Endogenously produced itaconate negatively regulates innate-driven cytokine production and drives global ubiquitination in human macrophages.

A wide variety of electrophilic derivatives of itaconate, the Kreb's cycle-derived metabolite, are immunomodulatory, yet these derivatives have overlapping and sometimes contradictory activities. Therefore, we generated a genetic system to interrogate the immunomodulatory functions of endogenously produced itaconate in human macrophages. Endogenous itaconate is driven by multiple innate signals restraining inflammatory cytokine production. Endogenous itaconate directly targets cysteine 13 in IRAK4 (disrupting IRAK4 autophosphorylation and activation), drives the degradation of nuclear factor κB, and modulates global ubiquitination patterns. As a result, cells unable to make itaconate overproduce inflammatory cytokines such as tumor necrosis factor alpha (TNFα), interleukin-6 (IL-6), and IL-1ß in response to these innate activators. In contrast, the production of interferon (IFN)ß, downstream of LPS, requires the production of itaconate. These data demonstrate that itaconate is a critical arbiter of inflammatory cytokine production downstream of multiple innate signaling pathways, laying the groundwork for the development of itaconate mimetics for the treatment of autoimmunity.

Metabolic Messengers: itaconate.

The metabolite itaconate has emerged as an important immunoregulator with roles in antibacterial defence, inhibition of inflammation and, more recently, as an inhibitory factor in obesity. Itaconate is one of the most upregulated metabolites in inflammatory macrophages. It is produced owing to the disturbance of the tricarboxylic acid cycle and the diversion of aconitate to itaconate via the enzyme aconitate decarboxylase 1. In immunology, initial studies concentrated on the role of itaconate in inflammatory macrophages where it was shown to be inhibitory, but this has expanded as the impact of itaconate on other cell types is starting to emerge. This review focuses on itaconate as a key immunoregulatory metabolite and describes its diverse mechanisms of action and its many impacts on the immune and inflammatory responses and in cancer. We also examine the clinical relevance of this immunometabolite and its therapeutic potential for immune and inflammatory diseases.

Itaconic acid inhibits nontuberculous mycobacterial growth in pH dependent manner while 4-octyl-itaconic acid enhances THP-1 clearance of nontuberculous mycobacteria in vitro.

Increasingly prevalent, nontuberculous mycobacteria (NTM) infections affect approximately 20% of people with cystic fibrosis (CF). Previous studies of CF sputum identified lower levels of the host metabolite itaconate in those infected with NTM. Itaconate can inhibit the growth of M. tuberculosis (MTB) in vitro via the inhibition of the glyoxylate cycle enzyme (ICL), but its impact on NTM is unclear. To test itaconic acid's (IA) effect on NTM growth, laboratory and CF clinical strains of Mycobacterium abscessus and Mycobacterium avium were cultured in 7H9 minimal media supplemented with 1-10 mM of IA and short-chain fatty acids (SCFA). M. avium and M. abscessus grew when supplemented with SCFAs, whereas the addition of IA (≥ 10 mM) completely inhibited NTM growth. NTM supplemented with acetate or propionate and 5 mM IA displayed slower growth than NTM cultured with SCFA and ≤ 1 mM of IA. However, IA's inhibition of NTM was pH dependent; as similar and higher quantities (100 mM) of pH adjusted IA (pH 7) did not inhibit growth in vitro, while in an acidic minimal media (pH 6.1), 1 to 5 mM of non-pH adjusted IA inhibited growth. None of the examined isolates displayed the ability to utilize IA as a carbon source, and IA added to M. abscessus isocitrate lyase (ICL) decreased enzymatic activity. Lastly, the addition of cell-permeable 4-octyl itaconate (4-OI) to THP-1 cells enhanced NTM clearance, demonstrating a potential role for IA/itaconate in host defense against NTM infections.

Neutrophil-macrophage communication via extracellular vesicle transfer promotes itaconate accumulation and ameliorates cytokine storm syndrome.

