Mitochondrial metabolism regulates macrophage biology

Mitochondria are critical for regulation of the activation, differentiation, and survival of macrophages and other immune cells. In response to various extracellular signals, such as microbial or viral infection, changes to mitochondrial metabolism and physiology could underlie the corresponding state of macrophage activation. These changes include alterations of oxidative metabolism, mitochondrial membrane potential, and tricarboxylic acid (TCA) cycling, as well as the release of mitochondrial reactive oxygen species (mtROS) and mitochondrial DNA (mtDNA) and transformation of the mitochondrial ultrastructure. Here, we provide an updated review of how changes in mitochondrial metabolism and various metabolites such as fumarate, succinate, and itaconate coordinate to guide macrophage activation to distinct cellular states, thus clarifying the vital link between mitochondria metabolism and immunity. We also discuss how in disease settings, mitochondrial dysfunction and oxidative stress contribute to dysregulation of the inflammatory response. Therefore, mitochondria are a vital source of dynamic signals that regulate macrophage biology to fine-tune immune responses. Mitochondria are critical for regulation of the activation, differentiation, and survival of macrophages and other immune cells. In response to various extracellular signals, such as microbial or viral infection, changes to mitochondrial metabolism and physiology could underlie the corresponding state of macrophage activation. These changes include alterations of oxidative metabolism, mitochondrial membrane potential, and tricarboxylic acid (TCA) cycling, as well as the release of mitochondrial reactive oxygen species (mtROS) and mitochondrial DNA (mtDNA) and transformation of the mitochondrial ultrastructure. Here, we provide an updated review of how changes in mitochondrial metabolism and various metabolites such as fumarate, succinate, and itaconate coordinate to guide macrophage activation to distinct cellular states, thus clarifying the vital link between mitochondria metabolism and immunity. We also discuss how in disease settings, mitochondrial dysfunction and oxidative stress contribute to dysregulation of the inflammatory response. Therefore, mitochondria are a vital source of dynamic signals that regulate macrophage biology to fine-tune immune responses. Macrophages safeguard tissue homeostasis and regulate inflammatory responses. To exert these varied functions, macrophages show high plasticity and adopt different activation states according to the stimulus signals. The Th1 cytokine interferon-γ (IFNγ) together with Toll-like receptor (TLR) ligands, including lipopolysaccharide (LPS), promotes classically activated proinflammatory macrophages (commonly known as M1-like macrophages), which secrete proinflammatory cytokines such as interleukin-6 (IL-6) and IL-1β to induce inflammatory responses and fight against infection; generate highly reactive oxygen species and nitrogen intermediates to gain efficient microbicidal and tumoricidal activities; and increase major histocompatibility complex (MHC)-I/II, CD80, and CD86 expression (1Orecchioni M. Ghosheh Y. Pramod A.B. Ley K. Macrophage polarization: Different gene signatures in M1(LPS+) vs. classically and M2(LPS-) vs. alternatively activated macrophages.Front. Immunol. 2019; 10: 1084Crossref PubMed Scopus (239) Google Scholar, 2Shapouri-Moghaddam A. Mohammadian S. Vazini H. Taghadosi M. Esmaeili S.A. Mardani F. Seifi B. Mohammadi A. Afshari J.T. Sahebkar A. 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The signals that drive macrophage inflammatory activation induce breaks in and rewire the TCA cycle by influencing expression of TCA cycle enzymes, IDH (isocitrate dehydrogenase) and SDH (succinate dehydrogenase), resulting in elevations in citrate and succinate, respectively. These signals also augment glycolysis (also known as Warburg Effect). In contrast, IL-4-activated macrophages maintain an unbroken TCA cycle and preferentially engage oxidative phosphorylation (OXPHOS) and fatty acid oxidation (FAO) for ATP production. OXPHOS in IL-4-activated macrophages is fueled by the oxidation of fatty acids and glutamine, which activates the peroxisome proliferator-activated receptor-γ (PPARγ) to mediate the induction of genes regulating alternative macrophage functions (8Batista-Gonzalez A. Vidal R. Criollo A. Carreño L.J. New insights on the role of lipid metabolism in the metabolic reprogramming of macrophages.Front. Immunol. 