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

by Selleckchem | Mar 19 2023

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 (pmTORSer2448), 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.

Comments：

The mechanistic target of rapamycin (mTOR) pathway plays a critical role in the pathogenesis of glioblastoma (GBM), a deadly brain tumor characterized by the presence of cancer stem cells (CSCs) that are resistant to conventional therapies. The loss of tumor suppressor gene PTEN, which leads to aberrant activation of the mTOR pathway, is a major contributing factor to GBM tumorigenesis.

In this study, the researchers investigated the effect of inhibiting the canonical PI3K/AKT/mTOR pathways on the regulation of stem cells in GBM. They treated GBM cells with inhibitors of mTORC1 (rapamycin), ATP binding mTORC1/2 (PP242), PI3K (LY294002), and MAPK (U0126) pathways, and found that the stem cell marker nestin and activated mTOR (pmTORSer2448) were expressed in a significant number of GBM tumors, with most tumor cells co-expressing both markers.

Treatment with rapamycin and LY294002 disrupted the neurospheres, and reduced the expression of stem cell marker NANOG. Additionally, the combination of rapamycin or PP242 with the differentiating agent All-trans-retinoic acid inhibited stem cell proliferation. The researchers also tested novel small molecule inhibitors of mTORC1/2, and found that Torin1 and Torin2 suppressed the proliferation of GBM CSCs, with Torin2 being the most effective in halting self-renewal and inhibiting cell proliferation. Torin2 was also able to eradicate tumor cells.

Overall, this study highlights the potential of mTOR pathway inhibition as a strategy for targeting GBM CSCs, and suggests that Torin2 may be a promising therapeutic agent for the treatment of GBM.