Cytokine storm syndrome (CSS) is a life-threatening systemic inflammatory syndrome involving innate immune hyperactivity triggered by various therapies, infections, and autoimmune conditions. However, the potential interplay between innate immune cells is not fully understood. Here, using poly I:C and lipopolysaccharide (LPS)-induced cytokine storm models, a protective role of neutrophils through the modulation of macrophage activation was identified in a CSS model. Intravital imaging revealed neutrophil-derived extracellular vesicles (NDEVs) in the liver and spleen, which were captured by macrophages. NDEVs suppressed proinflammatory cytokine production by macrophages when cocultured in vitro or infused into CSS models. Metabolic profiling of macrophages treated with NDEV revealed elevated levels of the anti-inflammatory metabolite, itaconate, which is produced from cis-aconitate in the Krebs cycle by cis-aconitate decarboxylase (Acod1, encoded by Irg1). Irg1 in macrophages, but not in neutrophils, was critical for the NDEV-mediated anti-inflammatory effects. Mechanistically, NDEVs delivered miR-27a-3p, which suppressed the expression of Suclg1, the gene encoding the enzyme that metabolizes itaconate, thereby resulting in the accumulation of itaconate in macrophages. These findings demonstrated that neutrophil-to-macrophage communication mediated by extracellular vesicles is critical for promoting the anti-inflammatory reprogramming of macrophages in CSS and may have potential implications for the treatment of this fatal condition.

The IRG1-itaconate axis protects from cholesterol-induced inflammation and atherosclerosis.

Atherosclerosis is fueled by a failure to resolve lipid-driven inflammation within the vasculature that drives plaque formation. Therapeutic approaches to reverse atherosclerotic inflammation are needed to address the rising global burden of cardiovascular disease (CVD). Recently, metabolites have gained attention for their immunomodulatory properties, including itaconate, which is generated from the tricarboxylic acid-intermediate cis-aconitate by the enzyme Immune Responsive Gene 1 (IRG1/ACOD1). Here, we tested the therapeutic potential of the IRG1-itaconate axis for human atherosclerosis. Using single-cell RNA sequencing (scRNA-seq), we found that IRG1 is up-regulated in human coronary atherosclerotic lesions compared to patient-matched healthy vasculature, and in mouse models of atherosclerosis, where it is primarily expressed by plaque monocytes, macrophages, and neutrophils. Global or hematopoietic Irg1-deficiency in mice increases atherosclerosis burden, plaque macrophage and lipid content, and expression of the proatherosclerotic cytokine interleukin (IL)-1ß. Mechanistically, absence of Irg1 increased macrophage lipid accumulation, and accelerated inflammation via increased neutrophil extracellular trap (NET) formation and NET-priming of the NLRP3-inflammasome in macrophages, resulting in increased IL-1ß release. Conversely, supplementation of the Irg1-itaconate axis using 4-octyl itaconate (4-OI) beneficially remodeled advanced plaques and reduced lesional IL-1ß levels in mice. To investigate the effects of 4-OI in humans, we leveraged an ex vivo systems-immunology approach for CVD drug discovery. Using CyTOF and scRNA-seq of peripheral blood mononuclear cells treated with plasma from CVD patients, we showed that 4-OI attenuates proinflammatory phospho-signaling and mediates anti-inflammatory rewiring of macrophage populations. Our data highlight the relevance of pursuing IRG1-itaconate axis supplementation as a therapeutic approach for atherosclerosis in humans.

Metabolic rewiring promotes anti-inflammatory effects of glucocorticoids.

