Glucose Starvation Blocks Translation at Multiple Levels
Authors: Chen-Song Zhang, D. Grahame Hardie, Sheng-Cai Lin
Journal: Cell Metabolism (2020)

Deficiency of glucose, even under sufficient amino acid supply, turns off translation and promotes catabolic processes to aid cell survival. A recent report by Yoon et al., 2020Yoon I. Nam M. Kim H.K. Moon H.S. Kim S. Jang J. Song J.A. Jeong S.J. Kim S.B. Cho S. et al.). Glucose-dependent control of leucine metabolism by leucyl-tRNA synthetase 1.Science. 2020; 376: 205-210Google Scholar shows that glucose is required for the full activity of the leucyl-tRNA synthetase LARS1 and maintains mTORC1 function via LARS1 to enhance translation. Glucose starvation abolishes both effects via phosphorylation of LARS1 by the AMPK-ULK1 signaling pathway. This study supports the idea that glucose starvation inhibits translation at multiple levels. Deficiency of glucose, even under sufficient amino acid supply, turns off translation and promotes catabolic processes to aid cell survival. A recent report by Yoon et al., 2020Yoon I. Nam M. Kim H.K. Moon H.S. Kim S. Jang J. Song J.A. Jeong S.J. Kim S.B. Cho S. et al.). Glucose-dependent control of leucine metabolism by leucyl-tRNA synthetase 1.Science. 2020; 376: 205-210Google Scholar shows that glucose is required for the full activity of the leucyl-tRNA synthetase LARS1 and maintains mTORC1 function via LARS1 to enhance translation. Glucose starvation abolishes both effects via phosphorylation of LARS1 by the AMPK-ULK1 signaling pathway. This study supports the idea that glucose starvation inhibits translation at multiple levels. Translation is an energy-consuming process that consumes at least two molecules of ATP and two of GTP to add each new amino acid to the growing polypeptide chain. Cells employ multiple regulatory mechanisms to ensure that this energy-consuming anabolic process can only take place when ample nutrients and growth factors are available. Availability of glucose, the major energy source for most cells, is known to be a prerequisite for translation. It has long been known that AMP-activated protein kinase (AMPK), a cellular sensor of energy and nutrients, is activated upon glucose starvation, in turn switching off the mammalian target of rapamycin complex 1 (mTORC1) pathway, which is critical for both the initiation and elongation steps of translation (Johanns et al., 2017Johanns M. Pyr Dit Ruys S. Houddane A. Vertommen D. Herinckx G. Hue L. Proud C.G. Rider M.H. Direct and indirect activation of eukaryotic elongation factor 2 kinase by AMP-activated protein kinase.Cell. Signal. 2017; 36: 212-221Crossref PubMed Scopus (32) Google Scholar). AMPK inhibits mTORC1 by dual mechanisms: (1) phosphorylation of tuberous sclerosis 2 (TSC2) (Inoki et al., 2003Inoki K. Zhu T. Guan K.L. TSC2 mediates cellular energy response to control cell growth and survival.Cell. 2003; 115: 577-590Abstract Full Text Full Text PDF PubMed Scopus (2744) Google Scholar), leading to inhibition of RHEB (Ras homolog enriched in brain), a lysosome-localized small GTPase that facilitates mTORC1 activation only in the GTP-bound state; and (2) phosphorylation of RAPTOR (regulatory-associated protein of mTOR), a component of the mTORC1 complex (Gwinn et al., 2008Gwinn D.M. Shackelford D.B. Egan D.F. Mihaylova M.M. Mery A. Vasquez D.S. Turk B.E. Shaw R.J. AMPK phosphorylation of raptor mediates a metabolic checkpoint.Mol. Cell. 2008; 30: 214-226Abstract Full Text Full Text PDF PubMed Scopus (2401) Google Scholar). In addition to these effects via mTORC1, AMPK also phosphorylates and activates eukaryotic elongation factor-2 (eEF2) kinase, which then triggers phosphorylation and inactivation of eEF2 itself to directly block the elongation step of translation (Johanns et al., 2017Johanns M. Pyr Dit Ruys S. Houddane A. Vertommen D. Herinckx G. Hue L. Proud C.G. Rider M.H. Direct and indirect activation of eukaryotic elongation factor 2 kinase by AMP-activated protein kinase.Cell. Signal. 