HIF-1α-PDK1 axis-induced active glycolysis plays an essential role in macrophage migratory capacity

Enhanced glycolysis contributes to the pathogenesis of experimental autoimmune neuritis

Background

With the recognition of the key roles of cellular metabolism in immunity, targeting metabolic pathway becomes a new strategy for autoimmune disease treatment. Guillain-Barré syndrome (GBS) is an acute immune-mediated inflammatory demyelinating disease of the peripheral nervous system, characterized by inflammatory cell infiltration. These inflammatory cells, including activated macrophages, Th1 cells, and Th17 cells, generally undergo metabolic reprogramming and rely mainly on glycolysis to exert functions. This study aimed to explore whether enhanced glycolysis contributed to the pathogenesis of experimental autoimmune neuritis (EAN), a classic model of GBS.

Methods

Preventive and therapeutic treatments with glycolysis inhibitor, 2-deoxy-d-glucose (2-DG), were applied to EAN rats. The effects of treatments were determined by clinical scoring, weighting, and tissue examination. Flow cytometry and ELISA were used to evaluate T cell differentiation, autoantibody level, and macrophage functions in vivo and in vitro.

Results

Glycolysis inhibition with 2-DG not only inhibited the initiation, but also prevented the progression of EAN, evidenced by the improved clinical scores, weight loss, inflammatory cell infiltration, and demyelination of sciatic nerves. 2-DG inhibited the differentiation of Th1, Th17, and Tfh cells but enhanced Treg cell development, accompanied with reduced autoantibody secretion. Further experiments in vitro proved glycolysis inhibition decreased the nitric oxide production and phagocytosis of macrophages and suppressed the maturation of dendritic cells (DC).

Conclusion

The effects of glycolysis inhibition on both innate and adaptive immune responses and the alleviation of animal clinical symptoms indicated that enhanced glycolysis contributed to the pathogenesis of EAN. Glycolysis inhibition may be a new therapy for GBS.

Metabolic Modulation in Macrophage Effector Function

Traditionally cellular respiration or metabolism has been viewed as catabolic and anabolic pathways generating energy and biosynthetic precursors required for growth and general cellular maintenance. However, growing literature provides evidence of a much broader role for metabolic reactions and processes in controlling immunological effector functions. Much of this research into immunometabolism has focused on macrophages, cells that are central in pro- as well as anti-inflammatory responses—responses that in turn are a direct result of metabolic reprogramming. As we learn more about the precise role of metabolic pathways and pathway intermediates in immune function, a novel opportunity to target immunometabolism therapeutically has emerged. Here, we review the current understanding of the regulation of macrophage function through metabolic remodeling.

The hypoxic tumour microenvironment

Cancer progression often benefits from the selective conditions present in the tumour microenvironment, such as the presence of cancer-associated fibroblasts (CAFs), deregulated ECM deposition, expanded vascularisation and repression of the immune response. Generation of a hypoxic environment and activation of its main effector, hypoxia-inducible factor-1 (HIF-1), are common features of advanced cancers. In addition to the impact on tumour cell biology, the influence that hypoxia exerts on the surrounding cells represents a critical step in the tumorigenic process. Hypoxia indeed enables a number of events in the tumour microenvironment that lead to the expansion of aggressive clones from heterogeneous tumour cells and promote a lethal phenotype. In this article, we review the most relevant findings describing the influence of hypoxia and the contribution of HIF activation on the major components of the tumour microenvironment, and we summarise their role in cancer development and progression.

A New Perspective on the Heterogeneity of Cancer Glycolysis

Tumors are dynamic metabolic systems which highly augmented metabolic fluxes and nutrient needs to support cellular proliferation and physiological function. For many years, a central hallmark of tumor metabolism has emphasized a uniformly elevated aerobic glycolysis as a critical feature of tumorigenecity. This led to extensive efforts of targeting glycolysis in human cancers. However, clinical attempts to target glycolysis and glucose metabolism have proven to be challenging. Recent advancements revealing a high degree of metabolic heterogeneity and plasticity embedded among various human cancers may paint a new picture of metabolic targeting for cancer therapies with a renewed interest in glucose metabolism. In this review, we will discuss diverse oncogenic and molecular alterations that drive distinct and heterogeneous glucose metabolism in cancers. We will also discuss a new perspective on how aberrantly altered glycolysis in response to oncogenic signaling is further influenced and remodeled by dynamic metabolic interaction with surrounding tumor-associated stromal cells.

