F-box proteins: deregulated cell cycle control and proteolysis in cancer

描述（由申请人提供）：泛素依赖性蛋白水解确保特定蛋白质功能在正确的时间、正确的地点以单向方式关闭。许多细胞周期调控蛋白的降解受两类泛素连接酶控制：SCF（Skp 1-Cull-F-box protein）复合物和后期促进复合物/环体（APC/C）。在人类中，有68种SCF连接酶，每种SCF连接酶的特征在于不同的F盒蛋白亚基，其通过将底物直接募集到连接酶的其余部分并最终募集到泛素缀合酶来提供特异性。尽管有大量的F盒蛋白，只有三个人SCF泛素连接酶（SCF skp 2，SCF？Trcp和SCFFbw 7）具有完善的功能和底物，其中许多参与细胞周期控制（例如，Cdc 25 A、细胞周期蛋白E、p21、p27和Wee 1）。鉴于细胞周期机制的重要功能，细胞周期调节因子的蛋白水解改变显然是癌细胞典型的无限制增殖的决定因素。值得注意的是，三个特征性的F盒蛋白，Skp 2是原癌基因的产物，Fbw 7是一种肿瘤抑制因子，和过度表达？Trcp有助于转化，至少在某些上皮组织中。在最初的9年中，CA 76584支持阐明了三种泛素连接酶复合物（SCF Skp 2，SCF？Trcp和APC/CCDh 1）通过降解调节CDK活性的癌症相关底物来控制细胞周期进程。此外，这些途径在癌症中发生的腐败也被揭示出来。新的初步研究表明，高水平的F-box蛋白F11和F12与Skp 2在人类癌症中的稳定性增加相关，并表明F11和F12作为癌蛋白发挥作用。基于这些结果，在CA 76584的第三周期下提出的新目标集中于泛素连接酶/细胞周期网络的失调及其在癌症中的参与的新层次：确定Emil和Skp 2的表达是否在肿瘤中失调，并研究癌细胞中Skp 2稳定性失调的机制（目标1）;利用组织培养系统和体内实验研究p53 2对癌症发展的贡献（目的2）;研究p53 2在癌细胞中的细胞周期功能并鉴定其生物学意义的底物（目的3）。随着泛素介导的细胞周期调节因子的蛋白水解机制的解开，该团队致力于将其基础研究结果与恶性转化的理解相结合。

DESCRIPTION (provided by applicant): Ubiquitin-dependent proteolysis ensures that specific protein functions are turned off at the right time, in the right place and in a unidirectional fashion. The degradation of many cell cycle regulatory proteins is controlled by two classes of ubiquitin ligases: the SCF (Skp1-Cull-F-box protein) complexes and the Anaphase Promoting Complex/Cyclosome (APC/C). In humans there are sixty-eight SCF ligases, each characterized by a different F-box protein subunit that provides specificity by directly recruiting the substrate to the rest of the ligase and, ultimately, to the ubiquitin conjugating enzyme. Despite the large number of F-box proteins, only three human SCF ubiquitin ligases (SCFskp2, SCF?Trcp and SCFFbw7) have well- established functions and substrates, many of which are involved in cell cycle control (e.g., Cdc25A, cyclin E, Emi1, p21, p27 and Wee1). Given the crucial function of the cell cycle machinery, altered proteolysis of cell cycle regulators is clearly a contributing determinant of the unrestrained proliferation typical of cancer cells. Significantly, of the three characterized F-box proteins, Skp2 is the product of a proto-oncogene, Fbw7 is a tumor suppressor, and overexpression of ?Trcp contributes to transformation, at least in certain epithelial tissues. During the first nine years, CA76584 supported the elucidation of the molecular and cellular mechanisms by which three ubiquitin ligase complexes (SCFSkp2, SCF?Trcp and APC/CCdh1) control cell cycle progression through the degradation of cancer-relevant substrates that regulate the activity of CDKs. Furthermore, the corruption of these pathways occurring in cancer was revealed. Novel preliminary studies show that high levels of the F-box proteins Emi1 and Emi2 correlate with an increased stability of Skp2 in human cancers, and suggest that Emi1 and Emi2 function as oncoproteins. Based on these results, the new aims proposed under the third cycle of CA76584 are focused on a new tier of deregulation of the ubiquitin ligase/cell cycle network and its involvement in cancer: To determine whether the expression of Emil and Emi2 is deregulated in tumors and to investigate the mechanisms deregulating Skp2 stability in cancer cells (Aim 1); To study the contribution of Emi2 to cancer development using tissue culture systems and in vivo experiments (Aim 2); To study the cell cycle functions of Emi2 in cancer cells and to identify its biologically significant substrates (Aim 3). As the mechanisms of the ubiquitin-mediated proteolysis of cell cycle regulators are unraveled, this team is committed to the integration of its basic research results with an understanding of malignant transformation.