Tumor Suppressor Function of Smad4 by Ubiquitin

描述(由申请人提供)：失去对转化生长因子-β的抗增殖反应通常被认为是肿瘤进展的一个重要步骤。这项应用的长期目标是了解转化生长因子-β抗增殖信号通路的改变如何导致人类疾病中生长控制的放松调控的分子基础。这一应用的总体策略是关注肿瘤抑制基因Smad4的泛素和相扑-1修饰，以及这些修饰在调节细胞生长和肿瘤发生中的生理功能。Smad4是一种肿瘤抑制因子。它是所有SMAD依赖反应的重要共同介质，因此在转化生长因子-β超家族信号转导中发挥核心作用。Smad4通过转录依赖机制抑制细胞增殖。然而，Smad4的调控机制仍有待阐明。这一假设的统一假设是，Smad4的生物学功能受泛素/蛋白酶体途径和SUMO化途径控制。为了验证这一假说，我们将研究Smad4泛素化和SUMO化的分子机制，它们在调节转化生长因子-β信号中的功能，以及在正常细胞和癌细胞中调节细胞生长的生理结果。提出了四个具体的目标：1.确定Smad4-Skp2相互作用、Smad4泛素化和蛋白酶体介导的降解的分子机制。2.检测Skp2基因沉默对Smad4稳定性和转化生长因子-β抗增殖反应的影响。3.研究Skp2和Smad4在人类肿瘤中的相反功能。4.了解Smad4在人类肿瘤中泛素化、SUMO化和稳定性之间的相互关系。这些研究将有助于建立转化生长因子-β介导的细胞生长控制和发育过程中Smad4活性的翻译后调控的工作理论，并有助于理解Smad4在人类癌症恶性转化和进展中的作用机制。研究结果可为合理设计疾病防治新的治疗方法提供依据。

DESCRIPTION (provided by applicant): Loss of the antiproliferative responsiveness to TGF-beta is often considered as a major step in tumor progression. The long-term objective of this application is to understand the molecular basis of how alterations in TGF-beta antiproliferative signaling pathways lead to deregulation of growth control in human diseases. The general strategy of this application is to focus on the ubiquitin and SUMO-1 modifications of tumor suppressor SMAD4 and the physiological functions of these modifications in the regulation of cell growth and tumorigenesis. SMAD4 is a tumor suppressor. It is an essential common mediator for all the SMAD-dependent responses, thus playing a central role in signaling of TGF-beta superfamily. SMAD4 inhibits cell proliferation through transcription-dependent mechanisms. However, the mechanism of how SMAD4 is regulated remains to be elucidated. The unifying hypothesis of this proposal is that the biological functions of SMAD4 are controlled by ubiquitin/proteasome pathway and SUMOylation pathway. To test this hypothesis, we will study the molecular mechanisms of SMAD4 ubiquitination and SUMOylation, their functions in the regulation of TGF-beta signaling, and the physiological outcomes in cell growth regulation in normal and cancer cells. Four Specific Aims are proposed: 1. Determine the molecular mechanisms for SMAD4-SKP2 interaction, SMAD4 ubiquitination and proteasome-mediated degradation. 2. Examine the effects of SKP2 gene silencing on SMAD4 stability and TGF-beta antiproliferative responses. 3. Investigate the opposing functions of SKP2 and SMAD4 in human cancers. 4. Understand the inter-relationship among ubiquitination, SUMOylation and stability of SMAD4 in human cancers. The proposed studies should help to establish a working theory for the posttranslational regulation of SMAD4 activity during TGF-beta-mediated cell growth control and development, and to understand the mechanisms of SMAD4 actions in malignant transformation and progression of human cancers. Finally, the results may provide a foundation for the rational design of novel therapeutic approaches for disease prevention and treatment.