Efficient phagocytosis requires triacylglycerol hydrolysis by adipose triglyceride lipase

Peroxisome proliferator-activated receptors: A key link between lipid metabolism and cancer progression

Lipids represent the essential components of membranes, serve as fuels for high-energy processes, and play crucial roles in signaling and cellular function. One of the key hallmarks of cancer is the reprogramming of metabolic pathways, especially abnormal lipid metabolism. Alterations in lipid uptake, lipid desaturation, de novo lipogenesis, lipid droplets, and fatty acid oxidation in cancer cells all contribute to cell survival in a changing microenvironment by regulating feedforward oncogenic signals, key oncogenic functions, oxidative and other stresses, immune responses, or intercellular communication. Peroxisome proliferator-activated receptors (PPARs) are transcription factors activated by fatty acids and act as core lipid sensors involved in the regulation of lipid homeostasis and cell fate. In addition to regulating whole-body energy homeostasis in physiological states, PPARs play a key role in lipid metabolism in cancer, which is receiving increasing research attention, especially the fundamental molecular mechanisms and cancer therapies targeting PPARs. In this review, we discuss how cancer cells alter metabolic patterns and regulate lipid metabolism to promote their own survival and progression through PPARs. Finally, we discuss potential therapeutic strategies for targeting PPARs in cancer based on recent studies from the last five years.

Triacylglycerol uptake and handling by macrophages: From fatty acids to lipoproteins

Macrophages are essential innate immune cells and form our first line of immune defense. Also known as professional phagocytes, macrophages interact and take up various particles, including lipids. Defective lipid handling can drive excessive lipid accumulation leading to foam cell formation, a key feature of various cardiometabolic conditions such as atherosclerosis, non-alcoholic fatty liver disease, and obesity. At the same time, intracellular lipid storage and foam cell formation can also be viewed as a protective and anti-lipotoxic mechanism against a lipid-rich environment and associated elevated lipid uptake. Traditionally, foam cell formation has primarily been linked to cholesterol uptake via native and modified low-density lipoproteins. However, other lipids, including non-esterified fatty acids and triacylglycerol (TAG)-rich lipoproteins (very low-density lipoproteins and chylomicrons), can also interact with macrophages. Recent studies have identified multiple pathways mediating TAG uptake and processing by macrophages, including endocytosis and receptor/transporter-mediated internalization and transport. This review will present the current knowledge of how macrophages take up different lipids and lipoprotein particles and address how TAG-rich lipoproteins are processed intracellularly. Understanding how macrophages take up and process different lipid species such as TAG is necessary to design future therapeutic interventions to correct excessive lipid accumulation and associated co-morbidities.

Cooperative lipolytic control of neuronal triacylglycerol by spastic paraplegia-associated enzyme DDHD2 and ATGL

Intracellular lipolysis-the enzymatic breakdown of lipid droplet-associated triacylglycerol (TAG)-depends on the cooperative action of several hydrolytic enzymes and regulatory proteins, together designated as lipolysome. Adipose triglyceride lipase (ATGL) acts as a major cellular TAG hydrolase and core effector of the lipolysome in many peripheral tissues. Neurons initiate lipolysis independently of ATGL via DDHD domain-containing 2 (DDHD2), a multifunctional lipid hydrolase whose dysfunction causes neuronal TAG deposition and hereditary spastic paraplegia. Whether and how DDHD2 cooperates with other lipolytic enzymes is currently unknown. In this study, we further investigated the enzymatic properties and functions of DDHD2 in neuroblastoma cells and primary neurons. We found that DDHD2 hydrolyzes multiple acylglycerols in vitro and substantially contributes to neutral lipid hydrolase activities of neuroblastoma cells and brain tissue. Substrate promiscuity of DDHD2 allowed its engagement at different steps of the lipolytic cascade: In neuroblastoma cells, DDHD2 functioned exclusively downstream of ATGL in the hydrolysis of sn-1,3-diacylglycerol (DAG) isomers but was dispensable for TAG hydrolysis and lipid droplet homeostasis. In primary cortical neurons, DDHD2 exhibited lipolytic control over both, DAG and TAG, and complemented ATGL-dependent TAG hydrolysis. We conclude that neuronal cells use noncanonical configurations of the lipolysome and engage DDHD2 as dual TAG/DAG hydrolase in cooperation with ATGL.

Metabolic and functional remodeling of colonic macrophages in response to high-fat diet-induced obesity

Little is known about the effects of high-fat diet (HFD)-induced obesity on resident colonic lamina propria (LP) macrophages (LPMs) function and metabolism. Here, we report that obesity and diabetes resulted in increased macrophage infiltration in the colon. These macrophages exhibited the residency phenotype CX3CR1hiMHCIIhi and were CD4-TIM4-. During HFD, resident colonic LPM exhibited a lipid metabolism gene expression signature that overlapped that used to define lipid-associated macrophages (LAMs). Via single-cell RNA sequencing, we identified a sub-cluster of macrophages, increased in HFD, that were responsible for the LAM signature. Compared to other macrophages in the colon, these cells were characterized by elevated glycolysis, phagocytosis, and efferocytosis signatures. CX3CR1hiMHCIIhi colonic resident LPMs had fewer lipid droplets (LDs) and decreased triacylglycerol (TG) content compared to equivalent cells in lean mice and exhibited increased phagocytic capacity, suggesting that HFD induces adaptive responses in LPMs to limit bacterial translocation.

Time-resolved single-cell RNAseq profiling identifies a novel Fabp5+ subpopulation of inflammatory myeloid cells with delayed cytotoxic profile in chronic spinal cord injury

Traumatic spinal cord injuries (SCI) are a group of highly debilitating pathologies affecting thousands annually, and adversely affecting quality of life. Currently, no fully restorative therapies exist, and SCI still results in significant personal, societal and financial burdens. Inflammation plays a major role in the evolution of SCI, with myeloid cells, including bone marrow derived macrophages (BMDMs) and microglia (MG) being primary drivers of both early secondary pathogenesis and delayed wound healing events. The precise role of myeloid cell subsets is unclear as upon crossing the blood-spinal cord barrier, infiltrating bone marrow derived macrophages (BMDMs) may take on the morphology of resident microglia, and upregulate canonical microglia markers, thus making the two populations difficult to distinguish. Here, we used time-resolved scRNAseq and transgenic fate-mapping to chart the transcriptional profiles of tissue-resident and -infiltrating myeloid cells in a mouse model of thoracic contusion SCI. Our work identifies a novel subpopulation of foam cell-like inflammatory myeloid cells with increased expression of Fatty Acid Binding Protein 5 (Fabp5) and comprise both tissue-resident and -infiltrating cells. Fabp5+ inflammatory myeloid cells display a delayed cytotoxic profile that is predominant at the lesion epicentre and extends into the chronic phase of SCI.

Impact of Lipid Metabolism on Macrophage Polarization: Implications for Inflammation and Tumor Immunity

Macrophage polarization is influenced by lipids, which also exert significant control over macrophage functions. Lipids and their metabolites are players in intricate signaling pathways that modulate macrophages' responses to pathogens, phagocytosis, ferroptosis, and inflammation. This review focuses on lipid metabolism and macrophage functions and addresses potential molecular targets for the treatment of macrophage-related diseases. While lipogenesis is crucial for lipid accumulation and phagocytosis in M1 macrophages, M2 macrophages likely rely on fatty acid β-oxidation to utilize fatty acids as their primary energy source. Cholesterol metabolism, regulated by factors such as SREBPs, PPARs, and LXRs, is associated with the cholesterol efflux capacity and the formation of foam cells (M2-like macrophages). Foam cells, which are targets for atherosclerosis, are associated with an increase in inflammatory cytokines. Lipolysis and fatty acid uptake markers, such as CD36, also contribute to the production of cytokines. Enhancing the immune system through the inhibition of lipid-metabolism-related factors can potentially serve as a targeted approach against tumor cells. Cyclooxygenase inhibitors, which block the conversion of arachidonic acid into various inflammatory mediators, influence macrophage polarization and have generated attention in cancer research.

