Pyruvate dehydrogenase operates as an intramolecular nitroxyl generator during macrophage metabolic reprogramming

M1 macrophages enter a glycolytic state when endogenous nitric oxide (NO) reprograms mitochondrial metabolism by limiting aconitase 2 and pyruvate dehydrogenase (PDH) activity. Here, we provide evidence that NO targets the PDH complex by using lipoate to generate nitroxyl (HNO). PDH E2-associated lipoate is modified in NO-rich macrophages while the PDH E3 enzyme, also known as dihydrolipoamide dehydrogenase (DLD), is irreversibly inhibited. Mechanistically, we show that lipoate facilitates NO-mediated production of HNO, which interacts with thiols forming irreversible modifications including sulfinamide. In addition, we reveal a macrophage signature of proteins with reduction-resistant modifications, including in DLD, and identify potential HNO targets. Consistently, DLD enzyme is modified in an HNO-dependent manner at Cys477 and Cys484, and molecular modeling and mutagenesis show these modifications impair the formation of DLD homodimers. In conclusion, our work demonstrates that HNO is produced physiologically. Moreover, the production of HNO is dependent on the lipoate-rich PDH complex facilitating irreversible modifications that are critical to NO-dependent metabolic rewiring.

Itaconic acid underpins hepatocyte lipid metabolism in non-alcoholic fatty liver disease in male mice

Itaconate, the product of the decarboxylation of cis-aconitate, regulates numerous biological processes. We and others have revealed itaconate as a regulator of fatty acid β-oxidation, generation of mitochondrial reactive oxygen species and the metabolic interplay between resident macrophages and tumors. In the present study, we show that itaconic acid is upregulated in human non-alcoholic steatohepatitis and a mouse model of non-alcoholic fatty liver disease. Male mice deficient in the gene responsible for itaconate production (immunoresponsive gene (Irg)-1) have exacerbated lipid accumulation in the liver, glucose and insulin intolerance and mesenteric fat deposition. Treatment of mice with the itaconate derivative, 4-octyl itaconate, reverses dyslipidemia associated with high-fat diet feeding. Mechanistically, itaconate treatment of primary hepatocytes reduces lipid accumulation and increases their oxidative phosphorylation in a manner dependent upon fatty acid oxidation. We propose a model whereby macrophage-derived itaconate acts in trans upon hepatocytes to modulate the liver's ability to metabolize fatty acids.

Nitric Oxide in Macrophage Immunometabolism: Hiding in Plain Sight

Nitric Oxide (NO) is a soluble endogenous gas with various biological functions like signaling, and working as an effector molecule or metabolic regulator. In response to inflammatory signals, immune myeloid cells, like macrophages, increase production of cytokines and NO, which is important for pathogen killing. Under these proinflammatory circumstances, called “M1”, macrophages undergo a series of metabolic changes including rewiring of their tricarboxylic acid (TCA) cycle. Here, we review findings indicating that NO, through its interaction with heme and non-heme metal containing proteins, together with components of the electron transport chain, functions not only as a regulator of cell respiration, but also a modulator of intracellular cell metabolism. Moreover, diverse effects of NO and NO-derived reactive nitrogen species (RNS) involve precise interactions with different targets depending on concentration, temporal, and spatial restrictions. Although the role of NO in macrophage reprogramming has been in evidence for some time, current models have largely minimized its importance. It has, therefore, been hiding in plain sight. A review of the chemical properties of NO, past biochemical studies, and recent publications, necessitates that mechanisms of macrophage TCA reprogramming during stimulation must be re-imagined and re-interpreted as mechanistic results of NO exposure. The revised model of metabolic rewiring we describe here incorporates many early findings regarding NO biochemistry and brings NO out of hiding and to the forefront of macrophages immunometabolism.

Glufosinate constrains synchronous and metachronous metastasis by promoting anti-tumor macrophages

Glutamine synthetase (GS) generates glutamine from glutamate and controls the release of inflammatory mediators. In macrophages, GS activity, driven by IL10, associates to the acquisition of M2-like functions. Conditional deletion of GS in macrophages inhibits metastasis by boosting the formation of anti-tumor, M1-like, tumor-associated macrophages (TAMs). From this basis, we evaluated the pharmacological potential of GS inhibitors in targeting metastasis, identifying glufosinate as a specific human GS inhibitor. Glufosinate was tested in both cultured macrophages and on mice bearing metastatic lung, skin and breast cancer. We found that glufosinate rewires macrophages toward an M1-like phenotype both at the primary tumor and metastatic site, countering immunosuppression and promoting vessel sprouting. This was also accompanied to a reduction in cancer cell intravasation and extravasation, leading to synchronous and metachronous metastasis growth inhibition, but no effects on primary tumor growth. Glufosinate treatment was well-tolerated, without liver and brain toxicity, nor hematopoietic defects. These results identify GS as a druggable enzyme to rewire macrophage functions and highlight the potential of targeting metabolic checkpoints in macrophages to treat cancer metastasis.

