More than just protein building blocks: how amino acids and related metabolic pathways fuel macrophage polarization

Abstract

Macrophages represent the first line of defence in innate immune responses and additionally serve important functions for the regulation of host inflammation and tissue homeostasis. The M1/M2 model describes the two extremes of macrophage polarization states, which can be induced by multiple stimuli, most notably by LPS/IFN-γ and IL-4/IL-13. Historically, the expression of two genes encoding for enzymes, which use the same amino acid as their substrate, iNOS and ARG1, has been used to define classically activated M1 (iNOS) and alternatively activated M2 (ARG1) macrophages. This 'arginine dichotomy' has recently become a matter of debate; however, in parallel with the emerging field of immunometabolism there is accumulating evidence that these two enzymes and their related metabolites are fundamentally involved in the intrinsic regulation of macrophage polarization and function. The aim of this review is to highlight recent advances in macrophage biology and immunometabolism with a specific focus on amino acid metabolism and their related metabolic pathways: iNOS/ARG1 (arginine), TCA cycle and OXPHOS (glutamine) as well as the one-carbon metabolism (serine, glycine).

Keywords: TCA cycle; arginase/iNOS; glutamine; immunometabolism; macrophage polarization; nitric oxide; oxidative phosphorylation; polyamines; serine; α-ketoglutarate.

Fig. 1

Amino acids and corresponding metabolic pathways shape M1 macrophage activation. The ETC and mitochondrial complexes are remodelled in response to LPS/IFN‐γ stimulation to increase mitochondrial ROS production and decrease respiration. Mitochondrial ROS has been shown to drive the inflammatory response of M1‐polarized macrophages. Arginine is converted via iNOS to citrulline and NO, which inhibits mitochondrial complex I and complex II and promotes the loss of mitochondrial complexes during late stages of M1 polarization. The TCA cycle also undergoes rapid changes during M1 polarization. In the early stage, production of itaconate leads to accumulation of succinate by inhibition of SDH, which stabilizes HIF‐1α and augments the inflammatory response. NO inhibits ACO2 and IDH, which leads to decreased carbon flux from citrate to α‐KG and, which triggers increased carbon entry from glutamine into the TCA cycle to fuel α‐KG and succinate anaplerosis. The ratio of succinate and α‐KG regulates M1 polarization via PHD‐dependent proline hydroxylation of IKKβ, which is important for activation of NF‐κB signalling. High levels of succinate in the early phase of M1 polarization favour a strong inflammatory response. The later phase is characterized by inhibition of PDHC and OGDC, which leads to a drastic reduction in the levels of citrate, itaconate and succinate. PDHC has been shown to be inhibited by NO‐mediated nitrosation of cysteine residues from DLD, a subunit, which also associates with OGDC. Thus, it is also likely that NO mediates the observed diminished flux from α‐KG to succinate in the late phase. Additionaly, glutamine‐derived α‐KG is important for the induction of endotoxin tolerance and GLS inhibition leads to increased mortality in the context of repeated LPS administrations. The SSP is also involved in M1 polarization by generation of glycine and driving the folate cycle, which is important for glutathione and SAM production. Glutathione production is important for optimal IL‐1β expression and SAM promotes H3K36 trimethylation of inflammatory genes to increase their expression.

Fig. 2

Amino acids and corresponding metabolic pathways shape M2 macrophage activation. In comparison to M1 macrophages, M2‐polarized cells exhibit an intact TCA cycle and enhanced OXPHOS. Furthermore, it is suggested that they prefer glutamine and fatty acids as energy sources in contrast to glucose. Thus, alternatively activated macrophages do not rely on glycolysis as long as they can fuel the TCA cycle with other substrates, for example glutamine. One hallmark of M2 macrophages is the expression of ARG1. This enzyme converts arginine to ornithine, therefore providing the substrate for subsequent polyamine synthesis. Interestingly, intracellular arginine is not only derived from simple environmental uptake, but can also be generated from phagocytosed apoptotic cells in the lysosomes. Its downstream metabolite putrescine leads to HuR‐mediated stabilization of Mcf2 mRNA, a guanine nucleotide exchange factor that activates Rac1. Subsequently, this results in increased actin polymerization and enhanced uptake of apoptotic bodies, a hallmark M2 macrophage function. Additionally, arginine‐derived spermidine is involved in the hypusination of eIF5A, a translation factor, that leadsto the efficient expression of proteins involved in the TCA cycle and in OXPHOS. Another important amino acid for macrophage alternative polarization is glutamine. This nutrient can serve as the precursor metabolite for α‐KG by glutaminolysis (enzymatic reactions involving GLS) that in turn regulates JMJD3‐mediated histone demethylation and thus the expression of M2‐associated genes. Furthermore, α‐KG can feed into the TCA cycle and thereby support enhanced OXPHOS. However, glutamine production via GS might also be critically involved in alternative macrophage activation. Therefore, glutamine per se could influence the phenotype of macrophages by yet unknown mechanisms. For example, this amino acid could serve as a nitrogen source for UDP‐GlcNac, which acts as a substrate for posttranslational modifications of essential M2 marker proteins such as CD206. Another important factor in alternative activation is PPAR‐γ signalling. PPAR‐γ is not only linked to a switch to glutamine metabolism, but also involved in upregulating OXPHOS.