Mammalian Target Of Rapamycin Signaling Regulates Cell Differentiation Through Notch Pathway

The receptor tyrosine kinase (RTK)-phosphatidylinositol 3-kinase (PI3K)-AKT-mammalian target of rapamycin (mTOR) pathway plays multiple roles in regulating normal cellular functions as well as tumorigenesis. It is involved in regulating cell growth, proliferation, survival and differentiation. Components of this pathway include proto-oncogenes, such as RTKs, PI3K, AKT and mTOR, and tumor suppressors, such as PTEN and TSC1/TSC2. It is one of the most frequently altered signaling networks in tumors. However, the precise mechanisms of mTOR activation induced tumorigenesis are less clear.mTOR signaling has been shown to be essential for cell differentiation under physiological conditions, while our results indicates that hyperactive mTOR inhibits differentiation. In this study, a series of mTOR hyperactivated MEFs were applied to examine the effect of mTOR pathway on cell differentiation.Notch is a transmembrane receptor. After binding with its ligands, the intracellular domain of Notch (NICD) is cleaved and translocated to the nucleus, where it binds to the co-transactivator CSL and activates target genes. Notch signaling regulates cell differentiation, proliferation, and survival, as well as oncogenic transformation. Therefore whether there is functional interaction between mTOR and Notch in tumor development warrants investigation.First, the adipocyte and muscle differentiation models were generated by introducing PPARγ(for adipogenesis) and MyoD (for myogenesis) genes into mTOR hyperactivated MEFs as well as the wild type MEFs. Then these cells were used for differentiation induction with or without rapamycin treatment. mTOR hyperactivation was found to impaire cell differentiation.Second, the relationship between mTOR and Notch signaling was identified. Our data suggest that mTOR complex 1 (mTORC1) is a positive regulator of Notch signaling, acting through up-regulation of the expression of Notch ligand Jagged1.Next, we found augmented Notch signaling by mTOR was responsible for the poor differentiated phenotype of mTOR hyperactivated MEFs.Finally, mTOR induced tumorigenesis has been shown to be mediated by Notch signaling, as the proliferation of mTOR hyperactive cells could be inhibited by Notch inhibitor, and the tumorigenic potential of the cells with activated mTOR signaling was compromised by Notch blockage.In conclusion, we have elucidated that oncogenically activated mTOR impaired cell differentiation at least partly due to the up-regulation of Notch signaling. Inhibition of cell differentiation might be an explanation for hyperactive mTOR induced tumorigenesis. Furthermore, this study suggested that components of both RTKs-PI3K-AKT-mTOR pathway and the Jag1-Notch axis could be the potential targets for anti-cancer therapy.