Burn Induces Browning of the Subcutaneous White Adipose Tissue in Mice and Humans

Metabolaging: a new geroscience perspective linking aging pathologies and metabolic dysfunction

With age, our metabolic systems undergo significant alterations, which can lead to a cascade of adverse effects that are implicated in both metabolic disorders, such as diabetes, and in the body's ability to respond to acute stress and trauma. To elucidate the metabolic imbalances arising from aging, we introduce the concept of "metabolaging." This framework encompasses the broad spectrum of metabolic disruptions associated with the hallmarks of aging, including the functional decline of key metabolically active organs, like the adipose tissue. By examining how these organs interact with essential nutrient-sensing pathways, "metabolaging" provides a more comprehensive view of the systemic metabolic imbalances that occur with age. This concept extends to understanding how age-related metabolic disturbances can influence the response to acute stressors, like burn injuries, highlighting the interplay between metabolic dysfunction and the ability to handle severe physiological challenges. Finally, we propose potential interventions that hold promise in mitigating the effects of metabolaging and its downstream consequences.

Emerging debates and resolutions in brown adipose tissue research

Until two decades ago, brown adipose tissue (BAT) was studied primarily as a thermogenic organ of small rodents in the context of cold adaptation. The discovery of functional human BAT has opened new opportunities to understand its physiological role in energy balance and therapeutic applications for metabolic disorders. Significantly, the role of BAT extends far beyond thermogenesis, including glucose and lipid homeostasis, by releasing mediators that communicate with other cells and organs. The field has made major advances by using new model systems, ranging from subcellular studies to clinical trials, which have also led to debates. In this perspective, we identify six fundamental issues that are currently controversial and comprise dichotomous models. Each side presents supporting evidence and, critically, the necessary methods and falsifiable experiments that would resolve the dispute. With this collaborative approach, the field will continue to productively advance the understanding of BAT physiology, appreciate the importance of thermogenic adipocytes as a central area of ongoing research, and realize the therapeutic potential.

Illusory movements for immobile patients with extensive burns (IMMOBILE): A randomized, controlled, cross-over trial

INTRODUCTION

Patients who have sustained extensive burns frequently exhibit substantial damage to skeletal muscle and associated complications. The rehabilitation of these patients can be challenging due to the nature of the injury and the subsequent complications. Nevertheless, there is a possibility that functional proprioceptive stimulation (illusory movements) may facilitate effective rehabilitation in patients with limited physiotherapy options. Nevertheless, this approach has yet to be tested in patients with burn injuries.

MATERIAL AND METHODOLOGY

A prospective, randomised, crossover trial was conducted at a burn centre in a tertiary teaching hospital. The objective was to assess the effects of illusory movements on energy metabolism, insulin sensitivity, and skeletal muscle biology in adult critically ill patients with deep burns covering 30 % or more of the total body surface area. Two 30-minute daily sessions of functional proprioceptive stimulation were administered in addition to the standard physical therapy or physical activity regimen. Subsequently, the patients proceeded to the next stage of the trial, which involved a two-week crossover period.

MEASUREMENTS AND MAIN RESULTS

Daily indirect calorimetry and calculation of nitrogen balance. Skeletal muscle biopsies from vastus lateralis for high resolution respirometry and euglycemic clamps to assess whole body glucose disposal were performed three times: at baseline and then fortnightly after each intervention period. The intervention was feasible and well tolerated in both early and late stages of burn disease. It did not change energy expenditure (mean change -33 [95 % CI: -292;+227] kcal .24 h-1, p = 0.79), nitrogen balance (+2.0 [95 % CI: -3.1;+7.1] g N .1.73 m-2 BSA .24 h-1), or insulin sensitivity (mean change of insulin-mediated glucose disposal -0.33 [95 % CI: -1.18;+0.53] mmol.h-1). At the cellular level, the intervention increased the capacity of mitochondria to synthesize ATP by aerobic phosphorylation and tended to increase mitochondrial coupling. Functional capacities of fatty acid oxidation and electron transfer chain complexes I, II, and IV were unaffected.

CONCLUSIONS

Compared to physical therapy alone, two daily sessions of functional proprioceptive stimulation in addition to usual physical therapy in patients with extensive burns did not change energy expenditure, insulin sensitivity, nitrogen balance, or energy substrate oxidation. At cellular level, the intervention improved the capacity of aerobic phosphorylation in skeletal muscle mitochondria. Clinical effects remain to be demonstrated in adequately powered trials.

