Mechanism of TGF-beta Signaling Termination

TGF-􀀁信号通路在正常上皮细胞中是一条主要的生长抑制通路，但在间充质来源的细胞中却促进增殖。因此，TGF-􀀁是一把双刃剑，在肿瘤早期既是抑瘤因子，又是肿瘤侵袭转移的重要促进因子。虽然TGF-􀀁生长抑制作用的丧失是癌症的一个标志，但TGF-􀀁信号的过量与肿瘤转移、纤维化、自身免疫性和心血管疾病有关。更好地理解TGF-􀀁信号通路的激活和终止的分子机制有助于改善这些疾病的前景。我们提出的研究重点是通过耦合去磷酸化核输出步骤来终止TGF-􀀁信号传导的分子机制。作为第一步，我们最近发现PPM1A是启动TGF-􀀁信号终止步骤的关键蛋白磷酸酶。现在，我们首次证明了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TGF-􀀁在细胞核中的信号传导。为了验证这一假设，我们已经开始了生物化学和细胞生物学的研究，以确定RanBP3如何调节TGF-􀀁-mediated下游信号通路的激活和生理反应。提出了两个具体目标：1。全面了解RanBP3如何控制Smad2/3的核出口；2. 阐明RanBP3如何特异性调控TGF􀀁在正常细胞和癌细胞中的反应。本研究不仅可以深入了解TGF-􀀁和RanBP3在生理条件下的作用机制，还可以为靶向TGF-􀀁预防和治疗癌症提供宝贵的信息。

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.