifests ingravescent clinical behavior. The intrinsic events that confer malignant properties on osteosarcoma cells have remained unclear, however. We previously established two lines of mouse osteosarcoma cells: AX cells, which are able to form tumors in syngeneic mice, and AXT cells, which were derived from such tumors and acquired an increased tumorigenic capacity during tumor development. We have now identified Igf2 mRNA-binding protein3 (Imp3) as a key molecule responsible for this increased tumorigenicity of AXT cells in vivo. Imp3 is consistently up-regulated in tumors formed by AX cells, and its expression in these cells was found to confer malignant properties such as anchorage-independent growth, loss of contact inhibition, and escape from anoikis in vitro. The expression level of Imp3 also appeared directly related to tumorigenic ability in vivo which is the critical determination for tumor-initiating cells. The effect of Imp3 on tumorigenicity of osteosarcoma cells did not appear to be mediated through Igf2-dependent mechanism. Our results implicate Imp3 as a key regulator of stem-like tumorigenic characteristics in osteosarcoma cells and as a potential therapeutic target for this malignancy.

Modulation of glucose metabolism by CD44 contributes to antioxidant status and drug resistance in cancer cells

An increased glycolytic flux accompanied by activation of the pentose phosphate pathway (PPP) is implicated in chemoresistance of cancer cells. In this study, we found that CD44, a cell surface marker for cancer stem cells, interacts with pyruvate kinase M2 (PKM2) and thereby enhances the glycolytic phenotype of cancer cells that are either deficient in p53 or exposed to hypoxia. CD44 ablation by RNA interference increased metabolic flux to mitochondrial respiration and concomitantly inhibited entry into glycolysis and the PPP. Such metabolic changes induced by CD44 ablation resulted in marked depletion of cellular reduced glutathione (GSH) and increased the intracellular level of reactive oxygen species in glycolytic cancer cells. Furthermore, CD44 ablation enhanced the effect of chemotherapeutic drugs in p53-deficient or hypoxic cancer cells. Taken together, our findings suggest that metabolic modulation by CD44 is a potential therapeutic target for glycolytic cancer cells that manifest drug resistance.

Fibroblast growth factor-2 is an important factor that maintains cellular immaturity and contributes to aggressiveness of osteosarcoma

Osteosarcoma is the most frequent, nonhematopoietic, primary malignant tumor of bone. Histopathologically, osteosarcoma is characterized by complex mixtures of different cell types with bone formation. The role of environmental factors in the formation of such a complicated tissue structure as osteosarcoma remains to be elucidated. Here, a newly established murine osteosarcoma model was used to clarify the roles of environmental factors such as fibroblast growth factor-2 (Fgf2) or leukemia-inhibitory factor (Lif) in the maintenance of osteosarcoma cells in an immature state. These factors were highly expressed in tumor environmental stromal cells, rather than in osteosarcoma cells, and they potently suppressed osteogenic differentiation of osteosarcoma cells in vitro and in vivo. Further investigation revealed that the hyperactivation of extracellular signal-regulated kinase (Erk)1/2 induced by these factors affected in the process of osteosarcoma differentiation. In addition, Fgf2 enhanced both proliferation and migratory activity of osteosarcoma cells and modulated the sensitivity of cells to an anticancer drug. The results of the present study suggest that the histology of osteosarcoma tumors which consist of immature tumor cells and pathologic bone formations could be generated dependent on the distribution of such environmental factors. The combined blockade of the signaling pathways of several growth factors, including Fgf2, might be useful in controlling the aggressiveness of osteosarcoma.

Establishment of a choriocarcinoma model from immortalized normal extravillous trophoblast cells transduced with HRASV12

Gestational choriocarcinoma is a malignant trophoblastic tumor. The development of novel molecular-targeted therapies is needed to reduce the toxicity of current multiagent chemotherapy and to treat successfully the chemoresistant cases. The molecular mechanisms underlying choriocarcinoma tumorigenesis remain uncharacterized, however, and appropriate choriocarcinoma animal models have not yet been developed. In this study, we established a choriocarcinoma model by inoculating mice with induced-choriocarcinoma cell-1 (iC³-1) cells, generated from HTR8/SVneo human trophoblastic cells retrovirally transduced with activated H-RAS (HRASV12). The iC³-1 cells exhibited constitutive activation of the mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) pathways and developed into lethal tumors in all inoculated mice. Histopathological analysis revealed that the tumors consisted of two distinct types of cells, reminiscent of syncytiotrophoblasts and cytotrophoblasts, as seen in the human choriocarcinoma. The tumors expressed HLA-G and cytokeratin (trophoblast markers) and hCG (a choriocarcinoma marker). Comparative analysis of gene expression profiles between iC³-1 cells and parental HTR8/SVneo cells revealed that iC³-1 cells expressed matrix metalloproteinases, epithelial-mesenchymal transition-related genes, and SOX3 at higher levels than parental trophoblastic cells. Administration of SOX3-specific short-hairpin RNA decreased SOX3 expression and attenuated the tumorigenic activity of iC³-1 cells, suggesting that SOX3 overexpression might be critically involved in the pathogenesis of choriocarcinoma. Our murine model represents a potent new tool for studying the pathogenesis and treatment of choriocarcinoma.