2019; 10: 2993Crossref PubMed Scopus (5) Google Scholar). Glucose oxidation, induced by the mTORC2-IRF4 signaling axis, also contributes to IL-4 mediated gene induction (9Huang S.C. Smith A.M. Everts B. Colonna M. Pearce E.L. Schilling J.D. Pearce E.J. Metabolic reprogramming mediated by the mTORC2-IRF4 signaling axis is essential for macrophage alternative activation.Immunity. 2016; 45: 817-830Abstract Full Text Full Text PDF PubMed Scopus (238) Google Scholar). Shifts in mitochondrial metabolism are closely linked to macrophage activation. In this review, we discuss the mechanistic underpinnings of differential mitochondrial metabolism in distinct macrophage activation states, discuss how they are induced and how they contribute to macrophage activation and biology. The type I inflammatory response usually starts when macrophages and other sentinel cells are activated by pathogen-associated molecular patterns (PAMPs), including microbial cell wall components, nucleic acids, and lipoproteins. Macrophage metabolism also undergoes dynamic changes during such activation. At the center of cellular metabolism is the mitochondria, which not only supplies energy but is also involved in biosynthesis and maintaining cellular redox and serves as a platform for various innate immunological signaling pathways (10Tur J. Vico T. Lloberas J. Zorzano A. Celada A. Macrophages and mitochondria: A critical interplay between metabolism, signaling, and the functional activity.Adv. Immunol. 2017; 133: 1-36Crossref PubMed Scopus (21) Google Scholar). Macrophage activation signals alter the activity of the electron transport chain (ETC) and the TCA cycle to influence multiple aspects of mitochondrial metabolism. They also induce an upregulation of glucose and glutamine utilization and a shift toward anabolic pathways. Aerobic glycolysis, induced by LPS-stimulated mammalian target of rapamycin (mTOR) and hypoxia-inducible factor 1-alpha (HIF-1α) pathways (11Cheng S.C. Quintin J. Cramer R.A. Shepardson K.M. Saeed S. Kumar V. Giamarellos-Bourboulis E.J. Martens J.H. Rao N.A. Aghajanirefah A. Manjeri G.R. Li Y. Ifrim D.C. Arts R.J. van der Veer B.M. et al.mTOR- and HIF-1α-mediated aerobic glycolysis as metabolic basis for trained immunity.Science. 2014; 345: 1250684Crossref PubMed Scopus (741) Google Scholar), is upregulated for ATP production while OXPHOS is repressed through multiple mechanisms, including the two breaks in the TCA cycle. One break results from decreased expression of IDH, the TCA cycle enzyme that converts citrate to α-ketoglutarate (α-KG), allowing for the cumulation of citrate, which can be redirected for generating itaconic acid or withdrawn for fatty acid biosynthesis (12Jha A.K. Huang S.C. Sergushichev A. Lampropoulou V. Ivanova Y. Loginicheva E. Chmielewski K. Stewart K.M. Ashall J. Everts B. Pearce E.J. Driggers E.M. Artyomov M.N. 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Consistently, oxidative metabolism is suppressed in LPS-tolerant macrophages, which are no longer able to produce inflammatory cytokines as a result of long-term LPS exposure (14Butcher S.K. O'Carroll C.E. Wells C.A. Carmody R.J. Toll-like receptors drive specific patterns of tolerance and training on restimulation of macrophages.Front. Immunol. 2018; 9: 933Crossref PubMed Scopus (28) Google Scholar). Note that while some characteristics of tolerant macrophages resemble that of the M2 macrophages, it would be an oversimplification to equate the two macrophage states, which differ in many aspects of metabolism, phenotype, and function (14Butcher S.K. O'Carroll C.E. Wells C.A. Carmody R.J. Toll-like receptors drive specific patterns of tolerance and training on restimulation of macrophages.Front. Immunol. 2018; 9: 933Crossref PubMed Scopus (28) Google Scholar). 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Jones R.G. et al.Itaconate links inhibition of succinate dehydrogenase with macrophage metabolic remodeling and regulation of inflammation.Cell Metab. 2016; 24: 158-166Abstract Full Text Full Text PDF PubMed Google Scholar, 37Mills E.L. Ryan D.G. Prag H.A. Dikovskaya D. Menon D. Zaslona Z. Jedrychowski M.P. Costa A.S.H. Higgins M. Hams E. Szpyt J. Runtsch M.C. King M.S. McGouran J.F. Fischer R. et al.Itaconate is an anti-inflammatory metabolite that activates Nrf2 via alkylation of KEAP1.Nature. 