Wei Qiou, Miskimins W Keith, Miskimins Robin
Division of Basic Biomedical Sciences, School of Medicine, University of South Dakota, Vermillion, South Dakota 57069, USA.
Mol Cell Biol. 2003 Jun;23(12):4035-45. doi: 10.1128/MCB.23.12.4035-4045.2003.
p27(Kip1) levels increase in many cells as they leave the cell cycle and begin to differentiate. The increase in p27(Kip1) levels generally precedes the expression of differentiation-specific genes. Previous studies from our laboratory showed that the overexpression of p27(Kip1) enhances myelin basic protein (MBP) promoter activity. This activation is specific to p27(Kip1). Additionally, inhibition of cyclin-dependent kinase activity alone is not sufficient to increase MBP expression. In this study, we focused on understanding how p27(Kip1) can activate gene transcription by using the MBP gene in oligodendrocytes as a model. We show that the enhancement of MBP promoter activity by p27(Kip1) is mediated by a proximal region of the MBP promoter that contains a conserved GC box binding sequence. This sequence binds transcription factors Sp1 and Sp3. Increased expression of p27(Kip1) increases the level of Sp1 promoter binding to the GC box but does not change the level of Sp3 binding. The binding of Sp1 to this element activates the MBP promoter. p27(Kip1) leads to increased Sp1 binding through a decrease in Sp1 protein turnover. Enhancement of MBP promoter activity by an increase in the level of p27(Kip1) involves a novel mechanism that is mediated through the stabilization and binding of transcription factor Sp1.
许多细胞在离开细胞周期并开始分化时，p27（Kip1）水平会升高。p27（Kip1）水平的升高通常先于分化特异性基因的表达。我们实验室之前的研究表明，p27（Kip1）的过表达会增强髓鞘碱性蛋白（MBP）启动子的活性。这种激活对p27（Kip1）具有特异性。此外，仅抑制细胞周期蛋白依赖性激酶活性不足以增加MBP的表达。在本研究中，我们以少突胶质细胞中的MBP基因为模型，着重了解p27（Kip1）如何激活基因转录。我们发现，p27（Kip1）对MBP启动子活性的增强作用是由MBP启动子的一个近端区域介导的，该区域包含一个保守的GC盒结合序列。这个序列结合转录因子Sp1和Sp3。p27（Kip1）表达的增加会提高Sp1与GC盒的启动子结合水平，但不会改变Sp3的结合水平。Sp1与该元件的结合会激活MBP启动子。p27（Kip1）通过降低Sp1蛋白的周转导致Sp1结合增加。p27（Kip1）水平的升高对MBP启动子活性的增强涉及一种新机制，该机制是通过转录因子Sp1的稳定和结合介导的。
