Sirtuin-dependent metabolic and epigenetic regulation of macrophages during tuberculosis

Abstract

Macrophages are the preeminent phagocytic cells which control multiple infections. Tuberculosis a leading cause of death in mankind and the causative organism Mycobacterium tuberculosis (MTB) infects and persists in macrophages. Macrophages use reactive oxygen and nitrogen species (ROS/RNS) and autophagy to kill and degrade microbes including MTB. Glucose metabolism regulates the macrophage-mediated antimicrobial mechanisms. Whereas glucose is essential for the growth of cells in immune cells, glucose metabolism and its downsteam metabolic pathways generate key mediators which are essential co-substrates for post-translational modifications of histone proteins, which in turn, epigenetically regulate gene expression. Herein, we describe the role of sirtuins which are NAD⁺-dependent histone histone/protein deacetylases during the epigenetic regulation of autophagy, the production of ROS/RNS, acetyl-CoA, NAD⁺, and S-adenosine methionine (SAM), and illustrate the cross-talk between immunometabolism and epigenetics on macrophage activation. We highlight sirtuins as emerging therapeutic targets for modifying immunometabolism to alter macrophage phenotype and antimicrobial function.

Keywords: SIRTUIN; autophagy; glycolysis; histone modifications; human macrophages; metabolism.

Figure 1

A diagram of glycolysis and glycolic enzymes involved in the regulation of immune responses.

Figure 2

Up-regulated production of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in M1- macrophages. Increased production of NADH from reduction of NAD⁺ via glycolysis fuels the electron transport chain (ETC) complex 1 to generate O₂ ^.-; NAD⁺ is continuously replenished by upregulated NAD⁺ de novo synthesis from tryptophan metabolism. Increased production of NADPH via the pentose-phosphate-pathway (PPP), which is also up-regulated in M1- type macrophages (MΦs), fuels the ETC to produce O₂ ^.-. O₂ ^.- is also generated via succinate dehydrogenase (SDH) which is coupled with FADH2/FAD redox reaction in complex II of ETC. In M1-MΦs, RNS is derived from nitric oxide (NO) which is produced by arginine metabolism through iNOS/NOS2. Mitochondrial ROS/RNS regulates phagocytosis, bacterial killing, and polarization towards M1-MΦs via ATG and MAPK activation. Additional symbols: 1,3-BPG, 1,3-bisphosphoglyceric acid; α-KG, alpha-ketoglutarate; ATG, Autophagy regulating gene; CIT, citrate; DHAP, dihydroxyacetone phosphate; F-1, 6-P, Fructose-1, 6-biphosphate; F-6-P, Fructose-6-phosphate; FUM, fumarate; G-3-P, glycerol-3-phosphate; G-6-P, glucose-6-phosphate; GABA, γ-aminobutyrate; GA, glutaminase; GABA-T, GABA transferase; GAD, glutamate decarboxylase; GDH, Glutamate dehydrogenase; Gln, glutamine; Glu, glutamate; GLUT1, glucose transporter protein type 1; iCIT, isocitrate; IDO, Indoleamine-pyrrole 2,3-dioxygenase; Kyn, kynurenine; Lac, lactate; MAL, malate; OAA, oxaloacetic acid Pyr, pyruvate; SSA, succinate semialdehyde; SSADH, succinate semialdehyde dehydrogenase; SUC-CoA, succinyl-CoA; SUC, succinate.

Figure 3

Impact of glucose metabolism on autophagy in macrophages during tuberculosis. (A) Regulation of autophagy by glucose homeostasis (Left: glucose starvation; Right: glucose repletion) dependent metabolic sensor kinases. Description of the scheme is referred to the text. (B) Glycolysis-promoted histone lactylation and acetylation during macrophage polarization. Upregulation of glycolysis in M1-Mϕs increases lactate production from pyruvate; however, excess of lactate provided exogenously pushes the equilibrium of the conversion between pyruvate and lactate further to the synthesis of citrate from pyruvate and the former is broken up to acetyl -CoA by ACLY; this results in the elevation of both global histone acetylation and acetylation of chromatins associated with the promoters of genes which promote polarization towards M2-Mϕs.

