REGULATION OF ACTIVIN FUNCTION BY SMAD7

激活素是转化生长因子β超家族中的一员，具有多种生物学活性。激活素功能调控的研究不仅对了解激活素的生理活性具有重要意义，而且有助于进一步研究激活素功能修饰对患者有益的潜在临床应用。我们的初步研究表明，Smad7是一种新的激活素信号的细胞内调节因子。本研究的目的是确定Smad7对激活素活性的调控及其在转录水平上对Smad7表达的调控的分子机制。我们将追求以下具体目标：(1)研究Smad7调节激活素信号转导的分子机制。我们将确定Smad7与激活素I型受体ALK-4的相互作用是否是Smad7抑制活性所必需的。我们将使用Smad7和ALK-4的一系列结构突变来确定这两种蛋白质的相互作用结构域。此外，我们将通过激活素I型受体的磷酸化来研究Smad7/ALK-4相互作用的调节。(2)确定Smad7对激活素介导的抗增殖活性的调节作用。我们将使用肝细胞和前列腺癌细胞作为模型系统来确定Smad7对这些细胞中激活素的生长抑制活性的调节作用。我们将确定激活素介导的细胞周期控制机制的潜在机制。然后，我们将确定Smad7的异位表达是否能够调节激活素对这些细胞的调节作用。(3)确定激活素信号对Smad7转录的调节作用。我们将确定激活素是否上调了这些细胞中的Smad7 mRNA。如果激活素能够在转录水平上调节Smad7的表达，我们将确定Smad7的启动子的特征，并确定启动子中介导激活素反应的DNA元件。总之，这些研究将在细胞和分子水平上为Smad7对激活素功能的调节作用提供有价值的信息。

Activin is a member of the TGF-beta superfamily of cytokines and exhibits a large variety of biological activities. Studies on the regulation of activin function are not only important for understanding the physiological activities of activin, but also helpful for future research on potential clinical application when modification of activin function is beneficial to the patient. Our preliminary studies showed that Smad7 is a novel intracellular regulator of activin signaling. The objective of this proposal is to determine the molecular mechanisms underlying the regulation of activin activity by Smad7 and the regulation of Smad7 expression at the transcriptional level. We will pursue the following specific aims: (1) To characterize the molecular mechanism by which Smad7 regulates activin signaling. We will determine if the interaction of Smad7 with activin type I receptor ALK-4 is required for the inhibitory activity of Smad7. We will use a series of structural mutants of Smad7 and ALK-4 to determine the interaction domains of these two proteins. Furthermore, we will examine the regulation of Smad7/ALK-4 interaction by the phosphorylation of the activin type I receptor. (2) To determine the regulatory effect of Smad7 on activin-mediated anti-proliferative activity. We will use hepatocytes and prostate cancer cells as a model system to determine the regulatory effect of Smad7 on the growth-inhibitory activity of activin in these cells. We will determine the mechanism underlying the activin-mediated regulation of cell cycle control machinery. We will then determine if ectopic expression of Smad7 is able to modulate this regulatory role of activin on these cells. (3) To determine the regulation of Smad7 transcription by activin signaling. We will determine if Smad7 mRNA is upregulated by activin in these cells. If activin were able to regulate Smad7 expression at the transcriptional level, we will characterize the promoter of Smad7 and identify the DNA element within the promoter that mediates the activin response. In conclusion, these studies would provide valuable information on the regulatory effect Smad7 on activin function at both the cellular and molecular levels.