Hyperactive browning and hypermetabolism: potentially dangerous element in critical illness

Brown/beige adipose tissue has attracted much attention in previous studies because it can improve metabolism and combat obesity through non-shivering thermogenesis. However, recent studies have also indicated that especially in critical illness, overactivated brown adipose tissue or extensive browning of white adipose tissue may bring damage to individuals mainly by exacerbating hypermetabolism. In this review, the phenomenon of fat browning in critical illness will be discussed, along with the potential harm, possible regulatory mechanism and corresponding clinical treatment options of the induction of fat browning. The current research on fat browning in critical illness will offer more comprehensive understanding of its biological characteristics, and inspire researchers to develop new complementary treatments for the hypermetabolic state that occurs in critically ill patients.

β-Adrenergic blockade attenuates adverse adipose tissue responses after burn

Severe burn injuries are defined by a prolonged hypermetabolic response characterized by increases in resting energy expenditure, systemic catabolism, and multi-organ dysfunction. The sustained elevation of catecholamines following a burn injury is thought to significantly contribute to this hypermetabolic response, leading to changes in adipose tissue such as increased lipolysis and the browning of subcutaneous white adipose tissue (WAT). Failure to mitigate these adverse changes within the adipose tissue has been shown to exacerbate the post-burn hypermetabolic response and lead to negative outcomes. Propranolol, a non-selective β-blocker, has been clinically administered to improve outcomes of pediatric and adult burn patients, but there is inadequate knowledge of its effects on the distinct adipose tissue depots. In this study, we investigated the adipose depot-specific alterations that occur in response to burn injury. Moreover, we explored the therapeutic effects of β-adrenoceptor blockade via the drug propranolol in attenuating these burn-induced pathophysiological changes within the different fat depots. Using a murine model of thermal injury, we show that burn injury induces endoplasmic reticulum (ER) stress in the epididymal (eWAT) but not in the inguinal (iWAT) WAT depot. Conversely, burn injury induces the activation of key lipolytic pathways in both eWAT and iWAT depots. Treatment of burn mice with propranolol effectively mitigated adverse burn-induced alterations in the adipose by alleviating ER stress in the eWAT and reducing lipolysis in both depots. Furthermore, propranolol treatment in post-burn mice attenuated UCP1-mediated subcutaneous WAT browning following injury. Overall, our findings suggest that propranolol serves as an effective therapeutic intervention to mitigate the adverse changes induced by burn injury, including ER stress, lipotoxicity, and WAT browning, in both adipose tissue depots. KEY MESSAGES: Burn injury adversely affects adipose tissue metabolism via distinct changes in both visceral and subcutaneous adipose depots. Propranolol, a non-selective β-adrenergic blocker, attenuates many of the adverse adipose tissue changes mediated by burn injury.

GRP75 triggers white adipose tissue browning to promote cancer-associated cachexia

Cachexia, which affects 50-80% of cancer patients, is a debilitating syndrome that leads to 20% of cancer-related deaths. A key feature of cachexia is adipose tissue atrophy, but how it contributes to the development of cachexia is poorly understood. Here, we demonstrate in mouse models of cancer cachexia that white adipose tissue browning, which can be a characteristic early-onset manifestation, occurs prior to the loss of body weight and skeletal muscle wasting. By analysing the proteins differentially expressed in extracellular vesicles derived from cachexia-inducing tumours, we identified a molecular chaperone, Glucose-regulated protein 75 (GRP75), as a critical mediator of adipocyte browning. Mechanistically, GRP75 binds adenine nucleotide translocase 2 (ANT2) to form a GRP75-ANT2 complex. Strikingly, stabilized ANT2 enhances its interaction with uncoupling protein 1, leading to elevated expression of the latter, which, in turn, promotes adipocyte browning. Treatment with withanone, a GRP75 inhibitor, can reverse this browning and alleviate cachectic phenotypes in vivo. Overall, our findings reveal a novel mechanism by which tumour-derived GRP75 regulates white adipose tissue browning during cachexia development and suggest a potential white adipose tissue-centred targeting approach for early cachexia intervention.

A Closer Look into White Adipose Tissue Biology and the Molecular Regulation of Stem Cell Commitment and Differentiation

White adipose tissue (WAT) makes up about 20-25% of total body mass in healthy individuals and is crucial for regulating various metabolic processes, including energy metabolism, endocrine function, immunity, and reproduction. In adipose tissue research, "adipogenesis" is commonly used to refer to the process of adipocyte formation, spanning from stem cell commitment to the development of mature, functional adipocytes. Although, this term should encompass a wide range of processes beyond commitment and differentiation, to also include other stages of adipose tissue development such as hypertrophy, hyperplasia, angiogenesis, macrophage infiltration, polarization, etc.… collectively, referred to herein as the adipogenic cycle. The term "differentiation", conversely, should only be used to refer to the process by which committed stem cells progress through distinct phases of subsequent differentiation. Recognizing this distinction is essential for accurately interpreting research findings on the mechanisms and stages of adipose tissue development and function. In this review, we focus on the molecular regulation of white adipose tissue development, from commitment to terminal differentiation, and examine key functional aspects of WAT that are crucial for normal physiology and systemic metabolic homeostasis.