Therapeutic Targeting of the Pyruvate Dehydrogenase Complex/Pyruvate Dehydrogenase Kinase (PDC/PDK) Axis in Cancer

The mitochondrial pyruvate dehydrogenase complex (PDC) irreversibly decarboxylates pyruvate to acetyl coenzyme A, thereby linking glycolysis to the tricarboxylic acid cycle and defining a critical step in cellular bioenergetics. Inhibition of PDC activity by pyruvate dehydrogenase kinase (PDK)-mediated phosphorylation has been associated with the pathobiology of many disorders of metabolic integration, including cancer. Consequently, the PDC/PDK axis has long been a therapeutic target. The most common underlying mechanism accounting for PDC inhibition in these conditions is post-transcriptional upregulation of one or more PDK isoforms, leading to phosphorylation of the E1α subunit of PDC. Such perturbations of the PDC/PDK axis induce a "glycolytic shift," whereby affected cells favor adenosine triphosphate production by glycolysis over mitochondrial oxidative phosphorylation and cellular proliferation over cellular quiescence. Dichloroacetate is the prototypic xenobiotic inhibitor of PDK, thereby maintaining PDC in its unphosphorylated, catalytically active form. However, recent interest in the therapeutic targeting of the PDC/PDK axis for the treatment of cancer has yielded a new generation of small molecule PDK inhibitors. Ongoing investigations of the central role of PDC in cellular energy metabolism and its regulation by pharmacological effectors of PDKs promise to open multiple exciting vistas into the biochemical understanding and treatment of cancer and other diseases.

Monocyte and macrophage immunometabolism in atherosclerosis

Atherosclerosis is characterized by chronic low grade inflammation of arteries that results in the development of lipid dense plaques. Chronic inflammation induced by Western-type diet is associated with the risk of developing atherosclerosis, and new insights shed light on the importance of metabolic and functional reprogramming in monocytes and macrophages for progression of atherosclerosis. This review aims to provide an overview of our current understanding into how the metabolic reprogramming of glucose, cholesterol, fatty acid, and amino acid metabolism in macrophages contributes to inflammation during atherosclerosis. Recent insights suggest that transcriptional and epigenetic adaptation within innate immune cells (termed trained immunity) play an important role in the pathogenesis of atherosclerosis. We propose that metabolic changes induced by pro-atherogenic lipoproteins partly mediate these changes in trained macrophages. Finally, we discuss the possibility of manipulating cellular metabolism of immune cells for targeted therapeutic intervention against atherosclerosis.

The Roles of Hypoxia Signaling in the Pathogenesis of Cardiovascular Diseases

The circulatory system distributes blood flow to each tissue and transports oxygen and nutrients. Peripheral circulation is required to maintain the physiological function in each tissue. Disturbance of circulation, therefore, decreases oxygen delivery, leading to tissue hypoxia which takes place in several cardiovascular disorders including atherosclerosis, pulmonary arterial hypertension and heart failure. While tissue hypoxia can be induced because of cardiovascular disorders, hypoxia signaling itself has a potential to modulate tissue remodeling processes or the severity of the cardiovascular disorders. Hypoxia inducible factor-1α (HIF-1α) and HIF-2α belongs to a group of transcription factors which mediate most of the cellular responses to hypoxia at a transcriptional level. We, and others, have reported that HIF-α signaling plays a critical role in the initiation or the regulation of inflammation. HIF-α signaling contributes to the tissue remodeling processes; thus it has a potential to become a therapeutic target. Elucidation of the molecular link, therefore, between hypoxia signaling and tissue remodeling will greatly help us to understand the pathophysiology of the cardiovascular disorders. The purpose of this review is to give a brief overview of the current understanding about the function HIF-α in inflammation processes especially by focusing on its roles in macrophages. In addition, the pathophysiological roles of hypoxia signaling for the development of cardiovascular disease will be discussed.