MicroRNA-22 Regulates the Pro-inflammatory Responses and M1 Polarization of Macrophages by Targeting GLUT1 and 4-1BBL

Many microRNAs (miRNAs) are selectively expressed in mammalian immune cells and have been linked to immune responses in host defense and autoimmune disease. In macrophages, miRNAs regulate cell metabolism by repressing the expression of genes such as transcription factors, enzymes, and metabolism-related molecules, as well as the expression of genes that impact inflammatory responses and phenotype determination. Previous studies showed that miR-22 plays a role in a variety of biological processes, such as cancer cell growth, cell survival, and cell expansion. In CD4 + T cells of inflammatory bowel disease patients, miR-22 is upregulated and regulates inflammasome-mediated responses. However, it has not yet been determined how miR-22 contributes to the activation of innate immune cells. In this study, we identified a mechanism of toll-like receptors- (TLR-) dependent miR-22 induction that regulates the downstream signaling pathway linking inflammatory responses and macrophage polarization. MiR-22 is induced via TLR-signaling, which regulates the induction of Slc2a1 (glucose transporter 1 and Glut1) and Tnfsf9 (tumor necrosis factor 9, 4-1BB ligand, and 4-1BBL) mRNAs that contribute to sustained inflammatory responses and the polarization of macrophages. Our observations support further efforts to explore a potential therapeutic strategy using miR-22 for the modulation of excessive macrophage activation for the treatment of inflammatory diseases.

Amyloid β Induces Lipid Droplet-Mediated Microglial Dysfunction in Alzheimer's Disease

Several microglia-expressed genes have emerged as top risk variants for Alzheimer's disease (AD). Impaired microglial phagocytosis is one of the main proposed outcomes by which these AD-risk genes may contribute to neurodegeneration, but the mechanisms translating genetic association to cellular dysfunction remain unknown. Here we show that microglia form lipid droplets (LDs) upon exposure to amyloid-beta (Aβ), and that their LD load increases with proximity to amyloid plaques in brains from human patients and the AD mouse model 5xFAD. LD formation is dependent upon age and disease progression and is more prominent in the hippocampus in mice and humans. Despite variability in LD load between microglia from male versus female animals and between cells from different brain regions, LD-laden microglia exhibited a deficit in Aβ phagocytosis. Unbiased lipidomic analysis identified a substantial decrease in free fatty acids (FFAs) and a parallel increase in triacylglycerols (TAGs) as the key metabolic transition underlying LD formation. We demonstrate that DGAT2, a key enzyme for the conversion of FFAs to TAGs, promotes microglial LD formation, is increased in microglia from 5xFAD and human AD brains, and that inhibiting DGAT2 improved microglial uptake of Aβ. These findings identify a new lipid-mediated mechanism underlying microglial dysfunction that could become a novel therapeutic target for AD.

Pre-exposure to Fe2O3 or TiO2 Nanoparticles Inhibits Subsequent Biological Uptake of 55Fe-Labeled Fe2O3 Nanoparticles

Aquatic organisms are frequently exposed to various nanoparticles (NPs) in the natural environment. Thus, studies of NP bioaccumulation should include organisms that have been previously exposed to NPs. Our study investigated the effects of pre-exposure of Tetrahymena thermophila (T. thermophila) to Fe₂O₃ or TiO₂ NPs on the protozoan's subsequent uptake of ⁵⁵Fe-labeled Fe₂O₃ (⁵⁵Fe₂O₃) NPs. Molecular mechanisms underlying the pre-exposure effects were explored in transcriptomic and metabolomic experiments. Pre-exposure to either NPs inhibited the subsequent uptake of ⁵⁵Fe₂O₃ NPs. The results of the transcriptomic experiment indicated that NP pre-exposure influenced the expression of genes related to phagosomes and lysosomes and physiological processes such as glutathione and lipid metabolism, which are closely associated with the endocytosis of ⁵⁵Fe₂O₃ NPs. The differentially expressed metabolites obtained from the metabolomic experiments showed an enrichment of energy metabolism and antioxidation pathways in T. thermophila pre-exposed to NPs. Together, these results demonstrate that the pre-exposure of T. thermophila to Fe₂O₃ or TiO₂ NPs inhibited the protozoan's subsequent uptake of ⁵⁵Fe₂O₃ NPs, possibly by mechanisms involving the alteration of endocytosis-related organelles, the induction of oxidative stress, and a lowering of the intracellular energy supply. Thus, NP pre-exposure represents a scenario which can inform increasingly realistic estimates of NP bioaccumulation.

Autophagy impairment is involved in midazolam-induced lipid droplet accumulation and consequent phagocytosis decrease in BV2 cells

An increasing number of experimental and clinical observation suggest that the use of anaesthetics is closely associated with postoperative central nervous system (CNS) complications, such as delirium and cognitive dysfunction. Brain energy rescue is an emerging therapeutic strategy for central nervous system disease (CNSDs). However, the effect of anaesthetics on nerve cell energy utilisation, especially microglia, and its potential effects on cell function still unclear. Elucidating the effects of anaesthetics on lipid droplets, which are specific lipid storage organs, and phagocytosis of microglia is crucial to discover a new therapeutic concept for postoperative CNS complications. Here, we studied the effects of the commonly used anaesthetic midazolam on lipid droplets and phagocytosis in immortalised microglial BV2 cells. Lipid droplets were assessed by flow cytometry and triglyceride quantification. The phagocytosis of BV2 cells was evaluated by detecting their phagocytosis by latex beads. Additionally, the autophagy of BV2 cells was evaluated by western blot and observation under microscopy. Our results showed that midazolam caused lipid droplet accumulation and reduced phagocytosis in BV2 cells, and inhibition of lipid droplet accumulation partially restored phagocytosis. Furthermore, midazolam blocks autophagic degradation by increasing phosphorylated TFEB in BV2 cells, inhibition of midazolam-increased phosphorylated TFEB might contribute to the improvement of autophagic flux by rapamycin. Moreover, promoting autophagy reverse the lipid droplet accumulation and phagocytosis decrease. This study suggests autophagy is a target for attenuating lipid droplet accumulation, normal degradation of lipid droplets is important for maintaining microglia phagocytosis and attenuating the side effects of midazolam on the CNS.

Current Understanding on the Role of Lipids in Macrophages and Associated Diseases

Lipid metabolism is the major intracellular mechanism driving a variety of cellular functions such as energy storage, hormone regulation and cell division. Lipids, being a primary component of the cell membrane, play a pivotal role in the survival of macrophages. Lipids are crucial for a variety of macrophage functions including phagocytosis, energy balance and ageing. However, functions of lipids in macrophages vary based on the site the macrophages are residing at. Lipid-loaded macrophages have recently been emerging as a hallmark for several diseases. This review discusses the significance of lipids in adipose tissue macrophages, tumor-associated macrophages, microglia and peritoneal macrophages. Accumulation of macrophages with impaired lipid metabolism is often characteristically observed in several metabolic disorders. Stress signals differentially regulate lipid metabolism. While conditions such as hypoxia result in accumulation of lipids in macrophages, stress signals such as nutrient deprivation initiate lipolysis and clearance of lipids. Understanding the biology of lipid accumulation in macrophages requires the development of potentially active modulators of lipid metabolism.