BURNS INDUCE ALTERATIONS IN THE ACYL PROTEOME OF MICE AND HUMANS

ABSTRACT

Hypermetabolic reprogramming triggered by thermal injury causes substantial morbidity and mortality. Despite the therapeutic potential of targeting this response, the underlying mechanisms remain poorly understood. Interestingly, protein S-acylation is a reversible posttranslational modification induced by metabolic alterations via DHHC acyltransferases. While this modification aids in the regulation of cellular functions, deregulated S-acylation contributes to various diseases by altering protein structure, stability, and localization. However, whether and how S-acylation may impact morbidity and mortality during postburn hypermetabolism is unknown. In this study, we discovered that alterations in the acyl proteome play a key role in mediating adverse outcomes that occur after burn injury. Using a murine model, we show that burn injury induces profound changes in the expression of various DHHC isoforms in metabolic organs central to regulating postburn hypermetabolism, the adipose tissue, and liver. This was accompanied by increased levels of S-acylated proteins in several pathways involved in mediating the adverse hypermetabolic response, including ER stress, lipolysis, and browning. In fact, similar results were also observed in adipose tissue from severely burned patients, as reflected by increased S-acylation of ERK1/2, eIF2a, ATGL, FGF21, and UCP1 relative to nonburn controls. Importantly, pharmacologically targeting this posttranslational modification using a nonselective DHHC inhibitor effectively attenuated burn-induced ER stress, lipolysis, and browning induction in an ex vivo explant model. Together, these findings suggest that S-acylation may facilitate the protein activation profile that drives burn-induced hypermetabolism and that targeting it could potentially be an effective strategy to restore metabolic function and improve outcomes after injury.

Trauma-associated extracellular histones mediate inflammation via a MYD88-IRAK1-ERK signaling axis and induce lytic cell death in human adipocytes

Despite advances in the treatment and care of severe physical injuries, trauma remains one of the main reasons for disability-adjusted life years worldwide. Trauma patients often suffer from disturbances in energy utilization and metabolic dysfunction, including hyperglycemia and increased insulin resistance. White adipose tissue plays an essential role in the regulation of energy homeostasis and is frequently implicated in traumatic injury due to its ubiquitous body distribution but remains poorly studied. Initial triggers of the trauma response are mainly damage-associated molecular patterns (DAMPs) such as histones. We hypothesized that DAMP-induced adipose tissue inflammation contributes to metabolic dysfunction in trauma patients. Therefore, we investigated whether histone release during traumatic injury affects adipose tissue. Making use of a murine polytrauma model with hemorrhagic shock, we found increased serum levels of histones accompanied by an inflammatory response in white adipose tissue. In vitro, extracellular histones induced an inflammatory response in human adipocytes. On the molecular level, this inflammatory response was mediated via a MYD88-IRAK1-ERK signaling axis as demonstrated by pharmacological and genetic inhibition. Histones also induced lytic cell death executed independently of caspases and RIPK1 activity. Importantly, we detected increased histone levels in the bloodstream of patients after polytrauma. Such patients might benefit from a therapy consisting of activated protein C and the FDA-approved ERK inhibitor trametinib, as this combination effectively prevented histone-mediated effects on both, inflammatory gene activation and cell death in adipocytes. Preventing adipose tissue inflammation and adipocyte death in patients with polytrauma could help minimize posttraumatic metabolic dysfunction.

Potential Role of Pig UCP3 in Modulating Adipocyte Browning via the Beta-Adrenergic Receptor Signaling Pathway

Adipose tissue plays an important role in regulating body temperature and metabolism, with white adipocytes serving as storage units for energy. Recent research focused on the browning of white adipocytes (beige adipocytes), causing thermogenesis and lipolysis. The process of browning is linked to the activation of uncoupling protein (UCP) expression, which can be mediated by the β3 adrenergic receptor pathway. Transcriptional factors, such as peroxisome proliferator activated receptor γ (PPARγ) and PPARγ coactivator 1 alpha, play vital roles in cell fate determination for fat cells. Beige adipocytes have metabolic therapeutic potential to combat diseases such as obesity, diabetes mellitus, and dyslipidemia, owing to their significant impact on metabolic functions. However, the molecular mechanisms that cause the induction of browning are unclear. Therefore, research using animal models and primary culture is essential to provide an understanding of browning for further application in human metabolic studies. Pigs have physiological similarities to humans; hence, they are valuable models for research on adipose tissue. This study demonstrates the browning potential of pig white adipocytes through primary culture experiments. The results show that upregulation of UCP3 gene expression and fragmentation of lipid droplets into smaller particles occur due to isoproterenol stimulation, which activates beta-adrenergic receptor signaling. Furthermore, PPARγ and PGC-1α were found to activate the UCP3 promoter region, similar to that of UCP1. These findings suggest that pigs undergo metabolic changes that induce browning in white adipocytes, providing a promising approach for metabolic research with potential implications for human health. This study offers valuable insights into the mechanism of adipocyte browning using pig primary culture that can enhance our understanding of human metabolism, leading to cures for commonly occurring diseases.

