Targeted cancer therapies represent a major advance in our fight against cancer. The long-term goal of our research is to develop novel and efficacious therapeutic regimens for cancer treatment based on our mechanistic studies on cancer biology. The current application aims specifically at enhancing the efficacy of mTOR (the mammalian target of rapamycin)-targeted cancer therapy. Rapamycin and its derivatives that specifically inhibit mTOR signaling are now being actively tested either alone or in combination with other drugs in phase l-ll oncology clinical trials. Activated mTOR leads to phosphorylation of p70 S6 kinase (p70S6K) and eukaryotic translation initiation factor 4E (elF4E) binding protein 1 (4E-BP1), and the subsequently enhanced translation of mRNAs. Thus, the phosphorylation states of p70S6K and 4E-BP1 have been widely used as functional readouts for mTOR inhibitors. However, our preliminary studies have revealed an exciting new finding that inhibition of mTOR by rapamycin rapidly increases phosphorylation of Akt and elF4E while suppressing the phosphorylation of p70S6K and 4E-BP1. These induced activations of Akt and elF4E appear to counteract the action of the rapamycin-induced mTOR inhibition. Thus, these novel findings may provide new opportunities for improving the mTOR-targeted cancer therapy. Our findings lead to the following hypotheses: 1) Inhibition of mTOR activates PI3K/Akt pathway through a mechanism that may involve protein phosphatase 2A (PP2A);2) an mTOR inhibitor increases elF4E phosphorylation via PI3K/Akt-mediated mechanism;and 3) activation of PI3K/Akt and elF4E counteracts mTOR inhibitors'anticancer efficacy, whereas co-targeting PI3K/Akt/elF4E activation while suppressing mTOR will enhance mTOR-targeted cancer therapy. To test these hypotheses, we will determine how an mTOR inhibitor causes PI3K/Akt activation (specific aim 1) and increases elF4E phosphorylation (specific aim 2) while suppressing mTOR. In addition, we will test the impact of PI3K/Akt/elF4E activation on mTOR inhibitor-mediated growth inhibition of human cancer cells (specific aim 3). The accomplishment of this proposal will reveal a novel biological pathway or paradigm that mTOR negatively regulates PI3K/Akt pathway including elF4E and develop new strategies to enhance the mTOR-targeted cancer therapy with immediate clinical gain and translation significance.
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