Inhibition of EGFR induces a c-MET-driven stem cell population in glioblastoma

Glioblastoma multiforme (GBM) is the most lethal form of primary brain tumors, characterized by highly invasive and aggressive tumors that are resistant to all current therapeutic options. GBMs are highly heterogeneous in nature and contain a small but highly tumorigenic and self-renewing population of stem or initiating cells (glioblastoma stem cells or GSCs). GSCs have been shown to contribute to tumor propagation and resistance to current therapeutic modalities. Recent studies of human GBMs have elucidated the genetic alterations common in these tumors, but much remains unknown about specific signaling pathways that regulate GSCs. Here we identify a distinct fraction of cells in a genetically engineered mouse model of EGFR-driven GBM that respond to anti-EGFR therapy by inducing high levels of c-MET expression. The MET-positive cells displayed clonogenic potential and long-term self-renewal ability in vitro and are capable of differentiating into multiple lineages. The MET-positive GBM cells are resistant to radiation and highly tumorigenic in vivo. Activation of MET signaling led to an increase in expression of the stemness transcriptional regulators Oct4, Nanog, and Klf4. Pharmacological inhibition of MET activity in GSCs prevented the activation of Oct4, Nanog, and Klf4 and potently abrogated stemness. Finally, the MET expressing cells were preferentially localized in perivascular regions of mouse tumors consistent with their function as GSCs. Together, our findings indicate that EGFR inhibition in GBM induces MET activation in GSCs, which is a functional requisite for GSCs activity and thus represents a promising therapeutic target.