The role of brown adipose tissue in mediating healthful longevity

There are two major subtypes of adipose tissue, i.e., white adipose tissue (WAT) and brown adipose tissue (BAT). It has been known for a long time that WAT mediates obesity and impairs healthful longevity. More recently, interest has focused on BAT, which, unlike WAT, actually augments healthful aging. The goal of this review is to examine the role of BAT in mediating healthful longevity. A major role for BAT and its related beige adipose tissue is thermogenesis, as a mechanism to maintain body temperature by producing heat through uncoupling protein 1 (UCP1) or through UCP1-independent thermogenic pathways. Our hypothesis is that healthful longevity is, in part, mediated by BAT. BAT protects against the major causes of impaired healthful longevity, i.e., obesity, diabetes, cardiovascular disorders, cancer, Alzheimer's disease, reduced exercise tolerance, and impaired blood flow. Several genetically engineered mouse models have shown that BAT enhances healthful aging and that their BAT is more potent than wild-type (WT) BAT. For example, when BAT, which increases longevity and exercise performance in mice with disruption of the regulator of G protein signaling 14 (RGS14), is transplanted to WT mice, their exercise capacity is enhanced at 3 days after BAT transplantation, whereas BAT transplantation from WT to WT mice also resulted in increased exercise performance, but only at 8 weeks after transplantation. In view of the ability of BAT to mediate healthful longevity, it is likely that a pharmaceutical analog of BAT will become a novel therapeutic modality.

Challenges in the Management of Large Burns

Large burns provoke profound pathophysiological changes. Survival rates of patients with large burns have improved significantly with the advancement of critical care and adaptation of early excision protocols. Nevertheless, care of large burn wounds remains challenging secondary to limited donor sites, prolonged time to wound closure, and immunosuppression. The development of skin substitutes and new grafting techniques decreased time to wound closure. Individually, these methods have limited success, but a combination of them may yield more successful outcomes. Early identification of patients with likely poor prognosis should prompt goals of care discussion and involvement of a palliative care team when possible.

The splicing factor SF3B1 is involved in brown adipocyte thermogenic activation

The ability of alternative splicing mechanisms to control gene expression is increasingly being recognized as relevant for adipose tissue function. The expression of SF3B1, a key component of the SF3B complex directly involved in spliceosome formation, was previously reported to be significantly induced in brown adipose tissue under cold-induced thermogenic activation. Here, we identify that noradrenergic cAMP-mediated thermogenic stimulation increases SF3B1 expression in brown and beige adipocytes. We further show that pladienolide-B, a drug that binds SF3B1 to inhibit pre-mRNA splicing by targeting the SF3B complex, down-regulates key components of the thermogenic machinery (e.g., UCP1 gene expression), differentially alters the expression of alternative splicing-regulated transcripts encoding molecular actors involved in the oxidative metabolism of brown adipocytes (e.g., peroxisome proliferator-activated receptor-gamma co-activator-alpha [PGC-1α] and cytochrome oxidase subunit 7a genes), and impairs the respiratory activity of brown adipocytes. Similar alterations were found in brown adipocytes with siRNA-mediated knockdown of SF3B1 protein levels. Our findings collectively indicate that SF3B1 is a key factor in the appropriate thermogenic activation of differentiated brown adipocytes. This work exemplifies the importance of splicing processes in adaptive thermogenesis and suggests that pharmacological tools, such as pladienolide-B, may be used to modulate brown adipocyte thermogenic activity.

Subcutaneous white adipose tissue independently regulates burn-induced hypermetabolism via immune-adipose crosstalk

Severe burns induce a chronic hypermetabolic state that persists well past wound closure, indicating that additional internal mechanisms must be involved. Adipose tissue is suggested to be a central regulator in perpetuating hypermetabolism, although this has not been directly tested. Here, we show that thermogenic adipose tissues are activated in parallel to increases in hypermetabolism independent of cold stress. Using an adipose tissue transplantation model, we discover that burn-derived subcutaneous white adipose tissue alone is sufficient to invoke a hypermetabolic response in a healthy recipient mouse. Concomitantly, transplantation of healthy adipose tissue alleviates metabolic dysfunction in a burn recipient. We further show that the nicotinic acetylcholine receptor signaling pathway may mediate an immune-adipose crosstalk to regulate adipose tissue remodeling post-injury. Targeting this pathway could lead to innovative therapeutic interventions to counteract hypermetabolic pathologies.

