The evolutionarily conserved Ser-Thr kinase mTOR plays a critical role in regulating many pathophysiological processes. Functional characterization of the mTOR signaling pathways, however, has been hampered by the paucity of known substrates. We used large-scale quantitative phospho-proteomics experiments to define the signaling networks downstream of mTORC1 and mTORC2. Characterization of one mTORC1 substrate, the growth factor receptor-bound protein 10 (Grb10), showed that mTORC1-mediated phosphorylation stabilized Grb10, leading to feedback inhibition of the phosphatidylinositol-3-kinase (PI3K) and extracellular signal-regulated, mitogen-activated protein kinase (ERK-MAPK) pathways. Grb10 expression is frequently downregulated in various cancers, and loss of Grb10 and loss of the well-established tumor suppressor phosphatase PTEN appear to be mutually exclusive events, suggesting that Grb10 might be a tumor suppressor regulated by mTORC1.

Different protein complexes form on newly spliced mRNA to ensure the accuracy and efficiency of eukaryotic gene expression. For example, the exon junction complex (EJC) plays an important role in mRNA surveillance. The EJC also influences the first, or pioneer round of protein synthesis through a mechanism that is poorly understood. We show that the nutrient-, stress-, and energy-sensing checkpoint kinase, mTOR, contributes to the observed enhanced translation efficiency of spliced over nonspliced mRNAs. We demonstrate that, when activated, S6K1 is recruited to the newly synthesized mRNA by SKAR, which is deposited at the EJC during splicing, and that SKAR and S6K1 increase the translation efficiency of spliced mRNA. Thus, SKAR-mediated recruitment of activated S6K1 to newly processed mRNPs serves as a conduit between mTOR checkpoint signaling and the pioneer round of translation when cells exist in conditions supportive of protein synthesis.

The p70 ribosomal protein S6 kinase 1 (S6K1) plays a key role in cell growth and proliferation by regulating insulin sensitivity, metabolism, protein synthesis, and cell cycle. Thus, deregulation of S6K contributes to the progression of type 2 diabetes, obesity, aging, and cancer. Considering the biological and clinical importance of S6K1, a complete understanding of its regulation is critical. One of the key motifs in the activation of S6K1 is a turn motif, but its regulation is not well understood. Here we provide evidence for two mechanisms of modulating turn motif phosphorylation and S6K1 activity. First, mammalian target of rapamycin regulates turn motif phosphorylation by inhibiting its dephosphorylation. Second, we unexpectedly found that glycogen synthase kinase (GSK)-3 promotes turn motif phosphorylation. Our studies show that mammalian target of rapamycin and GSK-3 cooperate to control the activity of S6K1, an important regulator of cell proliferation and growth. Our unexpected results provide a clear rationale for the development and use of drugs targeting GSK-3 to treat diseases such as diabetes, cancer, and age-related diseases that are linked to improper regulation of S6K1. M ammalian target of rapamycin (mTOR) is a central regulator of cell proliferation and growth. mTOR integrates signals from multiple inputs such as growth factors, stress, nutrients, and energy to regulate protein synthesis, cell cycle progression, actin organization, and autophagy (1). Because of the essential roles of mTOR in cell growth and metabolism, deregulation of mTOR is prominent in the development and progression of cancer and in metabolic diseases such as diabetes and obesity. Its deregulation can be caused by overexpression and/or overactivation of upstream effectors, or deletion of negative regulators.The 40S ribosomal protein S6 kinase (S6K) is a major substrate of mTOR and is a crucial effector of mTOR signaling (2). One of the S6K isoforms, S6K1, plays important roles in cell growth, proliferation, and cell differentiation by regulating ribosome biogenesis, protein synthesis, cell cycle progression, and metabolism (3-5). Recent studies suggest that deletion of S6K1 not only increases lifespan but also reduces the incidence of agerelated pathologic processes, including bone, immune, and motor dysfunction and insulin resistance (6, 7). Because of its important role in cell growth and insulin sensitivity, aberrant activation of S6K1 plays a major role in the progression of tumors, diabetes, obesity, and aging (2, 6, 7). Therefore, understanding the mechanism of S6K1 regulation will contribute to the ongoing efforts to develop novel drugs that provide effective treatments to combat diseases that are characterized by deregulation of the S6K signaling pathway.S6K1 belongs to the AGC kinase family, a subgroup of Ser/Thr protein kinases (PKs) that are related to PKA, PKG, and PKC. This group includes S6K1, Akt, p90 ribosomal S6K (RSK), mitogen-and stress-activated protein kinase, and several members of the PKC family, wh...

The mammalian Target of Rapamycin Complex 1 (mTORC1)-signaling system plays a critical role in the maintenance of cellular homeostasis by sensing and integrating multiple extracellular and intracellular cues. Therefore, uncovering the effectors of mTORC1 signaling is pivotal to understanding its pathophysiological effects. Here we report that the transcription factor forkhead/winged helix family k1 (Foxk1) is a mediator of mTORC1-regulated gene expression. Surprisingly, Foxk1 phosphorylation is increased upon mTORC1 suppression, which elicits a 14-3-3 interaction, a reduction of DNA binding, and nuclear exclusion. Mechanistically, this occurs by mTORC1-dependent suppression of nuclear signaling by the Foxk1 kinase, Gsk3. This pathway then regulates the expression of multiple genes associated with glycolysis and downstream anabolic pathways directly modulated by Foxk1 and/or by Foxk1-regulated expression of Hif-1α. Thus, Foxk1 mediates mTORC1-driven metabolic rewiring, and it is likely to be critical for metabolic diseases where improper mTORC1 signaling plays an important role.