Two-stage metabolic remodelling in macrophages in response to lipopolysaccharide and interferon-γ stimulation

β-Hydroxybutyrate Oxidation Promotes the Accumulation of Immunometabolites in Activated Microglia Cells

Metabolic regulation of immune cells has arisen as a critical set of processes required for appropriate response to immunological signals. While our knowledge in this area has rapidly expanded in leukocytes, much less is known about the metabolic regulation of brain-resident microglia. In particular, the role of alternative nutrients to glucose remains poorly understood. Here, we use stable-isotope (¹³C) tracing strategies and metabolomics to characterize the oxidative metabolism of β-hydroxybutyrate (BHB) in human (HMC3) and murine (BV2) microglia cells and the interplay with glucose in resting and LPS-activated BV2 cells. We found that BHB is imported and oxidised in the TCA cycle in both cell lines with a subsequent increase in the cytosolic NADH:NAD⁺ ratio. In BV2 cells, stimulation with LPS upregulated the glycolytic flux, increased the cytosolic NADH:NAD⁺ ratio and promoted the accumulation of the glycolytic intermediate dihydroxyacetone phosphate (DHAP). The addition of BHB enhanced LPS-induced accumulation of DHAP and promoted glucose-derived lactate export. BHB also synergistically increased LPS-induced accumulation of succinate and other key immunometabolites, such as α-ketoglutarate and fumarate generated by the TCA cycle. Finally, BHB upregulated the expression of a key pro-inflammatory (M1 polarisation) marker gene, NOS2, in BV2 cells activated with LPS. In conclusion, we identify BHB as a potentially immunomodulatory metabolic substrate for microglia that promotes metabolic reprogramming during pro-inflammatory response.

Selective inhibition of mitochondrial respiratory complexes controls the transition of microglia into a neurotoxic phenotype in situ

Microglia are tissue resident macrophages (innate immunity) and universal sensors of alterations in CNS physiology. In response to pathogen or damage signals, microglia feature rapid activation and can acquire different phenotypes exerting neuroprotection or neurotoxicity. Although transcriptional aspects of microglial phenotypic transitions have been described, the underlying metabolic reprogramming is widely unknown. Employing postnatal organotypic hippocampal slice cultures, we describe that microglia transformed into a mild reactive phenotype by single TLR4 stimulation with lipopolysaccharide (LPS), which was boosted into a severe neurotoxic phenotype by IFN-γ (LPS + INF-γ). The two reactive phenotypes associated with reduction of microglial homeostatic "surveillance" markers, increase of cytokine release (IL-6, TNF-α) as well as enhancement of tissue energy demand and lactate production. These reactive phenotypes differed in the pattern of inhibition of the respiratory chain in mitochondria, however. TLR4 stimulation induced succinate dehydrogenase (complex II) inhibition by the metabolite itaconate. By contrast, TLR4 + IFN-γ receptor stimulation mainly resulted in complex IV inhibition by nitric oxide (NO) that also associated with severe oxidative stress, neuronal dysfunction and death. Notably, pharmacological depletion of microglia or treatment with itaconate resulted in effective neuroprotection reflected by well-preserved cytoarchitecture and electrical network activity, i.e., neuronal gamma oscillations (30-70 Hz) that underlie higher cognitive functions in vivo. Our findings provide in situ evidence that (i) proinflammatory microglia can substantially alter brain energy metabolism and (ii) fine-tuning of itaconate and NO metabolism determines microglial reactivity, impairment of neural network function and neurodegeneration. These data add mechanistic insights into microglial activation, with relevance to disorders featuring neuroinflammation and to drug discovery.

Fever-Inducible Lipid Nanocomposite for Boosting Cancer Therapy through Synergistic Engineering of a Tumor Microenvironment

A fever-mimic response capable of recruiting reprogrammed macrophages holds great potential in the engineering of the tumor microenvironment (TME). Low-temperature photothermal therapy (LT-PTT) can maintain tumors at a fever-like temperature (<45 °C) temporarily; however, it still faces several challenges in efficient regulation of TME because of reciprocal cross-talk between the bypass pathways. Here, we report a synergistic engineering of TME through an enhanced activation of a fever-mimic response based on both LT-PTT and tumor vascular normalization. Such engineering is achieved by a fever-inducible lipid nanocomposite (GNR-T/CM-L), which produces mild heat (∼43 °C) and sequentially releases multicomponents to cooperatively upregulate interferon-gamma under NIR irradiation, forming a bidirectionally closed loop for downstream M1 tumor-associated macrophage polarization and promoting the inhibition of the tumor growth. In proof-of-concept studies, GNR-T/CM-L demonstrated efficient tumor ablation in breast tumor xenograft-bearing mice and significantly prolonged their survival period. It paves an avenue to precisely reprogram TME for efficient cancer therapy through synergistic pathways of creating fever-like responses in the tumor.

