Regulation of nutrient access by growth factors and mTOR

描述(申请人提供)：哺乳动物细胞的生长和存活受到来自其他细胞的外部信号可用性的限制。这种对生长因子的要求导致了组织动态平衡所需的协同的、相互依赖的细胞生长。我的假设是，与单细胞生物体不同，哺乳动物细胞缺乏自主吸收足够的细胞内营养以支持细胞生长和生存或执行细胞类型特定功能的能力。相反，由生长因子介导的信号可能调节细胞获得所需的胞外营养的能力，并通过这种方式控制细胞的生长和存活。相比之下，肿瘤细胞获得了赋予生长因子非依赖性的突变。与酵母等单细胞生物体一样，转化细胞的生长只受营养物质供应的限制。肿瘤转化的一个常见机制是在缺乏配体的情况下激活生长因子信号通路。磷脂酰肌醇3激酶(PI3K)被多种生长因子受体激活，该信号转导通路的激活与人类肿瘤的发生密切相关。Akt原癌基因位于PI3K下游。与生长因子驱动营养吸收的模型一致，Akt激酶活性刺激营养吸收和代谢，这一作用是其抗凋亡活性的关键。由于Akt依赖的营养物质摄取的增加需要mTOR蛋白激酶的活性，mTOR也可能通过PI3K信号转导途径在调节细胞对胞外营养物质的获取中发挥重要作用。为了支持这一观点，最近对PTEN缺失肿瘤的研究表明，mTOR抑制剂雷帕霉素对转化的细胞具有强大的抗增殖作用。此外，我的初步实验表明，mTOR在增加营养吸收和致癌转化中具有不依赖Akt的作用。尽管雷帕霉素作为一种抗肿瘤药物正在进行临床试验，但mTOR在细胞生长中的作用以及mTOR活性被调控的机制尚不完全清楚。利用生长因子撤除的体外模型，我将尝试：1)确定细胞表面营养转运体的丧失是否在生长因子撤除诱导的死亡中起直接作用，并阐明mTOR是否调节这一过程；2)确定mTOR在细胞存活中的作用；3)研究哺乳动物细胞中mTOR和PP2A磷酸酶之间的相互作用。

DESCRIPTION (provided by applicant): 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.