Lipid metabolism and storage in neuroglia: role in brain development and neurodegenerative diseases

The importance of neuroglia in maintaining normal brain function under physiological and pathological conditions has been supported by growing evidence in recent years. The most important issues regarding glial metabolism and function include the cooperation between glial populations and neurons, morphological and functional changes in pathological states, and the role in the onset and progression of neurodegenerative diseases. Although lipid accumulation and further lipid droplet production in neurodegenerative disease brain models have been observed for a long time, the dynamic development of brain lipid droplet research in recent years suggests its role in the development and progression of neurodegenerative diseases was previously underestimated. First recognized as organelles of lipid storage, lipid droplets (LDs) have emerged as an important organelle in metabolic diseases, inflammation, and host defense. Dynamic changes in lipid metabolism within neurons and glial cells resulting in lipid accumulation and lipid droplet formation are present in brain models of various neurodegenerative diseases, yet their role in the brain remains largely unexplored. This paper first reviews the metabolism and accumulation of several major lipids in the brain and discusses the regulation of lipid accumulation in different types of brain cells. We explore the potential role of intracellular lipid accumulation in the pathogenesis of neurodegeneration, starting from lipid metabolism and LDs biogenesis in glial cells, and discuss several pathological factors that promote lipid droplet formation, mainly focusing on oxidative stress, energy metabolism and glial cell-neuron coupling, which are closely related to the etiology and progression of neurodegenerative diseases. Finally, the directions and challenges of intracellular lipid metabolism in glial cells in neurodegeneration are discussed.

Cytosolic lipolysis in non-adipose tissues: energy provision and beyond

Cytosolic lipolysis is a well-defined biochemical process that plays important roles in the mobilization of stored neutral lipids. Lipid turnover, regulated by cytosolic lipolysis, has been extensively studied in adipose tissue, liver, and muscle. The storage and utilization of neutral lipids is a basic function of most, if not all, tissues and cells. In this review, we focus on the functions of cytosolic lipolysis mainly in non-adipose tissues and in several physiological processes, including cancer, longevity, and pathogen infection. The mechanisms underlying the impact of cytosolic lipolysis on these events will be discussed. Detailed understanding of cytosolic lipolysis in both adipose and non-adipose tissues will have implications for future clinical translation.

ABHD5-A Regulator of Lipid Metabolism Essential for Diverse Cellular Functions

The α/β-Hydrolase domain-containing protein 5 (ABHD5; also known as comparative gene identification-58, or CGI-58) is the causative gene of the Chanarin-Dorfman syndrome (CDS), a disorder mainly characterized by systemic triacylglycerol accumulation and a severe defect in skin barrier function. The clinical phenotype of CDS patients and the characterization of global and tissue-specific ABHD5-deficient mouse strains have demonstrated that ABHD5 is a crucial regulator of lipid and energy homeostasis in various tissues. Although ABHD5 lacks intrinsic hydrolase activity, it functions as a co-activating enzyme of the patatin-like phospholipase domain-containing (PNPLA) protein family that is involved in triacylglycerol and glycerophospholipid, as well as sphingolipid and retinyl ester metabolism. Moreover, ABHD5 interacts with perilipins (PLINs) and fatty acid-binding proteins (FABPs), which are important regulators of lipid homeostasis in adipose and non-adipose tissues. This review focuses on the multifaceted role of ABHD5 in modulating the function of key enzymes in lipid metabolism.

Approaches to Measuring the Activity of Major Lipolytic and Lipogenic Enzymes In Vitro and Ex Vivo

Since the 1950s, one of the goals of adipose tissue research has been to determine lipolytic and lipogenic activity as the primary metabolic pathways affecting adipocyte health and size and thus representing potential therapeutic targets for the treatment of obesity and associated diseases. Nowadays, there is a relatively large number of methods to measure the activity of these pathways and involved enzymes, but their applicability to different biological samples is variable. Here, we review the characteristics of mean lipogenic and lipolytic enzymes, their inhibitors, and available methodologies for assessing their activity, and comment on the advantages and disadvantages of these methodologies and their applicability in vivo, ex vivo, and in vitro, i.e., in cells, organs and their respective extracts, with the emphasis on adipocytes and adipose tissue.

Microplastics can aggravate the impact of ocean acidification on the health of mussels: Insights from physiological performance, immunity and byssus properties

Ocean acidification may increase the risk of disease outbreaks that would challenge the future persistence of marine organisms if their immune system and capacity to produce vital structures for survival (e.g., byssus threads produced by bivalves) are compromised by acidified seawater. These potential adverse effects may be exacerbated by microplastic pollution, which is forecast to co-occur with ocean acidification in the future. Thus, we evaluated the impact of ocean acidification and microplastics on the health of a mussel species (Mytilus coruscus) by assessing its physiological performance, immunity and byssus properties. We found that ocean acidification and microplastics not only reduced hemocyte concentration and viability due to elevated oxidative stress, but also undermined phagocytic activity of hemocytes due to lowered energy budget of mussels, which was in turn caused by the reduced feeding performance and energy assimilation. Byssus quality (strength and extensibility) and production were also reduced by ocean acidification and microplastics. To increase the chance of survival with these stressors, the mussels prioritized the synthesis of some byssus proteins (Mfp-4 and Mfp-5) to help maintain adhesion to substrata. Nevertheless, our findings suggest that co-occurrence of ocean acidification and microplastic pollution would increase the susceptibility of bivalves to infectious diseases and dislodgement risk, thereby threatening their survival and undermining their ecological contributions to the community.

Regulation of microglia phagocytosis and potential involvement of exercise

Microglia are considered the main phagocytic cells in the central nervous system, remodeling neural circuits by pruning synapses during development. Microglial phagocytosis is also a crucial process in maintaining adult brain homeostasis and clearing potential toxic factors, which are recognized to be associated with neurodegenerative and neuroinflammatory disorders. For example, microglia can engulf amyloid-β plaques, myelin debris, apoptotic cells, and extracellular harmful substances by expressing a variety of specific receptors on the cell surface or by reprogramming intracellular glucose and lipid metabolism processes. Furthermore, physical exercise has been implicated to be one of the non-pharmaceutical treatments for various nervous system diseases, which is closely related to neuroplasticity and microglia functions including proliferation, activation, and phagocytosis. This review focuses on the central regulatory mechanisms related to microglia phagocytosis and the potential role of exercise training in this process.

Immunolipidomics Reveals a Globoside Network During the Resolution of Pro-Inflammatory Response in Human Macrophages

Toll-like receptor 4 (TLR4)-mediated changes in macrophages reshape intracellular lipid pools to coordinate an effective innate immune response. Although this has been previously well-studied in different model systems, it remains incompletely understood in primary human macrophages. Here we report time-dependent lipidomic and transcriptomic responses to lipopolysaccharide (LPS) in primary human macrophages from healthy donors. We grouped the variation of ~200 individual lipid species measured by LC-MS/MS into eight temporal clusters. Among all other lipids, glycosphingolipids (glycoSP) and cholesteryl esters (CE) showed a sharp increase during the resolution phase (between 8h or 16h post LPS). GlycoSP, belonging to the globoside family (Gb3 and Gb4), showed the greatest inter-individual variability among all lipids quantified. Integrative network analysis between GlycoSP/CE levels and genome-wide transcripts, identified Gb4 d18:1/16:0 and CE 20:4 association with subnetworks enriched for T cell receptor signaling (PDCD1, CD86, PTPRC, CD247, IFNG) and DC-SIGN signaling (RAF1, CD209), respectively. Our findings reveal Gb3 and Gb4 globosides as sphingolipids associated with the resolution phase of inflammatory response in human macrophages.