Metabolic but not transcriptional regulation by PKM2 is important for natural killer cell responses

Natural Killer (NK) cells have an important role in immune responses to viruses and tumours. Integrating changes in signal transduction pathways and cellular metabolism is essential for effective NK cells responses. The glycolytic enzyme Pyruvate Kinase Muscle 2 (PKM2) has described roles in regulating glycolytic flux and signal transduction, particularly gene transcription. While PKM2 expression is robustly induced in activated NK cells, mice lacking PKM2 in NK cells showed no defect in NK cell metabolism, transcription or antiviral responses to MCMV infection. NK cell metabolism was maintained due to compensatory PKM1 expression in PKM2-null NK cells. To further investigate the role of PKM2, we used TEPP-46, which increases PKM2 catalytic activity while inhibiting any PKM2 signalling functions. NK cells activated with TEPP-46 had reduced effector function due to TEPP-46-induced increases in oxidative stress. Overall, PKM2-regulated glycolytic metabolism and redox status, not transcriptional control, facilitate optimal NK cells responses.

Trem2 Supports the Metabolic Program of Alternative Activated Macrophages

Metabolism is a key modulator of macrophage differentiation, polarization and effector responses. While M1 favors a glycolytic configuration resembling the “Warburg effect”, M2 is fueled by oxidative metabolism. Here we describe a previously unappreciated metabolic phenotype controlled by The Triggering Receptor Expressed on Myeloid Cells (TREM)-2, a key player in inflammation and innate immunity. Trem2−/− macrophages show lower TCA cycle metabolites compared to wild type, but maintenance of glycolytic intermediates. Interestingly, while we show Trem2 abrogation is beneficial in M1 and protective against MCSF withdrawal, we find Trem2−/− cells to have disrupted lipid utilization as they accumulate both long chain free fatty acids (FFA) and carnitine-conjugated lipids. Because lipid metabolism is key for M2 responses, we hypothesized that Trem2 supports a metabolic program to maintain M2 activation. Indeed, in M2 cell, Trem2 sustains mitochondrial respiratory capacity, CD36 expression and lipoprotein and FFA uptake. Lipidomics confirmed Trem2−/− M2 cells had decreased FFA and enhanced triglycerides. Moreover, Trem2−/− M2 cells show a distinct metabolic profile with enhanced glutamine utilization, suggesting compensatory mechanisms in lieu of FFA for mitochondrial respiration. Transcriptome profiling decisively shows dysregulated genes related to defense response and glycerolipid metabolism, linking the metabolic findings to effector responses. Finally, in a model of in vivo Th2 response, Trem2−/− mice failed to control infection and exhibit metabolic dysregulation. Our findings suggest Trem2 is required for reprograming of macrophages by supporting the energetic requirements for M2 function.

Elucidation of the respective roles of itaconate and nitric oxide in the nuclear accumulation and activation of Nrf2 in activated macrophages

Nrf2 is a transcription factor that, under basal conditions, is produced in the cytosol and is marked for degradation by its repressor Keap1. Oxidative stress causes Keap1 to release Nrf2, and it accumulates in the nucleus to upregulate certain antioxidant gene targets. Itaconate, the metabolic product of IRG1, has been suggested to play a role in alkylating Keap1, leading to the release of Nrf2. However, there is much ambiguity and even controversy surrounding this role of itaconate. Our study therefore seeks to definitively elucidate the role of itaconate in the regulation and activation of Nrf2. Here we utilize expression analysis and targeted metabolomics in bone marrow-derived macrophages isolated from varying murine backgrounds (Acod1−/−, Nos2−/−, and double knockouts). Despite previously published data suggesting a role for itaconate in Nrf2 activity, our data demonstrate a significant role of Nos2, but not itaconate, in Nrf2 activation, as Acod1−/− mice show little change in the activity of Nrf2 transcription targets, while Nos2−/− mice show severe impairment of Nrf2 gene target activity. For Nrf2 to properly upregulate its gene targets, it must be released from Keap1, phosphorylated, and then allowed to translocate to the nucleus; all three must occur. This may help explain why some studies have found itaconate to be important in Keap1’s regulation of Nrf2, while others demonstrate that this same itaconate mechanism produces comparatively little Nrf2 activity.

