RhoB Signaling in Cancer Suppression and Cell Suicide

描述(由申请人提供)：癌细胞克服了防止不适当的细胞增殖、运动和存活的凋亡保护措施。虽然在揭示细胞凋亡的基本机制方面已经取得了长足的进步，但对表征癌细胞病理生理学特征的凋亡信号的确切缺陷却知之甚少。此外，癌症中治疗性细胞死亡的机制通常不是很好地阐明。Rho蛋白正在成为癌症进展和治疗反应的重要参与者。这些Ras超家族的小GTP酶，控制着肌动蛋白的组织，也控制着许多其他以肌动蛋白为基础的细胞过程。我们实验室的遗传学研究已经证实，抗癌基因RhoB蛋白是肿瘤转化细胞中凋亡的关键调节因子。基因敲除小鼠的研究表明，RhoB对于发育过程中的生理性细胞凋亡是必不可少的，这意味着它在应激诱导的过程中发挥作用。RhoB不是一种肿瘤抑制因子，因为杂合子小鼠不会表现出更高的癌症发病率。然而，这类小鼠在遭受致癌“打击”(例如RAS突变)后更容易形成肿瘤。因此，RhoB在癌症中是一个负修饰基因，其性质与细胞死亡信号的作用有关。初步工作表明，RhoB是在暴露于法尼基转移酶抑制剂、DNA损伤剂或紫杉醇的情况下触发转化细胞选择性凋亡所必需的。我们假设RhoB激活了一条不依赖于P53的死亡途径，该途径限制了癌症，并被临床上有效的癌症疗法所招募。我们将在一个基因敲除的小鼠模型系统中测试这一假设，该系统已经在我们的实验室中得到了表征。具体地说，我们将(1)确定RhoB缺失对小鼠肿瘤敏感性和治疗反应的影响，(2)确定RhoB在转化的小鼠细胞凋亡中的作用，(3)研究RhoB触发细胞凋亡的效应机制。我们的提案有几个创新元素。我们将建立并研究RhoB基因的功能，该基因是癌症中的候选负性修饰基因。修饰基因是人们非常感兴趣的，因为它们强烈地影响癌症的易感性、进展和治疗反应。我们认为抗癌药物可能会招募癌症修饰基因的功能，这是一个新的概念。我们将研究一种独特的依赖于RhoB和非依赖于P53的细胞死亡机制，我们已经证明这种机制是以转化状态为条件的。最后，我们将测试新的信号模型，这些模型将Rho与G2/M期细胞周期事件以及与肌动蛋白调控系统接口的假定细胞死亡调节器联系起来。这些信号通路的特征提供了一个独特的机会，以获得对癌症病理生理学和治疗反应的新见解。

DESCRIPTION (provided by applicant): Cancer cells overcome apoptotic safeguards that prevent inappropriate cell proliferation, movement, and survival. While great strides have been made in unraveling the basic mechanisms of apoptosis, much less is known about the exact defects in apoptotic signaling that characterize cancer cell pathophysiology. Furthermore, mechanisms of therapeutic cell death in cancer are generally not very well elucidated. Rho proteins are emerging as important players in cancer progression and therapeutic response. These small GTPase of the Ras superfamily, control actin organization but also many other actin-based cellular processes. Genetic investigations in our laboratory have identified the antioncogenic RhoB protein as a key regulator of apoptosis in neoplastic transformed cells. Knockout mouse studies indicate that RhoB is dispensable for physiological apoptosis during development, implying it functions in stress-induced processes. RhoB is not a tumor suppressor, because nullizygous mice do not exhibit a higher incidence of cancer. However, such mice are more prone to tumor formation after they suffer an oncogenic 'hit' (e.g. Ras mutation). Thus, RhoB is a negative modifier gene in cancer, the properties of which are linked to a role in cell death signaling. Preliminary work indicates that RhoB is required to trigger selective apoptosis of transformed cells exposed to farnesyltransferase inhibitors, DNA damaging agents, or paclitaxel. We hypothesize that RhoB activates a p53-independent death pathway that limits cancer and that is recruited by clinically effective cancer therapeutics. We will test this hypothesis in a knockout mouse model system that has been characterized in our laboratory. Specifically, we will (1) define the effect of RhoB loss on tumor susceptibility and therapeutic response in mice, (2) establish the role of RhoB in apoptosis in transformed mouse cells, and (3) investigate the effector mechanisms through which RhoB triggers apoptosis. Our proposal has several innovative elements. We will establish and investigate the function of the RhoB gene, a candidate negative modifier gene in cancer. Modifier genes are of great interest as they strongly affect cancer susceptibility, progression, and therapeutic response. Our notion that cancer drugs may recruit the function of cancer modifier genes is a novel concept. We will study a unique RhoB-dependent and p53- independent cell death mechanism that we have shown to be conditional on transformation status. Lastly, we will test new signaling models that connect Rho to G2/M phase cell cycle events and to a putative cell death regulator that interfaces with actin regulatory systems. Characterization of these signaling pathways offers a unique opportunity to obtain new insights into cancer pathophysiology and therapeutic response.