Cell-intrinsic Wnt4 ligand regulates mitochondrial oxidative phosphorylation in macrophages

Macrophages respond to their environment by adopting a predominantly inflammatory or anti-inflammatory profile, depending on the context. The polarization of the subsequent response is regulated by a combination of intrinsic and extrinsic signals and is associated with alterations in macrophage metabolism. Although macrophages are important producers of Wnt ligands, the role of Wnt signaling in regulating metabolic changes associated with macrophage polarization remains unclear. Wnt4 upregulation has been shown to be associated with tissue repair and suppression of age-associated inflammation, which led us to generate Wnt4-deficient bone marrow–derived macrophages to investigate its role in metabolism. We show that loss of Wnt4 led to modified mitochondrial structure, enhanced oxidative phosphorylation, and depleted intracellular lipid reserves, as the cells depended on fatty acid oxidation to fuel their mitochondria. Further we found that enhanced lipolysis was dependent on protein kinase C–mediated activation of lysosomal acid lipase in Wnt4-deficient bone marrow–derived macrophages. Although not irreversible, these metabolic changes promoted parasite survival during infection with Leishmania donovani. In conclusion, our results indicate that enhanced macrophage fatty acid oxidation impairs the control of intracellular pathogens, such as Leishmania. We further suggest that Wnt4 may represent a potential target in atherosclerosis, which is characterized by lipid storage in macrophages leading to them becoming foam cells.

Adipose triglyceride lipase-mediated lipid catabolism is essential for bronchiolar regeneration

The lung airways are constantly exposed to inhaled toxic substances, resulting in cellular damage that is repaired by local expansion of resident bronchiolar epithelial club cells. Disturbed bronchiolar epithelial damage repair lies at the core of many prevalent lung diseases, including chronic obstructive pulmonary disease, asthma, pulmonary fibrosis, and lung cancer. However, it is still not known how bronchiolar club cell energy metabolism contributes to this process. Here, we show that adipose triglyceride lipase (ATGL), the rate-limiting enzyme for intracellular lipolysis, is critical for normal club cell function in mice. Deletion of the gene encoding ATGL, Pnpla2 (also known as Atgl), induced substantial triglyceride accumulation, decreased mitochondrial numbers, and decreased mitochondrial respiration in club cells. This defect manifested as bronchiolar epithelial thickening and increased airway resistance under baseline conditions. After naphthalene‑induced epithelial denudation, a regenerative defect was apparent. Mechanistically, dysfunctional PPARα lipid-signaling underlies this phenotype because (a) ATGL was needed for PPARα lipid-signaling in regenerating bronchioles and (b) administration of the specific PPARα agonist WY14643 restored normal bronchiolar club cell ultrastructure and regenerative potential. Our data emphasize the importance of the cellular energy metabolism for lung epithelial regeneration and highlight the significance of ATGL-mediated lipid catabolism for lung health.

Lipid Dyshomeostasis and Inherited Cerebellar Ataxia

Cerebellar ataxia is a form of ataxia that originates from dysfunction of the cerebellum, but may involve additional neurological tissues. Its clinical symptoms are mainly characterized by the absence of voluntary muscle coordination and loss of control of movement with varying manifestations due to differences in severity, in the site of cerebellar damage and in the involvement of extracerebellar tissues. Cerebellar ataxia may be sporadic, acquired, and hereditary. Hereditary ataxia accounts for the majority of cases. Hereditary ataxia has been tentatively divided into several subtypes by scientists in the field, and nearly all of them remain incurable. This is mainly because the detailed mechanisms of these cerebellar disorders are incompletely understood. To precisely diagnose and treat these diseases, studies on their molecular mechanisms have been conducted extensively in the past. Accumulating evidence has demonstrated that some common pathogenic mechanisms exist within each subtype of inherited ataxia. However, no reports have indicated whether there is a common mechanism among the different subtypes of inherited cerebellar ataxia. In this review, we summarize the available references and databases on neurological disorders characterized by cerebellar ataxia and show that a subset of genes involved in lipid homeostasis form a new group that may cause ataxic disorders through a common mechanism. This common signaling pathway can provide a valuable reference for future diagnosis and treatment of ataxic disorders.

Triglyceride breakdown from lipid droplets regulates the inflammatory response in macrophages

Lipid droplets (LDs) are ubiquitous organelles that play important roles in cellular energy homeostasis, tightly regulating the accumulation and release of lipids. In macrophages, lipids accumulate in LDs during inflammation. However, it is unclear how inflammatory activation promotes the accumulation of lipids in LDs, and how the dynamic between lipid accumulation and breakdown could drive or inhibit inflammation. Elucidating the role of lipid accumulation during inflammation may provide important knowledge to influence inflammatory processes during health and disease. We identify the importance of the hypoxia-inducible lipid droplet–associated protein and the intracellular adipose triglyceride lipase in the regulation of lipid accumulation and breakdown in inflammatory macrophages. Furthermore, we determine the regulatory effect of lipid breakdown from LDs in supporting inflammation.

In response to inflammatory activation by pathogens, macrophages accumulate triglycerides in intracellular lipid droplets. The mechanisms underlying triglyceride accumulation and its exact role in the inflammatory response of macrophages are not fully understood. Here, we aim to further elucidate the mechanism and function of triglyceride accumulation in the inflammatory response of activated macrophages. Lipopolysaccharide (LPS)-mediated activation markedly increased triglyceride accumulation in macrophages. This increase could be attributed to up-regulation of the hypoxia-inducible lipid droplet–associated (HILPDA) protein, which down-regulated adipose triglyceride lipase (ATGL) protein levels, in turn leading to decreased ATGL-mediated triglyceride hydrolysis. The reduction in ATGL-mediated lipolysis attenuated the inflammatory response in macrophages after ex vivo and in vitro activation, and was accompanied by decreased production of prostaglandin-E2 (PGE2) and interleukin-6 (IL-6). Overall, we provide evidence that LPS-mediated activation of macrophages suppresses lipolysis via induction of HILPDA, thereby reducing the availability of proinflammatory lipid precursors and suppressing the production of PGE2 and IL-6.

Adipose Triglyceride Lipase Deficiency Attenuates In Vitro Thrombus Formation without Affecting Platelet Activation and Bleeding In Vivo

According to genome-wide RNA sequencing data from human and mouse platelets, adipose triglyceride lipase (ATGL), the main lipase catalyzing triglyceride (TG) hydrolysis in cytosolic lipid droplets (LD) at neutral pH, is expressed in platelets. Currently, it is elusive to whether common lipolytic enzymes are involved in the degradation of TG in platelets. Since the consequences of ATGL deficiency in platelets are unknown, we used whole-body and platelet-specific (plat)Atgl-deficient (-/-) mice to investigate the loss of ATGL on platelet function. Our results showed that platelets accumulate only a few LD due to lack of ATGL. Stimulation with platelet-activating agonists resulted in comparable platelet activation in Atgl-/-, platAtgl-/-, and wild-type mice. Measurement of mitochondrial respiration revealed a decreased oxygen consumption rate in platelets from Atgl-/- but not from platAtgl-/- mice. Of note, global loss of ATGL was associated with an anti-thrombogenic phenotype, which was evident by reduced thrombus formation in collagen-coated channels in vitro despite unchanged bleeding and occlusion times in vivo. We conclude that genetic deletion of ATGL affects collagen-induced thrombosis without pathological bleeding and platelet activation.

Endogenous expression of Notch pathway molecules in human trabecular meshwork cells

PURPOSE

Cells in the trabecular meshwork sense and respond to a myriad of physical forces through a process known as mechanotransduction. Whilst the effect of substratum stiffness or stretch on TM cells have been investigated in the context of transforming growth factor (TGF-β), Wnt and YAP/TAZ pathways, the role of Notch signaling, an evolutionarily conserved pathway, recently implicated in mechanotransduction, has not been investigated in trabecular meshwork (TM) cells. Here, we compare the endogenous expression of Notch pathway molecules in TM cells from glaucomatous and non-glaucomatous donors, segmental flow regions, and when subjected to cyclical strain, or grown on hydrogels of varying rigidity.

