PI3K/mTOR signaling pathways in medulloblastoma

Abstract

Medulloblastoma is the most common malignant brain tumor in children. Recent studies have implicated sonic hedgehog (SHH) and insulin growth factor (IGF) as important mediators in deregulated pathways, which directly inactivate tuberous sclerosis complex, leading to activation of the serine/threonine kinase, mammalian target of rapamycin (mTOR). mTOR consists of two catalytic subunits of biochemically distinct complexes called mTORC1 and mTORC2. This study aims to further elucidate the role of the mTOR pathway, in the development of medulloblastoma, and assess the use of mTOR inhibitors as novel therapeutic agents. Medulloblastoma cells treated with mTORC1 inhibitor, rapamycin, down-regulated pERK expression initially; however ERK activation was evident upon prolonged treatment. Phosphorylation of mTORC1 substrate, p70S6K at thr389 was reduced by rapamycin and pretreatment with rapamycin abrogated platelet-derived growth factor (PDGF)-induced activation of S6K, as well as that of mTORC2 substrate pAKT(Ser473). Activation of AKT was decreased at 1, 3, and 6 h of treatment, but extended treatment with rapamycin increased expression of pAKT(Ser473). Expression of cyclic dependent kinase inhibitor, P27, decreased following PDGF and increased following rapamycin treatment, suggesting their respective impact on cell proliferation via cell cycle control. Cell proliferation was increased by 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment of medulloblastoma cells, while it was suppressed following treatment with rapamycin or U0126 (MEK1/2 inhibitor). pp242, a novel combined mTORC1/2 inhibitor, and rapamycin limited proliferation by reducing the S-Phase entry as assessed by EdU incorporation, while PDGF increased EdU incorporation. pp242 reduced the number of cells entering the S-phase to a greater extent than did rapamycin. Migration of medulloblastoma cells towards fibronectin was suppressed in a time-dependent manner after rapamycin treatment. These results indicate that the mTOR pathway is involved in the pathogenesis of medulloblastoma, and that targeting this pathway may provide a strategy for therapy of medulloblastoma.