Succinate is an inflammatory signal that induces IL-1β through HIF-1α

Abstract

Macrophages activated by the gram negative bacterial product lipopolysaccharide (LPS) switch their core metabolism from oxidative phosphorylation to glycolysis1. Inhibition of glycolysis with 2-deoxyglucose (2DG) suppressed LPS-induced Interleukin-1 beta (IL-1β) but not Tumour necrosis factor alpha (TNFα) in macrophages. A comprehensive metabolic map of LPS-activated macrophages revealed up-regulation of glycolytic and down-regulation of mitochondrial genes, which correlated directly with the expression profiles of altered metabolites. LPS strongly increased the TCA cycle intermediate succinate. Glutamine-dependent anerplerosis was the major source of succinate with the ‘Gamma-Aminobutyric Acid (GABA)-shunt’ pathway also playing a role. LPS-induced succinate stabilized Hypoxia-inducible factor 1α (HIF-1α), an effect inhibited by 2DG, with IL-1β as an important target. LPS also increases succinylation of several proteins. Succinate is therefore identified as a metabolite in innate immune signalling which leads to enhanced IL-1β production during inflammation.

Succinate: a metabolic signal in inflammation

“…Citrate is needed for fatty acid biosynthesis and, once in the cytosol, is cleaved by citrate lyase into acetylCoA and oxaloacetate, precursors for NO, ROS, and arachidonic acid (which is used to generate PGs). Although studies that demonstrate a direct role for citrate in inflammation are currently lacking, an elevation in citrate and a decrease in isocitrate was observed in LPS-activated macrophages [4], further supporting the role of this metabolite in LPS-induced responses. Citrate is therefore acting as a signal to produce key reactive oxygen intermediates for host defense and inflammation, as well as PGs, which are important mediators of inflammation.…”

“…In recent years it has become evident that immune cells have the ability to shift their metabolism under varying conditions and that this is essential for proper immune function [2]. Stimulation of DCs and macrophages can result in a decrease in oxidative phosphorylation, which is normally employed under resting conditions, with a concomitant increase in glycolysis and the pentose-phosphate pathway [3,4]. This switch to glycolysis rapidly generates ATP to maintain mitochondrial membrane potential and energy homeostasis, ultimately ensuring cell viability [5].…”

“…For example, HIF-1a activation favors differentiation of T lymphocytes into proinflammatory Th17 cells and attenuates regulatory T cell development [7]. Importantly, as well as global changes in metabolism that occur in activated immune cells, individual metabolites, including succinate, citrate, and NAD + , have been demonstrated to possess signaling capacity and to influence immunity [4,8,9]. These somewhat surprising findings allow for metabolic changes occurring in inflammatory cells to serve as signals to which cells respond and can contribute to the inflammatory process ( Figure 2).…”

“…We discuss the role played by these events in immunity. These expanding roles for metabolites suggest that TCA cycle intermediates may represent a novel class of regulators of inflammation acting as key signals in multiple biological processes.Citrate transportation regulates inflammatory responses An increase in metabolite transporters in mitochondria, in addition to various metabolites, was observed in a metabolic screen and microarray analysis of macrophages treated with lipopolysaccharide (LPS), a component of the outer membrane of Gram-negative bacteria [4]. This finding suggests that the transport of particular mediators from the mitochondria to the cytosol is required following LPS stimulation in macrophages.…”

Understanding complexity in the HIF signaling pathway using systems biology and mathematical modeling

“…Since HIF-regulating prolyl-4-hydroxylases utilise -ketoglutarate as a co-substrate and produce succinate during their catalytic activity, it is not surprising that the accumulation of succinate blocks these hydroxylases [27]. Indeed, loss-of-function mutations in succinate dehydrogenase were found to be inhibitory towards PHDs leading to the stabilisation of HIF- subunits and succinate was identified as the mediator of this effect [28,29]. Thus, PHDs can also integrate metabolism-dependent stimuli in the regulation of HIF- which, in return, induces a panel of adaptive genes forming a classical feedforward loop.…”