Metabolic reprogramming in macrophages and dendritic cells in innate immunity

Abstract

Activation of macrophages and dendritic cells (DCs) by pro-inflammatory stimuli causes them to undergo a metabolic switch towards glycolysis and away from oxidative phosphorylation (OXPHOS), similar to the Warburg effect in tumors. However, it is only recently that the mechanisms responsible for this metabolic reprogramming have been elucidated in more detail. The transcription factor hypoxia-inducible factor-1α (HIF-1α) plays an important role under conditions of both hypoxia and normoxia. The withdrawal of citrate from the tricarboxylic acid (TCA) cycle has been shown to be critical for lipid biosynthesis in both macrophages and DCs. Interference with this process actually abolishes the ability of DCs to activate T cells. Another TCA cycle intermediate, succinate, activates HIF-1α and promotes inflammatory gene expression. These new insights are providing us with a deeper understanding of the role of metabolic reprogramming in innate immunity.

Figure 1

The Warburg effect. (A) In resting cells, glucose is metabolized to pyruvate via glycolysis. Some pyruvate is converted to lactate, but most is directed to the TCA cycle via acetyl-CoA. The TCA cycle generates NADH, which donates electrons to the mitochondrial electron transport chain so that OXPHOS can progress. (B) In highly proliferative or tumour cells, the metabolic profile switches from OXPHOS to aerobic glycolysis, known as the Warburg effect. Mature innate immune cells also rely on glycolysis, although they do not proliferate after activation. The majority of the pyruvate generated by glycolysis is converted to lactate, and glycolytic intermediates build up, meeting the high energy demand of the cell. Glycolysis is the source of ATP in these cells, and also provides glucose-6-phosphate for nucleotide biosynthesis in the PPP.

Figure 2

Mechanisms of LPS-induced Warburg metabolism in macrophages or DCs. Upon LPS stimulation of TLR4, a range of metabolic changes occur in macrophages or DCs. (1) LPS activation upregulates iNOS expression, increasing the production of NO, which nitrosylates and thus inhibits target proteins in the mitochondrial electron transport chain, thereby dampening OXPHOS. (2) LPS activates mTOR, thereby increasing the translation of mRNA with 5′-TOP sequences, including HIF-1α mRNA. HIF-1α then increases expression of its target genes. (3) LPS increases expression of u-PFK2, an isoform of PFK2, thereby increasing levels of the metabolite F-2,6-BP. F-2,6-BP activates the glycolytic enzyme 6-phosphofructo-1-kinase. (4) Finally, LPS inhibits AMPK, resulting in decreased β-oxidation of fatty acids and mitochondrial biogenesis.

Figure 3

Roles of citrate and succinate in macrophage and DC activation by LPS. Metabolic intermediates of the TCA cycle, such as citrate and succinate, can act as signaling molecules in macrophages and DCs, even when TCA cycle activity is decreased. LPS increases expression of the citrate carrier, Slc25a1, which could lead to increased transport of citrate out of the mitochondria. Citrate is then metabolized to acetyl-CoA and oxaloacetate by ACLY. Acetyl-CoA is used in fatty acid synthesis and also provides acetyl groups for acetylation of histone proteins. The conversion of oxaloacetate to pyruvate generates NADPH, which serves as a substrate in both iNOS-catalyzed NO production and NADPH oxidase-catalyzed ROS generation. NO nitrosylates and inhibits components of the mitochondrial electron transport chain and thus inhibits OXPHOS. ROS can stabilize HIF1α, and thus promote glycolysis and sustain transcription of the pro-inflammatory cytokine IL-1β. Succinate also promotes HIF-1α stabilization by inhibiting PHD enzymes, which, when active, hydroxylate and increase the degradation of HIF-1α. Succinate is also used to succinylate proteins, a post-translational modification with as yet unknown consequences. Sources of succinate in an LPS-activated macrophage include glutamine metabolism (anaplerosis and the GABA shunt), and possibly the glyoxylate shunt.

Figure 4

Metabolic differences between M1 and M2 macrophages. M1 macrophages rely on glycolysis for ATP production and have increased levels of iNOS, HIF-1α and u-PFK2, while M2 macrophages are fueled by OXPHOS and have increased levels of Arg-1, AMPK and PFKFB1. M1 macrophages release pro-inflammatory cytokines such as IL-1β, while M2 macrophages are involved in the response to parasite infection, as well as in wound healing, and they release the anti-inflammatory cytokine IL-10. In fact, it is thought that a spectrum of macrophage activation exists, with different populations of macrophages exhibiting different inflammatory and metabolic phenotypes.