Specific and Complex Reprogramming of Cellular Metabolism in Myeloid Cells during Innate Immune Responses

Renewed interest in immune cell metabolism has led to the emergence of a research field aimed at studying the importance of metabolic processes for an effective immune response. In addition to the adaptive immune system, cells of the myeloid lineage have been shown to undergo robust metabolic changes upon activation. Whereas the specific metabolic requirements of myeloid cells after lipopolysaccharide/TLR4 stimulation have been extensively studied, recent evidence suggested that this model does not represent a metabolic blueprint for activated myeloid cells. Instead, different microbial stimuli, pathogens, or tissue microenvironments lead to specific and complex metabolic rewiring of myeloid cells. Here we present an overview of the metabolic heterogeneity in activated myeloid cells during health and disease. Directions for future research are suggested to ultimately provide new therapeutic opportunities. The uniqueness of metabolic signatures accompanying different conditions will require tailor-made interventions to ultimately modulate aberrant myeloid cell activation during disease.

HIF-1-mediated suppression of mitochondria electron transport chain function confers resistance to lidocaine-induced cell death

The local anesthetic lidocaine induces cell death by altering reactive oxygen species (ROS) generation and mitochondrial electron transport chain function. Because hypoxia-inducible factor 1 (HIF-1) is involved in determining oxygen metabolism and mitochondria function, we investigated the involvement of HIF-1 activity in lidocaine-induced cell death. We investigated the role of HIF activation on lidocaine-induced caspase activation and cell death in renal cell-derived RCC4 cells lacking functional von Hippel-Lindau (VHL) protein. We demonstrate that HIF-1 suppressed oxygen consumption and facilitated glycolysis in a pyruvate dehydrogenase kinase-1-dependent manner and that activation of HIF-1 conferred resistance to lidocaine-induced cell death. We also demonstrated that exogenous HIF-1 activation, through HIFα-hydroxylase inhibition or exposure to hypoxic conditions, alleviates lidocaine toxicity by suppressing mitochondria function and generating ROS, not only in RCC4 cells, but also in the neuronal SH-SY5Y cells. In conclusion, we demonstrate that HIF-1 activation due to VHL deletion, treatment with small molecule HIFα-hydroxylase inhibitors, and exposure to hypoxic conditions suppresses mitochondrial respiratory chain function and confers resistance to lidocaine toxicity.

Metabolic regulation of macrophages during tissue repair: insights from skeletal muscle regeneration

Macrophages are highly versatile cells that are involved both in the mounting and the resolution of inflammatory responses. Besides their properties in innate immunity to fight against pathogens, macrophages are essential for tissue repair, during which they adopt sequential inflammatory status. While the acquisition of some canonical polarized inflammatory statuses in vitro (M1/M2) is beginning to be understood at the molecular level, the regulation of macrophage skewing in vivo has been less investigated. Immunometabolism, in particular, is an emerging field, and most of the studies so far have investigated the control of macrophage polarization using in vitro set-ups. In this context, skeletal muscle regeneration is an excellent paradigm to study tissue repair, since the sequential steps of inflammatory response and tissue repair are well characterized. In this Review, after introducing macrophage populations and functions during skeletal muscle regeneration, we present the current knowledge on the metabolic regulation of macrophage inflammatory status, with particular emphasis on the comparison between in vitro and in vivo models of macrophage activation. We also discuss the metabolic regulation of macrophages in vivo during skeletal muscle regeneration.

Hypoxia-inducible factor 1 alpha is a poor prognostic factor and potential therapeutic target in malignant peripheral nerve sheath tumor

Background

Malignant peripheral nerve sheath tumor (MPNST) is a rare soft tissue sarcoma with poor prognosis. Hypoxia-inducible factor 1 (HIF-1) plays a crucial role in the cellular response to hypoxia and regulates the expression of multiple genes involved in tumor progression in various cancers. However, the importance of the expression of HIF-1α in MPNSTs is unclear.

