Department of Biochemistry and Molecular Biology, School of Life Sciences, University of Sussex, JMS Building, Falmer, Brighton, United Kingdom.
PLoS One. 2013 Aug 5;8(8):e71138. doi: 10.1371/journal.pone.0071138. Print 2013.
The protein kinase mammalian target of rapamycin (mTOR) regulates the phosphorylation and activity of several proteins that have the potential to control translation, including p70S6 kinase and the eIF4E binding proteins 4E-BP1 and 4E-BP2. In spite of this, in exponentially growing cells overall protein synthesis is often resistant to mTOR inhibitors. We report here that sensitivity of wild-type mouse embryonic fibroblasts (MEFs) to mTOR inhibitors can be greatly increased when the cells are subjected to the physiological stress imposed by hypertonic conditions. In contrast, protein synthesis in MEFs with a double knockout of 4E-BP1 and 4E-BP2 remains resistant to mTOR inhibitors under these conditions. Phosphorylation of p70S6 kinase and protein kinase B (Akt) is blocked by the mTOR inhibitor Ku0063794 equally well in both wild-type and 4E-BP knockout cells, under both normal and hypertonic conditions. The response of protein synthesis to hypertonic stress itself does not require the 4E-BPs. These data suggest that under certain stress conditions: (i) translation has a greater requirement for mTOR activity and (ii) there is an absolute requirement for the 4E-BPs for regulation by mTOR. Importantly, dephosphorylation of p70S6 kinase and Akt is not sufficient to affect protein synthesis acutely.
哺乳动物雷帕霉素靶蛋白（mTOR）是一种蛋白激酶，它能够调控多种蛋白的磷酸化和活性，这些蛋白在翻译调控中具有潜在作用，包括 p70S6 激酶和真核起始因子 4E 结合蛋白 4E-BP1 和 4E-BP2。尽管如此，在指数生长期的细胞中，整体蛋白合成通常对 mTOR 抑制剂具有抗性。我们在这里报告，当细胞受到高渗条件所施加的生理应激时，野生型小鼠胚胎成纤维细胞（MEFs）对 mTOR 抑制剂的敏感性可以大大提高。相比之下，在这些条件下，4E-BP1 和 4E-BP2 双敲除的 MEFs 中的蛋白合成仍然对 mTOR 抑制剂具有抗性。在正常和高渗条件下，mTOR 抑制剂 Ku0063794 同样能很好地阻断 p70S6 激酶和蛋白激酶 B（Akt）的磷酸化，在野生型和 4E-BP 敲除细胞中都是如此。蛋白合成对高渗应激本身的反应并不需要 4E-BPs。这些数据表明，在某些应激条件下：（i）翻译对 mTOR 活性有更高的需求；（ii）4E-BPs 对 mTOR 的调节是绝对必需的。重要的是，p70S6 激酶和 Akt 的去磷酸化不足以对蛋白合成产生急性影响。
