The role of efferocytosis-fueled macrophage metabolism in the resolution of inflammation.

The phagocytosis of dying cells by macrophages, termed efferocytosis, is a tightly regulated process that involves the sensing, binding, engulfment, and digestion of apoptotic cells. Efferocytosis not only prevents tissue necrosis and inflammation caused by secondary necrosis of dying cells, but it also promotes pro-resolving signaling in macrophages, which is essential for tissue resolution and repair following injury or inflammation. An important factor that contributes to this pro-resolving reprogramming is the cargo that is released from apoptotic cells after their engulfment and phagolysosomal digestion by macrophages. The apoptotic cell cargo contains amino acids, nucleotides, fatty acids, and cholesterol that function as metabolites and signaling molecules to bring about this re-programming. Here, we review efferocytosis-induced changes in macrophage metabolism that mediate the pro-resolving functions of macrophages. We also discuss various strategies, challenges, and future perspectives related to drugging efferocytosis-fueled macrophage metabolism as strategy to dampen inflammation and promote resolution in chronic inflammatory diseases.

Circadian disruption by shifting the light-dark cycle negatively affects bone health in mice.

The past decade, it has become evident that circadian rhythms within metabolically active tissues are very important for physical health. However, although shift work has also been associated with an increased risk of fractures, circadian rhythmicity has not yet been extensively studied in bone. Here, we investigated which genes are rhythmically expressed in bone, and whether circadian disruption by shifts in light-dark cycle affects bone turnover and structure in mice. Our results demonstrate diurnal expression patterns of clock genes (Rev-erbα, Bmal1, Per1, Per2, Cry1, Clock), as well as genes involved in osteoclastogenesis, osteoclast proliferation and function (Rankl, Opg, Ctsk), and osteocyte function (c-Fos) in bone. Weekly alternating light-dark cycles disrupted rhythmic clock gene expression in bone and caused a reduction in plasma levels of procollagen type 1 amino-terminal propeptide (P1NP) and tartrate-resistant acidic phosphatase (TRAP), suggestive of a reduced bone turnover. These effects coincided with an altered trabecular bone structure and increased cortical mineralization after 15 weeks of light-dark cycles, which may negatively affect bone strength in the long term. Collectively, these results show that a physiological circadian rhythm is important to maintain bone health, which stresses the importance of further investigating the association between shift work and skeletal disorders.

Disruption of circadian rhythm by alternating light-dark cycles aggravates atherosclerosis development in APOE*3-Leiden.CETP mice.

Disruption of circadian rhythm by means of shift work has been associated with cardiovascular disease in humans. However, causality and underlying mechanisms have not yet been established. In this study, we exposed hyperlipidemic APOE*3-Leiden.CETP mice to either regular light-dark cycles, weekly 6 hours phase advances or delays, or weekly alternating light-dark cycles (12 hours shifts), as a well-established model for shift work. We found that mice exposed to 15 weeks of alternating light-dark cycles displayed a striking increase in atherosclerosis, with an approximately twofold increase in lesion size and severity, while mice exposed to phase advances and delays showed a milder circadian disruption and no significant effect on atherosclerosis development. We observed a higher lesion macrophage content in mice exposed to alternating light-dark cycles without obvious changes in plasma lipids, suggesting involvement of the immune system. Moreover, while no changes in the number or activation status of circulating monocytes and other immune cells were observed, we identified increased markers for inflammation, oxidative stress, and chemoattraction in the vessel wall. Altogether, this is the first study to show that circadian disruption by shifting light-dark cycles directly aggravates atherosclerosis development.

Relevance of lipid metabolism for brown fat visualization and quantification.