Glucocorticoids represent the mainstay of therapy for a broad spectrum of immune-mediated inflammatory diseases. However, the molecular mechanisms underlying their anti-inflammatory mode of action have remained incompletely understood1. Here we show that the anti-inflammatory properties of glucocorticoids involve reprogramming of the mitochondrial metabolism of macrophages, resulting in increased and sustained production of the anti-inflammatory metabolite itaconate and consequent inhibition of the inflammatory response. The glucocorticoid receptor interacts with parts of the pyruvate dehydrogenase complex whereby glucocorticoids provoke an increase in activity and enable an accelerated and paradoxical flux of the tricarboxylic acid (TCA) cycle in otherwise pro-inflammatory macrophages. This glucocorticoid-mediated rewiring of mitochondrial metabolism potentiates TCA-cycle-dependent production of itaconate throughout the inflammatory response, thereby interfering with the production of pro-inflammatory cytokines. By contrast, artificial blocking of the TCA cycle or genetic deficiency in aconitate decarboxylase 1, the rate-limiting enzyme of itaconate synthesis, interferes with the anti-inflammatory effects of glucocorticoids and, accordingly, abrogates their beneficial effects during a diverse range of preclinical models of immune-mediated inflammatory diseases. Our findings provide important insights into the anti-inflammatory properties of glucocorticoids and have substantial implications for the design of new classes of anti-inflammatory drugs.

Itaconate in host inflammation and defense.

Immune cells undergo rapid and extensive metabolic changes during inflammation. In addition to contributing to energetic and biosynthetic demands, metabolites can also function as signaling molecules. Itaconate (ITA) rapidly accumulates to high levels in myeloid cells under infectious and sterile inflammatory conditions. This metabolite binds to and regulates the function of diverse proteins intracellularly to influence metabolism, oxidative response, epigenetic modification, and gene expression and to signal extracellularly through binding the G protein-coupled receptor (GPCR). Administration of ITA protects against inflammatory diseases and blockade of ITA production enhances antitumor immunity in preclinical models. In this article, we review ITA metabolism and its regulation, discuss its target proteins and mechanisms, and conjecture a rationale for developing ITA-based therapeutics to treat inflammatory diseases and cancer.

The impact of Dimethyl itaconate on c-Fos expression in the spinal cord in experimental pain models.

Itaconate has been found to have potent anti-inflammatory effects and is being explored as a potential treatment for inflammatory diseases. However, its ability to relieve nociception and the mechanisms behind it are not yet understood. Our research aims to investigate the nociception-relieving properties of dimethyl itaconate (DMI) in the formalin test and writhing test. In male Wistar rats, Itaconic acid was injected intraperitoneally (i.p.). The formalin test and writhing test were conducted to determine the nociceptive behaviors. The spinal cords were removed from the rats and analyzed for c-fos protein expression. The study found that administering DMI 10 and 20 mg/kg reduced nociception in formalin and writhing tests. Injection of formalin into the periphery of the body led to an increase in the expression of c-fos in the spinal cord, which was alleviated by DMI 20 mg/kg. Similarly, acetic acid injection into the peritoneal cavity caused an increase in c-fos expression in the spinal cord, which was then reduced by 20 mg/kg. According to our findings, DMI reduced nociception in rats during the formalin and writhing tests. One possible explanation for this outcome is that the decrease in c-fos protein expression may be attributed to the presence of DMI.

The Structure of the Cardiac Mitochondria Respirasome Is Adapted for the ß-Oxidation of Fatty Acids.

It is well known that in the heart and kidney mitochondria, more than 95% of ATP production is supported by the ß-oxidation of long-chain fatty acids. However, the ß-oxidation of fatty acids by mitochondria has been studied much less than the substrates formed during the catabolism of carbohydrates and amino acids. In the last few decades, several discoveries have been made that are directly related to fatty acid oxidation. In this review, we made an attempt to re-evaluate the ß-oxidation of long-chain fatty acids from the perspectives of new discoveries. The single set of electron transporters of the cardiac mitochondrial respiratory chain is organized into three supercomplexes. Two of them contain complex I, a dimer of complex III, and two dimers of complex IV. The third, smaller supercomplex contains a dimer of complex III and two dimers of complex IV. We also considered other important discoveries. First, the enzymes of the ß-oxidation of fatty acids are physically associated with the respirasome. Second, the ß-oxidation of fatty acids creates the highest level of QH2 and reverses the flow of electrons from QH2 through complex II, reducing fumarate to succinate. Third, ß-oxidation is greatly stimulated in the presence of succinate. We argue that the respirasome is uniquely adapted for the ß-oxidation of fatty acids. The acyl-CoA dehydrogenase complex reduces the membrane's pool of ubiquinone to QH2, which is instantly oxidized by the smaller supercomplex, generating a high energization of mitochondria and reversing the electron flow through complex II, which reverses the electron flow through complex I, increasing the NADH/NAD+ ratio in the matrix. The mitochondrial nicotinamide nucleotide transhydrogenase catalyzes a hydride (H-, a proton plus two electrons) transfer across the inner mitochondrial membrane, reducing the cytosolic pool of NADP(H), thus providing the heart with ATP for muscle contraction and energy and reducing equivalents for the housekeeping processes.