Large area fractional laser treatment of mouse skin increases energy expenditure

Fractional laser (FL) treatment is a common dermatologic procedure that generates arrays of microscopic treatment zones separated by intact tissue, promoting fast wound healing. Using a mouse model, we introduced a large area fractional laser treatment (LAFLT) method to study metabolic effects. Using two laser modalities, ablative FL (AFL) and non-ablative FL (NAFL), and exposing different percentages of mice's total body surface area (TBSA), we followed changes in metabolic parameters in real time using metabolic cages. Additionally, body composition, markers of inflammation, neurohormonal signaling, and browning of adipocytes were investigated. LAFLT, especially in high TBSA groups, had specific metabolic effects such as significantly increased average daily energy expenditure, increased fat mass loss, systemic browning of adipocytes, and inflammatory states, without compromising other organs. The ability of LAFLT to stimulate metabolism in a controlled way could develop into a promising therapeutic treatment to induce positive metabolic changes that replace or augment systemic drugs.

Characterization of Cell Type Abundance and Gene Expression Timeline from Burned Skin Bulk Transcriptomics by Deconvolution

Currently, no timeline of cell heterogeneity in thermally injured skin has been reported. In this study, we proposed an approach to deconvoluting cell type abundance and expression from skin bulk transcriptomics with cell type signature matrix constructed by combining independent normal skin and peripheral blood scRNA-seq datasets. Using CIBERSORTx group mode deconvolution, we identified perturbed cell type fractions and cell type-specific gene expression in three stages postthermal injury. We found an increase in cell proportions and cell type-specific gene expression perturbation of neutrophils, macrophages, and endothelial cells and a decrease in CD4+ T cells, keratinocytes, melanocyte, and fibroblast cells, and cell type-specific gene expression perturbation postburn injury. Keratinocyte, fibroblast, and macrophage up regulated genes were dynamically enriched in overlapping and distinct Gene Ontology biological processes including acute phase response, leukocyte migration, metabolic, morphogenesis, and development process. Down-regulated genes were enriched in Wnt signaling, mesenchymal cell differentiation, gland and axon development, epidermal morphogenesis, and fatty acid and glucose metabolic process. We noticed an increase in the expression of CCL7, CCL2, CCL20, CCR1, CCR5, CCXL8, CXCL2, CXCL3, MMP1, MMP8, MMP3, IL24, IL6, IL1B, IL18R1, and TGFBR1 and a decrease in expression of CCL27, CCR10, CCR6, CCR8, CXCL9, IL37, IL17, IL7, IL11R, IL17R, TGFBR3, FGFR1-4, and IGFR1 in keratinocytes and/or fibroblasts. The inferred timeline of wound healing and CC and CXC genes in keratinocyte was validated on independent dataset GSE174661 of purified keratinocytes. The timeline of different cell types postburn may facilitate therapeutic timing.

Single-nuclei RNA Profiling Reveals Disruption of Adipokine and Inflammatory Signaling in Adipose Tissue of Burn Patients

OBJECTIVE

We conducted a large-scale investigation of the systemic and adipose tissue-specific alterations in a clinical population of burn patients to identify factors that may influence hypermetabolism.

BACKGROUND

Previous research has identified chronic disturbances in adipose tissue inflammation, lipolysis, and browning, which may drive the perpetuation of hypermetabolism following the severe adrenergic stress of a burn injury. Given that adipose tissue is thought to be a central node in the regulation of systemic metabolism, we believe that systematically delineating the pathologic role of adipose tissue postburn, will lead to the identification of novel interventions to mitigate morbidity and mortality from severe burns.

METHODS

This was a single-institution cohort study, which obtained plasma and subcutaneous adipose tissue samples from severely burn adult patients over various time points during acute hospitalization. Whole-body clinical, metabolic, and inflammatory mediators were assessed in plasma, while genetic analyses through RT-qPCR and single-nuclei RNA sequencing were conducted in adipose tissue.

RESULTS

Systemic inflammation and adrenergic stress increase IL-6 signaling, lipolysis, browning, and adipokine dysfunction in the adipose tissue of adult burn patients, which may further propagate the long-term hypermetabolic response. Moreover, using single-nuclei RNA sequencing, we provide the first comprehensive characterization of alterations in the adipose tissue microenvironment occurring at acute and chronic stages postburn.

