life Review Tricarboxylic Acid (TCA) Cycle Intermediates: Regulators of Immune Responses Inseok Choi , Hyewon Son and Jea-Hyun Baek * School of Life Science, Handong Global University, Pohang, Gyeongbuk 37554, Korea; [email protected] (I.C.); [email protected] (H.S.) * Correspondence: [email protected]; Tel.: +82-54-260-1347 Abstract: The tricarboxylic acid cycle (TCA) is a series of chemical reactions used in aerobic organisms to generate energy via the oxidation of acetylcoenzyme A (CoA) derived from carbohydrates, fatty acids and proteins. In the eukaryotic system, the TCA cycle occurs completely in mitochondria, while the intermediates of the TCA cycle are retained inside mitochondria due to their polarity and hydrophilicity. Under cell stress conditions, mitochondria can become disrupted and release their contents, which act as danger signals in the cytosol. Of note, the TCA cycle intermediates may also leak from dysfunctioning mitochondria and regulate cellular processes. Increasing evidence shows that the metabolites of the TCA cycle are substantially involved in the regulation of immune responses. In this review, we aimed to provide a comprehensive systematic overview of the molecular mechanisms of each TCA cycle intermediate that may play key roles in regulating cellular immunity in cell stress and discuss its implication for immune activation and suppression. Keywords: Krebs cycle; tricarboxylic acid cycle; cellular immunity; immunometabolism 1. Introduction The tricarboxylic acid cycle (TCA, also known as the Krebs cycle or the citric acid Citation: Choi, I.; Son, H.; Baek, J.-H. Tricarboxylic Acid (TCA) Cycle cycle) is a series of chemical reactions used in aerobic organisms (pro- and eukaryotes) to Intermediates: Regulators of Immune generate energy via the oxidation of acetyl-coenzyme A (CoA) derived from carbohydrates, Responses. Life 2021, 11, 69. fatty acids and proteins. In the eukaryotic system, the TCA cycle occurs exclusively in the https://doi.org/10.3390/life11010069 matrix of the mitochondria, which are surrounded by two lipid bilayers and impermeable to polar solutes. The mitochondrial membrane separates energy-producing reactions from Received: 31 December 2020 energy-consuming ones and retains the TCA cycle intermediates, which are small, highly Accepted: 18 January 2021 polar, and electrophilic organic carboxylic acids, inside mitochondria. The intermediates of Published: 19 January 2021 the TCA cycle take several roles as metabolites in metabolisms, such as insulin secretion and fatty acid synthesis (FAS) within mitochondria. In addition, they can be transported out Publisher’s Note: MDPI stays neutral of mitochondria through various shuttle mechanisms to participate in various biological with regard to jurisdictional claims in processes. Of note, increasing evidence suggests that the TCA cycle intermediates play published maps and institutional affil- crucial roles in regulating cellular immunity. In cell stress conditions, the mitochondrial iations. membrane may be disrupted and release the TCA cycle intermediates into the cytosol, thereby impacting the cellular immunity (Figure1). Mitochondria are multifunctional organelles. First, they are known as the power- houses of the cell. Mitochondria are the organelles staging the most efficient metabolic Copyright: © 2021 by the authors. pathways, including the TCA cycle and oxidative phosphorylation supplying the entire Licensee MDPI, Basel, Switzerland. cell with energy in form of adenosine triphosphate (ATP) [1]. In addition, mitochondria This article is an open access article are critical in many other processes, such as detoxification, iron-sulfur cluster biogenesis, distributed under the terms and intracellular calcium homeostasis. Importantly, mitochondria are the central regulators of conditions of the Creative Commons intracellular innate immune responses to pathogens [2–7]. In line with this, they provide Attribution (CC BY) license (https:// stress mediators initiating cell death and autophagy, present adaptor molecules required creativecommons.org/licenses/by/ for the transduction of cellular innate immune signals [2,3], and release mitochondrial 4.0/). Life 2021, 11, 69. https://doi.org/10.3390/life11010069 https://www.mdpi.com/journal/life LifeLife 20212021,, 1111,, 69x FOR PEER REVIEW 22 of 1919 components (e.g. (e.g.,, mitochondrial DNA, mitochondrial RNA, RNA, cytochrome c, c, reactive oxy- gen species (ROS))(ROS)) into the cytosol as dangerdanger signalssignals [[4]4] (Figure(Figure1 1).). MitochondrialMitochondrial DNADNA and RNA may activate intracellular sensors sensing pathogenic DNA and RNA (e.g., the retinoicretinoic-acid-inducible-acid-inducible gene ( (RIG)-I-likeRIG)-I-like proteins, protein kinase R (PKR),(PKR), thethe stimulatorstimulator of interferon genes (STING),(STING), mitochondrial antiviral signaling protein (MAVS),(MAVS), cycliccyclic guanine monophosphate monophosphate (GMP (GMP)—adenosine) - adenosine monophosphate monophosphate (AMP (AMP)) synthase synthase [cGAS] [cGAS] and interferonand interferon--γ-inducibleγ-inducible protein protein 16 (IFI16 16 (IFI16)))) to detect to detect intracellular intracellular pathogens pathogens [2,3,8– [132,3]., 8Stud-–13]. iesStudies have havefound found that mitochondrial that mitochondrial DNADNA leaking leaking from fromdysfunctioning dysfunctioning or disrupted or disrupted mito- chondriamitochondria directly directly activate activatess STING STING providing providing a direct a direct pot potentialential link link between between mitochon- mitochon- drial disease andand inflammatoryinflammatory response response [3 [3,14,15],14,15]. A. A more more recent recent study study showed showed that, that, under un- pathologicder pathologic conditions, conditions, mitochondrial mitochondrial RNA RNA is released is released from from dysfunctioning dysfunctioning mitochondria, mitochon- driin turn,a, in turn, triggering triggering immune immune activation activation [13]. [13]. Under Under conditions conditions of cell of cell stress, stress, mitochondria mitochon- drireleasea release cytochrome cytochrome c and c and ROS ROS inducing inducing apoptosis apoptosis and and cell cell and and tissue tissue destruction destruction [4]. [4]. In Inaddition, addition, TCA TCA cycle cycle intermediates intermediates may may also also leak leak from from mitochondria mitochondria upon upon cell stresscell stress and regulate cellular immune and stress responses (Figure1). and regulate cellular immune and stress responses (Figure 1). Figure 1. Mitochondrial danger signals in cell stress.stress. In the the last last ten ten years, years, it ithas has been been increasingly increasingly recognized recognized that that cellular cellular metabolism metabolism con- trolscontrols immune immune cell functions, cell functions, and it and became it became evident evident that some that metabolic some metabolic pathways pathways activate, andactivate, others and suppress others suppress immune immuneresponses. responses. Innovative Innovative systems systemsbiology biologytools have tools revolu- have tionizedrevolutionized metabolic metabolic research research and andare arecurrently currently providing providing us us with with an an overwhelming amount of of data data and and new new insights, insights, which which need need to be to put be together put together to form to a form coherent a coherent whole. whole.In this review, In this review, we present we present findings findings from studies from studies focusing focusing on the onrole the of role TCA of cycle TCA inter- cycle mediatesintermediates in cellular in cellular immunity immunity and andprovide provide a comprehensive a comprehensive systematic systematic overview overview of the of immunthe immunologicologic features features of each of each TCA TCA cycle cycle intermediate intermediate (Figure (Figure 2). 2). 2. Acetyl-CoA Acetyl-CoA is a molecule produced in the breakdown of carbohydrates, proteins, and fatty acids. It feeds into the TCA cycle by combining with oxaloacetate to citrate. Ac- etyl-CoA is, strictly speaking, not an intermediate of the TCA cycle, Nonetheless, the in- tramitochondrial levels of acetyl-CoA are essential for sustaining TCA cycle activity. Fur- thermore, acetyl-CoA is a precursor for fatty acids, steroids, and certain amino acids, such as glutamate, proline, and arginine [16] and has the important function of providing an acetyl group for histone acetylation in the nucleus. Elevated histone acetylation was found to upregulate the expression of glycolytic enzymes, which are related to immune activa- tion [17]. Of note, acetyl-CoA is generated in multiple compartments, including cytosol. Therefore, it is difficult to estimate to what extent the mitochondrial portion of acetyl-CoA contributes to the expression of glycolytic enzymes. Particularly, the role of mitochondrial Life 2021, 11, x FOR PEER REVIEW 3 of 19 acetyl-CoA leaking from mitochondria during cellular stress in cellular immunity has re- mained elusive [18]. Interestingly, the cytosolic and nucleic levels of acetyl-CoA can in- crease, when citrate is transported from mitochondria into the cytosol by the dicarboxylate antiporter solute carrier family 25 (SLC25A1; mitochondrial citrate carrier, CIC) and con- verted into oxaloacetate and acetyl-CoA by ATP citrate lyase (ACLY). ACLY-derived cy- tosolic and nucleic acetyl-CoA, in turn, may drive histone acetylation affecting cellular functions [19–21]. The acetylation of lysine residues on