Metabolic Regulation of the NLRP3 Inflammasome : Infectious Microbes & Diseases

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Review Articles

Metabolic Regulation of the NLRP3 Inflammasome

Ye, Qizhen1; Chen, Sheng1,2; Wang, Di1

Editor(s): Wang, Fudi

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Infectious Microbes & Diseases 3(4):p 183-186, December 2021. | DOI: 10.1097/IM9.0000000000000057
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Abstract

Introduction

Inflammation is a complex biological response by the cells to microbial infection, external stimulation, and tissue damage.1 During the process of inflammation, inflammasomes, which are regarded as the threat-assessment organelles of the innate immune system, can sense multiple threatening signals, such as pathogen associated molecular patterns (PAMPs), damage associated molecular patterns (DAMPs), and a variety of metabolites. Among all the inflammasomes, the NOD-, LRR-and pyrin domain-containing protein 3 (NLRP3), can recruit and activate caspase-1 protein through the interaction of PYD, NACHT, and LRR domains to regulate secretion of various cytokines, cell death, and even post infection repair.2 This review focuses on the recent advances in our understanding about the intrinsic metabolic regulation of the NLRP3 inflammasome.

Activation of the NLRP3 inflammasome

NLRP3, as one of the most widely studied inflammasomes, was first discovered by Jürg Tschopp in 2002 and has been shown to act as a key player in inflammation and infection.3 Activation of the NLRP3 inflammasome requires two signals: “priming” and “activation.” A wide range of PAMPs and DAMPs can serve as the first “priming” signals. One of the best known PAMPs in this process is lipopolysaccharide (LPS) from Gram-negative bacteria, and upon its recognition by Toll-like receptor 4 (TLR4), cells transcriptionally upregulate the NLRP3 inflammasome components by activating nuclear factor-kappa B (NF-κB). The priming step also serves as the induction of post-transcriptional modification of NLRP3, which stabilizes NLRP3 in an auto-suppressed state.4,5 The second “activation” signal can be PAMPs, DAMPs, or other endogenous particulates and crystals such as uric acid.6 They can active the NLRP3 inflammasome through diverse upstream mechanisms such as potassium efflux, calcium flux, lysosomal disruption, mitochondrial reactive oxygen species (mtROS) production, the relocalization of cardiolipin as well as the release of oxidized mitochondrial DNA (Ox-mtDNA).7,8 After formation of the NLRP3 inflammasome, activated caspase-1 can cleave not only pro-IL-1β and pro-IL-18 to produce mature cytokines, but also Gasdermin D (GSDMD), whose cleaved N-terminal can form pores in the cytoplasmic membrane that lead to pyroptosis.9,10 During viral infection, viral RNA can also activate NLRP3 through mitochondrial antiviral signaling protein (MAVS) on the outer mitochondrial membrane (OMM) and further trigger a series of inflammatory responses (Figure 1). Recent studies also found that histone deacetylase 3 (HDAC3) can translocate to mitochondria and shape mitochondrial adaptation through deacetylation of the fatty acid oxidation enzyme mitochondrial trifunctional enzyme subunit-alpha (HADHA), leading to optimal NLRP3 inflammasome activation.11 Besides the connection with mitochondria, endoplasmic reticulum and the Golgi apparatus also play a significant role in the activation of the NLRP3 inflammasome.12,13 During this process, the dynein adaptor histone deacetylase 6 (HDAC6) mediates the retrograde transport on the microtubule, which delivers NLRP3 from the trans-Golgi network to the microtubule-organizing center, and the NLRP3 inflammasome achieves its further assembly with the centrosomal kinase NIMA-related kinase 7 (NEK7).14 As a serine/threonine kinase, NEK7 can bind to the leucine-rich repeat domain of NLRP3, and the NLRP3-NEK7 interaction is important for formation of the NLRP3-apoptosis-associated speck-like protein containing a CARD (ASC) complex and further activation,15,16 after which the activated NLRP3 inflammasome shows key roles in the secretion of cytokines and pyroptosis in inflammation.

