A key role of the mitochondrial citrate carrier (SLC25A1) in TNFα- and IFNγ-triggered inflammation

Abstract

The chronic induction of inflammation underlies multiple pathological conditions, including metabolic, autoimmune disorders and cancer. The mitochondrial citrate carrier (CIC), encoded by the SLC25A1 gene, promotes the export of citrate from the mitochondria to the cytoplasm, a process that profoundly influences energy balance in the cells. We have previously shown that SLC25A1 is a target gene for lipopolysaccharide signaling and promotes the production of inflammatory mediators. We now demonstrate that SLC25A1 is induced at the transcriptional level by two key pro-inflammatory cytokines, tumor necrosis factor-α (TNFα) and interferon-γ (IFNγ), and such induction involves the activity of the nuclear factor kappa B and STAT1 transcription factors. By studying the down-stream events following SLC25A1 activation during signals that mimic inflammation, we demonstrate that CIC is required for regulating the levels of nitric oxide and of prostaglandins by TNFα or IFNγ. Importantly, we show that the citrate exported from mitochondria via CIC and its downstream metabolic intermediate, acetyl-coenzyme A, are necessary for TNFα or IFNγ to induce nitric oxide and prostaglandin production. These findings provide the first line of evidence that the citrate export pathway, via CIC, is central for cytokine-induced inflammatory signals and shed new light on the relationship between energy metabolism and inflammation.

Metabolic Alterations Contribute to Enhanced Inflammatory Cytokine Production in Irgm1-deficient Macrophages

“…Although our data do not differentiate between increased synthesis of fatty acids and decreased flux through ␤-oxidation, it seems likely that a decrease in ␤-oxidation is involved. The pronounced reduction in citrate levels that we found in IFN-␥-primed, Irgm1-deficient macrophages is consistent with a decrease in ␤-oxidation that would otherwise replenish citrate levels for flux through the tricarboxylic acid cycle (38,85,86). The reduced citrate availability would probably have the effect of decreasing fatty acid synthesis as well as promoting glycolysis by reducing the inhibitory effect of citrate on phosphofructokinase (85).…”

“…The pronounced reduction in citrate levels that we found in IFN-␥-primed, Irgm1-deficient macrophages is consistent with a decrease in ␤-oxidation that would otherwise replenish citrate levels for flux through the tricarboxylic acid cycle (38,85,86). The reduced citrate availability would probably have the effect of decreasing fatty acid synthesis as well as promoting glycolysis by reducing the inhibitory effect of citrate on phosphofructokinase (85). It is plausible that Irgm1 could play a direct role in lipid homeostasis, because it is a dynamin-like GTPase that associates with specific intracellular membrane compartments, including the Golgi, where it may alter membrane trafficking (11,12,21,25,52,78,87).…”

Acetyl Coenzyme A: A Central Metabolite and Second Messenger

“…The acetyl-CoA concentration gradient across the inner mitochondrial membrane is influenced not only by the rate of acetyl-CoA synthesis and consumption in the cytosol and within mitochondria, but also by the activity of: (1) the citrate carrier (SLC25A1), which can be increased by pro-inflammatory transcription factors like NF-kB and signal transducer and activator of transcription 1 (STAT1) (Infantino et al, 2014); and (2) ACLY, which is regulated by several signal transducers, including Kirsten rat sarcoma viral oncogene homolog (KRAS) and v-akt murine thymoma viral oncogene homolog 1 (AKT1) (Berwick et al, 2002;Lee et al, 2014). Of note, the export of citrate from mitochondria creates the need for the anaplerotic replenishment of TCA cycle intermediates that regenerate oxaloacetate, meaning that the extracellular availability of glutamine and the metabolic flux through glutaminolysis also influence the relative abundance of mitochondrial and cytosolic acetyl-CoA .…”

Epoxyeicosatrienoic Acids Regulate Adipocyte Differentiation of Mouse 3T3 Cells, Via PGC-1α Activation, Which Is Required for HO-1 Expression and Increased Mitochondrial Function

“…Mitochondrial superoxide formation was determined by the uptake of MitoSox Red reagent (Invitrogen) in live cells, as previously described [37,38]. Briefly, cells were plated at a density of 10,000 cells/cm 2 in six-well plates with or without EET-A, 10 mM, overnight and then incubated with the MitoSox Red reagent (5 mM) in Hank's balanced salt solution for 10 min at 37°C protected from light.…”