Smad4 cooperates with lymphoid enhancer-binding factor 1/T cell-specific factor to increase c-myc expression in the absence of TGF-β signaling
Smad4 cooperates with lymphoid enhancer-binding factor 1/T cell-specific factor to increase c-myc expression in the absence of TGF-β signaling
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DOI：
10.1073/pnas.0604773103
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发表时间：
2006-12-05
影响因子：
11.1
通讯作者：
Hoffmann, F. Michael
中科院分区：
文献类型：
--
作者：
Lim, S. Kyun;Hoffmann, F. Michael
c-myc原癌基因是细胞增殖的关键调节因子，其表达在正常上皮细胞中响应于生长抑制细胞因子TGF-β而降低。Smad 4通过与Smad 3、E2 F4/5和p107在c-myc启动子上的TGF-β抑制元件（TIE）元件处形成复合物来介导TGF-β的这种抑制作用。相反，细胞增殖和c-myc表达增加响应Writ配体;这种作用是通过c-myc启动子LEF/TCF结合元件（TBE 1和TBE 2）上的淋巴增强子结合因子1/T细胞特异性因子（LEF/TCF）家族的转录因子介导的。我们报道了一种肽适配体，其设计用于抑制Smad 4和LEF/TCF之间的结合，从而降低HepG 2细胞的c-myc表达和生长速率。进一步的分析表明，在没有TGF-β的情况下，Smad 4与来自c-myc启动子的正调控元件TBE 1结合并激活c-myc启动子活性。TGF-β降低了Smad 4与阳性TBE 1 c-myc元件的结合，这与Smad 4响应TGF-β对c-myc表达的抑制作用一致。通过RNAi敲低降低Smad 4水平也降低了c-myc表达水平，并通过血清剥夺使肝细胞对细胞死亡敏感。两种不能介导TGF-β反应的肿瘤衍生突变Smad 4蛋白仍然能够与LEF 1合作激活c-myc启动子。这些结果支持了以前未报道的TGF-β独立的Smad 4与LEF/TCF合作激活c-myc表达的功能。
The c-myc protooncogene is a key regulator of cell proliferation whose expression is reduced in normal epithelial cells in response to the growth inhibitory cytokine TGF-beta. Smad4 mediates this inhibitory effect of TGF-beta by forming a complex with Smad3, E2F4/5, and p107 at the TGF-beta inhibitory element (TIE) element on the c-myc promoter. In contrast, cell proliferation and c-myc expression are increased in response to Writ ligands; this effect is mediated through the lymphoid enhancer-binding factor 1/T cell-specific factor (LEF/TCF) family of transcription factors on the c-myc promoter LEF/TCF-binding elements (TBE1 and TBE2). We report that a peptide aptamer designed to inhibit the binding between Smad4 and LEF/TCF reduced c-myc expression and the growth rate of HepG2 cells. Further analysis demonstrated that, in the absence of TGF-beta, Smad4 was bound to the positive regulatory element TBE1 from the c-myc promoter and activated c-myc promoter activity. Smad4 binding to the positive TBE1 c-myc element was reduced by TGF-beta, consistent with Smad4's inhibitory role on c-myc expression in response to TGF-beta. Reduction of Smad4 levels by RNAi knockdown also reduced c-myc expression levels and sensitized hepatocytes to cell death by serum deprivation. Two tumor-derived mutant Smad4 proteins that fail to mediate TGF-beta responses were still competent to cooperate with LEF1 to activate the c-myc promoter. These results support a previously unreported TGF-beta-independent function for Smad4 in cooperating with LEF/TCF to activate c-myc expression.