Defining the role of mTOR pathway in the regulation of stem cells of glioblastoma

Abstract

The mechanistic target of rapamycin (mTOR), a serine/threonine kinase, functions by forming two multiprotein complexes termed mTORC1 and mTORC2. Glioblastoma (GBM) is a uniformly fatal brain tumor that remains incurable partly due to the existence of untreatable cancer stem cells (CSC). The pathogenesis of GBM is largely due to the loss of the tumor suppressor gene PTEN, which is implicated in the aberrant activation of the mTOR pathway. The major cause of tumor recurrence, growth, and invasion is the presence of the unique population of CSC. Resistance to conventional therapies appears to be caused by both extensive genetic abnormalities and dysregulation of the transcription landscape. Consequently, CSCs have emerged as targets of interest in new treatment paradigms. Evidence suggests that inhibition of the mTOR pathway can also be applied to target CSCs. Here we explored the role of the mTOR pathway in the regulation of stem cells of GBM by treating them with inhibitors of canonical PI3K/AKT/mTOR pathways such as rapamycin (mTORC1 inhibitor), PP242 (ATP binding mTORC1/2 inhibitor), LY294002 (PI3K inhibitor), and MAPK inhibitor, U0126. A significant number of GBM tumors expressed stem cell marker nestin and activated mTOR (pmTOR^Ser2448), with most tumor cells co-expressing both markers. The expression of stem cell marker NANOG was suppressed following rapamycin treatment. The neurospheres were disrupted following rapamycin and LY294002 treatments. Rapamycin or PP242 along with differentiating agent All-trans-retinoic acid reduced stem cell proliferation. Treatment with novel small molecule inhibitors of mTORC1/2 demonstrated that Torin1 and Torin2 suppressed the proliferation of GBM CSC, while XL388 was less effective. Torin1 and XL388 delay the process of self-renewal as compared to controls, whereas Torin2 halted self-renewal. Torin2 was able to eradicate tumor cells. In conclusion, Torin2 effectively targeted CSCs of GBM by halting self-renewal and inhibiting cell proliferation, underscoring the use of Torin2 in the treatment of GBM.

Keywords: Glioblastoma; Stem cell; mTOR; mTORC1; mTORC2.