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GCN2 Sustains Mtorc1 Suppression Upon Amino Acid Deprivation by Inducing Sestrin2

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GCN2 Sustains Mtorc1 Suppression Upon Amino Acid Deprivation by Inducing Sestrin2 Downloaded from genesdev.cshlp.org on October 2, 2021 - Published by Cold Spring Harbor Laboratory Press RESEARCH COMMUNICATION proteins (4EBPs) to promote 5′ cap-dependent translation GCN2 sustains mTORC1 (Hara et al. 1998; Wang et al. 1998). In addition, mTORC1 suppression upon amino acid phosphorylates and inactivates unc-51-like autophagy-ac- tivating kinase 1 (ULK1), the initiating kinase of autoph- deprivation by inducing Sestrin2 agy, to inhibit autophagic protein degradation (Kim et al. 2011; Shang et al. 2011). Through these two mechanisms, Jiangbin Ye,1 Wilhelm Palm,1 Min Peng,2 1 2 2 mTORC1 enhances protein anabolism under amino acid- Bryan King, Tullia Lindsten, Ming O. Li, replete conditions. However, the mechanisms by which Constantinos Koumenis,3 and Craig B. Thompson1 mTORC1 senses amino acids remain elusive. When ami- 1 2 no acids are present, the Rag family of GTPases plays an Cancer Biology and Genetics Program, Immunology Program, essential role in recruiting mTORC1 to the surface of Memorial Sloan Kettering Cancer Center, New York, New York the lysosome (Kim et al. 2008; Sancak et al. 2008, 2010), 3 10065, USA; Department of Radiation Oncology, Perelman where another small GTPase, Rheb, stimulates the kinase School of Medicine at the University of Pennsylvania, activity of mTORC1. Several recent reports demonstrate Philadelphia, Pennsylvania 19104, USA that a family of stress response proteins, Sestrins, inhibit Mammalian cells possess two amino acid-sensing kinas- the Rag complex to disrupt mTORC1 localization to the es: general control nonderepressible 2 (GCN2) and mech- lysosome (Chantranupong et al. 2014; Parmigiani et al. 2014; Peng et al. 2014; Kim et al. 2015). However, how Ses- anistic target of rapamycin complex 1 (mTORC1). Their trins are regulated by amino acid availability and which combined effects orchestrate cellular adaptation to amino Sestrin family members are critical for this process re- acid levels, but how their activities are coordinated re- main unknown. mains poorly understood. Here, we demonstrate an im- In yeast and mammals, GCN2 is the direct sensor of portant link between GCN2 and mTORC1 signaling. amino acid deprivation (AAD). Upon starvation, un- Upon deprivation of various amino acids, activated charged tRNAs bind to GCN2 and stimulate its dimeriza- GCN2 up-regulates ATF4 to induce expression of the tion and autophosphorylation (Wek et al. 1989, 1995; stress response protein Sestrin2, which is required to sus- Diallinas and Thireos 1994; Dong et al. 2000; Qiu et al. tain repression of mTORC1 by blocking its lysosomal lo- 2002; Narasimhan et al. 2004). Activated GCN2 phos- α calization. Moreover, Sestrin2 induction is necessary for phorylates eIF2 to block translation initiation of most mRNAs (Dever et al. 1992). However, particular mRNAs cell survival during glutamine deprivation, indicating that contain an upstream ORF (uORF) cluster in their that Sestrin2 is a critical effector of GCN2 signaling that 5′ untranslated region (UTR) are efficiently translated regulates amino acid homeostasis through mTORC1 upon eIF2α phosphorylation, including the yeast tran- suppression. scription factor GCN4 (Hinnebusch 1984) and its mam- Supplemental material is available for this article. malian ortholog, ATF4 (Harding et al. 2000; Vattem and Wek 2004). Once ATF4 is up-regulated, it induces expres- Received July 27, 2015; revised version accepted sion of many genes involved in amino acid metabolism, October 22, 2015. including amino acid synthetases and transporters, amino acyl tRNA synthetases (Harding et al. 2003; Bunpo et al. 2009; Ye et al. 2010, 2012), and regulators of autophagy Proteins are the most abundant macromolecules in living (B’chir et al. 2013). Together, these ATF4 targets promote cells. The basic building blocks of proteins, amino acids, cellular adaptation to the amino acid shortage. are not only required for regular cellular functions but GCN2 activation leads to attenuated global translation, are also key determinants for cell survival. Eukaryotes autophagy induction, and growth arrest (Lehman et al. have evolved two major regulatory mechanisms for amino 2015), all of which act in opposition to mTORC1 activity. acid sensing to adapt to fluctuating amino acid levels in In this study, we discovered that GCN2 activation can the environment: the mechanistic target of rapamycin lead to sustained suppression of mTORC1 kinase activity. complex 1 (mTORC1) signaling pathway, which is acti- Upon AAD, GCN2 signaling transcriptionally up-regulat- vated in the presence of amino acids, and the general con- ed the stress response protein Sestrin2. Sestrin2 