Methods

The expression of HIF-1α was examined immunohistochemically in 82 MPNST specimens. Cell culture assays of human MPNST cells under normoxic and hypoxic conditions were used to evaluate the impact of anti-HIF-1α–specific siRNA inhibition on cell survival. A screening kit was employed to identify small molecules that inhibited HIF-1α.

Results

The nuclear expression of HIF-1α was positive in 75.6% of MPNST samples (62/82 cases). Positivity for HIF-1α was a significant poor prognostic factor both in univariate (P = 0.048) and multivariate (P ≤ 0.0001) analyses. HIF-1α knockdown abrogated MPNST cell growth, inducing apoptosis. Finally, chetomin, an inhibitor of HIF-1α, effectively inhibited the growth of MPNST cells and induced their apoptosis.

Conclusion

Inhibition of HIF-1α signaling is a potential treatment option for MPNSTs.

Hif-1α regulates macrophage-endothelial interactions during blood vessel development in zebrafish

Macrophages are known to interact with endothelial cells during developmental and pathological angiogenesis but the molecular mechanisms modulating these interactions remain unclear. Here, we show a role for the Hif-1α transcription factor in this cellular communication. We generated hif-1aa;hif-1ab double mutants in zebrafish, hereafter referred to as hif-1α mutants, and find that they exhibit impaired macrophage mobilization from the aorta-gonad-mesonephros (AGM) region as well as angiogenic defects and defective vascular repair. Importantly, macrophage ablation is sufficient to recapitulate the vascular phenotypes observed in hif-1α mutants, revealing for the first time a macrophage-dependent angiogenic process during development. Further substantiating our observations of vascular repair, we find that most macrophages closely associated with ruptured blood vessels are Tnfα-positive, a key feature of classically activated macrophages. Altogether, our data provide genetic evidence that Hif-1α regulates interactions between macrophages and endothelial cells starting with the mobilization of macrophages from the AGM.

The molecular mechanism regulating macrophage interaction with endothelial cells during development is unclear. Here, the authors show that in zebrafish mutation of hypoxia-inducible factor-1α impairs macrophage mobilization from the aorta-gonad-mesonephros, causing defects in angiogenesis and vessel repair.

Fine tuning of immunometabolism for the treatment of rheumatic diseases

All immune cells depend on specific and efficient metabolic pathways to mount an appropriate response. Over the past decade, the field of immunometabolism has expanded our understanding of the various means by which cells modulate metabolism to achieve the effector functions necessary to fight infection or maintain homeostasis. Harnessing these metabolic pathways to manipulate inappropriate immune responses as a therapeutic strategy in cancer and autoimmunity has received increasing scrutiny by the scientific community. Fine tuning immunometabolism to provide the desired response, or prevent a deleterious response, is an attractive alternative to chemotherapy or overt immunosuppression. The various metabolic pathways used by immune cells in rheumatoid arthritis, systemic lupus erythematosus and osteoarthritis offer numerous opportunities for selective targeting of specific immune cell subsets to manipulate cellular metabolism for therapeutic benefit in these rheumatologic diseases.

Macrophage Immunometabolism: Where Are We (Going)?

A growing number of findings highlight the crucial role of metabolic reprogramming in macrophage activation. Metabolic pathways are closely interconnected and recent literature demonstrates the need for glucose metabolism in anti-inflammatory as well as inflammatory macrophages. Moreover, fatty acid oxidation (FAO) not only supports anti-inflammatory responses as described formerly but also drives inflammasome activation in inflammatory macrophages. Hence, defining glycolysis as proinflammatory and FAO as anti-inflammatory may be an oversimplification. Here we review how the rapid growth of the immunometabolism field has improved our understanding of macrophage activation and at the same time has led to an increase in the appearance of contradictory observations. To conclude we discuss current challenges in immunometabolism and present crucial areas for future research.

Mitochondrial composition and function under the control of hypoxia

Hypoxia triggers several mechanisms to adapt cells to a low oxygen environment. Mitochondria are major consumers of oxygen and a potential source of reactive oxygen species (ROS). In response to hypoxia they exchange or modify distinct subunits of the respiratory chain and adjust their metabolism, especially lowering the citric acid cycle. Intermediates of the citric acid cycle participate in regulating hypoxia inducible factors (HIF), the key mediators of adaptation to hypoxia. Here we summarize how hypoxia conditions mitochondria with consequences for ROS-production and the HIF-pathway.