CONCLUSION

We provide novel insight toward the effect of burns on adipokine release, inflammatory signaling pathways, and adipose heterogeneity over the trajectory of acute and chronic stages.

Supraclavicular brown adipocytes originate from Tbx1+ myoprogenitors

Brown adipose tissue (BAT) dissipates energy as heat, contributing to temperature control, energy expenditure, and systemic homeostasis. In adult humans, BAT mainly exists in supraclavicular areas and its prevalence is associated with cardiometabolic health. However, the developmental origin of supraclavicular BAT remains unknown. Here, using genetic cell marking in mice, we demonstrate that supraclavicular brown adipocytes do not develop from the Pax3+/Myf5+ epaxial dermomyotome that gives rise to interscapular BAT (iBAT). Instead, the Tbx1+ lineage that specifies the pharyngeal mesoderm marks the majority of supraclavicular brown adipocytes. Tbx1Cre-mediated ablation of peroxisome proliferator-activated receptor gamma (PPARγ) or PR/SET Domain 16 (PRDM16), components of the transcriptional complex for brown fat determination, leads to supraclavicular BAT paucity or dysfunction, thus rendering mice more sensitive to cold exposure. Moreover, human deep neck BAT expresses higher levels of the TBX1 gene than subcutaneous neck white adipocytes. Taken together, our observations reveal location-specific developmental origins of BAT depots and call attention to Tbx1+ lineage cells when investigating human relevant supraclavicular BAT.

An Overview of Recent Developments in the Management of Burn Injuries

According to the World Health Organization (WHO), around 11 million people suffer from burns every year, and 180,000 die from them. A burn is a condition in which heat, chemical substances, an electrical current or other factors cause tissue damage. Burns mainly affect the skin, but can also affect deeper tissues such as bones or muscles. When burned, the skin loses its main functions, such as protection from the external environment, pathogens, evaporation and heat loss. Depending on the stage of the burn, the patient's condition and the cause of the burn, we need to choose the most appropriate treatment. Personalization and multidisciplinary collaboration are key to the successful management of burn patients. In this comprehensive review, we have collected and discussed the available treatment options, focusing on recent advances in topical treatments, wound cleansing, dressings, skin grafting, nutrition, pain and scar tissue management.

Propranolol Normalizes Metabolomic Signatures Thereby Improving Outcomes After Burn

OBJECTIVE AND BACKGROUND

Propranolol, a nonselective beta-receptor blocker, improves outcomes of severely burned patients. While the clinical and physiological benefits of beta-blockade are well characterized, the underlying metabolic mechanisms are less well defined. We hypothesized that propranolol improves outcomes after burn injury by profoundly modulating metabolic pathways.

METHODS

In this phase II randomized controlled trial, patients with burns ≥20% of total body surface area were randomly assigned to control or propranolol (dose given to decrease heart rate <100 bpm). Outcomes included clinical markers, inflammatory and lipidomic profiles, untargeted metabolomics, and molecular pathways.

RESULTS

Fifty-two severely burned patients were enrolled in this trial (propranolol, n=23 and controls, n=29). There were no significant differences in demographics or injury severity between groups. Metabolomic pathway analyses of the adipose tissue showed that propranolol substantially alters several essential metabolic pathways involved in energy and nucleotide metabolism, as well as catecholamine degradation ( P <0.05). Lipidomic analysis revealed that propranolol-treated patients had lower levels of proinflammatory palmitic acid ( P <0.05) and saturated fatty acids ( P <0.05) with an increased ratio of polyunsaturated fatty acids ( P <0.05), thus shifting the lipidomic profile towards an anti-inflammatory phenotype after burn ( P <0.05). These metabolic effects were mediated by decreased activation of hormone-sensitive lipase at serine 660 ( P <0.05) and significantly reduced endoplasmic reticulum stress by decreasing phospho-JNK ( P <0.05).

CONCLUSION

Propranolol's ability to mitigate pathophysiological changes to essential metabolic pathways results in significantly improved stress responses.

Mesenchymal stromal cells as conductors of adipose tissue remodeling

Adipose tissue exhibits a remarkable capacity to expand, contract, and remodel in response to changes in physiological and environmental conditions. Here, we describe recent advances in our understanding of how functionally distinct tissue-resident mesenchymal stromal cell subpopulations orchestrate several aspects of physiological and pathophysiological adipose tissue remodeling, with a particular focus on the adaptations that occur in response to changes in energy surplus and environmental temperature. The study of adipose tissue remodeling provides a vehicle to understand the functional diversity of stromal cells and offers a lens through which several generalizable aspects of tissue reorganization can be readily observed.