2018; 556: 113-117Crossref PubMed Scopus (446) Google Scholar). Itaconate influences oxidative metabolism by suppressing the activity of SDH, a key enzyme in TCA cycle and in Complex II of the ETC. Such activity of itaconate leads to succinate accumulation and decreases in oxygen consumption and contributes to its anti-inflammatory activity (32Lampropoulou V. Sergushichev A. Bambouskova M. Nair S. Vincent E.E. Loginicheva E. Cervantes-Barragan L. Ma X. Huang S.C. Griss T. Weinheimer C.J. 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Higgins M. Hams E. Szpyt J. Runtsch M.C. King M.S. McGouran J.F. Fischer R. et al.Itaconate is an anti-inflammatory metabolite that activates Nrf2 via alkylation of KEAP1.Nature. 2018; 556: 113-117Crossref PubMed Scopus (446) Google Scholar). In contrast, other studies suggest that effects of itaconate on inflammation are not Nrf2-dependent (40Sun K.A. Li Y. Meliton A.Y. Woods P.S. Kimmig L.M. Cetin-Atalay R. Hamanaka R.B. Mutlu G.M. Endogenous itaconate is not required for particulate matter-induced NRF2 expression or inflammatory response.Elife. 2020; 9e54877Crossref PubMed Scopus (14) Google Scholar). They reported that electrophilic properties of itaconate and its derivatives inhibit IκBζ protein induction through activating transcription factor 3, leading to selective inhibition of some TLR-inducible transcriptional responses (41Bambouskova M. Gorvel L. Lampropoulou V. Sergushichev A. Loginicheva E. Johnson K. Korenfeld D. Mathyer M.E. Kim H. Huang L.H. Duncan D. Bregman H. 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Itaconate: An emerging determinant of inflammation in activated macrophages.Immunol. Cell Biol. 2019; 97: 134-141PubMed Google Scholar). It has been shown that during inflammatory macrophage activation, the Krebs cycle metabolite succinate accumulates and enhances mitochondrial ROS production, acting as a signal to activate proinflammatory gene expression (43Tannahill G.M. Curtis A.M. Adamik J. Palsson-McDermott E.M. McGettrick A.F. Goel G. Frezza C. Bernard N.J. Kelly B. Foley N.H. Zheng L. Gardet A. Tong Z. Jany S.S. Corr S.C. et al.Succinate is an inflammatory signal that induces IL-1β through HIF-1α.Nature. 2013; 496: 238-242Crossref PubMed Scopus (1584) Google Scholar, 44Mills E.L. Kelly B. Logan A. Costa A.S.H. Varma M. Bryant C.E. Tourlomousis P. Däbritz J.H.M. Gottlieb E. Latorre I. Corr S.C. McManus G. Ryan D. Jacobs H.T. Szibor M. et al.Succinate dehydrogenase supports metabolic repurposing of mitochondria to drive inflammatory macrophages.Cell. 2016; 167: 457-470.e413Abstract Full Text Full Text PDF PubMed Scopus (681) Google Scholar). Succinate oxidation by succinate dehydrogenase (SDH) leads to HIF-1α stabilization through effects on reverse electron transport (RET) and PHD inhibition, leading to the induction of glycolytic genes and sustaining the glycolytic metabolism of inflammatory macrophages (43Tannahill G.M. Curtis A.M. Adamik J. Palsson-McDermott E.M. McGettrick A.F. Goel G. Frezza C. Bernard N.J. Kelly B. Foley N.H. Zheng L. Gardet A. Tong Z. Jany S.S. Corr S.C. et al.Succinate is an inflammatory signal that induces IL-1β through HIF-1α.Nature. 2013; 496: 238-242Crossref PubMed Scopus (1584) Google Scholar). The accumulation of succinate is further linked to the induction of a proinflammatory phenotype through autocrine stimulation of a receptor called succinate receptor 1 (SUCNR1) that activates inflammatory pathways by enhancing IL-1β production (45Littlewood-Evans A. Sarret S. Apfel V. Loesle P. Dawson J. Zhang J. Muller A. Tigani B. Kneuer R. Patel S. Valeaux S. Gommermann N. Rubic-Schneider T. Junt T. Carballido J.M. GPR91 senses extracellular succinate released from inflammatory macrophages and exacerbates rheumatoid arthritis.J. Exp. Med. 2016; 213: 1655-1662Crossref PubMed Scopus (182) Google Scholar). In intestinal Tuft cells of the gut that express high levels of SUCNR1, succinate has also been shown to activate microbiota-induced type 2 immunity in response to certain infectious agents (46Murphy M.P. O'Neill L.A.J. Krebs cycle reimagined: The emerging roles of succinate and itaconate as signal transducers.Cell. 2018; 174: 780-784Abstract Full Text Full Text PDF Pu