The growth and survival of mammalian cells is limited by the availability of extrinsic signals from other cells. This requirement for growth factors results in the cooperative, interdependent cellular growth required for tissue homeostasis. My hypothesis is that, unlike unicellular organisms, mammalian cells lack the autonomous ability to take up sufficient intracellular nutrients to support cellular growth and survival or to perform cell-type specific functions. Rather, signals mediated by growth factors may regulate the ability of cells to obtain required extracellular nutrients and in this way control cell growth and survival. Neoplastic cells, in contrast, have acquired mutations that confer growth factor-independence. Like unicellular organisms such as yeast, the growth of transformed cells is restrained only by the availability of nutrients. One common mechanism of neoplastic transformation is the activation of growth factor signaling pathways in the absence of ligand. Phosphatidylinositol 3 kinase (PI3K) is activated by multiple growth factor receptors and the activation of this signal transduction pathway is responsible for a large fraction of human cancers. The Akt proto-oncogene lies downstream of PI3K. Consistent with the model that growth factors drive nutrient uptake, Akt kinase activity stimulates nutrient uptake and metabolism, and this effect is critical for its anti-apoptotic activity. As Akt-dependent increases in nutrient uptake require the activity of the mTOR protein kinase, mTOR is also likely to play an important role in the regulation of cellular access to extracellular nutrients by the PI3K signal transduction pathway. In support of this idea, recent studies in PTEN-deleted tumors have shown that the mTOR inhibitor, rapamycin, has potent anti-proliferative effects on transformed cells. Furthermore, my preliminary experiments suggest an Akt-independent role for mTOR in increasing nutrient uptake and in oncogenic transformation. Despite ongoing clinical trials of rapamycin as an anti-neoplastic agent, the role of mTOR in cellular growth and the mechanisms by which mTOR activity is regulated are incompletely understood. Using in vitro models for growth factor withdrawal, I will attempt to: 1) determine whether the loss of nutrient transporters from the cell surface plays a direct role in growth factor withdrawal-induced death and clarify whether mTOR regulates this process, 2) define the role of mTOR in cell survival, and 3) investigate the interaction between mTOR and the PP2A phosphatase in mammalian cells.