TREML4 Promotes Inflammatory Programs in Human and Murine Macrophages and Alters Atherosclerosis Lesion Composition in the Apolipoprotein E Deficient Mouse

The Triggering Receptor Expressed on Myeloid cells-like 4 (TREML4) is a member of the TREM receptor family, known modulators of inflammatory responses. We have previously found that TREML4 expression positively correlates with human coronary arterial calcification (CAC). However, the role of TREML4 in the pathogenesis of cardiovascular disease remains incompletely defined. Since macrophages play a key role in inflammatory conditions, we investigated if activated macrophages selectively expressed TREML4 and found that carriage of either one of the eQTL SNP's previously associated with increased TREML4 expression conferred higher expression in human inflammatory macrophages (M1) compared to alternatively activated macrophages (M2). Furthermore, we found that TREML4 expression in human M1 dysregulated several inflammatory pathways related to leukocyte activation, apoptosis and extracellular matrix degradation. Similarly, murine M1 expressed substantial levels of Treml4, as did oxLDL treated macrophages. Transcriptome analysis confirmed that murine Treml4 controls the expression of genes related to inflammation and lipid regulation pathways, suggesting a possible role in atherosclerosis. Analysis of Apoe-/-/Treml4-/- mice showed reduced plaque burden and lesion complexity as indicated by decreased stage scores, macrophage content and collagen deposition. Finally, transcriptome analysis of oxLDL-loaded murine macrophages showed that Treml4 represses a specific set of genes related to carbohydrate, ion and amino acid membrane transport. Metabolomic analysis confirmed that Treml4 deficiency may promote a beneficial relationship between iron homeostasis and glucose metabolism. Together, our results suggest that Treml4 plays a role in the development of cardiovascular disease, as indicated by Treml4-dependent dysregulation of macrophage inflammatory pathways, macrophage metabolism and promotion of vulnerability features in advanced lesions.

Nitric oxide orchestrates metabolic rewiring in M1 macrophages by targeting aconitase 2 and pyruvate dehydrogenase

Profound metabolic changes are characteristic of macrophages during classical activation and have been implicated in this phenotype. Here we demonstrate that nitric oxide (NO) produced by murine macrophages is responsible for TCA cycle alterations and citrate accumulation associated with polarization. ¹³C tracing and mitochondrial respiration experiments map NO-mediated suppression of metabolism to mitochondrial aconitase (ACO2). Moreover, we find that inflammatory macrophages reroute pyruvate away from pyruvate dehydrogenase (PDH) in an NO-dependent and hypoxia-inducible factor 1α (Hif1α)-independent manner, thereby promoting glutamine-based anaplerosis. Ultimately, NO accumulation leads to suppression and loss of mitochondrial electron transport chain (ETC) complexes. Our data reveal that macrophages metabolic rewiring, in vitro and in vivo, is dependent on NO targeting specific pathways, resulting in reduced production of inflammatory mediators. Our findings require modification to current models of macrophage biology and demonstrate that reprogramming of metabolism should be considered a result rather than a mediator of inflammatory polarization.

Production of inflammatory mediators by M1-polarized macrophages is thought to rely on suppression of mitochondrial metabolism in favor of glycolysis. Refining this concept, here the authors define metabolic targets of nitric oxide as responsible for the mitochondrial rewiring resulting from polarization.

Abstract 1186: Regulation of inducible nitric oxide synthase at the single cell level modulates the inflammatory microenvironment