The Metabolic Basis of Immune Dysfunction Following Sepsis and Trauma

Critically ill, severely injured and high-risk surgical patients are vulnerable to secondary infections during hospitalization and after hospital discharge. Studies show that the mitochondrial function and oxidative metabolism of monocytes and macrophages are impaired during sepsis. Alternatively, treatment with microbe-derived ligands, such as monophosphoryl lipid A (MPLA), peptidoglycan, or β-glucan, that interact with toll-like receptors and other pattern recognition receptors on leukocytes induces a state of innate immune memory that confers broad-spectrum resistance to infection with common hospital-acquired pathogens. Priming of macrophages with MPLA, CPG oligodeoxynucleotides (CpG ODN), or β-glucan induces a macrophage metabolic phenotype characterized by mitochondrial biogenesis and increased oxidative metabolism in parallel with increased glycolysis, cell size and granularity, augmented phagocytosis, heightened respiratory burst functions, and more effective killing of microbes. The mitochondrion is a bioenergetic organelle that not only contributes to energy supply, biosynthesis, and cellular redox functions but serves as a platform for regulating innate immunological functions such as production of reactive oxygen species (ROS) and regulatory intermediates. This review will define current knowledge of leukocyte metabolic dysfunction during and after sepsis and trauma. We will further discuss therapeutic strategies that target leukocyte mitochondrial function and might have value in preventing or reversing sepsis- and trauma-induced immune dysfunction.

A small molecule G6PD inhibitor reveals immune dependence on pentose phosphate pathway

Glucose is catabolized by two fundamental pathways, glycolysis to make ATP and the oxidative pentose phosphate pathway to make NADPH. The first step of the oxidative pentose phosphate pathway is catalyzed by the enzyme glucose-6-phosphate dehydrogenase (G6PD). Here we develop metabolite reporter and deuterium tracer assays to monitor cellular G6PD activity. Using these, we show that the most widely cited G6PD antagonist, dehydroepiandosterone (DHEA), does not robustly inhibit G6PD in cells. We then identify a small molecule (G6PDi-1) that more effectively inhibits G6PD. Across a range of cultured cells, G6PDi-1 depletes NADPH most strongly in lymphocytes. In T cells but not macrophages, G6PDi-1 markedly decreases inflammatory cytokine production. In neutrophils, it suppresses respiratory burst. Thus, we provide a cell-active small molecule tool for oxidative pentose phosphate pathway inhibition, and use it to identify G6PD as a pharmacological target for modulating immune response.

Krebs Cycle Reborn in Macrophage Immunometabolism

A striking change has happened in the field of immunology whereby specific metabolic processes have been shown to be a critical determinant of immune cell activation. Multiple immune receptor types rewire metabolic pathways as a key part of how they promote effector functions. Perhaps surprisingly for immunologists, the Krebs cycle has emerged as the central immunometabolic hub of the macrophage. During proinflammatory macrophage activation, there is an accumulation of the Krebs cycle intermediates succinate and citrate, and the Krebs cycle–derived metabolite itaconate. These metabolites have distinct nonmetabolic signaling roles that influence inflammatory gene expression. A key bioenergetic target for the Krebs cycle, the electron transport chain, also becomes altered, generating reactive oxygen species from Complexes I and III. Similarly, alternatively activated macrophages require α-ketoglutarate-dependent epigenetic reprogramming to elicit anti-inflammatory gene expression. In this review, we discuss these advances and speculate on the possibility of targeting these events therapeutically for inflammatory diseases.