ABCG2 is an itaconate exporter that limits antibacterial innate immunity by alleviating TFEB-dependent lysosomal biogenesis.

Itaconate is a metabolite that synthesized from cis-aconitate in mitochondria and transported into the cytosol to exert multiple regulatory effects in macrophages. However, the mechanism by which itaconate exits from macrophages remains unknown. Using a genetic screen, we reveal that itaconate is exported from cytosol to extracellular space by ATP-binding cassette transporter G2 (ABCG2) in an ATPase-dependent manner in human and mouse macrophages. Elevation of transcription factor TFEB-dependent lysosomal biogenesis and antibacterial innate immunity are observed in inflammatory macrophages with deficiency of ABCG2-mediated itaconate export. Furthermore, deficiency of ABCG2-mediated itaconate export in macrophages promotes antibacterial innate immune defense in a mouse model of S. typhimurium infection. Thus, our findings identify ABCG2-mediated itaconate export as a key regulatory mechanism that limits TFEB-dependent lysosomal biogenesis and antibacterial innate immunity in inflammatory macrophages, implying the potential therapeutic utility of blocking itaconate export in treating human bacterial infections.

TREM2 macrophage promotes cardiac repair in myocardial infarction by reprogramming metabolism via SLC25A53.

Efferocytosis and metabolic reprogramming of macrophages play crucial roles in myocardial infarction (MI) repair. TREM2 has been proven to participate in phagocytosis and metabolism, but how it modulates myocardial infarction remains unclear. In this study, we showed that macrophage-specific TREM2 deficiency worsened cardiac function and impaired post-MI repair. Using RNA-seq, protein and molecular docking, and Targeted Metabolomics (LC-MS), our data demonstrated that macrophages expressing TREM2 exhibited decreased SLC25A53 transcription through the SYK-SMAD4 signaling pathway after efferocytosis, which impaired NAD+ transport into mitochondria, downregulated SLC25A53 thereby causing the breakpoint in the TCA cycle and subsequently increased itaconate production. In vitro experiments confirmed that itaconate secreted by TREM2+ macrophages inhibited cardiomyocyte apoptosis and promoted fibroblast proliferation. Conversely, overexpression of TREM2 in macrophages could improve cardiac function. In summary, our study reveals a novel role for macrophage-specific TREM2 in MI, connecting efferocytosis to immune metabolism during cardiac repair.

Itaconate promotes hepatocellular carcinoma progression by epigenetic induction of CD8+ T-cell exhaustion.

Itaconate is a well-known immunomodulatory metabolite; however, its role in hepatocellular carcinoma (HCC) remains unclear. Here, we find that macrophage-derived itaconate promotes HCC by epigenetic induction of Eomesodermin (EOMES)-mediated CD8+ T-cell exhaustion. Our results show that the knockout of immune-responsive gene 1 (IRG1), responsible for itaconate production, suppresses HCC progression. Irg1 knockout leads to a decreased proportion of PD-1+ and TIM-3+ CD8+ T cells. Deletion or adoptive transfer of CD8+ T cells shows that IRG1-promoted tumorigenesis depends on CD8+ T-cell exhaustion. Mechanistically, itaconate upregulates PD-1 and TIM-3 expression levels by promoting succinate-dependent H3K4me3 of the Eomes promoter. Finally, ibuprofen is found to inhibit HCC progression by targeting IRG1/itaconate-dependent tumor immunoevasion, and high IRG1 expression in macrophages predicts poor prognosis in HCC patients. Taken together, our results uncover an epigenetic link between itaconate and HCC and suggest that targeting IRG1 or itaconate might be a promising strategy for HCC treatment.

