Xie Wei, Jiang Peng, Miao Lin, Zhao Ying, Zhimin Zhai, Qing Li, Zhu Wei-guo, Wu Mian
Hefei National Laboratory for Physical Sciences at Microscale and School of Life Sciences, University of Science and Technology of China Hefei, Anhui, 230027, China.
Nucleic Acids Res. 2006 Apr 14;34(7):2046-55. doi: 10.1093/nar/gkl150. Print 2006.
Deregulated expression of E2F1 not only promotes S-phase entry but also induces apoptosis. Although it has been well documented that E2F1 is able to induce p53-dependent apoptosis via raising ARF activity, the mechanism by which E2F induces p53-independent apoptosis remains unclear. Here we report that E2F1 can directly bind to and activate the promoter of Smac/DIABLO, a mitochondrial proapoptotic gene, through the E2F1-binding sites BS2 (-542 approximately -535 bp) and BS3 (-200 approximately -193 bp). BS2 and BS3 appear to be utilized in combination rather than singly by E2F1 in activation of Smac/DIABLO. Activation of BS2 and BS3 are E2F1-specific, since neither E2F2 nor E2F3 is able to activate BS2 or BS3. Using the H1299 ER-E2F1 cell line where E2F1 activity can be conditionally induced, E2F1 has been shown to upregulate the Smac/DIABLO expression at both mRNA and protein levels upon 4-hydroxytamoxifen treatment, resulting in an enhanced mitochondria-mediated apoptosis. Reversely, reducing the Smac/DIABLO expression by RNA interference significantly diminishes apoptosis induced by E2F1. These results may suggest a novel mechanism by which E2F1 promotes p53-independent apoptosis through directly regulating its downstream mitochondrial apoptosis-inducing factors, such as Smac/DIABLO.
E2F1表达失调不仅会促进细胞进入S期，还会诱导细胞凋亡。尽管已有充分证据表明E2F1能够通过提高ARF活性诱导p53依赖性细胞凋亡，但E2F1诱导p53非依赖性细胞凋亡的机制仍不清楚。在此我们报告，E2F1可通过E2F1结合位点BS2（-542至-535 bp）和BS3（-200至-193 bp）直接结合并激活线粒体促凋亡基因Smac/DIABLO的启动子。在激活Smac/DIABLO过程中，E2F1似乎是同时利用BS2和BS3，而非单独使用其中一个。BS2和BS3的激活具有E2F1特异性，因为E2F2和E2F3均无法激活BS2或BS3。利用可条件性诱导E2F1活性的H1299 ER-E2F1细胞系，已证明在4-羟基他莫昔芬处理后，E2F1在mRNA和蛋白质水平上均上调Smac/DIABLO的表达，从而导致线粒体介导的细胞凋亡增强。相反，通过RNA干扰降低Smac/DIABLO的表达可显著减少E2F1诱导的细胞凋亡。这些结果可能提示了一种新机制，即E2F1通过直接调节其下游线粒体凋亡诱导因子（如Smac/DIABLO）来促进p53非依赖性细胞凋亡。
