The counterattack brother of lactic acid-itaconic acid! The article takes you to understand itaconic acid

Since the first report of lactylation modification in Nature in 2019, lactic acid, a "metabolic waste", has become a "treasure small molecule", which can be described as "unknown in ten years and well-known throughout the world". Little did they know that Itaconate (ITA), as an intermediate product of the tricarboxylic acid cycle (TCA), has the same fate as lactate, from being used for other purposes to discovering its important biological functions, especially in the field of immune regulation research. Today, the editor will take you to learn about the "past life" and "present life" of itaconic acid.

01. Discovery and development of itaconic acid

The discovery of itaconic acid can be traced back to the 1830s. At the beginning, it was mainly known as an important organic synthesis and chemical production raw material. In 1840, German scholar Gustav Crasso confirmed that itaconic acid was a product of the thermal decomposition reaction of aconitic acid, and rearranged the alphabetical order of aconite to name the product itaconic acid.

The development of itaconic acid has undergone a transformation from its initial industrial application to redefining its biological function. It was not until the past decade that scientists discovered the presence of itaconic acid in mammalian macrophages and that it is a metabolite with important immune regulatory functions. In 2013, Michelucci et al. revealed the biosynthetic pathway of itaconic acid in mammals. Since then, a large number of studies have emphasized the important role of itaconic acid in the critical link between immune response, metabolism, and inflammation, and the research enthusiasm continues to rise.

02. Application of Itaconic Acid in Different Disease Research

As an intermediate metabolite in the TCA cycle, itaconic acid regulates the interaction between metabolism, immunity, and inflammation, providing an alternative therapeutic strategy for treating immune inflammatory diseases. Lampropoulou et al. first proved that intravenous infusion of DMI can reduce the myocardial infarction area during ischemia in mice, and later published several papers, reporting the therapeutic benefits of ITA or its chemically modified derivatives in animal models of various inflammatory diseases, including antiviral and bacterial infections, to reduce alopecia areata, diabetes, fibrosis, heart transplantation, I/R injury, obesity, osteoarthritis, neurodegenerative diseases, sepsis, cancer, etc. (Table 1-2). These studies emphasize the broad potential of ITA based interventions to alleviate inflammation and related pathology in different disease contexts. In addition, recent studies have shown that in mouse models, the deletion of the IRG1 gene reduces tumor growth and enhances the efficacy of immunotherapy. In summary, these findings provide a convincing theoretical basis for exploring therapeutic strategies targeting ITA signaling in clinical settings.  

03. The biosynthesis and metabolism of itaconic acid

Itaconic acid is an intermediate product of the TCA cycle, which is decarboxylated by aconitine decarboxylase (ACOD1) encoded by immune response gene 1 (IRG1) to produce cis aconitine. Mammalian cells, mainly bone marrow cells, produce ITA as part of the immune response and use it to regulate inflammation. In cells that produce ITA, ITA catabolism can occur through a three-step reaction: first, ITA is converted to itaconic CoA, then itaconic CoA hydrates to form citrate CoA, and finally citrate CoA is cleaved to acetyl CoA and pyruvic acid, catalyzed by succinyl CoA synthase (SCS), methylglutaryl-CoA hydratase (AUH), and citrate lyase beta subunit like (CLYBL, also known as citrate CoA lyase), respectively. Although pyruvate can enter various metabolic pathways, including the TCA cycle (Figure 3), it is unclear whether ITA catabolism contributes to the energy and biosynthetic needs of immune cells during inflammation.

Structurally, itaconic acid is a five carbon dicarboxylic acid containing alpha, beta unsaturated olefins. It is structurally similar to other metabolites such as phosphoenolpyruvate (PEP), alpha ketoglutarate (α - KG), succinic acid, malonic acid, and fumaric acid, providing a structural basis for its competitive inhibition of key enzymes in glucose metabolism and participating in cellular metabolic reprogramming. The esterification derivatives of itaconic acid, 4-octyl itaconate (4-OI) and dimethyl itaconate (DMI), are commonly used to simulate their biological effects in vitro and in vivo due to their high membrane permeability and similar biological functions to itaconic acid. 4-OI can be converted into itaconic acid in the cell, providing more possibilities for the pharmaceutical development of itaconic acid and pointing the way for new therapies based on itaconic acid. 

04. Target and mechanism of action of itaconic acid

Unlike most other intermediate metabolites, ITA does not participate in metabolic pathways related to energy metabolism or biosynthesis, indicating that it may primarily function independently of intermediate metabolism. Over the past two decades, research has found that ITA produced in mammalian cells serves as a signaling molecule that regulates immune responses by binding to and affecting the functions of different target proteins (Figure 4). ① ITA inhibits the metabolic enzyme succinate dehydrogenase (SDH), leading to the accumulation of succinic acid and altering cellular metabolism. ② ITA and its derivatives cause covalent protein alkylation. The alkylation of KEAP1 E3 ubiquitin ligase inhibits KEAP1 mediated degradation of NRF2 and activates NRF2 mediated oxidative stress response. ③ ITA inhibits TET DNA dioxygenase, promotes DNA methylation, and regulates gene expression. ④ Secretory ITA acts as an agonist of G protein coupled receptor (GPCR) protein OXGR1, promoting the Ca2+signaling pathway. Understanding the regulation of these proteins by ITA binding is not only of great significance for ITA's function in immune regulation, but also for its potential therapeutic applications.

It is worth mentioning that due to the presence of electrophilic α, β - unsaturated carboxyl groups, itaconic acid can accept electron pairs from potential interacting partners. Therefore, a key mechanism of itaconic acid is to alkylate the cysteine residues of related proteins through Michael addition reaction, thereby regulating the function of the corresponding proteins. For example, by alkylating KEAP1 (Kelch like ECH associated protein 1) protein and activating the Nrf2 signaling pathway, inflammation and oxidative stress in macrophages can be alleviated. In addition to KEAP1, ITA or its derivatives can also alkylate other proteins (Table 3), including functionally validated ITA targeted proteins such as ICL 1; Metabolic enzymes GAPDH, ALDOA, and LDHA; And inflammasome protein 3 (NLRP3), GSDMD, JAK1, TFEB, and STING.