Immunoresponsive Gene 1 and Itaconate Inhibit Succinate Dehydrogenase to Modulate Intracellular Succinate Levels

TLDR

Metabolic reprogramming, including dynamic control of metabolites such as succinate, is a hallmark of innate immune responses in macrophages and other immune cells, with Irg1 induction leading to itaconate production from the TCA cycle. The study aims to uncover a regulatory pathway linking Irg1‑driven itaconate production to succinate accumulation during innate immune activation. The authors show that during LPS stimulation, itaconate and Irg1 expression strongly correlate with succinate, that itaconate inhibits succinate dehydrogenase to drive succinate accumulation, and that loss of itaconate in Irg1−/− macrophages reduces this accumulation, linking innate immunity, TCA metabolism, and electron transport chain function.

Abstract

Metabolic reprogramming is emerging as a hallmark of the innate immune response, and the dynamic control of metabolites such as succinate serves to facilitate the execution of inflammatory responses in macrophages and other immune cells. Immunoresponsive gene 1 (Irg1) expression is induced by inflammatory stimuli, and its enzyme product cis-aconitate decarboxylase catalyzes the production of itaconate from the tricarboxylic acid cycle. Here we identify an immunometabolic regulatory pathway that links Irg1 and itaconate production to the succinate accumulation that occurs in the context of innate immune responses. Itaconate levels and Irg1 expression correlate strongly with succinate during LPS exposure in macrophages and non-immune cells. We demonstrate that itaconate acts as an endogenous succinate dehydrogenase inhibitor to cause succinate accumulation. Loss of itaconate production in activated macrophages from Irg1−/− mice decreases the accumulation of succinate in response to LPS exposure. This metabolic network links the innate immune response and tricarboxylic acid metabolism to function of the electron transport chain.