The anti-cancer versus pro-tumor behavior of cancer tissues is dependent largely on tumor cell-macrophage interactions in the tumor/inflammatory microenvironment and is regulated by Nitric Oxide (NO). High inducible nitric oxide synthase (Nos2) is associated with poor prognosis in breast cancer. Previously, we found that murine macrophages can be activated by different inflammatory cytokines/LPS to produce distinct NO fluxes (entirely Nos2-derived), suggesting flux-specific biological ramifications of NO. However, the effects of these NO fluxes on M1 phenotype have not been delineated. LC/MS analysis of M1 stimulated, wild type (wt) and Nos2-/- macrophages showed that Nos2 was the only source of citrulline. Metabolic analyses and microscopy showed that flattened cell phenotype characteristic of M1 macrophages and mitochondrial respiration are Nos2 dependent and regulated in a NO flux-dependent manner, while proinflammatory cytokine profile and aerobic glycolysis are Nos2 independent. We show for the first time that induction of Nos2 expression occurs only in specific stimulated cells that also harbor depolarized mitochondria. NO production has been linked to decreased oxygen consumption in hypoxic environments. We utilized a novel, in vitro chamber system that forms cell-generated hypoxic and metabolic gradients in two-dimensions by restricting the diffusive exchange of oxygen and metabolites to a monolayer of cells in a small volume- analogous to diffusion between a capillary and nearby tissue. We investigated interactions between Nos2 in M1-stimulated macrophages and hypoxia and demonstrated that treatment with IFNγ+LPS increases Nos2 expression and alters the magnitude and spatial extent of hypoxic gradients. A modified scratch assay revealed that low doses (1-50μM) of NO increased and high doses (1000μM) inhibited the migratory capacity of 4T1 tumor cells. However, in vivo, Nos2-/- mice did not show difference in primary tumor or metastatic burden compared to wt mice but bone marrow derived macrophages (BMDMs) from wt tumor bearing mice produced significantly lower levels of NO compared to BMDMs from tumor bearing- Nos2-/- mice. In summary, we find that the right flux NO is required to tune the inflammatory microenvironment. Nos2 and citrulline are robust intracellular readouts of extracellular NO flux. Nos2 dependent and independent events cooperate to regulate inflammatory macrophages. Autocrine, single cell effects on metabolism build up to cause paracrine effects including alleviation of hypoxia. 4T1 primary tumor and lung metastasis were not Nos2-regulated but host Nos2 hampered the ability of BMDMs to respond to proinflammatory stimuli hinting at possible systemic effects of Nos2. Hence, to make a reliable prognostic prediction, it is important to know the exact NO flux, which cells within the tumor express Nos2 and what other cells associate with Nos2hi cells.

Citation Format: Veena Somasundaram, Caroline Gilmore, Erika M. Palmieri, Debashree Basudhar, Will Heinz, Robert Y. Cheng, Lisa A. Ridnour, Stephen J. Lockett, Daniel W. McVicar, David A. Wink. Regulation of inducible nitric oxide synthase at the single cell level modulates the inflammatory microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1186.

TREML4 affects lesion composition but not calcification in the Apolipoprotein E deficient model of experimental Atherosclerosis

The Triggering Receptor Expressed on Myeloid cells-like 4 (Treml4) is a member of the TREM receptor family which are recognized fine-tuners of the inflammatory response. We have previously found that Treml4 expression correlates to increased risk of Human Coronary Arterial Calcification and Acute Coronary Syndrome. Moreover, we show that carriage of either one of the eQTL SNPs previously identified to permit Treml4 expression, confers the highest treml4 expression in human inflammatory macrophages when compared to alternative activated macrophages. However, the exact role of Treml4 in the pathogenesis of coronary diseases remains incompletely defined. To elucidate the role of Treml4 in atherosclerosis, we investigated whether treml4 deficiency affected pathogenesis in the Apolipoprotein E knockout mouse (apoe−/−). We found that aortas from apoe−/− mice had increased treml4 expression when compared to wild type and that treatment with either LDL or oxLDL upregulated macrophage treml4 expression in vitro. After a western diet, overall plaque burden and calcification in the aortic sinus of apoe−/−/treml4−/− mice remained unchanged compared to controls. However, we found that treml4 deficiency resulted in less complex lesions as indicated by decreased foam cell content, plaque necrosis and collagen deposition as well as dysregulation of matrix metalloproteinases expression within the plaques. Moreover, less collagenous metaplasia was observed in the brachiocephalic artery of apoe−/−/treml4−/− mice. Taken together, our results suggest that Treml4 contributes to the inflammatory mechanisms and extracellular matrix regulation associated with lesion complexity without affecting lesion burden or calcification.

