Mori Shunsuke, Nada Shigeyuki, Kimura Hironobu, Tajima Shoji, Takahashi Yusuke, Kitamura Ayaka, Oneyama Chitose, Okada Masato
Department of Oncogene Research, Research Institute for Microbial Diseases, Osaka University, 3-1 Yamadaoka, Suita, Osaka, Japan.
Laboratory of Epigenetics, Institute for Protein Research, Osaka University, 3-2 Yamadaoka, Suita, Osaka, Japan.
PLoS One. 2014 Feb 18;9(2):e88891. doi: 10.1371/journal.pone.0088891. eCollection 2014.
The mechanistic target of rapamycin (mTOR) functions as a component of two large complexes, mTORC1 and mTORC2, which play crucial roles in regulating cell growth and homeostasis. However, the molecular mechanisms by which mTOR controls cell proliferation remain elusive. Here we show that the FoxO3a transcription factor is coordinately regulated by mTORC1 and mTORC2, and plays a crucial role in controlling cell proliferation. To dissect mTOR signaling, mTORC1 was specifically inactivated by depleting p18, an essential anchor of mTORC1 on lysosomes. mTORC1 inactivation caused a marked retardation of cell proliferation, which was associated with upregulation of cyclin-dependent kinase inhibitors (CDKIs). Although Akt was activated by mTORC1 inactivation, FoxO3a was upregulated via an epigenetic mechanism and hypophosphorylated at Ser314, which resulted in its nuclear accumulation. Consistently, mTORC1 inactivation induced downregulation of serum- and glucocorticoid-inducible kinase 1 (SGK1), the kinase responsible for Ser314 phosphorylation. Expression of FoxO3a mutated at Ser314 suppressed cell proliferation by inducing CDKI expression. SGK1 overexpression suppressed CDKI expression in p18-deficient cells, whereas SGK1 knockdown induced CDKI expression in wild-type cells, resulting in the suppression of cell proliferation. These results suggest that mTORC1, in coordination with mTORC2, controls cell proliferation by regulating FoxO3a gene expression and SGK1-mediated phosphorylation of FoxO3a at Ser314.
雷帕霉素的作用机制靶点（mTOR）作为两种大型复合物mTORC1和mTORC2的一个组成部分，在调节细胞生长和体内平衡中发挥着关键作用。然而，mTOR控制细胞增殖的分子机制仍不清楚。在此，我们表明叉头框蛋白O3a（FoxO3a）转录因子受mTORC1和mTORC2的协同调控，并在控制细胞增殖中起关键作用。为了剖析mTOR信号通路，通过耗尽p18（mTORC1在溶酶体上的一个必需锚定蛋白）特异性地使mTORC1失活。mTORC1失活导致细胞增殖明显迟缓，这与细胞周期蛋白依赖性激酶抑制剂（CDKIs）的上调有关。尽管Akt通过mTORC1失活而被激活，但FoxO3a通过一种表观遗传机制上调，并且在丝氨酸314位点低磷酸化，这导致其在细胞核内积累。一致地，mTORC1失活诱导血清和糖皮质激素诱导激酶1（SGK1）下调，SGK1是负责丝氨酸314磷酸化的激酶。在丝氨酸314位点突变的FoxO3a的表达通过诱导CDKI表达来抑制细胞增殖。SGK1过表达抑制p18缺陷细胞中的CDKI表达，而SGK1敲低诱导野生型细胞中的CDKI表达，从而导致细胞增殖受到抑制。这些结果表明，mTORC1与mTORC2协同作用，通过调节FoxO3a基因表达和SGK1介导的FoxO3a在丝氨酸314位点的磷酸化来控制细胞增殖。
