Article Figures & Data
Figures
- Fig. 1.
mTORC1 regulation by upstream stimuli. All positive regulators (green) and negative regulators (red) mTORC1 converge on TSC regulation. Growth factors control the binding of insulin receptor substrate proteins, which activate phosphoinositide 3-kinase (PI3K) and lead to phosphorylation of phosphatidylinositol 4,5-bisphosphate (PIP2) to phosphatidylinositol 3,4,5-triphosphate (PIP3). Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) negatively regulates this activation. PIP3 subsequently activates phosphoinositide-dependent kinase 1/2 (PDK1/2), which in turn activates plasma membrane-localized AKT. mTORC2 can also phosphorylate AKT. AKT activates mTORC1 through phosphorylation of TSC, which activates Rheb. TSC is inhibited through ERK by the Ras-Raf-Mek-Erk signaling cascade. Tumor necrosis factor α (TNFα) and hypoxia secondarily regulate mTORC1 through IκB kinase β (IKKβ) and DNA damage response 1 (REDD1), respectively. Wingless-type (Wnt) signaling inhibits TSC by inhibiting glycogen synthase kinase 3 (GSK3). Cellular stress and DNA damage trigger AMP-activated protein kinase activation through p53 and liver kinase B1 (LKB1), respectively, to inhibit mTORC1.
- Fig. 2.
mTORC1 senses two different amino acid signaling cascades. (A) The Rag-dependent signaling pathway is stimulated by amino acids such as Ala, His, Ser, Met, Thr, Val, Leu, and Arg and converges on the Rag GTPases. Following activation, RagA or RagB is GTP bound and heterodimerizes with RagC or RagD. Rag GTPase heterodimer directly interacts with the Raptor subunit of the mTORC1 at the lysosome. Finally, Rheb activates the mTORC1. (B) The Rag-independent signaling pathway is stimulated by the addition of amino acids such as Gln and Asn. The small GTPase adenosine diphosphate ribosylation factor 1 (Arf1), vacuolar H+-ATPase (v-ATPase), and Rheb are necessary components of the Rag GTPase independent pathway. The cycling of Arf1 to a GTP-bound state through guanine nucleotide exchange factor (GEF) and GDP-bound state through GTPase activating protein (GAP) promotes mTORC1 activation and lysosomal localization. v-ATPase is present at the lysosome and required for amino acid signaling to mTORC1 for both pathways.
- Fig. 3.
Gαs-coupled GPCR inhibition of mTORC1. Ligand binding activates GPCRs, resulting in the Gα protein switching from inactive GDP-bound state to active GTP-bound state, leading to a Gα subunit dissociation following conformational change from the Gβγ complex. The GTP-bound Gαs subunit can activate AC, which converts ATP to cAMP. Elevated cAMP activates PKA by binding to its regulatory subunit and releasing PKA’s catalytic subunits. AKAPs act as a scaffolding protein that assists in PKA localization to distinct compartments in the cell to facilitate signaling cascades. PKA mediated phosphorylation of Raptor at Ser 971 to inhibit mTORC1. Phosphodiesterase (PDE) negatively regulates the cAMP signaling by hydrolyzing cAMP to AMP.
Tables
- TABLE 1
GPCRs that Regulate mTORC1 Activity
5-Hydroxytryptamine (5-HT); α-2 adrenergic receptor (α2-AR); angiotensin II type I receptor (AT1R); Arg vasopressin receptor (AVP); β-1/β-2 adrenergic receptor (β1/β2-AR); calcium-sensing receptor (CaSR); C-X-C chemokine receptor 4 (CXCR4); dopamine D1 receptor (D1R); dopamine D2 receptor (D2R); glucagon receptor (GCGR); glucagon-like peptide 1 (GLP-1); GPCR 137B (GPR137B); 5-hydroxytryptamine receptor 6 (HTR6); kappa opioid receptor (KOR); metabotropic glutamate receptor (mGluR); muscarinic acetylcholine receptor M4 (M4 mAChR); orexin 1/2 receptor (OX1/2R); prostaglandin E2/F2-α receptor (PGE2/PGF2α); purinergic Receptor (P2Y12); taste receptor type 1 member 1/3 (T1R1/T1R3); thyroid stimulating hormone receptor (TSH); vasopressin V1 receptor (V1); opioid receptor mu 1; rat adrenal gland cells (PC-12); cell lines: lung adenocarcinoma (A549); bovine steroidogenic luteal cells (bLCs); esophageal squamous cell carcinoma (ESCC); Chinese hamster ovary cells (CHO); human pharyngeal squamous carcinoma (FaDu); haploid 1 cells (HAP1); human embryonic kidney 293T cells (HEK-293T); human pancreatic cancer (PANC-1); Henrietta Lacks cervical adenocarcinoma cells (HeLa); human primary fibroblasts (Hs68); mouse embryonic fibroblast (MEF); mouse insulinoma 6 (MIN6); mouse neuro2A neuroblastoma with FmK6H construct (N2A-FmK6H); mouse embryonic hypothalamus N41 cells (N41); and Rat pancreatic hybrid cells of NEDH and RINm5F (BRIN-BD11).