Lactate shuttling drives the browning of white adipose tissue after burn

High levels of plasma lactate are associated with increased mortality in critically injured patients, including those with severe burns. Although lactate has long been considered a waste product of glycolysis, it was recently revealed that it acts as a potent inducer of white adipose tissue (WAT) browning, a response implicated in mediating postburn cachexia, hepatic steatosis, and sustained hypermetabolism. Despite the clinical presentation of hyperlactatemia and browning in burns, whether these two pathological responses are linked is currently unknown. Here, we report that elevated lactate plays a causal signaling role in mediating adverse outcomes after burn trauma by directly promoting WAT browning. Using WAT obtained from human burn patients and mouse models of thermal injury, we show that the induction of postburn browning is positively correlated with a shift toward lactate import and metabolism. Furthermore, daily administration of l-lactate is sufficient to augment burn-induced mortality and weight loss in vivo. At the organ level, increased lactate transport amplified the thermogenic activation of WAT and its associated wasting, thereby driving postburn hepatic lipotoxicity and dysfunction. Mechanistically, the thermogenic effects of lactate appeared to result from increased import through MCT transporters, which in turn increased intracellular redox pressure, [NADH/NAD+], and expression of the batokine, FGF21. In fact, pharmacological inhibition of MCT-mediated lactate uptake attenuated browning and improved hepatic function in mice after injury. Collectively, our findings identify a signaling role for lactate that impacts multiple aspects of postburn hypermetabolism, necessitating further investigation of this multifaceted metabolite in trauma and critical illness.NEW & NOTEWORTHY To our knowledge, this study was the first to investigate the role of lactate signaling in mediating white adipose tissue browning after burn trauma. We show that the induction of browning in both human burn patients and mice is positively correlated with a shift toward lactate import and metabolism. Daily l-lactate administration augments burn-induced mortality, browning, and hepatic lipotoxicity in vivo, whereas pharmacologically targeting lactate transport alleviates burn-induced browning and improves liver dysfunction after injury.

[Application and research progress of permissive hypocaloric nutrition in nutritional therapy of severe burns]

Nutritional therapy plays an important role in the treatment of severe burns. With the deepening understanding of metabolic patterns and body responses after severe burns, the concepts and measures of nutritional therapy are also constantly developing and improving. Permissive hypocaloric nutrition is a nutritional management approach for critically ill patients, which generally refers to a nutritional administration method in which energy intake is lower than 70% of caloric requirement. This article aims to review the metabolic characteristics after severe burns, as well as the implementation timing, duration, target calories, and nutritional content of permissive hypocaloric nutrition, in order to provide reference for clinical decision-making by clinical physicians, improve the efficacy of nutritional treatment for severe burn patients, and improve patients' prognosis.

Cardiometabolic disease risk markers are increased following burn injury in children

Introduction

Burn injury in children causes prolonged systemic effects on physiology and metabolism leading to increased morbidity and mortality, yet much remains undefined regarding the metabolic trajectory towards specific health outcomes.

Methods

A multi-platform strategy was implemented to evaluate the long-term immuno-metabolic consequences of burn injury combining metabolite, lipoprotein, and cytokine panels. Plasma samples from 36 children aged 4-8 years were collected 3 years after a burn injury together with 21 samples from non-injured age and sex matched controls. Three different ¹H Nuclear Magnetic Resonance spectroscopic experiments were applied to capture information on plasma low molecular weight metabolites, lipoproteins, and α-1-acid glycoprotein.

Results

Burn injury was characterized by underlying signatures of hyperglycaemia, hypermetabolism and inflammation, suggesting disruption of multiple pathways relating to glycolysis, tricarboxylic acid cycle, amino acid metabolism and the urea cycle. In addition, very low-density lipoprotein sub-components were significantly reduced in participants with burn injury whereas small-dense low density lipoprotein particles were significantly elevated in the burn injured patient plasma compared to uninjured controls, potentially indicative of modified cardiometabolic risk after a burn. Weighted-node Metabolite Correlation Network Analysis was restricted to the significantly differential features (q <0.05) between the children with and without burn injury and demonstrated a striking disparity in the number of statistical correlations between cytokines, lipoproteins, and small molecular metabolites in the injured groups, with increased correlations between these groups.

Discussion

These findings suggest a 'metabolic memory' of burn defined by a signature of interlinked and perturbed immune and metabolic function. Burn injury is associated with a series of adverse metabolic changes that persist chronically and are independent of burn severity and this study demonstrates increased risk of cardiovascular disease in the long-term. These findings highlight a crucial need for improved longer term monitoring of cardiometabolic health in a vulnerable population of children that have undergone burn injury.

