Wang Chuangui, Hou Xinghua, Mohapatra Subhra, Ma Yihong, Cress W Douglas, Pledger W Jack, Chen Jiandong
Molecular Oncology Program, H. Lee Moffitt Cancer Center & Research Institute, Tampa, Florida 33612, USA.
J Biol Chem. 2005 Apr 1;280(13):12339-43. doi: 10.1074/jbc.C400536200. Epub 2005 Feb 14.
The E2F1 transcription factor is a critical regulator of cell cycle due to its ability to promote S phase entry. However, E2F1 overexpression also sensitizes cells to apoptosis and E2F1-null mice are predisposed to tumor development, suggesting that it also has properties of a growth suppressor. E2F1 transcription function is regulated by interaction with hypophosphorylated pRb. Cdk inhibitors such as p16INK4a and p27Kip1 inhibit pRb phosphorylation by the cyclin D/Cdk4 and cyclin E/Cdk2 complexes, thus keeping E2F1 in an inactive state. We found that E2F1 binds to the p27 promoter in vivo and activates p27 mRNA and protein expression. Depletion of endogenous E2F1 by siRNA causes a reduction in basal p27 expression level. Inhibition of endogenous p27 expression by siRNA increases E2F1 transcriptional activity and permits accelerated cell cycle progression by exogenous E2F1. These observations suggest that induction of p27 acts as a negative feedback mechanism for E2F1 and may also contribute to other functions of E2F1.
E2F1转录因子是细胞周期的关键调节因子，因为它能够促进细胞进入S期。然而，E2F1的过表达也会使细胞对凋亡敏感，且E2F1基因敲除的小鼠易患肿瘤，这表明它也具有生长抑制因子的特性。E2F1的转录功能通过与低磷酸化的pRb相互作用来调节。细胞周期蛋白依赖性激酶（Cdk）抑制剂，如p16INK4a和p27Kip1，可抑制细胞周期蛋白D/Cdk4和细胞周期蛋白E/Cdk2复合物对pRb的磷酸化，从而使E2F1保持无活性状态。我们发现，E2F1在体内与p27启动子结合，并激活p27的mRNA和蛋白表达。通过小干扰RNA（siRNA）耗尽内源性E2F1会导致基础p27表达水平降低。用siRNA抑制内源性p27表达会增加E2F1的转录活性，并使外源性E2F1加速细胞周期进程。这些观察结果表明，p27的诱导作为E2F1的负反馈机制，也可能有助于E2F1的其他功能。
