The PI3K/AKT/mTOR pathway has been implicated in many human cancers. However, the contribution and mechanism of mTOR in the tumor pathogenesis remains poorly understood. Over the last funding period, a wealth of genetic and biochemical studies have identified a key role of the TSC1/TSC2 gene products in the regulation of mTOR activity. Cell culture and animal models have confirmed that the loss of either one of these two tumor suppressor genes results in hyperactivation of mTOR. Consequently, patients with tuberous sclerosis (TSC) develop multiple tumor-like lesions that exhibit aberrant phenotypes in cell size (growth), proliferation, differentiation and migration. The focus of our research is to understand how the TSC/mTOR pathway affects these fundamental processes in tumor biology. To this end, we have recently identified a novel function of mTOR in regulating microtubule organization, and hence cell morphology as well as its role in protein transport. In cells lacking hamartin or tuberin, products of TSC1 and TSC2, certain plasma membrane proteins are mislocalized in the cytosol secondary to a defect in microtubule organization. Since microtubules play an important role in long-distant transport, cell polarity, migration and cytokinesis, we hypothesize that mTOR may affect these processes via a microtubule-dependent mechanism. In this proposal, we will examine the functional consequences of TSC/mTOR-dependent microtubule activity pertaining to its role in protein transport and cell polarity. We hypothesize that mTOR can affect plasma membrane targeting of hexose transporters such as Glut4 and cell adhesion molecules such as E-cadherin resulting in modulation of glucose uptake and cell-cell adhesion, respectively. Further, we hypothesize that mTOR can influence the establishment of cell polarity and therefore, directional migration. Disruption of these processes in cells with mTOR activation may contribute to many aspects of cancer biology.
The mechanisms by which mTOR activation contributes to the pathogenesis of tumors remain poorly defined. The purpose of this application is to gain further insights into a novel function of mTOR that we have identified, namely, its role in the regulation of microtubule organization. Microtubules are critical to many cellular functions and have been a target of chemotherapeutic agents in cancer therapy.
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