Front Immunol. 2020 Feb 18;11:131.
doi: 10.3389/fimmu.2020.00131
Background
Activation of macrophages with pro-inflammatory stimuli, also known as classical M1 activation, induces a profound shift in energetic metabolism characterized by aerobic glycolysis and decreased mitochondrial substrate oxidation. Lipid accumulation is another salient metabolic feature of phagocyte activation during infection and sterile inflammation. But whether lipids accumulating during classical macrophage activation originate from de novo fatty acid synthesis or from an exogenous source of lipid is still unclear. Lipid droplets originate from endoplasmic reticulum and contain a core of neutral lipid, namely triacylglycerol (TAG) and cholesterol ester. Lipids contained within lipid droplets can be used as substrate for ATP synthesis through β-oxidation and as precursors for membrane lipids, eicosanoids, and nuclear receptor ligands.
Methods
Cells: MafB/c-Maf double deficient (Maf-DKO) primary mouse macrophagesNeutral Lipid Staining: LipidTOX is a fluorescent dye specific to neutral lipids. Adipose differentiation-related protein (ADRP, also known as perilipin2 or adipophilin) is a marker of lipid droplets.liquid chromatography-mass spectrometry (LC-MS): monitored 13C incorporation into TAGResults
Maf-DKO cells polarize to M1 (iNOS/ TNF) and M2 (arginase-1) phenotypes when activated with IFNγ and IL-4, respectively.IFNγ up-regulates glucose uptake and lactate release, inhibits respiration, and induces triacylglycerol and lipid droplet accumulation.Glucose is not a relevant substrate for de novo synthesis of fatty acid contained in TAG of macrophages activated with IFNγ, but instead provides glycerol to the headgroup of TAG.Neutral lipids accumulating in activated macrophages originate from glucose-derived glycerol as well as directly incorporated externally-derived fatty acids, with no need for de novo fatty acid synthesis. Activation of bone marrow-derived macrophages with IFNγ induced lipid accumulation that was also dependent on external lipids.Macrophages oxidize fatty acid under basal conditions, and that inhibition of fatty acid oxidation or mitochondrial respiration is sufficient to increase neutral lipid content in non-activated macrophages, provided exogenous lipids are available.Inhibition of mitochondrial respiration by nitric oxide contributed to accumulation of neutral lipid upon activation with IFNγ.Conclusions
The authors established a new metabolic pathway in activated macrophages in which exogenous fatty acids are the primary source of acyl chains that are then esterified with de novo synthesized glycerol from glucose yielding TAG. In comparison to the previously assumed de novo fatty acid biosynthesis, this pathway is a more efficient way to store lipids since it requires minimal energy and overall metabolic activities. This lipogenesis pathway is fundamentally different from that in cancer cells that synthesize their own fatty acids from glutamine and glucose, and highlights that metabolism of activated macrophages and cancer cells might be more different than previously assumed.