Immune-responsive gene 1 protein links metabolism to immunity by catalyzing itaconic acid production

Abstract

Immunoresponsive gene 1 (Irg1) is highly expressed in mammalian macrophages during inflammation, but its biological function has not yet been elucidated. Here, we identify Irg1 as the gene coding for an enzyme producing itaconic acid (also known as methylenesuccinic acid) through the decarboxylation of cis-aconitate, a tricarboxylic acid cycle intermediate. Using a gain-and-loss-of-function approach in both mouse and human immune cells, we found Irg1 expression levels correlating with the amounts of itaconic acid, a metabolite previously proposed to have an antimicrobial effect. We purified IRG1 protein and identified its cis-aconitate decarboxylating activity in an enzymatic assay. Itaconic acid is an organic compound that inhibits isocitrate lyase, the key enzyme of the glyoxylate shunt, a pathway essential for bacterial growth under specific conditions. Here we show that itaconic acid inhibits the growth of bacteria expressing isocitrate lyase, such as Salmonella enterica and Mycobacterium tuberculosis. Furthermore, Irg1 gene silencing in macrophages resulted in significantly decreased intracellular itaconic acid levels as well as significantly reduced antimicrobial activity during bacterial infections. Taken together, our results demonstrate that IRG1 links cellular metabolism with immune defense by catalyzing itaconic acid production.

Early pregnancy metabolite profiling discovers a potential biomarker for the subsequent development of gestational diabetes mellitus

“…Interestingly, cis-aconitate levels were found to be higher in GDM cases in our study when compared to controls, verging on statistical significance. The expression of the immunoresponsive gene 1 is upregulated in macrophages in response to inflammation [7]. Itaconic acid's association with inflammation may demonstrate a potential role of inflammation in early pregnancy, in the development of GDM.…”

“…However, a recent study [7] investigated the gene, immunoresponsive gene 1, coding an enzyme essential for the production of itaconic acid in humans, through the decarboxylation of cis-aconitate. Interestingly, cis-aconitate levels were found to be higher in GDM cases in our study when compared to controls, verging on statistical significance.…”

Methylcitrate cycle defines the bactericidal essentiality of isocitrate lyase for survival of Mycobacterium tuberculosis on fatty acids

“…We were unable to stably express this GFP reporter in ICLdeficient Mtb, but overcame this limitation by using wild-type Mtb and itaconic acid, a specific and recently validated chemical inhibitor of Mtb's ICLs in intact bacteria (16). We found that, as predicted, itaconic acid specifically inhibited growth of wild-type Mtb on acetate and propionate, but not dextrose, in an ICLdependent manner, and elicited metabolomic changes similar to those observed with ICL-deficient Mtb (Figs.…”

Identification of an itaconic acid degrading pathway in itaconic acid producing Aspergillus terreus

“…For example, the first enzyme in Y. pestis, YpIct, could catalyze the conversion of itaconic acid into itaconyl-CoA with other CoA esters in addition to succinyl-CoA, while Ict from P. aeruginosa, PaIct, could only use succinyl-CoA as a specific CoA donor (Sasikaran et al 2014). For pathogens, such as Y. pestis and P. aeruginosa, the itaconic acid catabolic pathway is required for their survival inside the eukaryotic host, which can produce itaconic acid as one of the antimicrobial agents to protect them against pathogenic infections (Michelucci et al 2013).…”