induction trol nonderepressible 2 (GCN2) signaling pathway, which is required to block lysosomal recruitment and activation is activated by the absence of amino acids. of mTORC1. Furthermore, Sesn2 deletion sensitizes cells mTORC1 integrates environmental signals such as nu- to glutamine starvation-induced cell death, which is res- trients, growth factors, oxygen, and stress to regulate pro- cued by the mTORC1 inhibitor rapamycin, suggesting tein translation, metabolism, and cell growth (Laplante that Sestrin2 is not only a critical mediator of the two ami- and Sabatini 2012; Jewell et al. 2013; Shimobayashi and no acid-sensing machineries but also a key determinant of Hall 2014). Although growth factors stimulate mTORC1 cell fate during AAD. activity, amino acids are required for complete activation of mTORC1 (Hara et al. 1998). Activated mTORC1 phos- phorylates ribosomal S6 kinase (S6K) and eIF4E-binding © 2015 Ye et al. This article is distributed exclusively by Cold Spring Harbor Laboratory Press for the first six months after the full-issue publi- [Keywords: GCN2; mTORC1; Sestrin; amino acid deprivation] cation date (see http://genesdev.cshlp.org/site/misc/terms.xhtml). After Corresponding author: [email protected] six months, it is available under a Creative Commons License (Attribu- Article published online ahead of print. Article and publication date are tion-NonCommercial 4.0 International), as described at http:// online at http://www.genesdev.org/cgi/doi/10.1101/gad.269324.115. creativecommons.org/licenses/by-nc/4.0/. GENES & DEVELOPMENT 29:1–6 Published by Cold Spring Harbor Laboratory Press; ISSN 0890-9369/15; www.genesdev.org 1 Downloaded from genesdev.cshlp.org on October 2, 2021 - Published by Cold Spring Harbor Laboratory Press Ye et al. − − Results and Discussion sion was observed in both wild-type and Gcn2 / cells upon leucine depletion, mTORC1 activity did not GCN2 is required to inhibit mTORC1 kinase activity decrease in Gcn2−/− cells during prolonged starvation and lysosomal localization upon prolonged AAD (8–24 h) (Fig. 1B), suggesting that GCN2 is required for long-term mTORC1 suppression but not short-term sup- Genetic experiments have suggested that GCN2 activa- pression. Since ATF4 is a major downstream effector of tion can contribute to mTORC1 inhibition following leu- GCN2 that regulates amino acid homeostasis (Harding cine depletion (Anthony et al. 2004), but the molecular et al. 2003; Ye et al. 2010), we next tested whether ATF4 mechanism and the dynamics of the cross-talk between was required for mTORC1 suppression upon leucine dep- GCN2 and mTORC1 have not been identified. When − − rivation. Similar to what we observed in Gcn2 / cells, wild-type mouse embryonic fibroblasts (MEFs) were cul- − − Atf4 / cells displayed sustained S6K phosphorylation tured in leucine-free medium, an acute repression of upon leucine withdrawal (Fig. 1B), indicating that mTORC1 activity, as assessed by decreased S6K phos- GCN2-dependent mTORC1 suppression relies on ATF4. phorylation (T389), was detected within 30 min, followed To confirm that the acute mTORC1 regulation by leucine by a brief recovery phase. After 8 h, mTORC1 activity does not depend on GCN2–ATF4 signaling, wild-type, started decreasing again, with maximum repression − − − − Gcn2 / , and Atf4 / MEFs were starved in leucine-free reached by 24 h (Fig. 1A). ATF4 induction was monitored medium for 20 min, and then leucine was added back as a measure of GCN2 activity, which inversely correlated for another 20 min. All three cell lines were able to reacti- with mTORC1 activity during prolonged starvation (Fig. vate mTORC1 by leucine stimulation, indicating that 1A). To investigate whether GCN2 activation is required GCN2–ATF4 signaling is not required for the acute acti- for sustained mTORC1 suppression upon leucine deple- − − vation of mTORC1 by leucine (Supplemental Fig. S1). tion, wild-type and Gcn2 / MEFs were starved in leu- Since amino acids regulate mTORC1 activity by induc- cine-free medium for up to 24 h. Leucine depletion − − ing its recruitment to lysosomal membranes (Sancak et al. induced ATF4 in wild-type cells but not in Gcn2 / cells 2008, 2010), we examined whether the GCN2–ATF4 path- (Fig. 1B). Interestingly, although acute mTORC1 suppres- way was required to block lysosomal localization of mTORC1 during AAD. In wild-type MEFs cultured in full medium, mTOR was present in punctate structures that colocalized with the lysosomal membrane protein LAMP2. After 24 h of leucine starvation, mTOR was dis- tributed throughout the cytoplasm (Fig. 1C). In contrast, in both Gcn2−/− and Atf4−/− MEFs, mTOR remained in punctate structures upon leucine starvation. Together, these data show that the GCN2–ATF4 pathway represses mTORC1 activity by inhibiting its lysosomal localization. The GCN2–ATF4 pathway transcriptionally up-regulates Sestrin2 during AAD
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