Nitric oxide dictates the reprogramming of carbon flux during M1 macrophage polarization

Classical pro-inflammatory activation of macrophages is characterized by profound intracellular metabolic changes, with increased glycolytic usage of carbon, away from Oxidative Phosphorylation (OXPHOS). We previously demonstrated that Nitric Oxide (NO) levels induced in Bone Marrow Derived Macrophages (BMDMs) from Wild Type (WT) mice are necessary and sufficient for the repression of OXPHOS. Here we demonstrate that NO is also responsible for the “break” in the mitochondrial TCA cycle and citrate accumulation during LPS/IFNγ stimulation; macrophages that lack NO maintain indeed substantial levels of Oxygen Consumption Rates (OCR) and TCA cycle intermediates. Carbon tracing experiments in the presence of U13C-glucose show almost undetected labelled α-ketoglutarate from citrate in WT but a conserved pattern of heavy carbon fate in Nos2−/− macrophages. Moreover we found that mitochondrial respiration elicited through citrate was decreased in WT M1 macrophages, but isocitrate was a full substrate for complex I-dependent OCR suggesting suppression of metabolism at mitochondrial Aconitase (ACO2). Consistent with this data, we found ACO2 enzymatic activity blunted in WT vs Nos2−/−. In addition we observed that M1 macrophages reroute pyruvate away from Pyruvate Dehydrogenase (PDH) in an NO dependent manner since only WT show halted flux through PDH. Surprisingly, we demonstrate this mechanism to be independent on the activation of Hif1α and its suggested effect on limiting acetyl-coA for the TCA. With these data together we hypothesize that NO orchestrates macrophage metabolism during inflammation inhibiting OXPHOS by blocking Krebs Cycle, therefore depriving of substrates the mitochondrial electron transport chain.

Pharmacologic or Genetic Targeting of Glutamine Synthetase Skews Macrophages toward an M1-like Phenotype and Inhibits Tumor Metastasis

Glutamine-synthetase (GS), the glutamine-synthesizing enzyme from glutamate, controls important events, including the release of inflammatory mediators, mammalian target of rapamycin (mTOR) activation, and autophagy. However, its role in macrophages remains elusive. We report that pharmacologic inhibition of GS skews M2-polarized macrophages toward the M1-like phenotype, characterized by reduced intracellular glutamine and increased succinate with enhanced glucose flux through glycolysis, which could be partly related to HIF1α activation. As a result of these metabolic changes and HIF1α accumulation, GS-inhibited macrophages display an increased capacity to induce T cell recruitment, reduced T cell suppressive potential, and an impaired ability to foster endothelial cell branching or cancer cell motility. Genetic deletion of macrophagic GS in tumor-bearing mice promotes tumor vessel pruning, vascular normalization, accumulation of cytotoxic T cells, and metastasis inhibition. These data identify GS activity as mediator of the proangiogenic, immunosuppressive, and pro-metastatic function of M2-like macrophages and highlight the possibility of targeting this enzyme in the treatment of cancer metastasis.

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GS expression and activity are induced by M2 stimuli, especially under starvation

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Inhibition of GS activity skews M2 macrophages toward an M1-like phenotype

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Metabolic rewiring by GS loss favors immunostimulatory and antiangiogenic features

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GS ablation in macrophages blocks vessels, immunosuppression, and metastasis

Peritoneal tissue-resident macrophages are metabolically poised to engage microbes using tissue-niche fuels

The importance of metabolism in macrophage function has been reported, but the in vivo relevance of the in vitro observations is still unclear. Here we show that macrophage metabolites are defined in a specific tissue context, and these metabolites are crucially linked to tissue-resident macrophage functions. We find the peritoneum to be rich in glutamate, a glutaminolysis-fuel that is exploited by peritoneal-resident macrophages to maintain respiratory burst during phagocytosis via enhancing mitochondrial complex-II metabolism. This niche-supported, inducible mitochondrial function is dependent on protein kinase C activity, and is required to fine-tune the cytokine responses that control inflammation. In addition, we find that peritoneal-resident macrophage mitochondria are recruited to phagosomes and produce mitochondrially derived reactive oxygen species, which are necessary for microbial killing. We propose that tissue-resident macrophages are metabolically poised in situ to protect and exploit their tissue-niche by utilising locally available fuels to implement specific metabolic programmes upon microbial sensing.

Blockade of Glutamine Synthetase Enhances Inflammatory Response in Microglial Cells

AIMS

Microglial cells are brain-resident macrophages engaged in surveillance and maintained in a constant state of relative inactivity. However, their involvement in autoimmune diseases indicates that in pathological conditions microglia gain an inflammatory phenotype. The mechanisms underlying this change in the microglial phenotype are still unclear. Since metabolism is an important modulator of immune cell function, we focused our attention on glutamine synthetase (GS), a modulator of the response to lipopolysaccharide (LPS) activation in other cell types, which is expressed by microglia.

RESULTS

GS inhibition enhances release of inflammatory mediators of LPS-activated microglia in vitro, leading to perturbation of the redox balance and decreased viability of cocultured neurons. GS inhibition also decreases insulin-mediated glucose uptake in microglia. In vivo, microglia-specific GS ablation enhances expression of inflammatory markers upon LPS treatment. In the spinal cords from experimental autoimmune encephalomyelitis (EAE), GS expression levels and glutamine/glutamate ratios are reduced.