METHODS

Primary TM from glaucomatous (GTM), non-glaucomatous (NTM) donors, and from segmental flow regions [high flow (HF), low flow (LF)], were utilized between passages 2-6. Cells were (i) plated on tissue culture plastic, (ii) subjected to cyclical strain (6 h and 24 h), or (iii) cultured on 3 kPa and 80 kPa hydrogels. mRNA levels of Notch receptors/ligands/effectors in the TM cells was determined by qRT-PCR. Phagocytosis was determined as a function of substratum stiffness in NTM-HF/LF cells in the presence or absence of 100 nM Dexamethasone treatment.

RESULTS

Innate expression of Notch pathway genes were significantly overexpressed in GTM cells with no discernible differences observed between HF/LF cells in either NTM or GTM cells cultured on plastic substrates. With 6 h of cyclical strain, a subset of Notch pathway genes presented with altered expression. Expression of Notch receptors/ligands/receptors/inhibitors progressively declined with increasing stiffness and this correlated with phagocytic ability of NTM cells. Dexamethasone treatment decreased phagocytosis regardless of stiffness or cells isolated from segmental outflow regions.

CONCLUSIONS

We demonstrate here that the Notch expression in cultured TM cells differ intrinsically between GTM vs NTM, and by substratum cues (cyclical strain and stiffness). Of import, the most apparent differences in gene expression were observed as a function of substratum stiffness which closely followed phagocytic ability of cells. Interestingly, on soft substrates (mimicking normal TM stiffness) Notch expression and phagocytosis was highest, while both expression and phagocytosis was significantly lower on stiffer substrates (mimicking glaucomatous stiffness) regardless of DEX treatment. Such context dependent changes suggest Notch pathway may play differing roles in disease vs homeostasis. Studies focused on understanding the mechanistic role of Notch (if any) in outflow homeostasis are thus warranted.

Lipidomic Analyses Uncover Apoptotic and Inhibitory Effects of Pyrvinium Pamoate on Cholangiocarcinoma Cells via Mitochondrial Membrane Potential Dysfunction

Pyrvinium pamoate (PP), an FDA-approved anthelmintic drug, has been validated as a highly potent anti-cancer agent and patented recently as a potential chemotherapeutic drug for various cancers. The aims of this study were, therefore, to investigate the ability of PP in anti-proliferative activity and focused on the lipid profiles revealing the alteration of specific lipid species in the liver fluke Opisthorchis viverrini (Ov)-associated cholangiocarcinoma (CCA) cells. PP inhibited CCA cell viability through suppressing mitochondrial membrane potential (MMP) and ATP productions, leading to apoptotic cell death. Liquid chromatography-mass spectrometry combined with chemometrics was performed to investigate lipid alteration during PP-induced apoptosis. The lipidomic analyses showed the altered lipid signatures of CCA cell types including S-acetyldihydrolipoamide, methylselenopyruvate, and triglycerides that were increased in PP-treated CCA cells. In contrast, the levels of sphinganine and phosphatidylinositol were lower in the PP-treated group compared with its counterpart. The orthogonal partial-least squares regression analysis revealed that PP-induced MMP dysfunction, leading to remarkably reduced ATP level, was significantly associated with triglyceride (TG) accumulation observed in PP-treated CCA cells. Our findings indicate that PP could suppress the MMP function, which causes inhibition of CCA cell viability through lipid production, resulting in apoptotic induction in CCA cells. These findings provide an anti-cancer mechanism of PP under apoptotic induction ability that may serve as the alternative approach for CCA treatment.

The role of the immune system in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder where the accumulation of amyloid plaques and the formation of tau tangles are the prominent pathological hallmarks. Increasing preclinical and clinical studies have revealed that different components of the immune system may act as important contributors to AD etiology and pathogenesis. The recognition of misfolded Aβ and tau by immune cells can trigger a series of complex immune responses in AD, and then lead to neuroinflammation and neurodegeneration. In parallel, genome-wide association studies have also identified several immune related loci associated with increased - risk of AD by interfering with the function of immune cells. Other immune related factors, such as impaired immunometabolism, defective meningeal lymphatic vessels and autoimmunity might also be involved in the pathogenesis of AD. Here, we review the data showing the alterations of immune cells in the AD trajectory and seek to demonstrate the crosstalk between the immune cell dysfunction and AD pathology. We then discuss the most relevant research findings in regards to the influences of gene susceptibility of immune cells for AD. We also consider impaired meningeal lymphatics, immunometabolism and autoimmune mechanisms in AD. In addition, immune related biomarkers and immunotherapies for AD are also mentioned in order to offer novel insights for future research.

Microglial 'fat shaming' in development and disease

Neuronal-immune interactions are known to play crucial roles in brain development and homoeostasis. Of great relevance in this context are microglia, brain macrophages that phagocytose neurons that die during development, and many neurological disorders. Single-cell RNA sequencing methods have significantly advanced our understanding of microglial heterogeneity and transcriptional response to environmental changes. Here, we review recent work showing how microglia adopt a similar molecular signature during development and disease characterised by the expression of genes linked to phagocytosis and lipid uptake and metabolism. These studies show that in many neurodegenerative conditions, microglia accumulate cholesterols and lipid-rich debris, pointing to lipid processing and transport as promising targets for developing new therapeutical treatments against neurodegenerative disorders.

Heart Failure Syndrome With Preserved Ejection Fraction Is a Metabolic Cluster of Non-resolving Inflammation in Obesity

Heart failure with preserved ejection fraction (HFpEF) is an emerging disease with signs of nonresolving inflammation, endothelial dysfunction, and multiorgan defects. Moreover, based on the clinical signs and symptoms and the rise of the obesity epidemic, the number of patients developing HFpEF is increasing. From recent molecular and cellular studies, it becomes evident that HFpEF is not a single and homogenous disease but a cluster of heterogeneous pathophysiology with aging at the base of the pyramid. Obesity superimposed on aging drives the number of inflammatory pathways that intersect with metabolic dysfunction and suboptimal inflammation. Here, we compiled information on obesity-directed macrophage dysfunction that coincide with metabolic defects. Obesity-associated proinflammatory stimuli facilitates heart and interorgan inflammation in HFpEF. Furthermore, diversified mechanisms that drive heart failure urge the need of studying pervasive and unresolved inflammation in animal models to understand HFpEF. A broad and system-based approach will help to study major translational aspects of HFpEF, since no single animal model recapitulates all signs of differential HFpEF stages in the clinical setting. Here, we covered experimental models that target HFpEF and emphasized the advances observed with formyl peptide 2 (FPR2) receptor, a prime sensor that is important in inflammation-resolution signaling. Dysfunction of FPR2 led to the development of spontaneous obesity, impaired macrophage function, and triggered kidney fibrosis, providing evidence of multiorgan defects in HFpEF in an obesogenic aging experimental model.

Sphingolipid metabolism during Toll-like receptor 4 (TLR4)-mediated macrophage activation

Macrophage activation in response to stimulation of Toll-like receptor 4 (TLR4) provides a paradigm for investigating energy metabolism that regulates the inflammatory response. TLR4-mediated pro-inflammatory macrophage activation is characterized by increased glycolysis and altered mitochondrial metabolism, supported by selective amino acid uptake and/or usage. Fatty acid metabolism remains as a highly complex rewiring that accompanies classical macrophage activation. TLR4 activation leads to de novo synthesis of fatty acids, which flux into sphingolipids, complex lipids that form the building blocks of eukaryotic cell membranes and regulate cell function. Here, we review the importance of TLR4-mediated de novo synthesis of membrane sphingolipids in macrophages. We first highlight fatty acid metabolism during TLR4-driven macrophage immunometabolism. We then focus on the temporal dynamics of sphingolipid biosynthesis and emphasize the modulatory role of some sphingolipid species (i.e. sphingomyelins, ceramides and glycosphingolipids) on the pro-inflammatory and pro-resolution phases of LPS/TLR4 activation in macrophages.

