Critical Care Division and Meakins-Christie Laboratories, Faculty of Medicine, McGill University, Montreal, Quebec H3A 1A1, Canada.
J Biol Chem. 2012 Apr 27;287(18):14325-35. doi: 10.1074/jbc.M111.246785. Epub 2012 Mar 6.
Under conditions of reduced mitogen or nutritional substrate levels, the serine/threonine kinase target of rapamycin can augment the nuclear content of distinct transcription factors and promote the induction of stress response genes. In its latent (i.e., unphosphorylated) form, the transcription factor STAT1 regulates a subset of genes involved in immune modulation and apoptosis. Based on previous work indicating a functional relationship between mammalian target of rapamycin (mTOR) and the nuclear content of latent STAT1, we investigated the mechanism by which mTOR controls STAT1 nuclear import. By fluorescence confocal microscopy, inactivation of mTOR with rapamycin promoted the nuclear translocation of unphosphorylated STAT1, but not that of a STAT1 mutant incapable of binding its nuclear import adaptor karyopherin-α1 (KPNA1). By immunoprecipitation, KPNA1 was physically associated with mTOR and STAT1 in a complex that translocated to the nucleus in response to rapamycin. Although mTOR is not a kinase for KPNA1, the mTOR-associated phosphatase protein phosphatase 2A catalytic interacted directly with KPNA1 and regulated nuclear import of the mTOR-KPNA1 complex. KPNA1, or its interaction with STAT1, was required for the nuclear import of latent STAT1, transcriptional induction of the STAT1 gene, and caspase-3 activation under conditions of reduced mTOR activity (i.e. rapamycin, glucose starvation, serum withdrawal). Therefore, at low mitogen or nutrient levels, mTOR and protein phosphatase 2A catalytically control the constitutive nuclear import of latent STAT1 by KPNA1, which are key modulators of STAT1 expression and apoptosis.
在有丝分裂原或营养基质水平降低的情况下，雷帕霉素的丝氨酸/苏氨酸激酶靶标可以增加不同转录因子的核含量，并促进应激反应基因的诱导。在其潜伏（即未磷酸化）形式下，转录因子 STAT1 调节一组参与免疫调节和细胞凋亡的基因。基于先前的工作表明哺乳动物雷帕霉素靶标 (mTOR) 和潜伏 STAT1 的核含量之间存在功能关系，我们研究了 mTOR 控制 STAT1 核输入的机制。通过荧光共聚焦显微镜，雷帕霉素使 mTOR 失活促进了未磷酸化 STAT1 的核易位，但不能促进不能与核输入衔接蛋白 karyopherin-α1 (KPNA1) 结合的 STAT1 突变体的核易位。通过免疫沉淀，KPNA1 与 mTOR 和 STAT1 物理结合形成复合物，该复合物响应雷帕霉素而转位到核内。尽管 mTOR 不是 KPNA1 的激酶，但 mTOR 相关的磷酸酶蛋白磷酸酶 2A 催化直接与 KPNA1 相互作用，并调节 mTOR-KPNA1 复合物的核输入。在 mTOR 活性降低的情况下（即雷帕霉素、葡萄糖饥饿、血清剥夺），KPNA1 或其与 STAT1 的相互作用是潜伏 STAT1 的核输入、STAT1 基因的转录诱导和半胱天冬酶-3 激活所必需的。因此，在有丝分裂原或营养水平较低的情况下，mTOR 和蛋白磷酸酶 2A 催化通过 KPNA1 控制潜伏 STAT1 的组成型核输入，这是 STAT1 表达和细胞凋亡的关键调节剂。