Metabolite itaconate in host immunoregulation and defense.

Metabolic states greatly influence functioning and differentiation of immune cells. Regulating the metabolism of immune cells can effectively modulate the host immune response. Itaconate, an intermediate metabolite derived from the tricarboxylic acid (TCA) cycle of immune cells, is produced through the decarboxylation of cis-aconitate by cis-aconitate decarboxylase in the mitochondria. The gene encoding cis-aconitate decarboxylase is known as immune response gene 1 (IRG1). In response to external proinflammatory stimulation, macrophages exhibit high IRG1 expression. IRG1/itaconate inhibits succinate dehydrogenase activity, thus influencing the metabolic status of macrophages. Therefore, itaconate serves as a link between macrophage metabolism, oxidative stress, and immune response, ultimately regulating macrophage function. Studies have demonstrated that itaconate acts on various signaling pathways, including Keap1-nuclear factor E2-related factor 2-ARE pathways, ATF3-IκBζ axis, and the stimulator of interferon genes (STING) pathway to exert antiinflammatory and antioxidant effects. Furthermore, several studies have reported that itaconate affects cancer occurrence and development through diverse signaling pathways. In this paper, we provide a comprehensive review of the role IRG1/itaconate and its derivatives in the regulation of macrophage metabolism and functions. By furthering our understanding of itaconate, we intend to shed light on its potential for treating inflammatory diseases and offer new insights in this field.

Candidatus Accumulibacter use fermentation products for enhanced biological phosphorus removal.

Previous research suggested that two major groups of polyphosphate-accumulating organisms (PAOs), i.e., Ca. Accumulibacter and Tetrasphaera, play cooperative roles in enhanced biological phosphorus removal (EBPR). The fermentation of complex organic compounds by Tetrasphaera provides carbon sources for Ca. Accumulibacter. However, the viability of the fermentation products (e.g., lactate, succinate, alanine) as carbon sources for Ca. Accumulibacter and their potential effects on the metabolism of Ca. Accumulibacter were largely unknown. This work for the first time investigated the capability and metabolic details of Ca. Accumulibacter cognatus clade IIC strain SCUT-2 (enriched in a lab-scale reactor with a relative abundance of 42.8%) in using these fermentation products for EBPR. The enrichment culture was able to assimilate lactate and succinate with the anaerobic P release to carbon uptake ratios of 0.28 and 0.36 P mol/C mol, respectively. In the co-presence of acetate, the uptake of lactate was strongly inhibited, since two substrates shared the same transporter as suggested by the carbon uptake bioenergetic analysis. When acetate and succinate were fed at the same time, Ca. Accumulibacter assimilated two carbon sources simultaneously. Proton motive force (PMF) was the key driving force (up to 90%) for the uptake of lactate and succinate by Ca. Accumulibacter. Apart from the efflux of proton in symport with phosphate via the inorganic phosphate transport system, translocation of proton via the activity of fumarate reductase contributed to the generation of PMF, which agreed with the fact that PHV was a major component of PHA when lactate and succinate were used as carbon sources, involving the succinate-propionate pathway. Metabolic models for the usage of lactate and succinate by Ca. Accumulibacter for EBPR were built based on the combined physiological, biochemical, metagenomic, and metatranscriptomic analyses. Alanine was shown as an invalid carbon source for Ca. Accumulibacter. Instead, it significantly and adversely affected Ca. Accumulibacter-mediated EBPR. Phosphate release was observed without alanine uptake. Significant inhibitions on the aerobic phosphate uptake was also evident. Overall, this study suggested that there might not be a simply synergic relationship between Ca. Accumulibacter and Tetrasphaera. Their interactions would largely be determined by the kind of fermentation products released by the latter.

The yin and yang of itaconate metabolism and its impact on the tumor microenvironment.

