Tumor and Stem Cell Biology MET Signaling Regulates Glioblastoma Stem Cells Kyeung Min Joo 1,3 , Juyoun Jin 1,2 , Eunhee Kim 5 , Kang Ho Kim 1,2 , Yonghyun Kim 1 , Bong Gu Kang 1,2 , Youn-Jung Kang 1,2 , Justin D. Lathia 5 , Kwang Ho Cheong 6 , Paul H. Song 6 , Hyunggee Kim 4 , Ho Jun Seol 1,2 , Doo-Sik Kong 2 , Jung-Il Lee 2 , Jeremy N. Rich 5 , Jeongwu Lee 5 , and Do-Hyun Nam 1,2 Abstract 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. Introduction Glioblastoma multiforme (GBM) is the most common and lethal primary brain tumor with a median survival of 14.6 months despite maximal therapy (1). As included in classifi- cation criteria for GBMs, excessive and unstable blood vessel formation and tumor necrosis associated with hypoxia are key pathologic characteristics of GBMs (2). GBMs heavily infiltrate into the neighboring brain parenchyma and are almost uni- formly resistant to standard therapeutic regimes such as irradiation and chemotherapy (3). These biologic character- istics of GBMs are major reasons of lethality and need to be targeted for therapy. Cancer stem cell hypothesis posits that a subpopulation of cancer cells is highly enriched with tumorigenic potential (3–7). Compared with bulk tumor cells, glioblastoma stem cells (GSC) survive better against irradiation and chemotherapies, thereby contributing to therapeutic resistance and tumor recurrence (8, 9). In addition, GSCs frequently reside in peri- vascular and hypoxic regions, actively promoting angiogenesis and facilitating the survival in harsh environment (10–12). However, the underlying molecular pathways that govern these processes in GSCs are poorly understood. The MET receptor tyrosine kinase regulates cell growth and motility. Hepatocyte growth factor (HGF)/scatter factor is a cognate ligand for MET signaling (13, 14). MET pathway activation induces glioma cell proliferation, survival, and migration (15, 16). We and others reported that MET over- expression is associated with poor prognosis and tumor invasiveness in GBM patients (17, 18). Large-scale genomic studies confirmed frequent MET pathway activation and genomic amplification of MET in GBMs, indicating that aberrant activation of MET is an important genetic event in GBMs (19–21). As hypoxia directly induces MET expression and HGF induces expression of VEGF in glioma cells, MET signaling may be particularly important for tumor cell survival in hypoxia and co-option with angiogenesis (15, 22, 23). In addition, it was reported that MET signaling is a key mech- anism to maintain stem cell niche in brain (24). A recent study reported that MET signaling enhances GSC populations, sug- gesting a link between MET signaling and GSCs (25). The precise roles of MET signaling operated in GSCs, however, remain unclear (25). On the basis of this background, we hypothesized that MET signaling promotes self-renewal and therapeutic resistance of GSCs. By extensive in vivo and in vitro studies using a large number of freshly isolated patient-derived GBM cells, we provide evidence suggesting that MET signaling plays critical roles in GSC maintenance, migration, and resistance to radiation. Authors' Affiliations: 1 Cancer Stem Cell Research Center, 2 Department of Neurosurgery, Samsung Medical Center and Samsung Biomedical Research Institute, Sungkyunkwan University School of Medicine; 3 Depart- ment of Anatomy, Seoul National University College of Medicine; 4 School of Life Sciences and Biotechnology, Korea University, Seoul, Korea; 5 Department of Stem Cell Biology and Regenerative Medicine, Lerner Research Institute, Cleveland Clinic, Cleveland, Ohio; and 6 Therapeutic Antibody Group, Bio Lab, Samsung Advanced Institute of Technology, Samsung Electronics, Gyeonggi-do, Korea Note: Supplementary data for this article are available at Cancer Research Online (http://cancerres.aacrjournals.org/). Corresponding Authors: Jeongwu Lee, Department of Stem Cell Biology and Regenerative Medicine, Lerner Research Institute, Cleveland Clinic, 9500 Euclid Avenue, Cleveland, OH 44195. Phone: 216-444-9834; Fax: 216-636-5454; E-mail: leej7@ccf.org; and Do-Hyun Nam, Cancer Stem Cell Research Center, Department of Neurosurgery, Samsung Medical Center and Samsung Biomedical Research Institute, Sungkyunkwan University School of Medicine, Seoul, Korea. E-mail: nsnam@skku.edu doi: 10.1158/0008-5472.CAN-11-3760 Ó2012 American Association for Cancer Research. Cancer Research Cancer Res; 72(15) August 1, 2012 3828 Downloaded from http://aacrjournals.org/cancerres/article-pdf/72/15/3828/2670787/3828.pdf by guest on 22 June 2022