Lipid droplets and the host-pathogen dynamic: FATal attraction?

In the ongoing conflict between eukaryotic cells and pathogens, lipid droplets (LDs) emerge as a choke point in the battle for nutrients. While many pathogens seek the lipids stored in LDs to fuel an expensive lifestyle, innate immunity rewires lipid metabolism and weaponizes LDs to defend cells and animals. Viruses, bacteria, and parasites directly and remotely manipulate LDs to obtain substrates for metabolic energy, replication compartments, assembly platforms, membrane blocks, and tools for host colonization and/or evasion such as anti-inflammatory mediators, lipoviroparticles, and even exosomes. Host LDs counterattack such advances by synthesizing bioactive lipids and toxic nucleotides, organizing immune signaling platforms, and recruiting a plethora of antimicrobial proteins to provide a front-line defense against the invader. Here, we review the current state of this conflict. We will discuss why, when, and how LDs efficiently coordinate and precisely execute a plethora of immune defenses. In the age of antimicrobial resistance and viral pandemics, understanding innate immune strategies developed by eukaryotic cells to fight and defeat dangerous microorganisms may inform future anti-infective strategies.

HILPDA Uncouples Lipid Droplet Accumulation in Adipose Tissue Macrophages from Inflammation and Metabolic Dysregulation

Obesity leads to a state of chronic, low-grade inflammation that features the accumulation of lipid-laden macrophages in adipose tissue. Here, we determined the role of macrophage lipid-droplet accumulation in the development of obesity-induced adipose-tissue inflammation, using mice with myeloid-specific deficiency of the lipid-inducible HILPDA protein. HILPDA deficiency markedly reduced intracellular lipid levels and accumulation of fluorescently labeled fatty acids. Decreased lipid storage in HILPDA-deficient macrophages can be rescued by inhibition of adipose triglyceride lipase (ATGL) and is associated with increased oxidative metabolism. In diet-induced obese mice, HILPDA deficiency does not alter inflammatory and metabolic parameters, despite markedly reducing lipid accumulation in macrophages. Overall, we find that HILPDA is a lipid-inducible, physiological inhibitor of ATGL-mediated lipolysis in macrophages and uncouples lipid storage in adipose tissue macrophages from inflammation and metabolic dysregulation. Our data question the contribution of lipid droplet accumulation in adipose tissue macrophages in obesity-induced inflammation and metabolic dysregulation.

Transcriptomic and metabolomic insights into the variety of sperm storage in oviduct of egg layers

In birds, the sperm storage tubules (SST) are dispersed in uterovaginal junction (UVJ) and highly correlated with differential capacity of sperm storage (SS) in and among species with unspecified mechanisms. Here, the SS duration of 252 egg layer breeders was evaluated in 5 rounds with 3 phenotypic traits to screen high- and low-SS individuals, respectively, followed with transcriptome of UVJ tissues and metabolome of serum (high-SS vs. low-SS) to decipher the candidate genes and biochemical markers correlated with differential SS capacity. Histological characterization suggested slightly higher density of SST in UVJ (high-SS vs. low-SS). Transcriptome analyses identified 596 differentially expressed genes (336 upregulated vs. 260 downregulated), which were mainly enriched in gene ontology terms of homeostasis, steroid and lipid metabolism and hormone activity, and 12 significant pathways (P < 0.05) represented by calcium, steroid, and lipid metabolism. Immunohistochemical staining of GNAQ, ST6GAL1, ADFP, and PCNA showed similar distribution in UVJ tissues between 2 groups. Several candidates (HSD11B2, DIO2, AQP3, GNAQ, NANS, ST6GAL1) combined with 4 (11β-prostaglandin F2α, prostaglandin B1, 7α-hydroxytestosterone, and N-acetylneuraminic acid) of 40 differential metabolites enriched in serum metabolome were considered as regulators and biomarkers of SS duration in egg layer breeders. The integrated transcriptome and metabolome analyses of chicken breeder hens will provide novel insights for exploration and improvement of differential SS capacity in birds.

Mitochondrial Regulation of Microglial Immunometabolism in Alzheimer's Disease

Alzheimer’s disease (AD) is an age-associated terminal neurodegenerative disease with no effective treatments. Dysfunction of innate immunity is implicated in the pathogenesis of AD, with genetic studies supporting a causative role in the disease. Microglia, the effector cells of innate immunity in the brain, are highly plastic and perform a diverse range of specialist functions in AD, including phagocytosing and removing toxic aggregates of beta amyloid and tau that drive neurodegeneration. These immune functions require high energy demand, which is regulated by mitochondria. Reflecting this, microglia have been shown to be highly metabolically flexible, reprogramming their mitochondrial function upon inflammatory activation to meet their energy demands. However, AD-associated genetic risk factors and pathology impair microglial metabolic programming, and metabolic derailment has been shown to cause innate immune dysfunction in AD. These findings suggest that immunity and metabolic function are intricately linked processes, and targeting microglial metabolism offers a window of opportunity for therapeutic treatment of AD. Here, we review evidence for the role of metabolic programming in inflammatory functions in AD, and discuss mitochondrial-targeted immunotherapeutics for treatment of the disease.

Degradation of Photoreceptor Outer Segments by the Retinal Pigment Epithelium Requires Pigment Epithelium-Derived Factor Receptor (PEDF-R)

Purpose

To examine the contribution of pigment epithelium-derived factor receptor (PEDF-R) to the phagocytosis process. Previously, we identified PEDF-R, the protein encoded by the PNPLA2 gene, as a phospholipase A2 in the retinal pigment epithelium (RPE). During phagocytosis, RPE cells ingest abundant phospholipids and protein in the form of photoreceptor outer segment (POS) tips, which are then hydrolyzed. The role of PEDF-R in RPE phagocytosis is not known.

Methods

Mice in which PNPLA2 was conditionally knocked out (cKO) in the RPE were generated. Mouse RPE/choroid explants were cultured. Human ARPE-19 cells were transfected with siPNPLA2 silencing duplexes. POSs were isolated from bovine retinas. The phospholipase A2 inhibitor bromoenol lactone was used. Transmission electron microscopy, immunofluorescence, lipid labeling, pulse-chase experiments, western blots, and free fatty acid and β-hydroxybutyrate assays were performed.

Results

The RPE of the cKO mice accumulated lipids, as well as more abundant and larger rhodopsin particles, compared to littermate controls. Upon POS exposure, RPE explants from cKO mice released less β-hydroxybutyrate compared to controls. After POS ingestion during phagocytosis, rhodopsin degradation was stalled both in cells treated with bromoenol lactone and in PNPLA2-knocked-down cells relative to their corresponding controls. Phospholipase A2 inhibition lowered β-hydroxybutyrate release from phagocytic RPE cells. PNPLA2 knockdown also resulted in a decline in fatty acids and β-hydroxybutyrate release from phagocytic RPE cells.

Conclusions

PEDF-R downregulation delayed POS digestion during phagocytosis. The findings imply that the efficiency of RPE phagocytosis depends on PEDF-R, thus identifying a novel contribution of this protein to POS degradation in the RPE.

