Mechanism of TGF-beta Signaling Termination

转化生长因子-􀀁信号通路在正常上皮细胞中是一个主要的生长抑制通路，矛盾的是，它促进了间充质来源的细胞的增殖。因此，转化生长因子-􀀁是一把双刃剑，在早期肿瘤中既作为肿瘤抑制因子，又在癌症中作为肿瘤侵袭和转移的重要促进剂。虽然转化生长因子-􀀁生长抑制作用的丧失是癌症的一个标志，但转化生长因子-􀀁信号过度与肿瘤转移、纤维化、自身免疫和心血管疾病有关。更好地了解控制转化生长因子-􀀁信号通路激活和终止的分子机制将有助于改善这些疾病的前景。我们建议的研究集中在通过偶联去磷酸化核出口步骤终止转化生长因子-􀀁信号的分子机制。作为第一步，我们最近确定PPM1a是启动转化生长因子-􀀁信号终止步骤的关键蛋白磷酸酶。现在，我们第一次证明了PPM1A去磷酸化的Smad2/3可以通过依赖于Ran结合蛋白RanBP3的途径输出到细胞核外。基于这些发现，目前提出的统一假设是PPM1a和RanBP3的联合作用终止了细胞核内的转化生长因子-􀀁信号转导。为了验证这一假设，我们已经开始了生化和细胞生物学研究，以确定RanBP3如何调节转化生长因子-􀀁介导的下游信号通路和生理反应的激活。提出了两个具体的目标：1.全面了解RanBP3是如何控制Smad2/3的核输出的；2.阐明RanBP3如何在正常细胞和癌细胞中特异性地调节转化生长因子􀀁的反应。这些研究不仅可以深入了解转化生长因子-􀀁和RanBP3在生理条件下的作用机制，而且可以为靶向转化生长因子-􀀁在肿瘤防治中的应用提供宝贵的信息。

The TGF-􀀁signaling pathway represents a major growth inhibitory pathway in normal epithelial cells, and paradoxically, it promotes proliferation in cells of mesenchymal origins. Thus, TGF-􀀁 is a double-edge sword acting as both a tumor suppressor in early tumors and as a significant promoter of tumor invasion and metastasis in carcinomas. Whilst loss of TGF-􀀁growth inhibitory actions is a hallmark in cancer, excess of TGF-􀀁signaling has been associated with tumor metastasis, fibrotic, autoimmune and cardiovascular diseases. Improving the outlook for these diseases can benefit from a better understanding of the molecular mechanisms that govern the activation and termination of TGF-􀀁signaling pathway. Our proposed research is to focus on the molecular mechanisms underlying the termination of TGF-􀀁signaling by coupled dephosphorylationnuclear export steps. As a first step, we recently identified PPM1A as a critical protein phosphatase that initiates the TGF-􀀁signal termination step. Now we demonstrated, for the first time, that the dephosphorylated Smad2/3 by PPM1A is ready to be exported out of the nucleus through a pathway dependent of Ran-binding protein RanBP3. Based on these discoveries, the unifying hypothesis of the current proposal is that the combined actions of PPM1A and RanBP3 terminate TGF-􀀁signaling in the nucleus. To test this hypothesis, we have begun biochemical and cell biological studies to determine how RanBP3 regulates TGF-􀀁-mediated activation of downstream signaling pathways and physiological responses. Two specific aims are proposed: 1. To fully understand how RanBP3 controls the nuclear export of Smad2/3; 2. To elucidate how RanBP3 specifically regulates TGF􀀁responses in normal and cancer cells. The proposed studies should not only gain insights into the mechanisms of TGF-􀀁and RanBP3 actions under physiological conditions, but also provide invaluable information on targeting TGF-􀀁in cancer prevention and treatment.