2017; 36: 212-221Crossref PubMed Scopus (32) Google Scholar). The recent report by Yoon et al., 2020Yoon I. Nam M. Kim H.K. Moon H.S. Kim S. Jang J. Song J.A. Jeong S.J. Kim S.B. Cho S. et al.). Glucose-dependent control of leucine metabolism by leucyl-tRNA synthetase 1.Science. 2020; 376: 205-210Google Scholar adds a new tier to the regulation of translation by AMPK in response to glucose availability. They first reported a new mechanism in which leucyl-tRNA synthetase 1 (LARS1), which catalyzes the ATP-dependent ligation of L-leucine to its cognate tRNA, can accelerate the inhibition of mTORC1 in low glucose in an AMPK-dependent manner. LARS1 has been previously identified as a leucine sensor that binds to RAGD, a lysosome-localized Rag GTPase essential for the stimulation of mTORC1 by amino acids, thus promoting conversion of RAGD from the GTP- to the GDP-bound state (Han et al., 2012Han J.M. Jeong S.J. Park M.C. Kim G. Kwon N.H. Kim H.K. Ha S.H. Ryu S.H. Kim S. Leucyl-tRNA synthetase is an intracellular leucine sensor for the mTORC1-signaling pathway.Cell. 2012; 149: 410-424Abstract Full Text Full Text PDF PubMed Scopus (505) Google Scholar). RAGD:GDP subsequently promotes mTORC1 translocation to the lysosomal surface to allow its activation by leucine. Intriguingly, the affinity of LARS1 toward RagD, as well as its ability to activate mTORC1, was reduced in low glucose even in the presence of leucine, indicating that glucose may exert a dominant regulation of LARS1. Through RNAi screening, they identified AMPK and its substrate ULK1 (Unc-51 like autophagy activating kinase 1) as being involved in the suppression of the LARS-RagD pathway in low glucose. Mechanistically, ULK1, which is phosphorylated and activated by AMPK in low glucose (Egan et al., 2011Egan D.F. Shackelford D.B. Mihaylova M.M. Gelino S. Kohnz R.A. Mair W. Vasquez D.S. Joshi A. Gwinn D.M. Taylor R. et al.Phosphorylation of ULK1 (hATG1) by AMP-activated protein kinase connects energy sensing to mitophagy.Science. 2011; 331: 456-461Crossref PubMed Scopus (1566) Google Scholar, Kim et al., 2011Kim J. Kundu M. Viollet B. Guan K.L. AMPK and mTOR regulate autophagy through direct phosphorylation of Ulk1.Nat. Cell Biol. 2011; 13: 132-141Crossref PubMed Scopus (3606) Google Scholar), phosphorylates S391 and S720 on LARS1, reducing its affinity toward RagD. Yoon et al., 2020Yoon I. Nam M. Kim H.K. Moon H.S. Kim S. Jang J. Song J.A. Jeong S.J. Kim S.B. Cho S. et al.). Glucose-dependent control of leucine metabolism by leucyl-tRNA synthetase 1.Science. 2020; 376: 205-210Google Scholar also showed that phosphorylation of LARS1 by ULK1 reduces the binding of both ATP and leucine to LARS1, leading to an inhibition of leucine loading onto tRNA and thus limiting protein elongation. This finding introduces the additional concept that glucose starvation directly disrupts an early step in translation; i.e., aminoacylation of tRNA. As a result, increased free intracellular leucine (originating both from enhanced autophagy after AMPK activation and mTORC1 inhibition, and reduced translation) can then be used as an alternative carbon source for the catabolic production of ATP (via the TCA [tricarboxylic acid] cycle) in low glucose. Consistent with this, removal of leucine from glucose-free medium reduced ATP concentrations in rhabdomyosarcoma cells. This study reveals a finely tuned network that connects AMPK to LARS1, as well as mTORC1, to determine the catabolic or anabolic fate of leucine when availability of glucose is limiting. Highly relevant to these findings are earlier reports (Zhang et al., 2014Zhang C.S. Jiang B. Li M. Zhu M. Peng Y. Zhang Y.L. Wu Y.Q. Li T.Y. Liang Y. Lu Z. et al.The lysosomal v-ATPase-Ragulator complex is a common activator for AMPK and mTORC1, acting as a switch between catabolism and anabolism.Cell Metab. 