F1
Figure 1:
Summary of the activation of the NLRP3 inflammasome. The first signal, such as LPS, can transcriptionally up-regulate NF-κB by activating TLR4, leading to the production of pro-IL-1β and pro-IL-18. The second signal, such as PAMPs, DAMPs, and some RNA viruses, can activate NLRP3 through different mechanisms like potassium efflux, calcium flux, lysosomal disruption, mtROS production, and the release of Ox-mtDNA, leading to activation of the NLRP3 inflammasome and further activate caspase-1. The activated caspase-1 cleaves pro-IL-1β and pro-IL-18 into their mature forms and simultaneously cleaves GSDMD, which N-terminal fragments form pores in the cell membrane and release the IL-1β and IL-18 cytokines. DAMPs: damage associated molecular patterns; GSDMD: Gasdermin D; LPS: lipopolysaccharide; mtROS: mitochondrial reactive oxygen species; NLRP3: NOD-, LRR- and pyrin domain-containing protein 3; Ox-mtDNA: oxidized mitochondrial DNA; PAMPs: pathogen associated molecular patterns; TLR4: Toll-like receptor 4.

Role of glycolysis in NLRP3 inflammasome activation

It is widely recognized that the metabolic flux of glycolysis is increased in inflammatory activated macrophages.17 However, there are two different views on the role of glycolysis in NLRP3 inflammasome activation.18 One holds the point that glycolysis is necessary or even a positive modulator during activation of NLRP3, since the mammalian target of rapamycin complex 1 (mTORC1) kinase can increase the level of hexokinase 1 (HK1), which further directly actives NLRP3 and simultaneously causes increased glycolytic flux in macrophage.19 Inhibition or silencing of mTOR and HK1 may also lead to inhibition of the canonical NLRP3 responses.20 Besides, glycolysis can also change septicemic mortality in mice by promoting macrophage inflammasome activation. Mechanistically, a key enzyme in glucose metabolism, pyruvate kinase 2 (PKM2), can induce NLRP3 inflammasome activation during this process by promoting the phosphorylation of eukaryotic translation initiation factor 2 alpha kinase 2 (EIF2AK2), and in vivo administration of a PKM2 specific inhibitor can improve the survival rate of septic mice.21

On the other hand, other studies showed that inhibition of glycolysis can also promote NLRP3 activation. A competitive inhibitor of HK, N-acetylglucosamine-containing bacterial peptidoglycan can promote the release of HK from the OMM and then active the NLRP3 inflammasome.22 Moreover, another study showed that the inhibition of glyceraldehyde phosphate dehydrogenase (GAPDH), which is associated with high NAD+/NADH ratios and mtROS levels, can also lead to activation of the NLRP3.23 To conclude, the role of glycolysis in NLRP3 activation still remains ambiguous and deserves further studies. The present different conclusions may be the result of different experimental systems and conditions. As outlined by the example of the HK, it may play different roles as stimulus or pattern recognition receptor in the different NLRP3 activation pathways.19

Role of lipid metabolism in the NLRP3 inflammasome

Many lipids and their derivatives can regulate activation of the NLRP3 inflammasome. Bile acids derived from cholesterol decomposition can inhibit LPS-induced septic shock and can be used in the treatment of type 2 diabetes mellitus.5 Bile acids can induce the ubiquitination of NLRP3 through the up-regulation of protein kinase A (PKA) and inhibit its activation.24 Prostaglandin E2 (PGE2) also possesses the function to increase the level of intracellular cAMP and inhibits activation of NLRP3. However, the regulatory effect of PGE2 on PKA remains to be studied.25 Our data demonstrated that elevated fatty acid oxidation inhibits NLRP3 inflammasome activation coupled with sustained mitochondria fitness. Omega-3 unsaturated fatty acids also have inhibitory effects through G protein-coupled receptor 120 (GPR120).26 Also, an intermediate product of fatty acid oxidative decomposition, ketone body β-hydroxybutyrate (BHB), can specifically inhibit NLRP3-mediated inflammation in multiple inflammatory models by influencing potassium efflux, which can reduce ASC oligomerization. Furthermore, BHB can also decrease phosphorylation of NF-κB, leading to the inhibition of NLRP3 assembly in neutrophils.27,28 These lipids are not only related to the activation of NLRP3 during infections, but are also closely linked to humans’ diet, obesity, hunger degree and even affect diseases such as type 2 diabetes, suggesting a complicated relations between lipid metabolism, the NLRP3 inflammasome, and systemic metabolic status. Recent studies also showed that NLRP3 is closely linked with health-related factors like diet, since NLRP3-deficient mice are resistant to nephropathy and diet-induced metabolic syndrome symptoms,29 and there is also a high-fat diet model that can induce NLRP3-dependent insulin resistance and type 2 diabetes, which can be relieved by bile acid treatment.5 This reveals that the regulatory factors of NLRP3-mediated inflammation are complex and diverse, and even dietary factors like ketogenic diet can regulate the intensity of inflammation.

