Transcriptional regulation of the target genes of transforming growth factor-β and it's abnormalities in disease.

我们已经确定了一个Tie/E2F元件，它在血清诱导和转化生长因子-β诱导的抑制中对c-myc的转录调控至关重要。另一个由WNT信号激活的元件TBE3也在c-myc的转录调节区中被发现。转化生长因子β激活的Smad3结合Tie/E2F元件，使p300与E2F-4解离。在TbE3上，Tcf-4在与Smad3结合时释放β-catenin。而Lef-1可同时与β-连环蛋白和Smad3结合。因此，在β-β存在的情况下，转化生长因子-连环素激活的c-myc的转录活性被阻断，而在存在Lee-1的情况下，它是耐受的。这些结果表明，结肠癌中频繁出现的Lef-1表达的增强可能会取消转化生长因子-β诱导的c-myc的抑制。转化生长因子-β信号通路不能使该突变体磷酸化。Smad2D450E可以阻断共表达的野生型Smad2的磷酸化，但不能阻断Smad3的磷酸化，Smad2D450E也不能阻断Smad3和Smad4的结合。然而，Smad2D450E阻断了Smad3与其靶DNA的结合，并抑制了Smad3反应基因的表达。因此，我们建议Smad2D450E在核转位或核内阻断Smad3的功能。我们检测了c-Ski对转化生长因子-β抑制c-myc转录的影响。C-Ski可恢复转化生长因子-β抑制的c-myc转录活性。C-Ski的这种功能不能用已知的c-Ski的分子功能来解释。我们提出了c-Ski这个新颖的角色。C-Ski被认为是通过延长非活性Smad-c-Ski复合体与靶DNA的结合来竞争活性Smad复合体与其靶DNA的结合。

We have identified a TIE/E2F element that is critical for transcriptional regulation of c-myc in both serum-induced induction and TGF-β-induced suppression. Another element TBE3 that is activated by WNT signaling is also identified in the transcriptional regulatory region of c-myc. TGF-β-activated Smad3 binds a TIE/E2F element and dissociates p300 from E2F-4. On TBE3, TCF-4 releases β-catenin when binds Smad3. However, LEF-1 can bind both β-catenin and Smad3 at the same time. Therefore, transcriptional activity of c-myc activated by β-catenin is blocked by TGF-β in the presence of TCF-4 but it is resistant in the presence of LEE-1. These results suggested that enhanced, LEF-1 expression frequently observed in colon cancer might cancel TGF-β-induced repression of c-myc.Smad2D450E mutant was previously identified in colon caner. TGF-β signaling could not phosphorylate this mutant. We have shown that Smad2D450E can block phosphorylation of co-expressed wild-type Smad2 but not of Smad3, and that binding of Smad3 and Smad4 was not blocked by Smad2D450E either. However, Smad2D450E blocked the binding of Smad3 to it's target DNA and suppressed Smad3-responsive gene expression. Therefore, Smad2D450E is suggested to block Smad3 function either in the step of nuclear translocation or in the nucleus.We have examined the effects of c-Ski on the transcriptional repression of c-myc by TGF-β. c-Ski recovered transcriptional activity of c-myc suppressed by TGF-β. This function of c-Ski is not explained by known molecular function of c-Ski. We propose the novel role of c-Ski. c-Ski is suggested to compete the binding of an active Smad complex on their target DNA by extending the binding of an inactive Smad-c-Ski complex on the target DNA.