Chen Ceshi, Sun Xiaodong, Ran Qimei, Wilkinson Keith D, Murphy T J, Simons Jonathan W, Dong Jin-Tang
Department of Oncology and Hematology, Winship Cancer Institute, Emory University School of Medicine, 1365-C Clifton Road, Atlanta, GA, USA.
Oncogene. 2005 May 5;24(20):3319-27. doi: 10.1038/sj.onc.1208497.
Ubiquitin-mediated proteolysis plays a central role in controlling intracellular levels of essential regulatory molecules such as p53, cyclins, myc, BRCA1, HIF-1alpha, etc. The Kruppel-like factor 5 (KLF5) transcription factor regulates biological processes involved in carcinogenesis, angiogenesis, and smooth muscle cell differentiation. In carcinogenesis, KLF5's role has been indicated by frequent genetic deletion as well as functional studies. Here we show that KLF5 is an unstable protein with a short half-life. Destruction of KLF5 was prevented by each of the proteasome-specific inhibitors tested but not by an inhibitor for trypsin-like proteases and cysteine proteases or by a lysosome inhibitor in epithelial cells. Furthermore, KLF5 underwent ubiquitination, and deletion of a 56-amino-acid sequence adjacent to a known transactivation domain of KLF5 significantly reduced its ubiquitination and degradation. Interestingly, cancer cells appeared to be more active in KLF5 degradation than untransformed epithelial cells, yet their proteasome activity was not higher. These results suggest that KLF5 protein is degraded at least in part through ubiquitination-proteasome pathway, which may have become hyperactive for KLF5 in cancer cells.
泛素介导的蛋白水解在控制细胞内关键调节分子如p53、细胞周期蛋白、myc、BRCA1、HIF-1α等的水平方面发挥着核心作用。Kruppel样因子5（KLF5）转录因子调节参与致癌、血管生成和平滑肌细胞分化的生物学过程。在致癌过程中，KLF5的作用已通过频繁的基因缺失以及功能研究得到证实。在此我们表明KLF5是一种半衰期短的不稳定蛋白。在所测试的每种蛋白酶体特异性抑制剂均可阻止KLF5的降解，但胰蛋白酶样蛋白酶和半胱氨酸蛋白酶抑制剂或上皮细胞中的溶酶体抑制剂则不能。此外，KLF5发生了泛素化，并且删除KLF5已知反式激活结构域附近的一个56个氨基酸的序列可显著降低其泛素化和降解。有趣的是，癌细胞在KLF5降解方面似乎比未转化的上皮细胞更活跃，但其蛋白酶体活性并不更高。这些结果表明，KLF5蛋白至少部分通过泛素化-蛋白酶体途径降解，这在癌细胞中可能对KLF5变得过度活跃。
