Regulation of innate immune cell function by mTOR

Abstract

The innate immune system is central for the maintenance of tissue homeostasis and quickly responds to local or systemic perturbations by pathogenic or sterile insults. This rapid response must be metabolically supported to allow cell migration and proliferation and to enable efficient production of cytokines and lipid mediators. This Review focuses on the role of mammalian target of rapamycin (mTOR) in controlling and shaping the effector responses of innate immune cells. mTOR reconfigures cellular metabolism and regulates translation, cytokine responses, antigen presentation, macrophage polarization and cell migration. The mTOR network emerges as an integrative rheostat that couples cellular activation to the environmental and intracellular nutritional status to dictate and optimize the inflammatory response. A detailed understanding of how mTOR metabolically coordinates effector responses by myeloid cells will provide important insights into immunity in health and disease.

Figure 1. The mTOR pathway in innate immune cells.

In innate immune cells, the mammalian target of rapamycin (mTOR) network can be activated via multiple signals. Growth factors, Toll-like receptor (TLR) ligands or cytokines activate mTOR complex 1 (mTORC1) and mTORC2 through their cognate receptors. Receptor activation leads to the recruitment of class I phosphatidylinositol-3 kinases (PI3Ks) to the receptor complex by different adaptors including the small GTPase RAB8A in macrophages. PI3Ks generate phosphatidylinositol-3,4,5-trisphosphate (PtdInsP₃) as a second messenger to recruit and activate the serine-threonine kinases AKT1, AKT2 and AKT3 via phosphorylation on threonine 308 by phosphoinositide-dependent protein kinase 1 (PDPK1). This process is negatively regulated by phosphatase and tensin homologue (PTEN), which dephosphorylates PtdInsP₃. mTORC2 phosphorylates AKT on serine 473, which seems to be required for full activation and substrate specificity of AKT. In addition, mTORC2 phosphorylates protein kinase C (PKC) and serum and glucocorticoid-regulated kinase 1 (SGK1) to regulate important cellular processes such as cytoskeleton reorganization. Two main effectors of AKT are forkhead box O1 (FOXO1) and tuberous sclerosis 2 (TSC2). TSC2 forms a heterodimeric complex with TSC1 and inhibits mTORC1. Phosphorylation of TSC2 at threonine 1462 (Thr1462) by AKT inhibits its GTPase-activating protein (GAP) activity for the small GTPase RAS homologue enriched in brain (RHEB), which therefore remains in a GTP-bound active state and activates mTORC1 on the lysosome. In parallel to the PI3K-AKT pathway, the mitogen-activated protein kinases (MAPKs) p38α, and COT also activate mTORC1 via MK2- and ERK-mediated phosphorylation of TSC2, respectively,. AMP-activated protein kinase (AMPK) detects low cellular energy by sensing the AMP levels and inhibits mTORC1 by phosphorylating regulatory-associated protein of mTOR (RAPTOR) at serine 792 and TSC2 at serine 1387 that promotes the inhibitory function of the TSC1-TSC2 complex. mTORC1 senses amino acid sufficiency on the lysosome via RAG GTPases and the ragulator complex, which seems to be a prerequisite for mTORC1 activation by growth factors. Moreover, low levels of glucose-6-phosphate inactivate mTORC1 by inducing binding to hexokinase 2 (HK2). Binding of the lipid metabolite phosphatidic acid (PA) to mTORC1 is also a prerequisite for mTORC1 activation. mTORC1-dependent phosphorylation of a variety of downstream effectors, such as eIF4E-binding protein 1 (4E-BP1), 4E-BP2 and ribosomal protein S6 kinase 1 (S6K1), promotes protein synthesis.

Figure 2. Metabolic control by mTOR in innate immunity.

The mammalian target of rapamycin complex 1 (mTORC1) promotes glycolysis through hypoxia-inducible factor 1α (HIF1α) and MYC, which enhances glucose import by increased expression and surface translocation of glucose transporter 1 (GLUT1) and increased expression of glycolytic genes. Activation of mTORC1 induces mitochondrial biogenesis through the transcription factors PPARγ coactivator 1α (PGC1α) and ying yang 1 (YY1), and promotes cholesterol and fatty acid synthesis from the tricarboxylic acid (TCA) cycle by sterol regulatory element-binding proteins (SREBPs) and peroxisome proliferator-activated receptor-γ (PPARγ). Fatty acids are further metabolized to lipid mediators such as leukotrienes, prostaglandins and resolvins. Cholesterol and fatty acids are also used as building blocks for endoplasmic reticulum (ER) and Golgi synthesis, which can promote the secretion of pro-inflammatory cytokines. This is especially important in lipopolysaccharide-activated dendritic cells, which promote ER and Golgi synthesis predominantly through AKT. mTORC1 can also have a negative effect on mitochondrial respiration by inducing the expression of interferon (IFN) and nitric oxide (NO), which subsequently promote aerobic glycolysis. mTORC2 promotes metabolic reprogramming by activating MYC and AKT, which enhance the expression of various glycolytic enzymes. Lactate, the end product of aerobic glycolysis, can directly reprogramme macrophages and dendritic cells and reduce the expression of interleukin-12 (IL-12), while enhancing the production of IL-10.