Role of amino acids in NLRP3 inflammasome activation

Knowledge on whether amino acid metabolism controls NLRP3 activation is limited.18 However, it has been proven that glutamate is closely related to the inflammatory response mediated by NLRP3. With the presence of pro-inflammatory stimulation, the glutamate consumption in macrophages is increased through pathways like glutaminolysis, and leads to increased succinate production that is required for the expression of pro-IL-1β, further activating the NLRP3 inflammasome.30 Metabolic derivatives of glutamate such as pyruvate and citrate also affect the activation of NLRP3, but the specific mechanisms and effects still remain ambiguous.31 Similarly, other amino acids, such as alanine or serine, which can be metabolically converted to pyruvate, are also speculated to affect the activation of NLRP3 when glycolysis is inhibited.18 In conclusion, the regulation effect of amino acids and their derivatives in NLRP3 inflammasome activation under specific conditions remains to be studied.

Regulation of NLRP3 by mitochondrial metabolism

As an important organelle closely related to glycolysis and energy metabolism, mitochondria also play an important role in activation of the NLRP3 inflammasome, and the mechanism of mitochondrial regulation of the NLRP3 metabolism is also complex. On the one hand, studies have found that some ligands such as mtROS and mtDNA produced by mitochondria can diffuse to NLRP3 and promote its assembly and activation.18 Many NLRP3-mediated inflammations are accompanied by the production of mtROS, while scavengers of mtROS can inhibit these NLRP3-mediated inflammatory responses. Although mtROS can be used as a direct signal to stimulate the NLRP3 inflammasome, several reports demonstrated that mtROS is not required in the process of its activation.7 mtDNA from mitochondria also plays an important role in the activation process through direct or indirect interaction with NLRP3, and the increase of mtDNA levels can also lead to the activation of NLRP3.32 Another theory holds the point that the molecules presented on the surface of mitochondria can recruit and activate the NLRP3 inflammasome.18 With the presence of an activator, MAVS on the mitochondrial surface can react with NLRP3 and promote its translocation from the OMM to further realize NLRP3 activation.33 In addition, cardiolipin transferred from the inner mitochondrial membrane to the OMM can recruit NLRP3 in a mechanism independent of mtROS, which is considered necessary and sufficient in the activation of NLRP3.34 Apart from this, some metabolites produced by the tricarboxylic acid cycle of mitochondria also regulate the activation of NLRP3. Succinate can stabilize hypoxia-inducible transcription factor-1α (HIF-1α) to promote the activation of NLRP3,35 but with the action of succinate dehydrogenase, it will be converted to fumarate, like dimethyl fumarate, which can inhibit activation of the NLRP3 inflammasome.36 Recent researches also found that the Krebs cycle-derived metabolite itaconate and the derivative of itaconate, 4-octyl itaconate, can inhibit NLRP3 inflammasome activation through the inhibition of the “dicarboxypropylated” C548 on NLRP3 and the interaction between NLRP3 and NEK7.37 These points indicate that regulation of mitochondria and its metabolism affect the NLRP3 inflammasome and are closely related to the inflammatory response. Therefore, research on mitochondrial-metabolic regulation of NLRP3 is a promising direction for further studies.

Summary and prospect

This article reviews the metabolic regulation of NLRP3-mediated inflammation by glycolysis, fatty acids, amino acids, and mitochondria. Nutrition and intracellular metabolic reprogramming are generally involved in the regulation of inflammation. The nature and strength of these metabolic regulations are often related to the host's anti-infection effect and prognosis. It is also very important for metabolic regulation to maintain a relatively balanced and stable state. In addition, these metabolites are closely related to some diseases (diabetes), obesity, and dietary factors, which highlights their association with inflammation. At present, the regulatory effects and mechanisms of some metabolites on the NLRP3 inflammasome still remain unclear, but some metabolites have the potential to become drug targets for regulating the inflammatory response, which is worthy of further studies.

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Keywords:

inflammation; metabolic regulation; NLRP3 inflammasome

Copyright © 2021 the Author(s). Published by Wolters Kluwer Health, Inc.