Cutting Edge: STING Induces ACLY Activation and Metabolic Adaptations in Human Macrophages through TBK1.

The 2'3'-cyclic GMP-AMP (cGAMP) synthase (cGAS)-stimulator of IFN genes (STING) pathway can sense infection and cellular stress by detecting cytosolic DNA. Upon ligand binding, cGAS produces the cyclic dinucleotide messenger cGAMP, which triggers its receptor STING. Active STING initiates gene transcription through the transcription factors IFN regulatory factor 3 (IRF3) and NF-κB and induces autophagy, but whether STING can cause changes in the metabolism of macrophages is unknown. In this study, we report that STING signaling activates ATP-citrate lyase (ACLY) by phosphorylation in human macrophages. Using genetic and pharmacologic perturbation, we show that STING targets ACLY via its prime downstream signaling effector TANK (TRAF family member-associated NF-κB activator)-binding kinase 1 (TBK1). We further identify that TBK1 alters cellular metabolism upon cGAMP treatment. Our results suggest that STING-mediated metabolic reprogramming adjusts the cellular response to DNA sensing in addition to transcription factor activation and autophagy induction.

Mesaconate is synthesized from itaconate and exerts immunomodulatory effects in macrophages.

Since its discovery in inflammatory macrophages, itaconate has attracted much attention due to its antimicrobial and immunomodulatory activity1-3. However, instead of investigating itaconate itself, most studies used derivatized forms of itaconate and thus the role of non-derivatized itaconate needs to be scrutinized. Mesaconate, a metabolite structurally very close to itaconate, has never been implicated in mammalian cells. Here we show that mesaconate is synthesized in inflammatory macrophages from itaconate. We find that both, non-derivatized itaconate and mesaconate dampen the glycolytic activity to a similar extent, whereas only itaconate is able to repress tricarboxylic acid cycle activity and cellular respiration. In contrast to itaconate, mesaconate does not inhibit succinate dehydrogenase. Despite their distinct impact on metabolism, both metabolites exert similar immunomodulatory effects in pro-inflammatory macrophages, specifically a reduction of interleukin (IL)-6 and IL-12 secretion and an increase of CXCL10 production in a manner that is independent of NRF2 and ATF3. We show that a treatment with neither mesaconate nor itaconate impairs IL-1ß secretion and inflammasome activation. In summary, our results identify mesaconate as an immunomodulatory metabolite in macrophages, which interferes to a lesser extent with cellular metabolism than itaconate.

Fumarate is a terminal electron acceptor in the mammalian electron transport chain.

For electrons to continuously enter and flow through the mitochondrial electron transport chain (ETC), they must ultimately land on a terminal electron acceptor (TEA), which is known to be oxygen in mammals. Paradoxically, we find that complex I and dihydroorotate dehydrogenase (DHODH) can still deposit electrons into the ETC when oxygen reduction is impeded. Cells lacking oxygen reduction accumulate ubiquinol, driving the succinate dehydrogenase (SDH) complex in reverse to enable electron deposition onto fumarate. Upon inhibition of oxygen reduction, fumarate reduction sustains DHODH and complex I activities. Mouse tissues display varying capacities to use fumarate as a TEA, most of which net reverse the SDH complex under hypoxia. Thus, we delineate a circuit of electron flow in the mammalian ETC that maintains mitochondrial functions under oxygen limitation.

GLP-2 Is Locally Produced From Human Islets and Balances Inflammation Through an Inter-Islet-Immune Cell Crosstalk.

Glucagon-like peptide-1 (GLP-1) shows robust protective effects on ß-cell survival and function and GLP-1 based therapies are successfully applied for type-2 diabetes (T2D) and obesity. Another cleavage product of pro-glucagon, Glucagon-like peptide-2 (GLP-2; both GLP-1 and GLP-2 are inactivated by DPP-4) has received little attention in its action inside pancreatic islets. In this study, we investigated GLP-2 production, GLP-2 receptor (GLP-2R) expression and the effect of GLP-2R activation in human islets. Isolated human islets from non-diabetic donors were exposed to diabetogenic conditions: high glucose, palmitate, cytokine mix (IL-1ß/IFN-γ) or Lipopolysaccharide (LPS) in the presence or absence of the DPP4-inhibitor linagliptin, the TLR4 inhibitor TAK-242, the GLP-2R agonist teduglutide and/or its antagonist GLP-2(3-33). Human islets under control conditions secreted active GLP-2 (full-length, non-cleaved by DPP4) into the culture media, which was increased by combined high glucose/palmitate, the cytokine mix and LPS and highly potentiated by linagliptin. Low but reproducible GLP-2R mRNA expression was found in all analyzed human islet isolations from 10 donors, which was reduced by pro-inflammatory stimuli: the cytokine mix and LPS. GLP-2R activation by teduglutide neither affected acute or glucose stimulated insulin secretion nor insulin content. Also, teduglutide had no effect on high glucose/palmitate- or LPS-induced dysfunction in cultured human islets but dampened LPS-induced macrophage-dependent IL1B and IL10 expression, while its antagonist GLP-2(3-33) abolished such reduction. In contrast, the expression of islet macrophage-independent cytokines IL6, IL8 and TNF was not affected by teduglutide. Medium conditioned by teduglutide-exposed human islets attenuated M1-like polarization of human monocyte-derived macrophages, evidenced by a lower mRNA expression of pro-inflammatory cytokines, compared to vehicle treated islets, and a reduced production of itaconate and succinate, marker metabolites of pro-inflammatory macrophages. Our results reveal intra-islet production of GLP-2 and GLP-2R expression in human islets. Despite no impact on ß-cell function, local GLP-2R activation reduced islet inflammation which might be mediated by a crosstalk between endocrine cells and macrophages.