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

描述（由申请人提供）：泛素依赖的蛋白质水解确保在正确的时间，正确的位置和单向的方式关闭特定的蛋白质功能。许多细胞周期调节蛋白的降解是由两类泛素连接酶控制的：SCF （Skp1-Cull-F-box蛋白）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，每一种都有不同的F-box蛋白亚基，通过直接将底物募集到连接酶的其余部分，并最终募集到泛素结合酶，从而提供特异性。尽管有大量的F-box蛋白，但只有三种人SCF泛素连接酶(SCFskp2， SCF？Trcp和SCFFbw7)具有良好的功能和底物，其中许多与细胞周期控制有关（例如，Cdc25A, cyclin E, Emi1, p21， p27和Wee1）。考虑到细胞周期机制的关键功能，细胞周期调节因子蛋白水解的改变显然是癌细胞无限制增殖的决定性因素。值得注意的是，在三种表征的F-box蛋白中，Skp2是原癌基因的产物，Fbw7是肿瘤抑制因子，并且？至少在某些上皮组织中，Trcp有助于转化。在最初的9年里，CA76584支持了三种泛素连接酶复合物(SCFSkp2， SCF？Trcp和APC/CCdh1)通过降解调节CDKs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破坏也被揭示出来。新的初步研究表明，高水平的F-box蛋白Emi1和Emi2与人类癌症中Skp2稳定性的增加相关，并表明Emi1和Emi2作为癌蛋白起作用。基于这些结果，CA76584第三周期下提出的新目标集中在泛素连接酶/细胞周期网络的新一层解除调控及其在癌症中的参与：确定肿瘤中Emil和Emi2的表达是否解除调控，并研究癌细胞中Skp2稳定性的解除调控机制（Aim 1）；利用组织培养系统和体内实验研究Emi2对癌症发展的贡献（目的2）；研究Emi2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揭示，该团队致力于将其基础研究成果与对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.