Metabolic flux analysis and fluxomics-driven determination of reaction free energy using multiple isotopes

Metabolite concentrations, fluxes, and free energies constitute the basis for understanding and controlling metabolism. Mass spectrometry and stable isotopes are integral tools in quantifying these metabolic features. For absolute metabolite concentration and flux measurement, ¹³C internal standards and tracers have been the gold standard. In contrast, no established methods exist for comprehensive thermodynamic quantitation under physiological environments. Recently, using high-resolution mass spectrometry and multi-isotope tracing, flux quantitation has been increasingly adopted in broader metabolism. The improved flux quantitation led to determination of Gibbs free energy of reaction (ΔG) in central carbon metabolism using a relationship between reaction reversibility and thermodynamic driving force. Here we highlight recent advances in multi-isotope tracing for metabolic flux and free energy analysis.

Itaconate modulates tricarboxylic acid and redox metabolism to mitigate reperfusion injury

OBJECTIVES

Cerebral ischemia/reperfusion (IR) drives oxidative stress and injurious metabolic processes that lead to redox imbalance, inflammation, and tissue damage. However, the key mediators of reperfusion injury remain unclear, and therefore, there is considerable interest in therapeutically targeting metabolism and the cellular response to oxidative stress.

METHODS

The objective of this study was to investigate the molecular, metabolic, and physiological impact of itaconate treatment to mitigate reperfusion injuries in in vitro and in vivo model systems. We conducted metabolic flux and bioenergetic studies in response to exogenous itaconate treatment in cultures of primary rat cortical neurons and astrocytes. In addition, we administered itaconate to mouse models of cerebral reperfusion injury with ischemia or traumatic brain injury followed by hemorrhagic shock resuscitation. We quantitatively characterized the metabolite levels, neurological behavior, markers of redox stress, leukocyte adhesion, arterial blood flow, and arteriolar diameter in the brains of the treated/untreated mice.

RESULTS

We demonstrate that the "immunometabolite" itaconate slowed tricarboxylic acid (TCA) cycle metabolism and buffered redox imbalance via succinate dehydrogenase (SDH) inhibition and induction of anti-oxidative stress response in primary cultures of astrocytes and neurons. The addition of itaconate to reperfusion fluids after mouse cerebral IR injury increased glutathione levels and reduced reactive oxygen/nitrogen species (ROS/RNS) to improve neurological function. Plasma organic acids increased post-reperfusion injury, while administration of itaconate normalized these metabolites. In mouse cranial window models, itaconate significantly improved hemodynamics while reducing leukocyte adhesion. Further, itaconate supplementation increased survival in mice experiencing traumatic brain injury (TBI) and hemorrhagic shock.

CONCLUSIONS

We hypothesize that itaconate transiently inhibits SDH to gradually "awaken" mitochondrial function upon reperfusion that minimizes ROS and tissue damage. Collectively, our data indicate that itaconate acts as a mitochondrial regulator that controls redox metabolism to improve physiological outcomes associated with IR injury.

Macrophage activation as an archetype of mitochondrial repurposing

Mitochondria are metabolic organelles essential not only for energy transduction, but also a range of other functions such as biosynthesis, ion and metal homeostasis, maintenance of redox balance, and cell signaling. A hallmark example of how mitochondria can rebalance these processes to adjust cell function is observed in macrophages. These innate immune cells are responsible for a remarkable breadth of processes including pathogen elimination, antigen presentation, debris clearance, and wound healing. These diverse, polarized functions often include similarly disparate alterations in the metabolic phenotype associated with their execution. In this chapter, mitochondrial bioenergetics and signaling are viewed through the lens of macrophage polarization: both classical, pro-inflammatory activation and alternative, anti-inflammatory activation are associated with substantive changes to mitochondrial metabolism. Emphasis is placed on recent evidence that aims to clarify the essential - rather than associative - mitochondrial alterations, as well as accumulating data suggesting a degree of plasticity within the metabolic phenotypes that can support pro- and anti-inflammatory functions.

Metabolic regulation of epigenetic remodeling in immune cells

Immune cells are capable of sensing various signals in the microenvironment and turning on specific immune functions in response. The appropriate transition of immune cells into diverse functional states, which is crucial for immunity, involves complex and well-regulated changes in transcriptional program. Accumulating evidence shows that epigenetic remodeling plays a central role in mediating the transcriptional program for immune cell activation and immunological memory. Concurrently, immune cells undergo significant metabolic reprogramming during immune response. Here we review recent studies that demonstrate shifts in metabolic state can orchestrate immune cell functions through its impact on epigenetic remodeling, and the microenvironment can exert its influence on immune cells through the metabolic regulation of epigenetics. We also discuss the systems biology approaches that enabled these discoveries.