Regulation of mTORC1 by Upstream Stimuli

Abstract

The mammalian target of rapamycin (mTOR) is an evolutionary conserved Ser/Thr protein kinase that senses multiple upstream stimuli to control cell growth, metabolism, and autophagy. mTOR is the catalytic subunit of mTOR complex 1 (mTORC1). A significant amount of research has uncovered the signaling pathways regulated by mTORC1, and the involvement of these signaling cascades in human diseases like cancer, diabetes, and ageing. Here, we review advances in mTORC1 regulation by upstream stimuli. We specifically focus on how growth factors, amino acids, G-protein coupled receptors (GPCRs), phosphorylation, and small GTPases regulate mTORC1 activity and signaling.

Keywords: G-protein coupled receptors; amino acids; and autophagy; cell growth; kinases; mTORC1; metabolism; phosphorylation; small GTPases.

Figure 1

Components of mTOR complex 1 (mTORC1) and mTORC2. Left- Core components of mTORC1 are mammalian target of rapamycin (mTOR) (kinase), Raptor (substrate recognizing component), and mLST8 (positive regulator). Other reported mTORC1 components are PRAS40 (negative regulator) and DEP-domain-containing mTOR-interacting protein (DEPTOR) (negative regulator). Five main downstream pathways are shown. The phosphorylation of S6 kinase 1 (S6K1) and 4EBP1 by mTORC1 regulates protein translation. The phosphorylation of ULK1 by mTORC1 regulates autophagy. mTORC1 also regulates lipid synthesis by phosphorylating S6K1 or Lipin1 to control SREBP, lysosome biogenesis by phosphorylating TFEB, and growth factor signaling by phosphorylating Grb10. Right- Core components of mTORC2 are mTOR (kinase), Rictor (substrate recognizing component), and mLST8 (positive regulator). Other complex components include mSin1 (positive regulator), Protor1/2 (positive regulator), and DEPTOR (negative regulator). mTORC2 regulated processes include cytoskeletal remodeling by phosphorylating PKC; cell survival, growth, and proliferation by phosphorylating Akt; and ion transport by phosphorylating SGK.

Figure 2

The mTOR upstream signaling network. Upstream regulators of mTOR signaling. Positive regulators of mTORC1 are shown in turquoise and negative regulators are shown in orange. Growth factors activate PI3K though the binding of IRS proteins. PI3K then phosphorylates PIP₂ to PIP₃ which then activates PDK1/2. Akt, containing a specific PIP₂ and PIP₃ PH domain, localizes to the plasma membrane and then subsequently activates through PDK1 phosphorylation. Akt promotes mTORC1 activity through the phosphorylation of TSC, subsequently activating Rheb. mTORC2 also phosphorylates Akt. The Ras-Raf-Mek-Erk signaling cascade leads to the inhibition of TSC through Erk or Rsk. mTORC1 activity is also controlled by Wnt signaling, TNFα through IKKβ, hypoxia through REDD1, and DNA damage through p53. Energy stress activates negative regulators such as LKB1 and AMPK to inhibit mTORC1. Rac-α Ser/Thr-protein kinase (Akt also known as PKB); AMP-activated protein kinase (AMPK); epidermal growth factor receptor (EGFR); extracellular signal-related kinase (Erk); GTPase activating protein (GAP); growth factor receptor-bound protein 2 (Grb2); glycogen synthase kinase 3 (GSK3); IκB kinase β (IKKβ); insulin receptor substrate (IRS); liver kinase B1 (LKB1); mitogen-activated protein kinase (MAPK); MAPK/ERK kinase (MEK); protein 53 (p53); pleckstrin homology (PH); phosphoinositide-dependent kinase 1/2 (PDK1/2); phosphoinositide 3-kinase (PI3K); phosphatidylinositol 4,5-bisphosphate (PIP₂); phosphatidylinositol 3,4,5-triphosphate (PIP₃); phosphatase and tensin homolog (PTEN); rapidly accelerated fibrosarcoma (Raf); rat sarcoma (Ras); DNA damage response 1 (REDD1); Ras homolog enriched in brain (Rheb); p90 ribosomal S6 kinase (Rsk); son of sevenless homolog (SOS); tumor necrosis factor α (TNFα); tuberous sclerosis complex (TSC); wingless-type (Wnt).

