Mechanism of TGF-beta Signaling Termination

TGF-β1信号通路代表正常上皮细胞中的主要生长抑制通路，并且矛盾的是，它促进间充质来源的细胞中的增殖。因此，TGF-β是一把双刃剑，既作为早期肿瘤中的肿瘤抑制因子，又作为癌中肿瘤侵袭和转移的重要促进因子。虽然TGF-β 1生长抑制作用的丧失是癌症的标志，但过量的TGF-β1信号传导与肿瘤转移、纤维化、自身免疫和心血管疾病有关。改善这些疾病的前景可以受益于更好地理解控制TGF-β1信号通路的激活和终止的分子机制。我们的研究主要集中在TGF-β1信号通路通过去磷酸化和核输出步骤终止的分子机制上。作为第一步，我们最近鉴定了PPM 1A作为启动TGF-β1信号终止步骤的关键蛋白磷酸酶。现在，我们首次证明，由PPM 1A脱磷酸化的Smad 2/3准备通过依赖于Ran结合蛋白RanBP 3的途径输出到细胞核外。基于这些发现，当前提议的统一假设是PPM 1A和RanBP 3的联合作用终止细胞核中的TGF-β1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研究，以确定RanBP 3如何调节TGF-β介导的下游信号通路和生理反应的激活。提出了两个具体目标：1。充分了解RanBP 3如何控制Smad 2/3的核出口; 2.阐明RanBP 3如何特异性调节正常和癌细胞中的TGF β1反应。这些研究不仅可以深入了解TGF-β1和RanBP 3在生理条件下的作用机制，还可以为靶向TGF-β 1在癌症预防和治疗中提供宝贵的信息。􀀁

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.