Epiregulin enhances tumorigenicity by activating the ERK/MAPK pathway in glioblastoma

Glioma is the most common primary tumor of the central nervous system, accounting for ∼30%, and is classified by the World Health Organization (WHO) into 4 clinical grades, from I to IV. The most aggressive form of glioma is glioblastoma multiforme (GBM), with a 5-year survival rate of ∼8%.1,2 Surgical resectability is the most important prognostic factor, as effects of additional chemotherapy and radiotherapy are limited. Temozolomide (TMZ) is an alkylating agent used in the treatment of malignant gliomas, including GBM.3 In a study of 573 patients with newly diagnosed, histologically confirmed GBM randomly assigned to receive radiotherapy alone or radiotherapy plus continuous daily TMZ, statistically significant survival benefits were shown in patients given TMZ.4 However, the prognosis for most patients with GBM remains dismal, with a median survival of only 14.6 months.4 A greater understanding of the biological mechanisms for GBM oncogenesis will contribute to the development of targeted therapies that can improve patient outcome. The genome-wide analysis performed by The Cancer Genome Atlas has shown that the most frequent genetic abbreviations were identified in the signaling pathways involving receptor tyrosine kinase, phosphatidylinositol-3 kinase (PI3K), p53, and retinoblastoma protein.5 In addition, it has been suggested that GBM can be classified into 3 subgroups: proneuronal, proliferative, and mesenchymal.6 Although various targeted molecular agents have been used either as single agents or in combination therapy for GBM, few have been reported to be effective in phase II trials so far.7 Therefore, identification of new molecular targets is still of paramount importance. The cancer stem cell hypothesis proposes that tumors are driven by subpopulations of tumor cells with stem cell–like properties.8 Cancer stem cells have been isolated in multiple tumor types, including GBM. Several molecules, such as cluster of differentiation (CD )133, sex determining region Y–box 2 (Sox2), CD15, integrin-α6, and the L1 cell adhesion molecule, have been proposed as markers for cancer-initiating cells.9–16 Especially, by promoter analysis for CD133, the pathway of epidermal growth factor receptor (EGFR)/extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK)–ERK has been shown to be involved in CD133 gene expression through Ets-family transcription factors.17 Considering the evaluation for their prognostic value, none of them was proven to be clinically useful in large-scale studies. Given the heterogeneity of GBM, further investigations are necessary to identify the treatment-resistant cell population, as these occasionally overlap with cancer-initiating cell properties. It is essential to develop tailored treatments to target this population with increased tumorigenic potential.18 In this study, through single subcutaneous passage in mice, we have developed highly aggressive variants of human GBM cell lines LN443 and U373, which showed increased tumor growth, colony-forming potential, sphere-forming potential, and invasiveness compared with the parental cell lines. Using DNA microarray analysis, we identified a novel molecular mechanism for the pathogenicity of GBM and explored new therapeutic agents that can be used for this disease.