Figure 4

Sirtuin5 plays a vital role in arginine metabolism during macrophage activation and polarization. Arginine is converted into citrulline to release NO in M1-MΦs where iNOS/NOS2 is up-regulated, whereas arginine is converted into ornithine in M2-MΦs where ARG1 is up-regulated. In addition, arginine metabolism is regulated by glutamine metabolism which is involved in the urea cycle by N-acetylglutamate (NAG). NAG which is an allosteric activator and is required for the initial and rate-limiting enzyme of the urea cycle, carbamoyl phosphate synthetase 1 (CPS1). The formation of this unique co-substrate from glutamate and acetyl Coenzyme-A is catalyzed by NAG synthase (NAGS). Sirtuin-5 (SIRT5) desuccinates and activates CPS1 to promote the formation of carbamoyl phosphate from ammonia. Carbamoyl phosphate can modify ornithine to form citrulline through the enzyme ornithine transcarbomoylase (OTC). Citrulline can react with aspartate facilitated by the catalytic enzyme arginosuccinate synthetase (ASS1) to form arginosuccinate, which can return to arginine and fumarate through argininosuccinate lyase (ASL). Both aspartate and ornithine can be acetylated to form acetylated aspartate (NAA) and acetylated ornithine (NAO). Asymmetric di-methylated arginine (ADMA/R_me2) can be hydrolyzed by enzyme dimethylarginine dimethylaminohydrolase (DDAH) into citrulline and dimethylamine. In bacteria, arginine biosynthesis can start with glutamate acetylation and a set of bacterium-specific catalytic enzymes (ArgA-H) are involved. Additional Symbols: NAT8L, N-acetyltransferase 8 like; PRMT, Protein arginine methyltransferase.

Figure 5

Regulation of Histone acetylation and deacetylation by metabolism-generated acetyl-CoA and NAD⁺. Acetyl-CoA is an essential co-substrate of histone acetyltransferase (HAT) that is mainly generated from glycolysis and fatty-aid β-oxidation; it is required for histone acetylation, amino acid acetylation including forming n-acetyl-aspartate (NAA), n-acetyl-glutamate (NAG), and n-acetyl-ornithine (NAO), and fatty-acid synthesis. NAD⁺ is an essential co-substrate of NAD⁺-dependent histone deacetylases that includes Sirtuin proteins. NAD⁺ is consumed by glycolysis (2 molecules) and TCA cycle (3 molecules), whereas it is regenerated from NADH oxidation via conversion of pyruvate into lactate and through ETC complex I. NAD⁺ is biosynthesized from quinolinic acid, the end product of tryptophan metabolism, catalyzed by the rate-limiting enzyme quinolinate phosphoribosyl transferase (QPRT). The NAD⁺ de novo biosynthesis pathway is coupled with and regulated by the NAD⁺ salvage pathway. Regulation of NAD⁺ usage and production in MΦs controls Sirt deacetylase activity, and hence, histone acetylation level. Additional Symbols: ACLY, ATP-citrate lyase; NAM, niacinamide; NAMPT, nicotinamide phosphoribosylransferase; NMN, nicotinamide mononucleotide; NMNAT, nicotinamide nucleotide adenylyltransferase; Orn, ornithine; PHGDH, phosphoglycerate dehydrogenase; p-Pyr, phosphopyruvate.

Figure 6

Histone methylation through one-carbon metabolism and serine biosynthesis in macrophages. Serine is biosynthesized from 3-phophoglycerol (3PG), an intermediate of glycolysis, by phosphoglycerol dehydrogenase (PHGDH) to form phosphopyruvate (p-Pyr) and catalyzed by phosphoserine aminotransferase (PSAT) to form phosphoserine (p-Ser) and then phosphoserine phosphate (PSPH) to form serine. With the aid of catalytic enzyme serine hydroxymethyltransferase (SHMT), serine is further converted into glycine donating one-carbon (a methyl group) to the tetrahydrofolate (THF) in the folate cycle to form sequentially 5, 10-methylenetetrahydrofolate (5,10-meTHF) and 5-methyl-tetrahydrofolate (meTHF); the methyl group of the latter is transferred to homocysteine (Hcy) to form methionine (Met) and S-adenosyl-methionine (SAM). SAM is the co-substrate of methyltransferases for DNA and histone methylation. Methionine and serine can also be respectively delivered from extracellular environment to the cells by their transporters, L-type amino acid transporter/solute carrier family member 5 (LAT1/SLC7A5) and alanine/serine/cysteine/threonine transporter 1 (ASCT1). The methyl transfer from meTHF to Hcy needs methionine synthesis (MS/MTR) and its co-substrate vitamin B₁₂. In M1-MΦs, elevated nitric oxide (NO) poisons vitamin B₁₂ causing deactivation of MTR and the disruption of one-carbon metabolism, resulting in reduced Met and SAM for histones/DNA methylation. In Mycobacterium tuberculosis MTB) infected MΦs, independent of vitamin B₁₂, the pathogen can bypass the one-carbon metabolic pathway to synthesize methionine and SAM through homoserine, a product of aspartate metabolic pathway.