Rheb GTPase as chemotherapeutic target for brain tumors

描述(申请人提供)：PTEN肿瘤抑制基因的缺失经常与脑肿瘤的恶性进展有关。这种脂质磷酸酶在调节PI3K/Akt信号通路中起关键作用，PI3K/Akt信号通路通过TSC2激活mTOR/S6K通路。我们最近发现TSC2肿瘤抑制因子是Ras样GTPase Rheb的GAP(GTPase激活蛋白；关闭开关)，并证明Rheb激活mTOR/S6K通路。这表明，一旦PTEN或TSC2丢失，Rheb将成为结构性激活的。事实上，我们发现在PTEN缺失的胶质母细胞瘤细胞中，PTEN重新表达后，Rheb-GTP水平降低了一半。有趣的是，Rheb是法尼化的，其生物活性被先前设计用于阻断RAS作用的法尼基转移酶抑制剂(FTI)所抑制。我们的中心假设是，PTEN缺失导致的RHEB活性失控促进了异常细胞生长，从而促进了肿瘤的进展。我们进一步提出，这种转化活性可以被FTI减弱。为了验证这一假设，在目标1中，我们将证明显性抑制Rheb突变体Rheb RNAi和FTIs可以抑制S6激酶激活和人胶质瘤细胞的增殖。在目标2中，我们将使用异种移植和颅内小鼠肿瘤模型来确定这些方法抑制肿瘤生长的能力。在目标3中，我们将使用我们建立的检测方法来确定负责启动Rheb的鸟嘌呤核苷酸交换因子(Global)。确定这一全球环境基金和调节它的途径将有助于更好地了解Rheb的细胞功能，并确定更多扰乱其活动的途径。我们还将确定与Rheb相关的GTPase Rheb2是否受到TSC2和GEF的类似调控，以及它是否以类似于Rheb的方式发挥作用。总之，这些研究将为Rheb的激活机制及其在细胞生长调节/肿瘤发生中的作用提供有价值的新信息。他们将解决FTI的非RAS作用的分子基础，并展示靶向Rheb在癌症治疗中的价值。

DESCRIPTION (provided by applicant): Loss of the PTEN tumor suppressor is frequently associated with the malignant progression of brain tumors. This lipid phosphatase plays a key role in regulating PI3K/Akt signaling which, through TSC2, activates the mTOR/S6K pathway. We recently identified the TSC2 tumor suppressor as a GAP (GTPase activating protein; off switch) for the Ras-like GTPase, Rheb, and demonstrated that Rheb activates the mTOR/ S6K pathway. This suggested that upon PTEN or TSC2 loss, Rheb will become constitutively activated. Indeed, we find Rheb-GTP levels are halved upon re-expression of PTEN in PTEN-deficient glioblastoma cells. Interestingly, Rheb is farnesylated and its biological activity is inhibited by the farnesyl transferase inhibitors (FTIs) previously designed to block Ras action. Our central hypothesis is that deregulated Rheb activity, resulting from PTEN loss, promotes abnormal cell growth that contributes to tumor progression. We further propose that this transforming activity can be attenuated by FTIs. To test this hypothesis, in Aim 1, we will demonstrate that dominant inhibitory Rheb mutants, Rheb RNAi and FTIs can inhibit S6 kinase activation and the proliferation of human glioma cells. In Aim 2, we will use xenograft and intracranial mouse tumor models to determine the ability of these approaches to inhibit tumor growth. In Aim 3, we will use our established assays to identify the guanine nucleotide exchange factor (GEF) responsible for turning Rheb on. Identification of this GEF and the pathway that regulates it will provide a better understanding of Rheb's cellular function and identify additional avenues to disrupt its activity. We will also determine if the Rheb-related GTPase, Rheb2, is similarly regulated by TSC2 and GEFs and if it functions in a similar manner to Rheb. Together these studies will provide valuable new information on the mechanisms of Rheb activation and its role in cell growth regulation/ tumorigenesis. They will address the molecular basis of non-Ras action of FTIs and demonstrate the value of targeting Rheb in cancer therapy.