PURPOSE OF REVIEW: Brown adipose tissue (BAT) is an emerging target to combat cardiometabolic disorders as it can take up substantial amounts of glucose and lipids from the circulation for heat production. This review focuses on new concepts in BAT physiology and discusses the need for new techniques to determine BAT activity in humans. RECENT FINDINGS: Mouse studies showed that BAT activation selectively increases oxidation of lipids over glucose, by recruiting fatty acids from intracellular triglycerides. To replenish these intracellular lipid stores, brown adipocytes take up both glucose and triglyceride-derived fatty acids, resulting in attenuation of dyslipidaemia, insulin resistance and atherosclerosis. Clinical studies identified the involvement of the ß3-adrenergic receptor in BAT activation and demonstrated that human BAT activation also selectively increases lipid oxidation. Notably, insulin resistance during ageing or weight gain reduces the capacity of BAT to internalize glucose, without reducing fatty acid uptake or oxidative metabolism. SUMMARY: Preclinical studies established BAT as an important target to combat cardiometabolic disorders and elucidated underlying mechanisms whereas clinical studies identified therapeutic handles. Development of novel lipid-based PET-CT tracers and identification of translational biomarkers of BAT activity are required as alternatives to [F]fluorodeoxyglucose PET-CT to accelerate clinical development of BAT-activating therapeutic strategies.

Efferocytosis-induced lactate enables the proliferation of pro-resolving macrophages to mediate tissue repair.

The clearance of apoptotic cells by macrophages (efferocytosis) prevents necrosis and inflammation and activates pro-resolving pathways, including continual efferocytosis. A key resolution process in vivo is efferocytosis-induced macrophage proliferation (EIMP), in which apoptotic cell-derived nucleotides trigger Myc-mediated proliferation of pro-resolving macrophages. Here we show that EIMP requires a second input that is integrated with cellular metabolism, notably efferocytosis-induced lactate production. Lactate signalling via GPR132 promotes Myc protein stabilization and subsequent macrophage proliferation. This mechanism is validated in vivo using a mouse model of dexamethasone-induced thymocyte apoptosis, which elevates apoptotic cell burden and requires efferocytosis to prevent inflammation and necrosis. Thus, EIMP, a key process in tissue resolution, requires inputs from two independent processes: a signalling pathway induced by apoptotic cell-derived nucleotides and a cellular metabolism pathway involving lactate production. These findings illustrate how seemingly distinct pathways in efferocytosing macrophages are integrated to carry out a key process in tissue resolution.

PFKFB2-mediated glycolysis promotes lactate-driven continual efferocytosis by macrophages.

Resolving-type macrophages prevent chronic inflammation by clearing apoptotic cells through efferocytosis. These macrophages are thought to rely mainly on oxidative phosphorylation, but emerging evidence suggests a possible link between efferocytosis and glycolysis. To gain further insight into this issue, we investigated molecular-cellular mechanisms involved in efferocytosis-induced macrophage glycolysis and its consequences. We found that efferocytosis promotes a transient increase in macrophage glycolysis that is dependent on rapid activation of the enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 2 (PFKFB2), which distinguishes this process from glycolysis in pro-inflammatory macrophages. Mice transplanted with activation-defective PFKFB2 bone marrow and then subjected to dexamethasone-induced thymocyte apoptosis exhibit impaired thymic efferocytosis, increased thymic necrosis, and lower expression of the efferocytosis receptors MerTK and LRP1 on thymic macrophages compared with wild-type control mice. In vitro mechanistic studies revealed that glycolysis stimulated by the uptake of a first apoptotic cell promotes continual efferocytosis through lactate-mediated upregulation of MerTK and LRP1. Thus, efferocytosis-induced macrophage glycolysis represents a unique metabolic process that sustains continual efferocytosis in a lactate-dependent manner. The differentiation of this process from inflammatory macrophage glycolysis raises the possibility that it could be therapeutically enhanced to promote efferocytosis and resolution in chronic inflammatory diseases.

Macrophage-targeted nanomedicine for the diagnosis and treatment of atherosclerosis.

Nanotechnology could improve our understanding of the pathophysiology of atherosclerosis and contribute to the development of novel diagnostic and therapeutic strategies to further reduce the risk of cardiovascular disease. Macrophages have key roles in atherosclerosis progression and, therefore, macrophage-associated pathological processes are important targets for both diagnostic imaging and novel therapies for atherosclerosis. In this Review, we highlight efforts in the past two decades to develop imaging techniques and to therapeutically manipulate macrophages in atherosclerotic plaques with the use of rationally designed nanoparticles. We review the latest progress in nanoparticle-based imaging modalities that can specifically target macrophages. Using novel molecular imaging technology, these modalities enable the identification of advanced atherosclerotic plaques and the assessment of the therapeutic efficacy of medical interventions. Additionally, we provide novel perspectives on how macrophage-targeting nanoparticles can deliver a broad range of therapeutic payloads to atherosclerotic lesions. These nanoparticles can suppress pro-atherogenic macrophage processes, leading to improved resolution of inflammation and stabilization of plaques. Finally, we propose future opportunities for novel diagnostic and therapeutic strategies and provide solutions to challenges in this area for the purpose of accelerating the clinical translation of nanomedicine for the treatment of atherosclerotic vascular disease.