The tumor microenvironment (TME) consists of a network of metabolically interconnected tumor and immune cell types. Macrophages influence the metabolic composition within the TME, which directly impacts the metabolic state and drug response of tumors. The accumulation of oncometabolites, such as succinate, fumarate, and 2-hydroxyglutarate, represents metabolic vulnerabilities in cancer that can be targeted therapeutically. Immunometabolites are emerging as metabolic regulators of the TME impacting immune cell functions and cancer cell growth. Here, we discuss recent discoveries on the potential impact of itaconate on the TME. We highlight how itaconate influences metabolic pathways relevant to immune responses and cancer cell proliferation. We also consider the therapeutic implications of manipulating itaconate metabolism as an immunotherapeutic strategy to constrain tumor growth.

Itaconate-producing neutrophils regulate local and systemic inflammation following trauma.

Modulation of the immune response to initiate and halt the inflammatory process occurs both at the site of injury as well as systemically. Due to the evolving role of cellular metabolism in regulating cell fate and function, tendon injuries that undergo normal and aberrant repair were evaluated by metabolic profiling to determine its impact on healing outcomes. Metabolomics revealed an increasing abundance of the immunomodulatory metabolite itaconate within the injury site. Subsequent single-cell RNA-Seq and molecular and metabolomic validation identified a highly mature neutrophil subtype, not macrophages, as the primary producers of itaconate following trauma. These mature itaconate-producing neutrophils were highly inflammatory, producing cytokines that promote local injury fibrosis before cycling back to the bone marrow. In the bone marrow, itaconate was shown to alter hematopoiesis, skewing progenitor cells down myeloid lineages, thereby regulating systemic inflammation. Therapeutically, exogenous itaconate was found to reduce injury-site inflammation, promoting tenogenic differentiation and impairing aberrant vascularization with disease-ameliorating effects. These results present an intriguing role for cycling neutrophils as a sensor of inflammation induced by injury - potentially regulating immune cell production in the bone marrow through delivery of endogenously produced itaconate - and demonstrate a therapeutic potential for exogenous itaconate following tendon injury.

Protective effects of the succinate/SUCNR1 axis on damaged hepatocytes in NAFLD.

OBJECTIVE: Succinate and succinate receptor 1 (SUCNR1) are linked to fibrotic remodeling in models of non-alcoholic fatty liver disease (NAFLD), but whether they have roles beyond the activation of hepatic stellate cells remains unexplored. We investigated the succinate/SUCNR1 axis in the context of NAFLD specifically in hepatocytes. METHODS: We studied the phenotype of wild-type and Sucnr1-/- mice fed a choline-deficient high-fat diet to induce non-alcoholic steatohepatitis (NASH), and explored the function of SUCNR1 in murine primary hepatocytes and human HepG2 cells treated with palmitic acid. Lastly, plasma succinate and hepatic SUCNR1 expression were analyzed in four independent cohorts of patients in different NAFLD stages. RESULTS: Sucnr1 was upregulated in murine liver and primary hepatocytes in response to diet-induced NASH. Sucnr1 deficiency provoked both beneficial (reduced fibrosis and endoplasmic reticulum stress) and detrimental (exacerbated steatosis and inflammation and reduced glycogen content) effects in the liver, and disrupted glucose homeostasis. Studies in vitro revealed that hepatocyte injury increased Sucnr1 expression, which when activated improved lipid and glycogen homeostasis in damaged hepatocytes. In humans, SUCNR1 expression was a good determinant of NAFLD progression to advanced stages. In a population at risk of NAFLD, circulating succinate was elevated in patients with a fatty liver index (FLI) ≥60. Indeed, succinate had good predictive value for steatosis diagnosed by FLI, and improved the prediction of moderate/severe steatosis through biopsy when added to an FLI algorithm. CONCLUSIONS: We identify hepatocytes as target cells of extracellular succinate during NAFLD progression and uncover a hitherto unknown function for SUCNR1 as a regulator of hepatocyte glucose and lipid metabolism. Our clinical data highlight the potential of succinate and hepatic SUCNR1 expression as markers to diagnose fatty liver and NASH, respectively.

Membrane Potential-requiring Succinate Dehydrogenase Constitutes the Key to Propionate Oxidation and Is Unique to Syntrophic Propionate-oxidizing Bacteria.

