FoxO3a Functions as a Key Integrator of Cellular Signals That Control Glioblastoma Stem-like Cell Differentiation and Tumorigenicity

Glioblastoma is one of the most aggressive types of human cancer, with invariable and fatal recurrence even after multimodal intervention, for which cancer stem-like cells (CSLCs) are now being held responsible. Our recent findings indicated that combinational inhibition of phosphoinositide-3-kinase/Akt/mammalian target of rapamycin (mTOR) and mitogen-activated protein/extracellular signal-regulated kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) pathways effectively promotes the commitment of glioblastoma CSLCs to differentiation and thereby suppresses their tumorigenicity. However, the mechanism by which these two signaling pathways are coordinated to regulate differentiation and tumorigenicity remains unknown. Here, we identified FoxO3a, a common phosphorylation target for Akt and ERK, as a key transcription factor that integrates the signals from these path-ways. Combinational blockade of both the pathways caused nuclear accumulation and activation of FoxO3a more efficiently than blockade of either alone, and promoted differentiation of glioblastoma CSLCs in a FoxO3a expression-dependent manner. Furthermore, the expression of a constitutively active FoxO3a mutant lacking phosphorylation sites for both Akt and ERK was sufficient to induce differentiation and reduce tumorigenicity of glioblastoma CSLCs. These findings suggest that FoxO3a may play a pivotal role in the control of differentiation and tumorigenicity of glioblastoma CSLCs by the PI3K/Akt/mTOR and MEK/ERK signaling pathways, and also imply that developing methods targeting effective FoxO3a activation could be a potential approach to the treatment of glioblastoma. STEM CELLS 2011;29:1327-1337