Castration of Male Mice Induces Metabolic Remodeling of the Heart.

Androgen deprivation therapy of prostate cancer, which suppresses serum testosterone to castrate levels, is associated with increased risk of heart failure. Here we tested the hypothesis that castration alters cardiac energy substrate uptake, which is tightly coupled to the regulation of cardiac structure and function. Short-term (3-4 weeks) surgical castration of male mice reduced the relative heart weight. While castration did not affect cardiac function in unstressed conditions, we observed reductions in heart rate, stroke volume, cardiac output, and cardiac index during pharmacological stress with dobutamine in castrated vs sham-operated mice. Experiments using radiolabeled lipoproteins and glucose showed that castration shifted energy substrate uptake in the heart from lipids toward glucose, while testosterone replacement had the opposite effect. There was increased expression of fetal genes in the heart of castrated mice, including a strong increase in messenger RNA and protein levels of ß-myosin heavy chain (MHC), the fetal isoform of MHC. In conclusion, castration of male mice induces metabolic remodeling and expression of the fetal gene program in the heart, in association with a reduced cardiac performance during pharmacological stress. These findings may be relevant for the selection of treatment strategies for heart failure in the setting of testosterone deficiency.

Sexual Dimorphism in Transcriptional and Functional Glucocorticoid Effects on Mouse Skeletal Muscle.

Muscle atrophy is common in patients with increased glucocorticoid exposure. Glucocorticoid effects are often sex-specific, and while different glucocorticoid responses between male and female subjects are reported, it is unclear why this is. In this study, we evaluated the effects of corticosterone and synthetic glucocorticoid treatment on muscle atrophy in male and female mice. We found that corticosterone treatment reduced grip strength in female mice only, whereas muscle mass was reduced in both sexes. Skeletal muscle transcriptional responses to corticosterone treatment were more pronounced and widespread in male mice. Synthetic glucocorticoid treatment reduced grip strength in both sexes, while female mice were more sensitive to muscle atrophy than male mice. To evaluate the role of androgens, chemically-castrated male mice were treated with synthetic glucocorticoids. We observed additively reduced muscle mass, but did not observe any interaction effects. Although sex differences in glucocorticoid responses in skeletal muscle are partly influenced by androgen signaling, further studies are warranted to fully delineate the underlying mechanisms.

Chronobiology and Chronotherapy of Osteoporosis.

Physiological circadian (ie, 24-hour) rhythms are critical for bone health. Animal studies have shown that genes involved in the intrinsic molecular clock demonstrate potent circadian expression patterns in bone and that genetic disruption of these clock genes results in a disturbed bone structure and quality. More importantly, circulating markers of bone remodeling show diurnal variation in mice as well as humans, and circadian disruption by, eg, working night shifts is associated with the bone remodeling disorder osteoporosis. In this review, we provide an overview of the current literature on rhythmic bone remodeling and its underlying mechanisms and identify critical knowledge gaps. In addition, we discuss novel (chrono)therapeutic strategies to reduce osteoporosis by utilizing our knowledge on circadian regulation of bone. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Deficiency of macrophage PHACTR1 impairs efferocytosis and promotes atherosclerotic plaque necrosis.