Figure 3. mTOR in dendritic cells (DCs) and its impact on T cell activation.

a| Toll-like receptor (TLR) stimulation regulates interleukin-10 (IL-10) and IL-12 production in a complex manner through mTOR complex 1 (mTORC1) and mTORC2. The AKT-mTORC1 network positively regulates IL-10 expression by at least three non-mutually exclusive mechanisms. mTORC1 induces the phosphorylation of signal transducer and activator of transcription 3 (STAT3) via currently unknown kinases. mTOR or ribosomal protein S6 kinase 1 (S6K1) phosphorylate programmed death cell protein 4 (PDCD4), which subsequently gets degraded by the proteasome. The transcription factor twist-related protein 2 (TWIST2) is released from PDCD4 and translocates to the nucleus to induce the expression of MAF. In addition, AKT phosphorylates and inhibits glycogen synthase kinase 3 (GSK3), which is then unable to phosphorylate and inhibit the transcriptional activity of cAMP response element-binding protein (CREB). Hence, mTORC1 promotes the expression of IL-10 via STAT3, MAF and CREB. mTORC1 decreases the activation of nuclear factor-κB (NF-κB) and the expression of IL-12. In addition, mTORC2 negatively regulates IL-12 production by inducing an AKT-dependent inactivation of forkhead box O1 (FOXO1), which usually promotes NF-κB activation. b| The expression of type I interferon production by DCs is regulated by S6Ks and eIF4E-binding proteins (4E-BPs) downstream of mTORC1. Phosphorylation of 4E-BP1 and 4E-BP2 by mTORC1 releases the eukaryotic translation initiation factor 4E (eIF4E), which promotes the translation of IFN regulatory factor 7 (IRF7) mRNA. In addition, S6K1 and S6K2 phosphorylate IRF7 to activate IFNα and IFNβ production, which is supported by IRF5. c| Lipopolysaccharide (LPS) activates mTORC1 and inhibits the transcription of new MHC class II mRNAs mediated by IRF4 and class II coactivator (CIITA). mTORC1 activation also limits antigen processing by decreasing autophagy, which contributes to the presentation of endogenous and exogenous antigen loading on MHC class I and class II molecules. mTORC1 promotes the expression of programmed cell death ligand 1 (PDL1), which limits T cell activation. mTORC1 positively regulates the production of CC-chemokine ligand 2 (CCL2) and negatively regulates signalling by CC-chemokine receptor 7 (CCR7), thereby affecting the migration of DCs to tissues and secondary lymphoid organs.

Figure 4. mTOR in neutrophils, mast cells and natural killer (NK) cells.

a| Chemotactic migration of neutrophils and mast cells is controlled by G-protein coupled receptor (GPCR)-mediated activation of mTOR complex 2 (mTORC2). mTORC2 activates RAS homologue (RHO) GTPases and AKT2 to regulate F-actin assembly at the leading edge. mTORC2 is also critical for tail retraction by phosphorylating protein kinase C βII (PKCβII). PKCβII then activates adenylyl cyclase 9 (AC9), which converts ATP into cAMP to activate RHOA. RHOA induces myosin II phosphorylation and tail retraction. b | mTORC1 controls the translation of preformed cyclooxygenase 2 (COX2) mRNA in LPS- or formyl-methionyl-leucyl-phenylalanine (fMLP)-activated neutrophils to induce the expression of prostaglandins that promote vasodilation. Soluble interleukin-6 receptor-α (sIL-6Rα) is synthesized via mTORC1. sIL-6Rα translocates to endothelial cells and induces a switch from neutrophil to mononuclear leukocyte recruitment to initiate the resolution of inflammation. mTORC1 promotes LPS-mediated neutrophil extracellular trap (NET) formation by translational control of hypoxia-inducible factor 1α (HIF1α) expression. Activation of mTORC1 can also negatively influence fMLP-stimulated NET formation by inhibiting autophagy. c | Activation of the high affinity receptor for IgE (FcεRI), prostaglandin E₂ receptor (PGE₂R) or the KIT receptor for stem cell factor (SCF) results in rapid phosphatidylinositol 3-kinase (PI3K)-dependent activation of mTORC1 and mTORC2 in mast cells. mTORC1 is required for mast cell survival and the expression of IL-8 and tumour necrosis factor (TNF) following FcεRI stimulation. However, constitutive activation of mTORC1 by loss of tuberous sclerosis 1 (TSC1) is detrimental and increases apoptosis by enhancing the expression of p53, which reduces the expression of B cell lymphoma 2 (BCL-2). mTORC2 promotes PGE₂-mediated chemotaxis, CC-chemokine ligand 2 (CCL2) expression and reactive oxygen species (ROS) production. TSC1 promotes mast cell degranulation and histamine production independent of mTORC1. RICTOR contributes to FcεRI-induced degranulation independent of mTORC2. d | IL-15 activates mTORC1 through PI3K, phosphoinositide-dependent protein kinase 1 (PDPK1) and AKT in NK cells. mTORC1 promotes NK cell proliferation and the acquisition of cytotoxicity through the induction of interferon-γ (IFNγ) and granzyme B production. mTORC1-mediated cytotoxicity requires metabolic reprogramming and enhanced glycolysis.