GPCR G-Protein Model/Cell Line mTORC1 Activity Reference α2-AR Gα(i/o) PC-12FaDuCervical cancer cell lines (HeLa, Caski, C-33A and SiHa) Increase;Decrease;Decrease (Wu and Wong, 2005)(Choi et al., 2018)(Wang et al., 2020) AVP Gα(s) Primary hepatocytes Decrease (Jewell et al., 2019) β1/β2-AR Gα(s) Various cell lines, mouse brain & liver3T3-L1 adipocytes, mouse brown adipose tissueHuman TSC ± fibroblasts DecreaseIncreaseIncrease (Jewell et al., 2019)(Liu et al., 2016)(Le et al., 2018) CXCR4 Gα(i/o) Gefitinib resistant A549 Increase (Jung et al., 2013) GCGR Gα(s) Primary hepatocytesMouse pancreatic α-cellsRat hepatocytes,H4IIE, HepG2 Decrease (Jewell et al., 2019)(Solloway et al., 2015)(Welles et al., 2020) GLP-1 Gα(s) BRIN-BD11 Increase (Carlessi et al., 2017) GPR137B unknown MEF, HEK-293T/E, HAP1, HeLa, Hs68, zebrafish Increase (Gan et al., 2019) HTR6 Gα(s) Mouse prefrontal cortexMouse hippocampal tissueRat dorsal spinal cord Increase (Meffre et al., 2012)(Teng et al., 2019)(Martin et al., 2020) KOR Gα(i/o) CD-1 male mice, N2A-FmK6H cells Increase (Liu et al., 2019) M4 mAChR Gα(i/o) PC-12 Increase (Wu and Wong, 2006) mGluR Gα(i/o) Primary neuronal Increase (Ma et al., 2010) OX1/2R Gα(s) HEK-293T, N41, MEF Increase (Wang et al., 2014) PGE2 Gα(s) PANC-1 Increase (Chang et al., 2015) PGF2α Gα(s) bLCs Increase (Arvisais et al., 2006) P2Y12 Gα(i/o) Human platelets Increase (Moore et al., 2014) T1R1/T1R3 Gα(i/o) MIN6 Increase (Wauson et al., 2012) TSH Gα(s) Rat thyroid, CHO Increase (Suh et al., 2003) V1 Gα(s) Rat mesangial cells Increase (Ghosh et al., 2001) GPRC6A Gαi, Gαq HEK293A, liver, PC-3 Increase (Ye et al., 2019) BRS-3 Gαq 3T3 fibroblast cell Increase (Dong et al., 2020) CaSR Gα (i/o) cystic kidney epithelial cells Increase (Di Mise et al., 2018) US28 Gαq glioblastoma cells Increase (de Wit et al., 2016) 5-HT7 Gα12 endothelial cells Increase (Banskota et al., 2016) AT1R Gαq ESCC Increase (Li et al., 2016a) HTR2B Gα (i/o) PDAC cells Increase (Jiang et al., 2017) D1R Gαq nucleus accumbens of mice Increase (Sutton and Caron, 2015) OPRM1 Gα(i/o) HEK293 cells Increase (Wang et al., 2014) DRD4 Gα(i/o) GBM stem cells Decrease (Dolma et al., 2016) DRD3 Gα(i/o) HEK293T, HeLa, COS-7 cells Decrease (Barroso-Chinea et al., 2020; Li et al., 2016b) D2R Gα(i/o) mesencephalic neurons Decrease (Fasano et al., 2008) CNR2 Gα(i/o) Neural progenitor cell Increase (Palazuelos et al., 2012) CNR1 Gα(i/o) Glial cells Increase (Díaz-Alonso et al., 2015) PDE Inhibitors Target Substrate Reference Rolipram PDE4 cAMP (Kim et al., 2017) Apremilast PDE4 cAMP (Schett et al., 2010) Crisaborole PDE4 cAMP (Paller et al., 2016) Roflumilast PDE4 cAMP (Baye, 2012) Cilomilast PDE4 cAMP (Compton et al., 2001) Theophylline NonselectivePDE4 cAMP (Barnes, 2013) Dipyridamole NonselectivePDE6, PDE8 cAMP (Gillespie and Beavo, 1989) Zaprinast NonselectivePDE6 cAMP (Zhang et al., 2005) PDE, phosphodiesterase.
