mTORC1-activated S6K1 phosphorylates Rictor on threonine 1135 and regulates mTORC2 signaling

Institute for Research in Immunology and Cancer, Department of Pathology and Cell Biology, Faculty of Medicine, Université de Montréal, Montreal, Quebec, Canada.
Molecular and Cellular Biology (Impact Factor: 4.78). 12/2009; 30(4):908-21. DOI: 10.1128/MCB.00601-09
Source: PubMed


The mammalian target of rapamycin (mTOR) is a conserved Ser/Thr kinase that forms two functionally distinct complexes important for nutrient and growth factor signaling. While mTOR complex 1 (mTORC1) regulates mRNA translation and ribosome biogenesis, mTORC2 plays an important role in the phosphorylation and subsequent activation of Akt. Interestingly, mTORC1 negatively regulates Akt activation, but whether mTORC1 signaling directly targets mTORC2 remains unknown. Here we show that growth factors promote the phosphorylation of Rictor (rapamycin-insensitive companion of mTOR), an essential subunit of mTORC2. We found that Rictor phosphorylation requires mTORC1 activity and, more specifically, the p70 ribosomal S6 kinase 1 (S6K1). We identified several phosphorylation sites in Rictor and found that Thr1135 is directly phosphorylated by S6K1 in vitro and in vivo, in a rapamycin-sensitive manner. Phosphorylation of Rictor on Thr1135 did not affect mTORC2 assembly, kinase activity, or cellular localization. However, cells expressing a Rictor T1135A mutant were found to have increased mTORC2-dependent phosphorylation of Akt. In addition, phosphorylation of the Akt substrates FoxO1/3a and glycogen synthase kinase 3 alpha/beta (GSK3 alpha/beta) was found to be increased in these cells, indicating that S6K1-mediated phosphorylation of Rictor inhibits mTORC2 and Akt signaling. Together, our results uncover a new regulatory link between the two mTOR complexes, whereby Rictor integrates mTORC1-dependent signaling.

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Available from: Philippe P. Roux, Sep 05, 2014
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    • "mTORC2 activity appears to be regulated by shared and distinct mechanisms compared to mTORC1. For instance, while TSC1/2 can regulate both mTORC1 and mTORC2 function,5354 S6K1 has been shown to direct mTORC2 activity.5556 Importantly, mTORC2 substrates are unique from mTORC1 substrates and include: AKT, SGK1 and PKCα.25 "
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    • "Because the major role of the Rictor-containing mTORC2 complex is thought to be the phosphorylation of AKT, we hypothesized that these results were due to changes in AKT phosphorylation. Consistent with previous findings [32-34], Raptor knockdown increased AKT phosphorylation, and Rictor knockdown decreased AKT phosphorylation (Figure 5C). Therefore, the effect of mTOR containing complexes on RWPE-ERG cell migration can be explained indirectly by changes to pAKT levels, rather than by a direct role. "
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    • "Pro-survival rather than anticancer effects of Rapalogs likely results from disruption of the mTORC1-dependent negative feedback loop to mTORC2 and IRS-1/PI3K. Particularly, mTORC1-activated S6K1 phosphorylates Rictor and/or Insulin receptor substrate (IRS)-1, thus inhibiting mTORC2 and PI3K/Akt signaling, respectively [51, 52]. More recent findings indicate that mTORC1 also phosphorylates Growth Factor Receptor Bound Protein 10 (Grb10), leading to accumulation of Grb10 and negative feedback inhibition of PI3K and the Microtubule-associated protein kinase/Extracellular-signal regulated kinase (MAPK/ERK) pathway [53]. "
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