Propionate oxidation in Pelotomaculum thermopropionicum is performed under a thermodynamic limit. The most energetically unfavorable reaction in the propionate oxidation pathway is succinate oxidation. Based on previous genomic and transcriptomic ana-lyses, succinate oxidation in P. thermopropionicum under propionate-oxidizing conditions is conducted by the membrane-bound forms of two succinate dehydrogenases (SDHs). We herein examined the activity of SDH, the mechanisms underlying the succinate oxidation reaction in P. thermopropionicum, and the importance of the protein sequences of related genes. SDH activity was highly localized to the membrane fraction. An ana-lysis of the soluble fraction revealed that fumarate reductase received electrons from NADH, suggesting the involvement of membrane-bound SDH in propionate oxidation. We utilized an uncoupler and inhibitors of adenosine triphosphate (ATP) synthase and membrane-bound SDH to investigate whether the membrane potential of P. thermopropionicum supports propionate oxidation alongside hydrogen production. These chemicals inhibited hydrogen production, indicating that membrane-bound SDH requires a membrane potential for succinate oxidation, and this membrane potential is maintained by ATP synthase. In addition, the phylogenetic distribution of the flavin adenine dinucleotide-binding subunit and conserved amino acid sequences of the cytochrome b subunit of SDHs in propionate-oxidizing bacteria suggests that membrane-bound SDHs possess specific conserved amino acid residues that are strongly associated with efficient succinate oxidation in syntrophic propionate-oxidizing bacteria.

4-octyl itaconate as a metabolite derivative inhibits inflammation via alkylation of STING.

The Krebs cycle-derived metabolite itaconate, whose production is catalyzed by immune response gene 1 (IRG1), has potential to link immunity and metabolism in activated macrophages through alkylation or competitive inhibition of target proteins. In support of this, our previous study demonstrated that the stimulator of interferon genes (STING) signaling platform functions as a hub in macrophage immunity and has a profound impact on the prognosis of sepsis. Interestingly, we find that itaconate, an endogenous immunomodulator, can significantly inhibit the activation of STING signaling. Moreover, 4-octyl itaconate (4-OI), which is a permeable itaconate derivative, can alkylate cysteine sites 65, 71, 88, and 147 of STING, thereby inhibiting its phosphorylation. Furthermore, itaconate and 4-OI inhibit the production of inflammatory factors in sepsis models. Our results broaden the knowledge on the role of the IRG1-itaconate axis in immunomodulation and highlight itaconate and its derivatives as potential therapeutic agents in sepsis.

The IRG1-Itaconate axis: A regulatory hub for immunity and metabolism in macrophages.

Metabolism could be served as a guiding force for immunity, and macrophages undergo drastic metabolic reprogramming during inflammatory processes, including enhancing glycolysis and reshaping the tricarboxylic acid cycle (TCA) cycle. The disrupted TCA cycle facilitates itaconate accumulation, consistent with the significant up-regulation of immune response gene 1 (IRG1) in activated macrophages. IRG1 catalyzes the decarboxylation of cis-aconitate to synthesize itaconate, and notably, the IRG1-Itaconate axis has excellent potential to link macrophages' immunity and metabolism. Here, we review vital molecules that affect the activation of the IRG1-Itaconate axis, including interferon regulatory factor 1/9 (IRF1/9), transcription 1 and 3 (STAT1/3), CCAAT enhancer-binding protein ß (C/EBPß), and the protein kinase C (PKC). We then focus on how the IRG1-Itaconate axis regulates the inflammatory pathway in macrophages, proposed to involve kelch-like ECH-associated protein 1 (Keap1), NOD-, LRR- and pyrin domain-containing 3 (NLRP3), gasdermin D (GSDMD), activating transcription factor 3 (ATF3), receptor-interacting protein kinase-3 (RIPK3), et al. In addition, we provide an overview of the way the axis participates in the metabolism of macrophages. Eventually, we summarize current connections between the IRG1-Itaconate axis and inflammatory diseases, bringing light to new therapeutic opportunities in inflammatory diseases.