Lin Xia, Liang Min, Liang Yao-Yun, Brunicardi F Charles, Feng Xin-Hua
Michael E. DeBakey Department of Surgery, Baylor College of Medicine, Houston, Texas 77030, USA.
J Biol Chem. 2003 Aug 15;278(33):31043-8. doi: 10.1074/jbc.C300112200. Epub 2003 Jun 17.
Tumor suppressor Smad4/DPC4 is a central intracellular signal transducer for transforming growth factor-beta (TGF-beta) signaling. We recently reported that transcriptional potential of Smad4 was regulated by SUMOylation in transfected HeLa cells (1), but the precise mechanism and function of Smad4 SUMOylation in TGF-beta signaling remain to be elucidated. Here, we describe the regulation of TGF-beta signaling by SUMOylation through the control of Smad4 metabolic stability and subcellular localization. We found that SUMO-1 overexpression strongly increases Smad4 levels, while inhibition of SUMOylation by small interfering RNA (siRNA)-mediated knockdown of the E2 enzyme Ubc9 reduces endogenous Smad4 levels. Concomitantly, SUMO-1 overexpression enhances and Ubc9 knockdown reduces levels of intranuclear Smad4, growth inhibitory response, as well as transcriptional responses to TGF-beta. Comparison of wild type and mutant forms of Smad4 for SUMOylation, ubiquitination, and half-life allows the conclusion that SUMO-1 modification serves to protect Smad4 from ubiquitin-dependent degradation and consequently enhances the growth inhibitory and transcriptional responses of Smad4.
肿瘤抑制因子Smad4/DPC4是转化生长因子-β（TGF-β）信号传导的核心细胞内信号转导分子。我们最近报道，在转染的HeLa细胞中，Smad4的转录潜能受SUMO化调控（1），但Smad4 SUMO化在TGF-β信号传导中的精确机制和功能仍有待阐明。在此，我们描述了通过控制Smad4的代谢稳定性和亚细胞定位，SUMO化对TGF-β信号传导的调控。我们发现，SUMO-1的过表达显著增加Smad4的水平，而通过小干扰RNA（siRNA）介导的E2酶Ubc9敲低来抑制SUMO化，则会降低内源性Smad4的水平。同时，SUMO-1的过表达增强，而Ubc9的敲低降低了核内Smad4的水平、生长抑制反应以及对TGF-β的转录反应。对Smad4野生型和突变型进行SUMO化、泛素化及半衰期的比较后得出结论，SUMO-1修饰可保护Smad4免受泛素依赖性降解，从而增强Smad4的生长抑制和转录反应。
