Mechanisms of Ras-Induced Non-Apoptotic Cell Death in Glioblastoma

描述（由申请方提供）：拟定研究的主要目标是确定激活的Ras蛋白诱导人胶质母细胞瘤细胞中新型非凋亡性细胞死亡的分子信号传导途径。众所周知，神经胶质瘤对凋亡性死亡具有抗性。因此，这项研究的长期目标是发现可以在治疗背景下操纵的分子靶点，以激活这种类型癌症的非凋亡性死亡。这些研究是由以下观察结果引起的：胶质母细胞瘤细胞中活化的H-Ras或K-Ras的异位表达导致细胞质空泡的积累，最终破坏细胞活力。空泡与自噬体不同，可能来自晚期内体或溶酶体隔室。活性Ras在9种不同的人神经胶质瘤细胞系中产生类似的作用，并且在Ras条件性表达的稳定的胶质母细胞瘤细胞系中再现了表型。Ras的不寻常作用取决于其膜缔合，但不依赖于其对众所周知的效应物如Raf或PI3K的刺激。初步研究表明，激活的Rac1，而不是RhoA或Cdc42，可以模拟激活的Ras的作用，而显性负Rac1阻断Ras诱导的表型。这些发现导致中心假设，Ras激活Rac1依赖的效应通路在胶质母细胞瘤，造成致命的破坏内体或溶酶体形态发生。为了验证这一假设，研究将集中在四个特定的目标：（目标1）我们将定义有缺陷的细胞器和运输事件，有助于人类胶质母细胞瘤细胞的非凋亡性死亡。这将包括识别参与液泡生物发生的膜隔室和描绘囊泡运输的变化。(Aim 2）我们将评估Ras和Rac 1表达对小鼠胶质母细胞瘤异种移植物的生长和活力的影响，使用已建立的用于蛋白质条件表达的细胞系。(Aim 3）我们将鉴定激活的Ras引发空泡表型所需的特异性Ras效应子途径，重点是Ras调节的核苷酸交换因子（例如，Tiam1、RasGRF），其激活Rac1信号传导。这将涉及显性负突变体的使用，RNAi介导的基因沉默，以及Ras结合伴侣的亲和分离和蛋白质组学表征。(Aim 4）我们将确定胶质母细胞瘤中Rac 1信号通路和内-溶酶体运输机制之间的下游分子联系。这些研究将集中于Rac1与Rab GTP酶和磷酸肌醇4-磷酸5 '-激酶的相互作用，这些相互作用调节早期和晚期内吞途径中的运输。这些研究将有助于更好地了解Ras和Rac1信号通路与内溶酶体功能之间的联系，并将提供有关一种新的、知之甚少的非凋亡性死亡形式的新知识，这种形式对脑肿瘤的治疗具有潜在的临床意义。

DESCRIPTION (provided by applicant): The main goal of the proposed studies is to define the molecular signaling pathways whereby activated Ras proteins induce a novel form of non-apoptotic cell death in human glioblastoma cells. Gliomas are notoriously resistant to apoptotic death. Therefore, the long-term goal of this research is to uncover molecular targets that can be manipulated in a therapeutic context to activate non-apoptotic death in this type of cancer. These studies were prompted by the observation that ectopic expression of activated H-Ras or K-Ras in glioblastoma cells causes accumulation of cytoplasmic vacuoles that ultimately disrupt cell viability. The vacuoles are distinct from autophagosomes and may arise from late endosome or lysosome compartments. Active Ras produces similar effects in nine different human glioma cell lines, and the phenotype is recapitulated in stable glioblastoma cell lines where Ras is conditionally expressed. The unusual effects of Ras depend on its membrane association, but are independent from its stimulation of well- known effectors like Raf or PI3K. Preliminary studies indicate that activated Rac1, but not RhoA or Cdc42, can mimic the effects of activated Ras, whereas dominant-negative Rac1 blocks the Ras-induced phenotype. These findings lead to the central hypothesis that Ras activates Rac1-dependent effector pathways in glioblastoma to cause lethal disruptions of endosome or lysosome morphogenesis. To test this hypothesis, studies will focus on four specific aims: (Aim 1) We will define the defective organelles and trafficking events that contribute to non-apoptotic death in human glioblastoma cells. This will include identification of membrane compartments involved in the biogenesis of the vacuoles and delineation of alterations in vesicular trafficking. (Aim 2) We will evaluate the effects of Ras and Rac1 expression on the growth and viability of glioblastoma xenografts in mice, using established cell lines for conditional expression of the proteins. (Aim 3) We will identify the specific Ras effector pathway(s) required for activated Ras to elicit the vacuolar phenotype, focusing on Ras-regulated nucleotide exchange factors (e.g., Tiam1, RasGRF) that activate Rac1 signaling. This will involve the use of dominant-negative mutants, RNAi-mediated gene silencing, and affinity isolation and proteomic characterization of Ras binding partners. (Aim 4) We will define the downstream molecular connections between Rac1 signaling pathway(s) and the endo-lysosomal trafficking machinery in glioblastoma. These studies will focus on Rac1 interactions with Rab GTPases and phosphoinositide 4-phosphate 5'-kinases, which regulate trafficking in early and late endocytic pathways. These studies will contribute to a better understanding of connections between Ras and Rac1 signaling pathways and endo-lysosomal function, and they will provide new knowledge about a novel and poorly understood form of non-apoptotic death with potential clinical significance for treatment of brain tumors.