Cyclopamine-Mediated Hedgehog Pathway Inhibition Depletes Stem-Like Cancer Cells in Glioblastoma

¹ Department of Pathology, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA
² Department of Neurosurgery, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA
³ Department of Oncology, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA
⁴ Department of Biotechnology and Biosciences, University of Milan Bicocca, Milan 20126, Italy
⁵ Istituto Nazionale Neurologico "Carlo Besta", Milan 20133, Italy
⁶ Department of Oncology, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA; Department of Neurosurgery, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA
⁷ Department of Pathology, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA; Department of Oncology, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA

^* To whom correspondence should be addressed. E-mail: ceberha{at}jhmi.edu.

	Abstract

Brain tumors can arise following deregulation of signaling pathways normally activated during brain development, and may derive from neural stem cells. Given the requirement for Hedgehog in non-neoplastic stem cells, we investigated if Hedgehog blockade could target the stem-like population in glioblastoma (GBM). We found that Gli1, a key Hedgehog pathway target, was highly expressed in 5 of 19 primary GBMs and in 4 of 7 GBM cell lines. Shh ligand was expressed in some primary tumors, and in GBM-derived neurospheres, suggesting a potential mechanism for pathway activation. Hedgehog pathway blockade by cyclopamine caused a 40-60% reduction in growth of adherent glioma lines highly expressing Gli1, but not in those lacking evidence of pathway activity. When GBM-derived neurospheres were treated with cyclopamine, then dissociated and seeded in media lacking the inhibitor, no new neurospheres formed, suggesting the clonogenic cancer stem cells had been depleted. Consistent with this hypothesis, the stem-like fraction in gliomas marked by both aldehyde dehydrogenase activity and Hoechst dye excretion (side population) was significantly reduced or eliminated by cyclopamine. In contrast, we found that radiation treatment of our GBM neurospheres increased the percentage of these stem-like cells, suggesting that this standard therapy preferentially targets better-differentiated neoplastic cells. Most importantly, viable GBM cells injected intracranially following Hedgehog blockade were no longer able to form tumors in athymic mice, indicating a cancer stem cell population critical for ongoing growth had been removed.

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