mTORC1-Mediated Cell Proliferation, But Not Cell Growth, Controlled by the 4E-BPs

Department of Biochemistry and Goodman Cancer Research Centre, McGill University, Montreal, Quebec H3A 1A3, Canada.
Science (Impact Factor: 31.48). 05/2010; 328(5982):1172-6. DOI: 10.1126/science.1187532
Source: PubMed

ABSTRACT The mammalian target of rapamycin complex 1 (mTORC1) integrates mitogen and nutrient signals to control cell proliferation
and cell size. Hence, mTORC1 is implicated in a large number of human diseases—including diabetes, obesity, heart disease,
and cancer—that are characterized by aberrant cell growth and proliferation. Although eukaryotic translation initiation factor
4E–binding proteins (4E-BPs) are critical mediators of mTORC1 function, their precise contribution to mTORC1 signaling and
the mechanisms by which they mediate mTORC1 function have remained unclear. We inhibited the mTORC1 pathway in cells lacking
4E-BPs and analyzed the effects on cell size, cell proliferation, and cell cycle progression. Although the 4E-BPs had no effect
on cell size, they inhibited cell proliferation by selectively inhibiting the translation of messenger RNAs that encode proliferation-promoting
proteins and proteins involved in cell cycle progression. Thus, control of cell size and cell cycle progression appear to
be independent in mammalian cells, whereas in lower eukaryotes, 4E-BPs influence both cell growth and proliferation.

Download full-text


Available from: Nahum Sonenberg, Jul 05, 2015
1 Follower
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The eIF4E-binding proteins (4E-BPs) represent a diverse class of translation inhibitors that are often deregulated in cancer cells. 4E-BPs inhibit translation by competing with eIF4G for binding to eIF4E through an interface that consists of canonical and non-canonical eIF4E-binding motifs connected by a linker. The lack of high-resolution structures including the linkers, which contain phosphorylation sites, limits our understanding of how phosphorylation inhibits complex formation. Furthermore, the binding mechanism of the non-canonical motifs is poorly understood. Here, we present structures of human eIF4E bound to 4E-BP1 and fly eIF4E bound to Thor, 4E-T, and eIF4G. These structures reveal architectural elements that are unique to 4E-BPs and provide insight into the consequences of phosphorylation. Guided by these structures, we designed and crystallized a 4E-BP mimic that shows increased repressive activity. Our studies pave the way for the rational design of 4E-BP mimics as therapeutic tools to decrease translation during oncogenic transformation. Copyright © 2015 Elsevier Inc. All rights reserved.
    Molecular Cell 02/2015; 57(6). DOI:10.1016/j.molcel.2015.01.017 · 14.46 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The larval period of the Drosophila life cycle is characterized by immense growth. In nutrient rich conditions, larvae increase in mass approximately two hundred-fold in five days. However, upon nutrient deprivation, growth is arrested. The prevailing view is that dietary amino acids drive this larval growth by activating the conserved insulin/PI3 kinase and Target of rapamycin (TOR) pathways and promoting anabolic metabolism. One key anabolic process is protein synthesis. However, few studies have attempted to measure mRNA translation during larval development or examine the signaling requirements for nutrient-dependent regulation. Our work addresses this issue. Using polysome analyses, we observed that starvation rapidly (within thirty minutes) decreased larval mRNA translation, with a maximal decrease at 6–18 hours. By analyzing individual genes, we observed that nutrient-deprivation led to a general reduction in mRNA translation, regardless of any starvation-mediated changes (increase or decrease) in total transcript levels. Although sugars and amino acids are key regulators of translation in animal cells and are the major macronutrients in the larval diet, we found that they alone were not sufficient to maintain mRNA translation in larvae. The insulin/PI3 kinase and TOR pathways are widely proposed as the main link between nutrients and mRNA translation in animal cells. However, we found that genetic activation of PI3K and TOR signaling, or regulation of two effectors – 4EBP and S6K – could not prevent the starvation-mediated translation inhibition. Similarly, we showed that the nutrient stress-activated eIF2α kinases, GCN2 and PERK, were not required for starvation-induced inhibition of translation in larvae. These findings indicate that nutrient control of mRNA translation in larvae is more complex than simply amino acid activation of insulin and TOR signaling.
    10/2014; DOI:10.1242/bio.20149407
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The activity of S6 kinases (S6K) is highly induced in cancer cells highlighting an essential role in carcinogenesis. The S6K family has two members: S6K1 and S6K2 which bear common as well as distinct features. In an attempt to identify S6K2 unique sequence features compared to S6K1, we applied extensive bioinformatic analysis and motif search approaches. Interestingly, we identified 14 unique protein signatures which are present in proteins directly connected to chromatin and/or involved in transcription regulation. Using chromatin binding assay, we biochemically showed that S6K2 is bound to chromatin as well as nuclear matrix cellular fractions in HEK293 cells. The presence of S6K2 in chromatin fractions raised the possibility that it may be in close proximity to a number of chromatin substrates. For that, we then searched for S6K phosphorylation consensus sites RXRXXT/S in mammalian proteins using the SWISS-PROT database. Interestingly, we identified some potential phosphorylation sites in histone H3 (Thr45). Using in vitro kinase assays and siRNA-based knockdown strategy; we confirmed that S6K2 but not S6K1 or AKT is essential for histone H3-Thr45 phosphorylation in HEK293 cells. Furthermore, we show that the nuclear localisation sequence in the S6K2 C-terminus is essential for this modification. We have found that, H3-Thr45 phosphorylation correlates to S6K activation in response to mitogens and TPA-induced cell differentiation of leukaemic cell lines U937, HL60 and THP1. Overall, we demonstrate that S6K2 is a novel kinase that can phosphorylate histone H3 at position Thr45, which may play a role during cell proliferation and/or differentiation. J. Cell. Biochem. 115: 1048–1062, 2014. © 2013 Wiley Periodicals, Inc.
    Journal of Cellular Biochemistry 06/2014; 115(6). DOI:10.1002/jcb.24566 · 3.37 Impact Factor