Efferocytosis, the process through which apoptotic cells (ACs) are cleared through actin-mediated engulfment by macrophages, prevents secondary necrosis, suppresses inflammation, and promotes resolution. Impaired efferocytosis drives the formation of clinically dangerous necrotic atherosclerotic plaques, the underlying etiology of coronary artery disease (CAD). An intron of the gene encoding PHACTR1 contains rs9349379 (A>G), a common variant associated with CAD. As PHACTR1 is an actin-binding protein, we reasoned that if the rs9349379 risk allele G causes lower PHACTR1 expression in macrophages, it might link the risk allele to CAD via impaired efferocytosis. We show here that rs9349379-G/G was associated with lower levels of PHACTR1 and impaired efferocytosis in human monocyte-derived macrophages and human atherosclerotic lesional macrophages compared with rs9349379-A/A. Silencing PHACTR1 in human and mouse macrophages compromised AC engulfment, and Western diet-fed Ldlr-/- mice in which hematopoietic Phactr1 was genetically targeted showed impaired lesional efferocytosis, increased plaque necrosis, and thinner fibrous caps - all signs of vulnerable plaques in humans. Mechanistically, PHACTR1 prevented dephosphorylation of myosin light chain (MLC), which was necessary for AC engulfment. In summary, rs9349379-G lowered PHACTR1, which, by lowering phospho-MLC, compromised efferocytosis. Thus, rs9349379-G may contribute to CAD risk, at least in part, by impairing atherosclerotic lesional macrophage efferocytosis.

Testosterone reduces metabolic brown fat activity in male mice.

Brown adipose tissue (BAT) burns substantial amounts of mainly lipids to produce heat. Some studies indicate that BAT activity and core body temperature are lower in males than females. Here we investigated the role of testosterone and its receptor (the androgen receptor; AR) in metabolic BAT activity in male mice. Castration, which renders mice testosterone deficient, slightly promoted the expression of thermogenic markers in BAT, decreased BAT lipid content, and increased basal lipolysis in isolated brown adipocytes. Further, castration increased the core body temperature. Triglyceride-derived fatty acid uptake, a proxy for metabolic BAT activity in vivo, was strongly increased in BAT from castrated mice (4.5-fold increase vs sham-castrated mice) and testosterone replacement reversed the castration-induced increase in metabolic BAT activity. BAT-specific AR deficiency did not mimic the castration effects in vivo and AR agonist treatment did not diminish the activity of cultured brown adipocytes in vitro, suggesting that androgens do not modulate BAT activity via a direct, AR-mediated pathway. In conclusion, testosterone is a negative regulator of metabolic BAT activity in male mice. Our findings provide new insight into the metabolic actions of testosterone.

Loss of glucocorticoid rhythm induces an osteoporotic phenotype in female mice.

Glucocorticoid (GC)-induced osteoporosis is a widespread health problem that is accompanied with increased fracture risk. Detrimental effects of anti-inflammatory GC therapy on bone have been ascribed to the excess in GC exposure, but it is unknown whether there is also a role for disruption of the endogenous GC rhythm that is inherent to GC therapy. To investigate this, we implanted female C57Bl/6J mice with slow-release corticosterone (CORT) pellets to blunt the rhythm in CORT levels without inducing hypercortisolism. Flattening of CORT rhythm reduced cortical and trabecular bone volume and thickness, whilst bone structure was maintained in mice injected with supraphysiologic CORT at the time of their endogenous GC peak. Mechanistically, mice with a flattened CORT rhythm showed disrupted circadian gene expression patterns in bone, along with changes in circulating bone turnover markers indicative of a negative balance in bone remodelling. Indeed, double calcein labelling of bone in vivo revealed a reduced bone formation in mice with a flattened CORT rhythm. Collectively, these perturbations in bone turnover and structure decreased bone strength and stiffness, as determined by mechanical testing. In conclusion, we demonstrate for the first time that flattening of the GC rhythm disrupts the circadian clock in bone and results in an osteoporotic phenotype in mice. Our findings indicate that at least part of the fracture risk associated with GC therapy may be the consequence of a disturbed GC rhythm, rather than excess GC exposure alone, and that a dampened GC rhythm may contribute to the age-related risk of osteoporosis.

A physiological glucocorticoid rhythm is an important regulator of brown adipose tissue function.