Abstract 409: Analysis of invasion patterns in an induced cancer stem cell model of malignant brain tumor

Background: The invasive phenotype of malignant brain tumors is a major cause for their recurrence and resistance to therapy. It has been suggested that tumor cells recapitulate the migration patterns of glial progenitors. However, it is still unclear which cells acquire a migratory potential, at which stage they acquire it and whether and how invasion patterns change during tumorigenesis and treatment. The present study aims to define the characteristics of infiltrating cells and the patterns of invasion of genetically-induced brain tumor-initiating cells (BTICs) in the syngeneic mouse adult brain.

Methods: We have established a mouse malignant brain tumor model by overexpressing RASV12 in neural stem cells/multipotent progenitor stem cells derived from the subventricular zone of mice with a homozygous deletion of the Ink4a/ARF locus. Orthotopic implantation of these BTICs into 6-week-old wild type mice resulted in formation of highly invasive, hypervascular, serially transplantable glioblastoma-like tumors with a 100% penetrance and a 5-week median survival. Fifteen mice were sacrificed at one-week intervals (n=3) and fixed brain sections were analyzed for onset and direction of cell migration. A second series of 10 mice were sacrificed at two-day intervals (n=2), and the live brains were sliced, cultured and motility and infiltration patterns were analyzed by timelapse microscopy, cell tracking and 3D reconstruction.

Results: Pathological analysis revealed that cellular migration was detectable at one week post-injection, with both movement along fiber tracts, as well as perivascular trajectories. During the later stages of tumorigenesis, invasive foci were located mostly around blood vessels and the invasive front coincided with VEGFR and HO-1 expression.

Timelapse microscopy confirmed motility of tumor cells as early as two days post- injection. Movement along blood vessels was quick and directed away from the tumor, while intraparenchymal movement was more saltatory, with repeated extension and retraction of leading processes, pausing and turning. Cells which exited the tumor early sometimes tested several routes before taking one and then exhibited a to-and-fro movement, creating paths for other cells. Motility was not affected by cell number and cells from secondary and tertiary tumors retained the infiltrative characteristics. Furthermore, co-injection of dsRed-labeled nestin-positive and GFP-labeled GFAP-positive tumor cells showed that differentiation status seems to affect infiltration patterns more than motility in itself.

Conclusion: Our results show that invasion is one of the earliest events in tumorigenesis. Moreover, the once established infiltration paths might facilitate further invasion at later stages. We therefore suggest that, to efficiently prevent recurrence, migration should be considered as a therapeutic target from the time of diagnosis.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 409.

Arsenic trioxide augments Chk2/p53-mediated apoptosis by inhibiting oncogenic Wip1 phosphatase

The oncogenic Wip1 phosphatase (PPM1D) is induced upon DNA damage in a p53-dependent manner and is required for inactivation or suppression of DNA damage-induced cell cycle checkpoint arrest and of apoptosis by dephosphorylating and inactivating phosphorylated Chk2, Chk1, and ATM kinases. It has been reported that arsenic trioxide (ATO), a potent cancer chemotherapeutic agent, in particular for acute promyelocytic leukemia, activates the Chk2/p53 pathway, leading to apoptosis. ATO is also known to activate the p38 MAPK/p53 pathway. Here we show that phosphatase activities of purified Wip1 toward phosphorylated Chk2 and p38 in vitro are inhibited by ATO in a dose-dependent manner. Furthermore, DNA damage-induced phosphorylation of Chk2 and p38 in cultured cells is suppressed by ectopic expression of Wip1, and this Wip1-mediated suppression can be restored by the presence of ATO. We also show that treatment of acute promyelocytic leukemia cells with ATO resulted in induction of phosphorylation and activation of Chk2 and p38 MAPK, which are required for ATO-induced apoptosis. Importantly, this ATO-induced activation of Chk2/p53 and p38 MAPK/p53 apoptotic pathways can be enhanced by siRNA-mediated suppression of Wip1 expression, further indicating that ATO inhibits Wip1 phosphatase in vivo. These results exemplify that Wip1 is a direct molecular target of ATO.

Chk2 kinase is required for methylglyoxal-induced G2/M cell-cycle checkpoint arrest: implication of cell-cycle checkpoint regulation in diabetic oxidative stress signaling

Methylglyoxal (MG) is a reactive endogenous metabolite that is produced from the process of degradation of triose-phosphates. Under hyperglycemic conditions the rate of MG formation increases as a result of elevated concentrations of precursors. It has been established that MG elicits oxidative stress signaling, leading to the activation of MAP kinases, p38 MAPK and JNK, yet it remains largely unknown about a role of cell-cycle checkpoint regulation in MG-induced signaling. Here, we show that checkpoint kinases, Chk1 and Chk2, as well as their upstream ATM kinase are phosphorylated and activated following MG treatment of cultured cells. This MG-induced activation of Chk1 and Chk2 were inhibited by either aminoguanidine (AG), an inhibitor of production of advanced glycation end products (AGEs) or N-acetyl-l-cysteine (NAC), an anti-oxidant in dose dependent manners, indicating that oxidative stress via AGEs is involved critically in the activation of Chk1 and Chk2 by MG. Furthermore, it was found that cell-cycle synchronized cells exhibited G(2)/M checkpoint arrest following MG treatment, and that siRNA-mediated knock-down of Chk2, but not Chk1, results in a failure of MG-induced G(2)/M arrest. Thus, the results indicate a critical role for Chk2 in MG-induced G(2)/M cell-cycle checkpoint arrest.