Discordant hepatic fatty acid oxidation and triglyceride hydrolysis leads to liver disease

To extract energy from stored lipids, fatty acids must first be liberated from triglyceride before their β-oxidation in mitochondria in a coordinated and stepwise manner. To determine the independent and interdependent roles of hepatic triglyceride hydrolysis and fatty acid oxidation, mice were generated with a liver-specific defect in triglyceride hydrolysis (Atgl^L–/–), fatty acid oxidation (Cpt2^L–/–), or both (double knockout). The loss of either gene resulted in the compensatory increase in the other, demonstrating their coordination. The loss of individual components of fatty acid catabolism (carnitine palmitoyl transferase 2 [Cpt2], adipose triglyceride lipase [Atgl], and Pparα) resulted in largely independent effects on hepatocyte morphology, intermediary metabolism, and gene expression in response to fasting. However, high-fat feeding revealed the interdependent role of Atgl and Cpt2, as the loss of only one of the genes resulted in steatosis (fatty liver) but the loss of both components resulted in significant steatohepatitis (inflammation and fibrosis). Lipolysis and β-oxidation are intimately linked within a continuous pathway, and disruption of their coordination leads to unique cellular and molecular phenotypes that ultimately result in liver disease.

The role of peroxisome proliferator-activated receptors (PPAR) in immune responses

Peroxisome proliferator-activated receptors (PPARs) are fatty acid-activated transcription factors of nuclear hormone receptor superfamily that regulate energy metabolism. Currently, three PPAR subtypes have been identified: PPARα, PPARγ, and PPARβ/δ. PPARα and PPARδ are highly expressed in oxidative tissues and regulate genes involved in substrate delivery and oxidative phosphorylation (OXPHOS) and regulation of energy homeostasis. In contrast, PPARγ is more important in lipogenesis and lipid synthesis, with highest expression levels in white adipose tissue (WAT). In addition to tissues regulating whole body energy homeostasis, PPARs are expressed in immune cells and have an emerging critical role in immune cell differentiation and fate commitment. In this review, we discuss the actions of PPARs in the function of the innate and the adaptive immune system and their implications in immune-mediated inflammatory conditions.

Microglia and lipids: how metabolism controls brain innate immunity

Microglia are universal sensors of alterations in CNS physiology. These cells integrate complex molecular signals and undergo comprehensive phenotypical remodeling to adapt inflammatory responses. In the last years, single-cell analyses have revealed that microglia exhibit diverse phenotypes during development, growth and disease. Emerging evidence suggests that such phenotype transitions are mediated by reprogramming of cell metabolism. Indeed, metabolic pathways are distinctively altered in activated microglia and are central nodes controlling microglial responses. Microglial lipid metabolism has been specifically involved in the control of microglial activation and effector functions, such as migration, phagocytosis and inflammatory signaling, and minor disturbances in microglial lipid handling associates with altered brain function in disorders featuring neuroinflammation. In this review, we explore new and relevant aspects of microglial metabolism in health and disease. We give special focus on how different branches of lipid metabolism, such as lipid sensing, synthesis and oxidation, integrate and control essential aspects of microglial biology, and how disturbances in these processes associate with aging and the pathogenesis of, for instance, multiple sclerosis and Alzheimer's disease. Finally, challenges and advances in microglial lipid research are discussed.

Triacylglycerol synthesis enhances macrophage inflammatory function

Foamy macrophages, which have prominent lipid droplets (LDs), are found in a variety of disease states. Toll-like receptor agonists drive triacylglycerol (TG)-rich LD development in macrophages. Here we explore the basis and significance of this process. Our findings indicate that LD development is the result of metabolic commitment to TG synthesis on a background of decreased fatty acid oxidation. TG synthesis is essential for optimal inflammatory macrophage activation as its inhibition, which prevents LD development, has marked effects on the production of inflammatory mediators, including IL-1β, IL-6 and PGE2, and on phagocytic capacity. The failure of inflammatory macrophages to make PGE2 when TG-synthesis is inhibited is critical for this phenotype, as addition of exogenous PGE2 is able to reverse the anti-inflammatory effects of TG synthesis inhibition. These findings place LDs in a position of central importance in inflammatory macrophage activation.

As macrophages switch to a proinflammatory gylcolytic state they start to generate triglyceride-rich lipid droplets, but what function these droplets have in this context is not clear. Here the authors show that this triglyceride synthesis is requisite for prostaglandin E2 production and subsequent inflammatory activation.

Phospholipids containing ether-bound hydrocarbon-chains are essential for efficient phagocytosis and neutral lipids of the ester-type perturb development in Dictyostelium

Lipids are the building blocks for cellular membranes; they provide signalling molecules for membrane dynamics and serve as energy stores. One path of their synthesis is initiated by glycerol-3-phosphate acyltransferase (GPAT), which in Dictyostelium resides on the endoplasmic reticulum. When an excess of fatty acids is present, it redistributes to storage organelles, the lipid droplets. Mutants, where the GPAT was eliminated by homologous recombination, produce fewer lipid droplets and are almost devoid of triacylglycerols (TAG), rendering them more resistant to cell death and cell loss in the developmental stages preceding fruiting body formation. The enzyme most closely related to GPAT is called FARAT, because it combines a fatty acyl-reductase (FAR) and an acyltransferase (AT) domain in its sequence. The protein is confined to the lumen of the peroxisome, where it transfers a fatty acid to dihydroxyacetone-phosphate initiating the synthesis of ether lipids, later completed at the endoplasmic reticulum. A mutant lacking FARAT produces lipid droplets that are devoid of the storage lipid monoalkyl-diacyl-glycerol (MDG), but the efficiency of spore formation in the developmental cycle is largely unaltered. Instead, these mutants are strongly impaired in phagocytosis of yeast particles, which is attributed to reduced synthesis of membrane phospholipids containing ether-linked chains.

Metabolic Reprogramming of Macrophages in Atherosclerosis: Is It All about Cholesterol?

Hypercholesterolemia contributes to the chronic inflammatory response during the progression of atherosclerosis, in part by favoring cholesterol loading in macrophages and other immune cells. However, macrophages encounter a substantial amount of other lipids and nutrients after ingesting atherogenic lipoprotein particles or clearing apoptotic cells, increasing their metabolic load and impacting their behavior during atherosclerosis plaque progression. This review examines whether and how fatty acids and glucose shape the cellular metabolic reprogramming of macrophages in atherosclerosis to modulate the onset phase of inflammation and the later resolution stage, in which the balance is tipped toward tissue repair.

Classical Activation of Macrophages Leads to Lipid Droplet Formation Without de novo Fatty Acid Synthesis

Altered lipid metabolism in macrophages is associated with various important inflammatory conditions. Although lipid metabolism is an important target for therapeutic intervention, the metabolic requirement involved in lipid accumulation during pro-inflammatory activation of macrophages remains incompletely characterized. We show here that macrophage activation with IFNγ results in increased aerobic glycolysis, iNOS-dependent inhibition of respiration, and accumulation of triacylglycerol. Surprisingly, metabolite tracing with ¹³C-labeled glucose revealed that the glucose contributed to the glycerol groups in triacylglycerol (TAG), rather than to de novo synthesis of fatty acids. This is in stark contrast to the otherwise similar metabolism of cancer cells, and previous results obtained in activated macrophages and dendritic cells. Our results establish a novel metabolic pathway whereby glucose provides glycerol to the headgroup of TAG during classical macrophage activation.