Figure 3

Amino acid sensing by mTORC1. (A) The Rag-dependent signaling pathway. Ala, Arg, His, Leu, Met, Ser, Thr, and Val can filter through upstream sensors in order to activate the Rag GTPases. Under sufficient amino acid conditions, the Rag GTPase heterodimer (GTP-RagA or RagB and GDP-RagC or RagD) interacts with mTORC1 at the lysosome, where Rheb resides. The Ragulator (consisting of p18, p14, MP1, C7orf59 and HBXIP) then anchors the Rag proteins to the lysosome and acts as a GEF for RagA and RagB. The FLCN-FNIP complex is a GAP for RagC and RagD. The v-ATPase, which is required for amino acid signaling to mTORC1, then binds to the Ragulator. KICSTOR (consisting of KPTN, ITFG2, C12orf66, and SZT2) anchors GATOR1 (consisting of DEPDC5, NPRL2 and NPRL3), the GAP for RagA and RagB, to the lysosome. GATOR2 (consisting of SEC13, SEH1L, WDR24, WDR59, and MIOS) inhibits GATOR1, through an unknown mechanism. Sestrin2 and CASTOR1 bind to GATOR2, preventing the inhibition of GATOR1 by GATOR2. Leu and Arg bind to sensors Sestrin2 and CASTOR1, respectively, which blocks Sestrin2-GATOR2 and CASTOR1-GATOR2 from interacting. (B) The Rag-independent signaling pathway. Only Gln and Asn activate mTORC1. The only known components that are required are the v-ATPase, Rheb, and the small GTPase Arf1. The cycling of Arf1 between a GTP- and a GDP-bound state promotes mTORC1 activation and lysosomal localization through an unknown mechanism. Adenosine diphosphate ribosylation factor 1 (Arf1); cellular Arg sensor for mTORC1 subunit 1 (CASTOR1); DEP domain containing 5 (DEPDC5); folliculin (FLCN); folliculin interacting protein (FNIP); GTPase-activating protein activity toward Rags (GATOR1); guanine nucleotide exchange factor (GEF); GTPase activating protein (GAP); hepatitis B virus X-interacting protein (HBXIP); integrin α FG-GAP repeat containing 2 (ITFG2); Kaptin (KPTN); meiosis regulator for oocyte development (MIOS); MEK partner 1 (MP1); mTOR complex 1 (mTORC1); NPR2 like, GATOR1 complex subunit (NPRL2); NPR3 like, GATOR1 complex subunit (NPRL3); vacuolar H+-ATPase (v-ATPase); Ras homolog enriched in brain (Rheb); SEH1-like nucleoporin (SEH1L); solute carrier family 38 member 9 (SLC38A9); seizure threshold 2 (SZT2); tuberous sclerosis complex (TSC); WD repeat-containing protein 24 (WDR24); WD repeat-containing protein 59 (WDR59).

Figure 4

GPCR inhibition of mTORC1. Activation of Gα_s-coupled GPCRs inhibit mTORC1 through the activation of PKA. The GPCR effector is a heterotrimeric G-protein made of three subunits (Gα, Gβ and Gγ). G-proteins are inactive in the GDP-bound state. GPCRs function as a receptor-catalyzed GEF to activate Gα subunit and separate them from the Gβγ dimer through conformational change. GTP hydrolyzing to GDP is the rate limiting step for Gα activity. Gα GDP-bound subunit then rejoins the βγ dimer until the next activating cycle. The GTP-Gα_s subunit can interact and turn on AC. AC is able to convert ATP cAMP. G-protein signaling induces cAMP, activating second messenger kinases such as PKA. PKA is a holoenzyme made of two regulatory subunits (R) and two catalytic subunits (C). R subunits have cAMP binding motifs. Two molecules of cAMP bind each R subunit to release and activate the C subunit of PKA. A well-known phosphorylation target of PKA is CREB at Ser 133. PKA can phosphorylate Raptor on Ser 791 to inhibit mTORC1 activity. adenylate cyclase (AC); 3′,5′-cyclic adenosine monophosphate (cAMP); G-protein coupled receptors (GPCRs); protein kinase A (PKA).