OBJECTIVE: Brown adipose tissue (BAT) displays a strong circadian rhythm in metabolic activity, but it is unclear how this rhythm is regulated. As circulating levels of corticosterone coincide with the rhythm of triglyceride-derived fatty acid (FA) uptake by BAT, we investigated whether corticosterone regulates BAT circadian rhythm. METHODS: Corticosterone levels were flattened by implanting mice with subcutaneous corticosterone-releasing pellets, resulting in constant circulating corticosterone levels. RESULTS: Flattened corticosterone rhythm caused a complete loss of circadian rhythm in triglyceride-derived fatty acid uptake by BAT. This effect was independent of glucocorticoid receptor expression in (brown) adipocytes and was not caused by deregulation of clock gene expression or overexposure to glucocorticoids, but rather seemed mediated by reduced sympathetic innervation of BAT. In a mouse model of hyperlipidemia and metabolic syndrome, long-term experimental flattening of corticosterone - and thus rhythm in BAT function - resulted in adiposity. CONCLUSIONS: This study highlights that a physiological rhythm in glucocorticoids is an important regulator of BAT function and essential for the maintenance of metabolic health.

Time for Novel Strategies to Mitigate Cardiometabolic Risk in Shift Workers.

Circadian disruption induced by shift work is robustly associated with obesity, diabetes, and cardiovascular disease in humans. Less well-known are the mechanisms underlying these associations, and the effectiveness of strategies to reduce cardiometabolic risk in the shift work population. In this review, the different ways in which shift work can deteriorate cardiometabolic health, and how to use this information to reflect on various risk-mitigating strategies, is discussed. While individual strategies appear promising in animal studies, the multifactorial disease risk in shift workers likely requires a multidisciplinary approach. Therefore, the need for individually-tailored combined lifestyle interventions, that could be essential in reducing cardiometabolic disorders in the large population of shift workers in our 24/7 society, is argued.

Continuous Light Does Not Affect Atherosclerosis in APOE*3-Leiden.CETP Mice.

Artificial light exposure is associated with dyslipidemia in humans, which is a major risk factor for the development of atherosclerotic cardiovascular disease. However, it remains unclear whether artificial light at night can exacerbate atherosclerosis. In this study, we exposed female APOE*3-Leiden.CETP mice, a well-established model for human-like lipid metabolism and atherosclerosis, to either a regular light-dark cycle or to constant bright light for 14 weeks. Mice exposed to constant light demonstrated a minor reduction in food intake, without any effect on body weight, body composition, or the weight of metabolic organs. Constant light increased the plasma levels of proatherogenic non-high-density lipoprotein (HDL) cholesterol but did not increase the size or severity of atherosclerotic lesions in the aortic root. Mice exposed to constant light did show lower immune cell counts, which could explain the absence of an effect of atherosclerosis despite increased non-HDL cholesterol levels. Behavioral analysis demonstrated variability in the response of mice to the light intervention. Constant light completely blunted behavioral rhythms in some mice, while others extended their behavioral period. However, rhythm strength was not an important determinant of atherosclerosis. Altogether, these results demonstrate that constant bright light does not affect atherosclerosis in APOE*3-Leiden.CETP mice. Whether artificial light exposure contributes to cardiovascular disease risk in humans remains to be investigated.

Liposomal Delivery Improves the Efficacy of Prednisolone to Attenuate Renal Inflammation in a Mouse Model of Acute Renal Allograft Rejection.

BACKGROUND: Systemic exposure to high-dose corticosteroids effectively combats acute rejection after kidney transplantation, but at the cost of substantial side effects. In this study, a murine acute renal allograft rejection model was used to investigate whether liposomal-encapsulated prednisolone (LP) facilitates local exposure to enhance its therapeutic effect. METHODS: Male BalbC recipients received renal allografts from male C57BL/6J donors. Recipients were injected daily with 5 mg/kg cyclosporine A and received either 10 mg/kg prednisolone (P), or LP intravenously on day 0, 3, and 6, or no additional treatment. Functional magnetic resonance imaging (fMRI) was performed on day 6 to study allograft perfusion and organs were retrieved on day 7 for further analysis. RESULTS: Staining of polyethylene-glycol-labeled liposomes and high performance liquid chromatography analysis revealed accumulation in the LP treated allograft. LP treatment induced the expression of glucocorticoid responsive gene Fkbp5 in the allograft. Flow-cytometry of allografts revealed liposome presence in CD45 cells, and reduced numbers of F4/80 macrophages, and CD3 T-lymphocytes upon LP treatment. Banff scoring showed reduced interstitial inflammation and tubulitis and fMRI analysis revealed improved allograft perfusion in LP versus NA mice. CONCLUSIONS: Liposomal delivery of prednisolone improved renal bio-availability, increased perfusion and reduced cellular infiltrate in the allograft, when compared with conventional prednisolone. Clinical studies should reveal if treatment with LP results in improved efficacy and reduced side effects in patients with renal allograft rejection.