INNOVATION

Recently, metabolism has been highlighted as mediator of immune cell function through the discovery of mechanisms that (behind these metabolic changes) modulate the inflammatory response. The present study shows for the first time a metabolic mechanism mediating microglial response to a proinflammatory stimulus, pointing to GS activity as a master modulator of immune cell function and thus unraveling a potential therapeutic target.

CONCLUSIONS

Our study highlights a new role of GS in modulating immune response in microglia, providing insights into the pathogenic mechanisms associated with inflammation and new strategies of therapeutic intervention. Antioxid. Redox Signal. 26, 351-363.

SLC25A26 overexpression impairs cell function via mtDNA hypermethylation and rewiring of methyl metabolism

Cancer cells down-regulate different genes to give them a selective advantage in invasiveness and/or metastasis. The SLC25A26 gene encodes the mitochondrial carrier that catalyzes the import of S-adenosylmethionine (SAM) into the mitochondrial matrix, required for mitochondrial methylation processes, and is down-regulated in cervical cancer cells. In this study we show that SLC25A26 is down-regulated due to gene promoter hypermethylation, as a mechanism to promote cell survival and proliferation. Furthermore, overexpression of SLC25A26 in CaSki cells increases mitochondrial SAM availability and promotes hypermethylation of mitochondrial DNA, leading to decreased expression of key respiratory complex subunits, reduction of mitochondrial ATP and release of cytochrome c. In addition, increased SAM transport into mitochondria leads to impairment of the methionine cycle with accumulation of homocysteine at the expense of glutathione, which is strongly reduced. All these events concur to arrest the cell cycle in the S phase, induce apoptosis and enhance chemosensitivity of SAM carrier-overexpressing CaSki cells to cisplatin.

Clinical implications from proteomic studies in neurodegenerative diseases: lessons from mitochondrial proteins

Mitochondria play a key role in eukaryotic cells, being mediators of energy, biosynthetic and regulatory requirements of these cells. Emerging proteomics techniques have allowed scientists to obtain the differentially expressed proteome or the proteomic redox status in mitochondria. This has unmasked the diversity of proteins with respect to subcellular location, expression and interactions. Mitochondria have become a research 'hot spot' in subcellular proteomics, leading to identification of candidate clinical targets in neurodegenerative diseases in which mitochondria are known to play pathological roles. The extensive efforts to rapidly obtain differentially expressed proteomes and unravel the redox proteomic status in mitochondria have yielded clinical insights into the neuropathological mechanisms of disease, identification of disease early stage and evaluation of disease progression. Although current technical limitations hamper full exploitation of the mitochondrial proteome in neurosciences, future advances are predicted to provide identification of specific therapeutic targets for neurodegenerative disorders.

Acetylation of human mitochondrial citrate carrier modulates mitochondrial citrate/malate exchange activity to sustain NADPH production during macrophage activation

The mitochondrial citrate-malate exchanger (CIC), a known target of acetylation, is up-regulated in activated immune cells and plays a key role in the production of inflammatory mediators. However, the role of acetylation in CIC activity is elusive. We show that CIC is acetylated in activated primary human macrophages and U937 cells and the level of acetylation is higher in glucose-deprived compared to normal glucose medium. Acetylation enhances CIC transport activity, leading to a higher citrate efflux from mitochondria in exchange with malate. Cytosolic citrate levels do not increase upon activation of cells grown in deprived compared to normal glucose media, indicating that citrate, transported from mitochondria at higher rates from acetylated CIC, is consumed at higher rates. Malate levels in the cytosol are lower in activated cells grown in glucose-deprived compared to normal glucose medium, indicating that this TCA intermediate is rapidly recycled back into the cytosol where it is used by the malic enzyme. Additionally, in activated cells CIC inhibition increases the NADP+/NADPH ratio in glucose-deprived cells; this ratio is unchanged in glucose-rich grown cells due to the activity of the pentose phosphate pathway. Consistently, the NADPH-producing isocitrate dehydrogenase level is higher in activated glucose-deprived as compared to glucose rich cells. These results demonstrate that, in the absence of glucose, activated macrophages increase CIC acetylation to enhance citrate efflux from mitochondria not only to produce inflammatory mediators but also to meet the NADPH demand through the actions of isocitrate dehydrogenase and malic enzyme.