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Hypoxia provokes changes in mitochondrial morphology, metabolism, and respiration.

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Hypoxia calls forth changes in redox signaling.

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HIF-signaling is linked to mitochondrial metabolism and ROS formation.

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Hypoxia adjusts ETC complex formation, activity, respiration, and ROS formation.

Hypoxia inducible factors are dispensable for myeloid cell migration into the inflamed mouse eye

Hypoxia inducible factors (HIFs) are ubiquitously expressed transcription factors important for cell homeostasis during dynamic oxygen levels. Myeloid specific HIFs are crucial for aspects of myeloid cell function, including their ability to migrate into inflamed tissues during autoimmune disease. This contrasts with the concept that accumulation of myeloid cells at ischemic and hypoxic sites results from a lack of chemotactic responsiveness. Here we seek to address the role of HIFs in myeloid trafficking during inflammation in a mouse model of human uveitis. We show using mice with myeloid-specific Cre-deletion of HIFs that myeloid HIFs are dispensable for leukocyte migration into the inflamed eye. Myeloid-specific deletion of Hif1a, Epas1, or both together, had no impact on the number of myeloid cells migrating into the eye. Additionally, stabilization of HIF pathways via deletion of Vhl in myeloid cells had no impact on myeloid trafficking into the inflamed eye. Finally, we chemically induce hypoxemia via hemolytic anemia resulting in HIF stabilization within circulating leukocytes to demonstrate the dispensable role of HIFs in myeloid cell migration into the inflamed eye. These data suggest, contrary to previous reports, that HIF pathways in myeloid cells during inflammation and hypoxia are dispensable for myeloid cell tissue trafficking.

PDK1 promotes the inflammatory progress of fibroblast-like synoviocytes by phosphorylating RSK2

This study investigated the role of PDK1 in inflammatory response which is initiated by TNF-α and analyzed the association between PDK1 and RSK2. TNF-α were added into MH7A cells to induce inflammation condition. Through overexpressing or suppressing PDK1 in MH7A cells, the role of PDK1 in cell invasiveness and inflammatory factors was determined. Levels of MMPs protein and inflammatory cytokines were assessed with PDK1 siRNA and TNF-α treatment. Inhibition of RSK2 was used to investigate the function of RSK2 on PDK1-induced inflammation. The phosphorylation of RSK2 was detected when PDK1 was inhibited. Luciferase reporter assay was performed to detect the transcriptional activity of NF-κB. We found highly expressed PDK1 could promote cell invasion and secretion of IL-1β and IL-6 in MH7A cells. Inhibition of RSK2 reduced the PDK1-induced cell invasion and cytokines secretion in MH7A cells. In response to TNF-α, PDK1 could phosphorylate RSK2 and activated RSK2, then promoting the activation of NF-κB. This may be a possible therapeutic option of rheumatoid arthritis.

The bacillary and macrophage response to hypoxia in tuberculosis and the consequences for T cell antigen recognition

Mycobacterium tuberculosis is a facultative anaerobe and its characteristic pathological hallmark, the granuloma, exhibits hypoxia in humans and in most experimental models. Thus the host and bacillary adaptation to hypoxia is of central importance in understanding pathogenesis and thereby to derive new drug treatments and vaccines.

Hypoxia-inducible factors: key regulators of myeloid cells during inflammation

Hypoxia is a prominent characteristic of many acute or chronic inflammatory diseases, and exerts significant influence on their progression. Macrophages and neutrophils are major cellular components of innate immunity and contribute not only to O2 deprivation at the site of inflammation, but also alter many of their functions in response to hypoxia to either facilitate or suppress inflammation. Hypoxia stabilizes HIF-αs in macrophages and neutrophils, and these O2-sensitive transcription factors are key regulators of inflammatory responses in myeloid cells. In this review, we will summarize our current understanding of the role of HIF-αs in shaping macrophage and neutrophil functions in the pathogenesis and progression of multiple inflammatory diseases.