Triacylglycerol synthesis enhances macrophage inflammatory function

Abstract

Macrophages are integral to most tissues. Foam cells, macrophages with lipid droplets (LDs) which are stores of triacylglycerols (TGs) and cholesterol esters (CEs), are found in various disease states 1 . LDs can act as energy stores since TG lipolysis releases fatty acids (FAs) for mitochondrial oxidation (FAO), a process that relies on long-chain FA conversion into acylcarnitines by the enzyme Cpt1a 2 . However, in macrophages, proinflammatory signals result in diminished FAO and increased TG synthesis with LD development 3,4 . We explored the significance of LDs in cells that do not utilize FAO. We show that macrophages stimulated with lipopolysaccharide (LPS) plus interferon-g (IFNg) accumulate TGs in LDs, and long-chain acylcarnitines. In these cells, inhibition of TG synthesis results in diminished LD development, and increased long chain acylcarnitine levels, suggesting that FA fate is balanced between TG and acylcarnitine synthesis. Nevertheless, TG-synthesis is required for inflammatory macrophage function, since its inhibition negatively affects production of proinflammatory IL-1b, IL-6 and PGE2, and phagocytic capacity, and protects against LPS-induced shock in vivo. Failure to make PGE2 is critical for this phenotype, since exogenous PGE2 reverses the anti-inflammatory effects of TG-synthesis inhibition. These findings place LDs in a position of central functional importance in inflammatory macrophages. EJP and ELP are Founders of Rheos Medicines MethodsMice and in vivo experiments C57BL/6 mice (RRID: IMSR_JAX:000664) were from The Jackson Laboratory, and were maintained in specific pathogen-free conditions under protocols approved by the animal care committee of the Regierungspräsidium Freiburg, Germany, in compliance with all relevant ethical regulations. Animals were 6-8 weeks old when used. They were euthanized by carbon dioxide asphyxiation followed by cervical

Inhibition of granuloma triglyceride synthesis imparts control ofMycobacterium tuberculosisthrough curtailed inflammatory responses

“…Importantly, eicosanoid production and action seems to be regulated at the transcript level upon inhibiting triglyceride synthesis. This is in agreement with previous studies in murine bone marrow derived macrophages where inhibition of DGAT1 in M1 polarized macrophages led to decrease in levels of prostaglandins via expression of the gene ptges (Castoldi et al, 2020). Previously, prostaglandin production by IFNγ polarized, Mtb infected BMDMs was also shown to be partially triglyceride dependent (Knight et al, 2018).…”

“…Previous studies have demonstrated that in both human and mouse macrophages, triglyceride synthesis by the enzyme DGAT1 enhances the pro-inflammatory response to both M. tuberculosis and LPS (Castoldi et al, 2020;Jaisinghani et al, 2018). However, the inflammatory responses in vivo in a granulomatous disease could be the sum total of contribution from various cell types and signals.…”

“…Reduction in triglyceride levels by silencing of DGAT1 in human macrophages led to reduction of the inflammatory response to Mtb infection (Jaisinghani et al, 2018). More recently, LPS stimulated DGAT1 activity was also found to be important for the inflammatory response of murine macrophages (Castoldi et al, 2020). Triglycerides are a potential source of arachidonic acids, and macrophage lipid droplets harbour lipases and cyclo-oxygenases which can catalyze stepwise conversion of this esterified arachidonic acid to eicosanoids (Dvorak et al, 1992;Menon et al, 2019).…”

Cell‐intrinsic metabolic regulation of mononuclear phagocyte activation: Findings from the tip of the iceberg

“…It emphasized that although glucose carbon enters mitochondria and is used to produce citrate, a break occurs after this point in the TCA cycle, at least in part due to reduced expression of isocitrate dehydrogenase (Idh), that substantially diminishes the production of α-ketoglutarate (α-KG) from glucose carbon ( Figure 1). This favors the accumulation and export of citrate for FAS, which allows membrane synthesis, 10 as well as lipid droplet development that, if inhibited, has marked effects in macrophage inflammatory potential 32 (Figure 1). Moreover, the parallel induction of expression of aconitate decarboxylase 1 (encoded by Acod1, also referred to as Irg1) allows the use of (iso) citrate carbon, via aconitate, for high output synthesis of the metabolite itaconate (Figure 1).…”

Norepinephrine promotes triglyceride storage in macrophages via beta2‐adrenergic receptor activation

“…29 Others have demonstrated that lipid droplets can act as reservoirs for precursor molecules required for pro-inflammatory mediator synthesis, and therefore increased macrophage triglyceride storage can heighten inflammatory response. [30][31][32] Support for the lipid-mediator pool hypothesis comes from data obtained from macrophages lacking Hypoxia Inducible Lipid Droplet Associated (HILPDA) protein, a potent endogenous inhibitor of ATGL. 33 Hilpda-deficient macrophages cannot store triglycerides, leading to impaired prostaglandin E2 production.…”