Molecular Oncology Programme, Centro Nacional de Investigaciones Oncológicas, Melchor Fernández Almagro 3, Madrid, Spain.
EMBO J. 2010 Mar 17;29(6):1091-104. doi: 10.1038/emboj.2010.7. Epub 2010 Feb 11.
We have used mouse embryonic fibroblasts (MEFs) devoid of Ras proteins to illustrate that they are essential for proliferation and migration, but not for survival, at least in these cells. These properties are unique to the Ras subfamily of proteins because ectopic expression of other Ras-like small GTPases, even when constitutively active, could not compensate for the absence of Ras proteins. Only constitutive activation of components of the Raf/Mek/Erk pathway was sufficient to sustain normal proliferation and migration of MEFs devoid of Ras proteins. Activation of the phosphatidylinositol 3-kinase (PI3K)/PTEN/Akt and Ral guanine exchange factor (RalGEF)/Ral pathways, either alone or in combination, failed to induce proliferation or migration of Rasless cells, although they cooperated with Raf/Mek/Erk signalling to reproduce the full response mediated by Ras signalling. In contrast to current hypotheses, Ras signalling did not induce proliferation by inducing expression of D-type Cyclins. Rasless MEFs had normal levels of Cyclin D1/Cdk4 and Cyclin E/Cdk2. However, these complexes were inactive. Inactivation of the pocket proteins or knock down of pRb relieved MEFs from their dependence on Ras signalling to proliferate.
我们使用缺乏 Ras 蛋白的鼠胚胎成纤维细胞 (MEFs) 来说明，Ras 蛋白对细胞的增殖和迁移是必需的，但对生存不是必需的，至少在这些细胞中是这样。这些特性是 Ras 蛋白亚家族所特有的，因为其他 Ras 样小 GTPase 的异位表达，即使是组成性激活的，也不能弥补 Ras 蛋白的缺失。只有 Raf/Mek/Erk 途径的组成性激活成分足以维持缺乏 Ras 蛋白的 MEFs 的正常增殖和迁移。PI3K/PTEN/Akt 和 Ral 鸟嘌呤交换因子 (RalGEF)/Ral 途径的激活，无论是单独还是联合激活，都不能诱导 Ras 缺失细胞的增殖或迁移，尽管它们与 Raf/Mek/Erk 信号通路合作，再现了由 Ras 信号通路介导的完整反应。与当前的假设相反，Ras 信号通路不是通过诱导 D 型细胞周期蛋白的表达来诱导增殖的。缺乏 Ras 的 MEFs 具有正常水平的 Cyclin D1/Cdk4 和 Cyclin E/Cdk2。然而，这些复合物是无活性的。口袋蛋白的失活或 pRb 的敲低使 MEFs 不再依赖 Ras 信号通路来增殖。