DAGL-Beta Functions as a PUFA-Specific Triacylglycerol Lipase in Macrophages

Here, we apply quantitative chemical proteomics and untargeted lipidomics to assign a polyunsaturated fatty acid (PUFA)-specific triacylglycerol (TAG) lipase activity for diacylglycerol lipase-beta (DAGLβ) in macrophages. We demonstrate that DAGLβ but not DAGLα is expressed and active in bone marrow-derived macrophages (BMDMs) as determined by activity-based protein profiling analysis of SILAC BMDMs. Genetic disruption of DAGLβ resulted in accumulation of cellular TAGs composed of PUFA but not saturated/low unsaturated fatty acid counterparts, which is recapitulated in wild-type macrophages treated with a DAGLβ-selective inhibitor. Biochemical assays with synthetic substrates confirm PUFA-TAGs as authentic DAGLβ substrates. In summary, our findings identify DAGLβ as a PUFA-specific TAG lipase in primary macrophages.

Lipid-droplet-accumulating microglia represent a dysfunctional and proinflammatory state in the aging brain

Microglia become progressively activated and seemingly dysfunctional with age, and genetic studies have linked these cells to the pathogenesis of a growing number of neurodegenerative diseases. Here we report a striking buildup of lipid droplets in microglia with aging in mouse and human brains. These cells, which we call 'lipid-droplet-accumulating microglia' (LDAM), are defective in phagocytosis, produce high levels of reactive oxygen species and secrete proinflammatory cytokines. RNA-sequencing analysis of LDAM revealed a transcriptional profile driven by innate inflammation that is distinct from previously reported microglial states. An unbiased CRISPR-Cas9 screen identified genetic modifiers of lipid droplet formation; surprisingly, variants of several of these genes, including progranulin (GRN), are causes of autosomal-dominant forms of human neurodegenerative diseases. We therefore propose that LDAM contribute to age-related and genetic forms of neurodegeneration.

Signs of Deregulated Gene Expression Are Present in Both CD14+ and CD14- PBMC From Non-Obese Men With Family History of T2DM

AIM

Development of type 2 diabetes (T2DM) is associated with disturbances in immune and metabolic status that may be reflected by an altered gene expression profile of peripheral blood mononuclear cells (PBMC). To reveal a potential family predisposition to these alterations, we investigated the regulation of gene expression profiles in circulating CD14⁺ and CD14^- PBMC in fasting conditions and in response to oral glucose tolerance test (OGTT) in glucose tolerant first-degree relatives (FDR) of T2DM patients and in control subjects.

MATERIALS AND METHODS

This work is based on the clinical study LIMEX (NCT03155412). Non-obese 12 non-diabetic (FDR), and 12 control men without family history of diabetes matched for age and BMI underwent OGTT. Blood samples taken before and at the end of OGTT were used for isolation of circulating CD14⁺ and CD14^- PBMC. In these cells, mRNA levels of 94 genes related to lipid and carbohydrate metabolism, immunity, and inflammation were assessed by qPCR.

RESULTS

Irrespectively of the group, the majority of analyzed genes had different mRNA expression in CD14⁺ PBMC compared to CD14^- PBMC in the basal (fasting) condition. Seven genes (IRS1, TLR2, TNFα in CD14⁺ PBMC; ABCA1, ACOX1, ATGL, IL6 in CD14^- PBMC) had different expression in control vs. FDR groups. OGTT regulated mRNA levels of nine genes selectively in CD14⁺ PBMC and of two genes (ABCA1, PFKL) selectively in CD14^-PBMC. Differences in OGTT-induced response between FDR and controls were observed for EGR2, CCL2 in CD14⁺ PBMC and for ABCA1, ACOX1, DGAT2, MLCYD, and PTGS2 in CD14^- PBMC.

CONCLUSION

This study revealed a different impact of glucose challenge on gene expression in CD14⁺ when compared with CD14^- PBMC fractions and suggested possible impact of family predisposition to T2DM on basal and OGTT-induced gene expression in these PBMC fractions. Future studies on these putative alterations of inflammation and lipid metabolism in fractionated PBMC in larger groups of subjects are warranted.

Systemic Lupus Erythematosus: Pathogenesis at the Functional Limit of Redox Homeostasis

Systemic lupus erythematosus (SLE) is a disease characterized by the production of autoreactive antibodies and cytokines, which are thought to have a major role in disease activity and progression. Immune system exposure to excessive amounts of autoantigens that are not efficiently removed is reported to play a significant role in the generation of autoantibodies and the pathogenesis of SLE. While several mechanisms of cell death-based autoantigenic exposure and compromised autoantigen removal have been described in relation to disease onset, a significant association with the development of SLE can be attributed to increased apoptosis and impaired phagocytosis of apoptotic cells. Both apoptosis and impaired phagocytosis can be caused by hydrogen peroxide whose cellular production is enhanced by exposure to endogenous hormones or environmental chemicals, which have been implicated in the pathogenesis of SLE. Hydrogen peroxide can cause lymphocyte apoptosis and glutathione depletion, both of which are associated with the severity of SLE. The cellular accumulation of hydrogen peroxide is facilitated by the myriad of stimuli causing increased cellular bioenergetic activity that enhances metabolic production of this toxic oxidizing agent such as emotional stress and infection, which are recognized SLE exacerbating factors. When combined with impaired cellular hydrogen peroxide removal caused by xenobiotics and genetically compromised hydrogen peroxide elimination due to enzymatic polymorphic variation, a mechanism for cellular accumulation of hydrogen peroxide emerges, leading to hydrogen peroxide-induced apoptosis and impaired phagocytosis, enhanced autoantigen exposure, formation of autoantibodies, and development of SLE.

Dicer in Macrophages Prevents Atherosclerosis by Promoting Mitochondrial Oxidative Metabolism

BACKGROUND

Alternative macrophage activation, which relies on mitochondrial oxidative metabolism, plays a central role in the resolution of inflammation and prevents atherosclerosis. Moreover, macrophages handle large amounts of cholesterol and triglycerides derived from the engulfed modified lipoproteins during atherosclerosis. Although several microRNAs regulate macrophage polarization, the role of the microRNA-generating enzyme Dicer in macrophage activation during atherosclerosis is unknown.

METHODS

To evaluate the role of Dicer in atherosclerosis, Apoe^-/- mice with or without macrophage-specific Dicer deletion were fed a high-fat diet for 12 weeks. Anti-argonaute 2 RNA immunoprecipitation chip and RNA deep sequencing combined with microRNA functional screening were performed in the Dicer wild-type and knockout bone marrow-derived macrophages to identify the individual microRNAs and the mRNA targets mediating the phenotypic effects of Dicer. The role of the identified individual microRNA and its target in atherosclerosis was determined by tail vein injection of the target site blockers in atherosclerotic Apoe^-/- mice.

RESULTS

We show that Dicer deletion in macrophages accelerated atherosclerosis in mice, along with enhanced inflammatory response and increased lipid accumulation in lesional macrophages. In vitro, alternative activation was limited whereas lipid-filled foam cell formation was exacerbated in Dicer-deficient macrophages as a result of impaired mitochondrial fatty acid oxidative metabolism. Rescue of microRNA (miR)-10a, let-7b, and miR-195a expression restored the oxidative metabolism in alternatively activated Dicer-deficient macrophages. Suppression of ligand-dependent nuclear receptor corepressor by miR-10a promoted fatty acid oxidation, which mediated the lipolytic and anti-inflammatory effect of Dicer. miR-10a expression was negatively correlated to the progression of atherosclerosis in humans. Blocking the interaction between ligand-dependent nuclear receptor corepressor and miR-10a by target site blockers aggravated atherosclerosis development in mice.

CONCLUSIONS

Dicer plays an atheroprotective role by coordinately regulating the inflammatory response and lipid metabolism in macrophages through enhancing fatty acid-fueled mitochondrial respiration, suggesting that promoting Dicer/miR-10a-dependent metabolic reprogramming in macrophages has potential therapeutic implications to prevent atherosclerosis.