Protein Phosphatase 2A Negatively Regulates Eukaryotic Initiation Factor 4E Phosphorylation and eIF4F Assembly through Direct Dephosphorylation of Mnk and eIF4E
Protein Phosphatase 2A Negatively Regulates Eukaryotic Initiation Factor 4E Phosphorylation and eIF4F Assembly through Direct Dephosphorylation of Mnk and eIF4E
复制标题
DOI：
10.1593/neo.10704
复制
发表时间：
2010-10-01
期刊：
影响因子：
4.8
通讯作者：
Sun, Shi-Yong
中科院分区：
文献类型：
--
作者：
Li, Yikun;Yue, Ping;Sun, Shi-Yong
真核翻译起始因子4E （eIF4E）在人类癌症中经常过度表达，并与细胞转化、肿瘤发生和转移进展有关。众所周知，Mnks可以磷酸化eIF4E。蛋白磷酸酶2A （Protein phosphatase 2A, PP2A）作为肿瘤抑制因子发挥作用，此前有人认为它可以调节eIF4E的磷酸化。然而，PP2A如何调节eIF4E磷酸化尚未完全解决。在本研究中，我们不仅验证了PP2A在调控eIF4E磷酸化中的作用，而且揭示了这一过程的机制。使用冈田酸或PP2A小干扰RNA （siRNA）抑制PP2A会增加eIF4E的磷酸化，这可以通过Mnk抑制剂CGP57380的存在或Mnk基因的缺失来消除。因此，Mnks参与了pp2a介导的eIF4E磷酸化调控。此外，去磷酸化实验显示PP2A可以直接使Mnk1和eIF4E去磷酸化。m(7)GTP下拉实验检测到，与对照sirna转染细胞相比，PP2A sirna转染细胞中eIF4G和phospho-eIF4E增加，4EBP-1减少，表明eIF4F复合物的帽结合增加。因此，冈田酸处理或PP2A敲除增加了c-Myc和Mcl-1的水平，这是已知由帽依赖翻译机制调节的蛋白质。综上所述，我们得出结论，PP2A通过Mnk和eIF4E的去磷酸化，负调控eIF4E磷酸化和eIF4F复合物组装，从而提示PP2A发挥其肿瘤抑制功能的新机制。
The eukaryotic translation initiation factor 4E ( eIF4E) is frequently overexpressed in human cancers and is associated with cellular transformation, tumorigenesis, and metastatic progression. It is known that Mnks can phosphorylate eIF4E. Protein phosphatase 2A (PP2A) functions as a tumor suppressor, and it was previously suggested to regulate eIF4E phosphorylation. However, how PP2A regulates eIF4E phosphorylation has not been fully addressed. In this study, we have not only validated the role of PP2A in regulation of eIF4E phosphorylation but also demonstrated the mechanism underlying this process. Inhibition of PP2A using either okadaic acid or PP2A small interfering RNA (siRNA) increased eIF4E phosphorylation, which could be abolished by the presence of the Mnk inhibitor CGP57380 or deficiency of Mnk genes. Thus, Mnks are involved in PP2A-mediated regulation of eIF4E phosphorylation. Moreover, a dephosphorylation assay revealed that PP2A could directly dephosphorylate Mnk1 and eIF4E. m(7)GTP pull-down assay detected more eIF4G and phospho-eIF4E and less 4EBP-1 in PP2A siRNA-transfected cells than in control siRNA-transfected cells, indicating an increased cap binding of eIF4F complex. Accordingly, okadaic acid treatment or PP2A knockdown increased the levels of c-Myc and Mcl-1, which are proteins known to be regulated by a cap-dependent translation mechanism. Taken together, we conclude that PP2A negatively regulates eIF4E phosphorylation and eIF4F complex assembly through dephosphorylation of Mnk and eIF4E, thus suggesting a novel mechanism by which PP2A exerts its tumor-suppressive function.