Time-restricted feeding improves adaptation to chronically alternating light-dark cycles.

Disturbance of the circadian clock has been associated with increased risk of cardio-metabolic disorders. Previous studies showed that optimal timing of food intake can improve metabolic health. We hypothesized that time-restricted feeding could be a strategy to minimize long term adverse metabolic health effects of shift work and jetlag. In this study, we exposed female FVB mice to weekly alternating light-dark cycles (i.e. 12 h shifts) combined with ad libitum feeding, dark phase feeding or feeding at a fixed clock time, in the original dark phase. In contrast to our expectations, long-term disturbance of the circadian clock had only modest effects on metabolic parameters. Mice fed at a fixed time showed a delayed adaptation compared to ad libitum fed animals, in terms of the similarity in 24 h rhythm of core body temperature, in weeks when food was only available in the light phase. This was accompanied by increased plasma triglyceride levels and decreased energy expenditure, indicating a less favorable metabolic state. On the other hand, dark phase feeding accelerated adaptation of core body temperature and activity rhythms, however, did not improve the metabolic state of animals compared to ad libitum feeding. Taken together, restricting food intake to the active dark phase enhanced adaptation to shifts in the light-dark schedule, without significantly affecting metabolic parameters.

Androgens modulate glucocorticoid receptor activity in adipose tissue and liver.

Glucocorticoid signaling is context-dependent, and in certain scenarios glucocorticoid receptors (GR) are able to engage with other members of the nuclear receptor subfamily. Glucocorticoid signaling can exert sexually dimorphic effects, suggesting a possible interaction with androgen sex hormones. We therefore set out to determine the crosstalk between glucocorticoids and androgens in metabolic tissues including white adipose tissue, liver and brown adipose tissue. Thereto we exposed male C57BL/6J mice to elevated levels of corticosterone in combination with an androgen receptor (AR) agonist or an AR antagonist. Systemic and local glucocorticoid levels were determined by mass spectrometry, tissue expression of glucocorticoid-responsive genes and protein was measured by RT-qPCR and Western blot, respectively. To evaluate crosstalk in vitro, cultured white and brown adipocytes were exposed to a combination of corticosterone and an androgen agonist. We found that AR agonism potentiated transcriptional response to GR in vitro in white and brown adipocytes and in vivo in white and brown adipose tissue. Conversely, AR antagonism substantially attenuated glucocorticoid signaling in white adipose tissue and liver. In white adipose tissue this effect could partially be attributed to decreased 11B-hydroxysteroid dehydrogenase type 1-mediated glucocorticoid regeneration upon AR antagonism. In liver, attenuated GR activity was independent of active glucocorticoid ligand levels. We conclude that androgen signaling modulates GR transcriptional output in a tissue-specific manner.

The GPR120 agonist TUG-891 promotes metabolic health by stimulating mitochondrial respiration in brown fat.

Brown adipose tissue (BAT) activation stimulates energy expenditure in human adults, which makes it an attractive target to combat obesity and related disorders. Recent studies demonstrated a role for G protein-coupled receptor 120 (GPR120) in BAT thermogenesis. Here, we investigated the therapeutic potential of GPR120 agonism and addressed GPR120-mediated signaling in BAT We found that activation of GPR120 by the selective agonist TUG-891 acutely increases fat oxidation and reduces body weight and fat mass in C57Bl/6J mice. These effects coincided with decreased brown adipocyte lipid content and increased nutrient uptake by BAT, confirming increased BAT activity. Consistent with these observations, GPR120 deficiency reduced expression of genes involved in nutrient handling in BAT Stimulation of brown adipocytes in vitro with TUG-891 acutely induced O2 consumption, through GPR120-dependent and GPR120-independent mechanisms. TUG-891 not only stimulated GPR120 signaling resulting in intracellular calcium release, mitochondrial depolarization, and mitochondrial fission, but also activated UCP1. Collectively, these data suggest that activation of brown adipocytes with the GPR120 agonist TUG-891 is a promising strategy to increase lipid combustion and reduce obesity.