Runyan Constance E, Hayashida Tomoko, Hubchak Susan, Curley Jessica F, Schnaper H William
Department of Pediatrics, Northwestern University, Chicago, Illinois 60611, USA.
J Biol Chem. 2009 Sep 11;284(37):25181-9. doi: 10.1074/jbc.M109.032847. Epub 2009 Jul 20.
By inducing epithelial-to-mesenchymal transition (EMT), transforming growth factor-beta (TGF-beta) promotes cancer progression and fibrosis. Here we show that expression of the TGF-beta receptor-associated protein, SARA (Smad anchor for receptor activation), decreases within 72 h of exposure to TGF-beta and that this decline is both required and sufficient for the induction of several markers of EMT. It has been suggested recently that expression of the TGF-beta signaling mediators, Smad2 and Smad3, may have different functional effects, with Smad2 loss being more permissive for EMT progression. We find that the loss of SARA expression leads to a concomitant decrease in Smad2 expression and a disruption of Smad2-specific transcriptional activity, with no effect on Smad3 signaling or expression. Further, the effects of inducing the loss of Smad2 mimic those of the loss of SARA, enhancing expression of the EMT marker, smooth muscle alpha-actin. Smad2 mRNA levels are not affected by the loss of SARA. However, the ubiquitination of Smad2 is increased in SARA-deficient cells. We therefore examined the E3 ubiquitin ligase Smurf2 and found that although Smurf2 expression was unaltered in SARA-deficient cells, the interaction of Smad2 and Smurf2 was enhanced. These results describe a significant role for SARA in regulating cell phenotype and suggest that its effects are mediated through modification of the balance between Smad2 and Smad3 signaling. In part, this is achieved by enhancing the association of Smad2 with Smurf2, leading to Smad2 degradation.
通过诱导上皮-间质转化（EMT），转化生长因子-β（TGF-β）促进癌症进展和纤维化。在此我们表明，TGF-β受体相关蛋白SARA（受体激活的Smad锚定蛋白）的表达在暴露于TGF-β的72小时内降低，并且这种下降对于诱导几种EMT标志物既是必需的也是充分的。最近有人提出，TGF-β信号转导介质Smad2和Smad3的表达可能具有不同的功能效应，Smad2缺失对EMT进展的影响更为宽松。我们发现SARA表达的缺失导致Smad2表达随之降低以及Smad2特异性转录活性的破坏，而对Smad3信号传导或表达没有影响。此外，诱导Smad2缺失的效应模拟了SARA缺失的效应，增强了EMT标志物平滑肌α-肌动蛋白的表达。Smad2 mRNA水平不受SARA缺失的影响。然而，在SARA缺陷细胞中Smad2的泛素化增加。因此我们检测了E3泛素连接酶Smurf2，发现尽管Smurf2在SARA缺陷细胞中的表达未改变，但Smad2与Smurf2的相互作用增强。这些结果描述了SARA在调节细胞表型中的重要作用，并表明其作用是通过改变Smad2和Smad3信号传导之间的平衡来介导的。部分原因是通过增强Smad2与Smurf2的结合，导致Smad2降解来实现的。
