Essential role of the Hedgehog signaling pathway in human glioma-initiating cells

Recent findings have demonstrated that malignant tumors, including glioblastoma multiforme, contain cancer-initiating cells (also known as cancer stem cells), which self-renew and are malignant, with features of tissue-specific stem cells. As these cells are resistant to irradiation and anti-cancer drugs, it is important to characterize them and find targeting therapies. In this study, we established two primary human glioma cell lines from anaplastic oligodendroglioma and glioblastoma multiforme. These lines were enriched in glioma-initiating cells, as just 10 cells formed malignant glioma when injected into mouse brain. We used these cell lines to examine the roles of the Notch, Hedgehog and Wnt signaling pathways, which are involved in stem-cell maintenance and tumorigenesis, to determine which of these pathways are crucial to glioma-initiating cells and their regulation. Here we show that the Hedgehog pathway is indispensable for glioma-initiating cell proliferation and tumorigenesis; the Hedgehog signaling inhibitors prevented glioma-initiating cell proliferation, while signaling inhibitors for Notch or Wnt did not. Overexpression of Gli2 Delta C, a C-terminal-truncated form of Gli2 that antagonizes Gli transcription factor functions, blocked glioma-initiating cell proliferation in culture and tumorigenesis in vivo. Knockdown of the Gli down-stream factor Cdc2 also prevented glioma-initiating cell proliferation. Taken together, these results show that the Hedgehog -> Gli -> Cdc2 signaling cascade plays a role in the proliferation and malignancy of glioma-initiating cells. (Cancer Sci 2011; 102: 1306-1312)