Shanghai Stem Cell Institute, Institutes of Medical Sciences, Shanghai Jiao Tong University School of Medicine, Shanghai 200025, China.
J Biol Chem. 2011 Dec 16;286(50):43370-82. doi: 10.1074/jbc.M111.225185. Epub 2011 Oct 19.
Embryonic stem cells (ESCs) can undergo unlimited self-renewal and retain pluripotent developmental potential. The unique characteristics of ESCs, including a distinct transcriptional network, a poised epigenetic state, and a specific cell cycle profile, distinguish them from somatic cells. However, the molecular mechanisms underlying these special properties of ESCs are not fully understood. Here, we report that nucleolin, a nucleolar protein highly expressed in undifferentiated ESCs, plays an essential role for the maintenance of ESC self-renewal. When nucleolin is knocked down by specific short hairpin RNA (shRNA), ESCs display dramatically reduced cell proliferation rate, increased cell apoptosis, and G(1) phase accumulation. Down-regulation of nucleolin also leads to evident ESC differentiation as well as decreased self-renewal ability. Interestingly, expression of pluripotency markers (Oct4 and Nanog) is unaltered in these differentiated cells. Mechanistically, depletion of nucleolin up-regulates the p53 protein level and activates the p53-dependent pathway, at least in part, via increasing p53 protein stability. Silencing of p53 rescues G(1) phase accumulation and apoptosis caused by nucleolin deficiency entirely, although it partially blocks abnormal differentiation in nucleolin-depleted ESCs. It is noteworthy that knocking down nucleolin in NIH3T3 cells affected cell survival and proliferation in a much milder way, despite the comparable silencing efficiency obtained in ESCs and NIH3T3 cells. Collectively, our data demonstrate that nucleolin is a critical regulator of ESC self-renewal and that suppression of the p53-dependent pathway is the major molecular mechanism underlying functions of nucleolin in ESCs.
胚胎干细胞（ESCs）可以无限自我更新并保持多能性发育潜能。ESCs 的独特特征，包括独特的转录网络、平衡的表观遗传状态和特定的细胞周期特征，将其与体细胞区分开来。然而，ESCs 这些特殊性质的分子机制尚未完全阐明。在这里，我们报告称，核仁蛋白 nucleolin 在未分化的 ESCs 中高度表达，对于维持 ESC 自我更新至关重要。当用特异性短发夹 RNA (shRNA) 敲低 nucleolin 时，ESCs 的细胞增殖率显著降低，细胞凋亡增加，G1 期积累。核仁蛋白 nucleolin 的下调也会导致明显的 ESC 分化以及自我更新能力下降。有趣的是，这些分化细胞中的多能性标志物（Oct4 和 Nanog）的表达没有改变。从机制上讲，nucleolin 的耗竭会上调 p53 蛋白水平并激活 p53 依赖性途径，至少部分是通过增加 p53 蛋白稳定性来实现的。沉默 p53 可完全挽救因 nucleolin 缺乏而导致的 G1 期积累和凋亡，尽管它部分阻止了 nucleolin 耗竭的 ESC 中的异常分化。值得注意的是，尽管在 ESCs 和 NIH3T3 细胞中获得的沉默效率相当，但在 NIH3T3 细胞中敲低 nucleolin 对细胞存活和增殖的影响要轻得多。总之，我们的数据表明 nucleolin 是 ESC 自我更新的关键调节因子，抑制 p53 依赖性途径是 nucleolin 在 ESCs 中发挥作用的主要分子机制。
