The role of itaconate in host defense and inflammation

Abstract

Macrophages exposed to inflammatory stimuli including LPS undergo metabolic reprogramming to facilitate macrophage effector function. This metabolic reprogramming supports phagocytic function, cytokine release, and ROS production that are critical to protective inflammatory responses. The Krebs cycle is a central metabolic pathway within all mammalian cell types. In activated macrophages, distinct breaks in the Krebs cycle regulate macrophage effector function through the accumulation of several metabolites that were recently shown to have signaling roles in immunity. One metabolite that accumulates in macrophages because of the disturbance in the Krebs cycle is itaconate, which is derived from cis-aconitate by the enzyme cis-aconitate decarboxylase (ACOD1), encoded by immunoresponsive gene 1 (Irg1). This Review focuses on itaconate's emergence as a key immunometabolite with diverse roles in immunity and inflammation. These roles include inhibition of succinate dehydrogenase (which controls levels of succinate, a metabolite with multiple roles in inflammation), inhibition of glycolysis at multiple levels (which will limit inflammation), activation of the antiinflammatory transcription factors Nrf2 and ATF3, and inhibition of the NLRP3 inflammasome. Itaconate and its derivatives have antiinflammatory effects in preclinical models of sepsis, viral infections, psoriasis, gout, ischemia/reperfusion injury, and pulmonary fibrosis, pointing to possible itaconate-based therapeutics for a range of inflammatory diseases. This intriguing metabolite continues to yield fascinating insights into the role of metabolic reprogramming in host defense and inflammation.

Figure 1. The structures of itaconate, similar metabolites, and its derivatives.

Itaconate is a five-carbon dicarboxylic acid with an α,β-unsaturated alkene, making it mildly electrophilic. Structurally, itaconate is similar to several metabolites, including succinate, malonate, phosphoenolpyruvate, and fumarate. For instance, through structural similarity to succinate and malonate, itaconate can competitively inhibit succinate dehydrogenase and prevent the oxidation of succinate to fumarate (53, 54). Similarly, the alkene group allows itaconate to act as a Michael acceptor and react with cysteine residues in a similar manner to fumarate (11). Commonly used derivatives of itaconate include 4-octyl itaconate (OI) and dimethyl itaconate (DI), which are useful because of their high membrane permeability. While DI is not metabolized to itaconate (109), there is evidence that OI may be converted into itaconate intracellularly by esterases (11, 77, 78).

Figure 2. The effect of itaconate on bacteria and viruses.

Upon exposure to pathogens, Irg1 is induced, mediating the production of Krebs cycle–derived itaconate within mitochondria (7). In neurons, Irg1 is induced through ZBP1, RIPK1/3, and IRF1 to restrict Zika viral replication through inhibition of SDH (49). Itaconate is also produced to inhibit the growth of bacteria such as Mycobacterium tuberculosis and Salmonella (28, 33). A notable antimicrobial mechanism is the inhibition of bacterial isocitrate lyase (ICL), which blocks the glyoxylate shunt that is required for optimal growth and pathogenicity (19, 21). The breakdown product of itaconate, itaconyl-CoA, is also an inhibitor of methylmalonyl-CoA mutase (MCM) in bacteria (e.g., M. tuberculosis), thereby blocking propionyl-CoA–dependent bacterial growth (28). Recently, itaconate was shown to be delivered to Salmonella-containing vacuoles, which limits bacterial growth. Mechanistically, this was found to occur through the Rab32 GTPase and its exchange factor, BLOC3, which interact with ACOD1 to target itaconate to bacteria contained within vacuoles (33). Although itaconate is antimicrobial, some pathogens, such as Pseudomonas aeruginosa and Staphylococcus aureus, have developed ways to exploit itaconate to fuel biofilm formation (43, 47). Itaconate causes membrane stress of P. aeruginosa and inhibits aldolase (and glycolysis) in S. aureus, both of which lead to the production of extracellular polysaccharides (EPSs), which further induce Irg1 expression and itaconate synthesis in a positive-feedback manner. EPSs also facilitate the formation of bacterial biofilms, promoting growth and survival of P. aeruginosa and S. aureus.

Figure 3. The immunoregulatory properties of itaconate.

Irg1 is induced by LPS in a TRIF-dependent manner, leading to itaconate production (7, 73). Taking evidence from itaconate derivatives or from Irg1-deficient macrophages, several targets of itaconate, including succinate dehydrogenase (SDH), have been identified, which prevent the oxidation of succinate to fumarate and decrease mtROS production (53, 54). Itaconate also exits the mitochondria, where it has numerous antiinflammatory effects (11). A key mechanism of itaconate is the modification of thiol-reactive cysteines, many of which have been identified by proteomic screens. Targets include the glycolytic enzymes aldolase A (ALDOA; ref. 76), lactate dehydrogenase A (LDHA; ref. 11), glyceraldehyde-3-phosphate dehydrogenase (GAPDH; ref. 78), and the NLRP3 inflammasome, which will prevent processing of IL-1β, IL-18, and GSDMD. Nrf2 and activating transcription factor 3 (ATF3) have also been identified as possible targets (11, 69). Additionally, in alveolar macrophages, itaconate has been shown to repress the severity of lung fibrosis (106). Finally, itaconate has also been reported to boost type I IFN signaling by an undetermined mechanism (17).