2014; 20: 526-540Abstract Full Text Full Text PDF PubMed Scopus (241) Google Scholar, Zhang et al., 2017Zhang C.S. Hawley S.A. Zong Y. Li M. Wang Z. Gray A. Ma T. Cui J. Feng J.W. Zhu M. et al.Fructose-1,6-bisphosphate and aldolase mediate glucose sensing by AMPK.Nature. 2017; 548: 112-116Crossref PubMed Scopus (237) Google Scholar) showing that glucose starvation simultaneously switches on AMPK and switches off mTORC1 on the lysosomal surface. In low glucose, decreased levels of the glycolytic intermediate fructose-1,6-bisphosphate (FBP) are sensed by the glycolytic enzyme aldolase, which cleaves FBP into phosphotrioses. Aldolase that is not occupied by FBP inhibits the vacuolar-type H+-ATPase (v-ATPase) on the lysosomal surface, leading to a change in its interaction with the pentameric Ragulator complex. Under these circumstances, AXIN (either AXIN1 or AXIN2), in complex with LKB1, docks onto the v-ATPase-Ragulator complex, triggering phosphorylation and activation of the lysosomal pool of AMPK by LKB1. The translocation of AXIN to lysosomal surfaces also facilitates release of mTORC1 from the lysosome, thereby inhibiting mTORC1 at the same time. Consistent with this, when AXIN1 is depleted in MEFs (in which AXIN2 is not expressed), mTORC1 inhibition becomes much slower after glucose removal. AXIN1 exerts this effect by facilitating the GAP activity of the Ragulator complex toward the RagC GTPase. Therefore, we would argue that high glucose does not activate mTORC1, rather it prevents inhibition of mTORC1 via its dissociation from the lysosome. It is important to stress that glucose starvation can still render mTORC1 inactive even in AMPK null cells, albeit at a slower rate (Zhang et al., 2014Zhang C.S. Jiang B. Li M. Zhu M. Peng Y. Zhang Y.L. Wu Y.Q. Li T.Y. Liang Y. Lu Z. et al.The lysosomal v-ATPase-Ragulator complex is a common activator for AMPK and mTORC1, acting as a switch between catabolism and anabolism.Cell Metab. 2014; 20: 526-540Abstract Full Text Full Text PDF PubMed Scopus (241) Google Scholar). In support for a critical effect of AXIN on the GAP activity of Ragulator, a "constitutively active" RAG, i.e., the GTP-constitutive binding mutant of RAGA or RAGB, completely blocks the inhibition of mTORC1 in low glucose, despite intact AMPK activation (Efeyan et al., 2013Efeyan A. Zoncu R. Chang S. Gumper I. Snitkin H. Wolfson R.L. Kirak O. Sabatini D.D. Sabatini D.M. Regulation of mTORC1 by the Rag GTPases is necessary for neonatal autophagy and survival.Nature. 2013; 493: 679-683Crossref PubMed Scopus (276) Google Scholar, Zhang et al., 2014Zhang C.S. Jiang B. Li M. Zhu M. Peng Y. Zhang Y.L. Wu Y.Q. Li T.Y. Liang Y. Lu Z. et al.The lysosomal v-ATPase-Ragulator complex is a common activator for AMPK and mTORC1, acting as a switch between catabolism and anabolism.Cell Metab. 2014; 20: 526-540Abstract Full Text Full Text PDF PubMed Scopus (241) Google Scholar). Therefore, the AXIN-dependent and AMPK-independent mechanisms can operate in parallel to control the activity of mTORC1 (Figure 1). It would be interesting in the future to compare the dynamics of translational inhibition following knockout of the various components of the AXIN-AMPK-ULK1 pathway. In summary, it is remarkable that glucose, as the most abundant cellular nutrient, exerts so many regulatory roles to control translation. S.-C.L. was supported by grants from NSFC ( 31730058 ) and MOST of China ( 2016YFA0502001 ). D.G.H. was supported by an Investigator Award from the Wellcome Trust ( 097726 ).