Complex role of mTOR signaling pathway in glioblastoma and its stem cells

Abstract

Mechanistic target of rapamycin (mTOR: aka mammalian target of rapamycin), a serine threonine kinase, functions by forming two multiprotein complexes designated mTORC1 and mTORC2. This signaling cascade of PI3K/AKT/mTOR is often upregulated due to frequent loss of the tumor suppressor PTEN, a phosphatase that functions antagonistically to PI3K. mTORC1 is sensitive to nutrients and mTORC2 is regulated via PI3K and growth factor signaling. Aberrant signaling of mTOR is shown to be associated with tumorigenesis of numerous malignancies including glioblastoma (GBM). mTORC1 and mTORC2 activate downstream substrates that execute cellular and metabolic functions. Experimental models have provided evidence of the existence of cancer stem cells (CSCs), also known as tumor-initiating cells within the tumor mass, that may play an active role in development, progression and reformation of GBM. In addition, presence of highly infiltrative CSCs in the peritumoral region of GBM may appear to play an important role in recurrence of disease. Since rapamycin and its analogues are less effective in treatment of GBM, the use of ATP-competitive dual inhibitors of mTORC1 and mTORC2 have been increasingly investigated. These attempt to suppress GBM growth by pharmacodynamically inhibiting phosphorylation of the mTORC1 substrates S6K Ser235/236 and 4E-BP1 Thr37/46. These inhibitors also cause down-regulation of mTORC2 substrate AKT Ser473. These reactions result in reduction of cell growth and migration. Notably, these inhibitors of mTOR also alter self-renewal and growth of CSC of GBM. The aim of this review is to reiterate the use of mTOR inhibitors in the treatment of GBM and its stem cells associated with progression and recurrence of the disease. In addition, understanding the peritumor area of GBM is a crucial means to control the recurrence of the disease.