Human thermogenic adipose tissue

Human thermogenic adipose tissue has long been touted as a promising therapeutic target for obesity and its associated metabolic diseases. Here, we provide a brief overview of the current knowledge of in vivo human thermogenic adipose tissue metabolism. We explore the evidence provided by retrospective and prospective studies describing the association of brown adipose tissue (BAT) [¹⁸F]fluorodeoxyglucose accumulation and various cardiometabolic risk factors. Although these studies have been invaluable in generating hypothesis, it has also raised some questions about the reliability of this method as an indicator of BAT thermogenic capacity. We discuss the evidence in support of human BAT functioning as a local thermogenic organ and energy sink, as an endocrine organ, and as a biomarker of adipose tissue health.

Role of medicinal herbs and phytochemicals in post burn management

Burn management is a natural and distinctly programmed process involving overlapping phases of hemostasis, inflammation, proliferation and remodeling. Burn wound healing involves initiation of inflammation, re-epithelialization, granulation, neovascularization and wound contraction. Despite the availability of multiple preparations for management of burn wound, there is dire need for efficacious alternative agents. Current approaches for burn wound management include pharmaceutical agents and antibiotics. However, high cost of synthetic drugs and accelerated resistance to antibiotics is challenging for both developed and developing nations. Among alternative options, medicinal plants have been a biocompatible, safe and affordable source of preventive/curative approaches. Due to cultural acceptance and patient compliance, there has been a focus on the use of botanical drugs and phytochemicals for burn wound healing. Keeping in consideration of medicinal herbs and phytochemicals as suitable therapeutic/adjuvant agents for burn wound management, this review highlights therapeutic potential of 35 medicinal herbs and 10 phytochemicals. Among these, Elaeis guineensis, Ephedra ciliate and Terminalia avicennioides showed better burn wound healing potential with varied mechanisms such as modulation of TNF-alpha, inflammatory cytokines, nitric oxide, eicosanoids, ROS and leukocyte response. Phytochemicals (oleanolic acid, ursolic acid, kirenol) also showed promising role in burn wound management though various pathways involving such as down regulation of TNF-alpha, IL-6 and inflammatory mediators including plasma proteases and arachidonic acid metabolites. This review provides a pavement for therapeutic/adjuvant use of potential botanical drugs and novel druggable phyto-compounds to target skin burn injury with diverse mechanisms, affordability and safety profile.

Adipose tissue at single-cell resolution

Adipose tissue exhibits remarkable plasticity with capacity to change in size and cellular composition under physiological and pathophysiological conditions. The emergence of single-cell transcriptomics has rapidly transformed our understanding of the diverse array of cell types and cell states residing in adipose tissues and has provided insight into how transcriptional changes in individual cell types contribute to tissue plasticity. Here, we present a comprehensive overview of the cellular atlas of adipose tissues focusing on the biological insight gained from single-cell and single-nuclei transcriptomics of murine and human adipose tissues. We also offer our perspective on the exciting opportunities for mapping cellular transitions and crosstalk, which have been made possible by single-cell technologies.

Effect of a collagen-enriched beverage with or without omega-3 fatty acids on wound healing, metabolic biomarkers, and adipokines in patients with major burns

BACKGROUND & AIMS

This study investigated the effects of collagen hydrolysate and omega-3 fatty acids (FAs) on the rate and quality of wound healing, metabolic disorders, and adipose-derived peptides in patients with major burns.

METHODS

In this randomized clinical trial, 66 patients with 20-45% deep partial or full-thickness burns were randomly assigned to three groups to receive either a beverage containing collagen (40 gr/d), collagen (40 gr/d) plus 3 gr/d omega-3 (ω-3) FAs, or placebo for four weeks. Wound healing rate, Vancouver scar scale (VSS), as well as baseline, weeks two and three serum concentrations of adiponectin, fibroblast growth factor 21 (FGF21), neuregulin 4 (NRG4), transforming growth factor (TGF)-β1, and pre-albumin/hs-CRP ratio were assessed.

RESULTS

The wound healing rate during the weeks post-burn (p = 0.006 and p = 0.01), and days of 95% (21.3 ± 6.8 and 22.9 ± 8.7 vs. 34.3 ± 14.8 days, p = 0.003 and p = 0.03) and complete (26 ± 7.7 and 27.4 ± 9.4 vs. 41.1 ± 16.6 days, p = 0.003 and p = 0.01) wound healing were significantly better with Collagen and Collagen. ω-3 compared to the placebo group. The VSS was significantly lower, indicated better scar status, in the both intervention groups compared to the placebo (p = 0.02 and p = 0.01). Wound healing outcomes were not statistically different between the Collagen and Collagen. ω-3 groups. Hs-CRP/pre-albumin ratio was significantly lower in the Collagen. ω-3 than the placebo group at week three (1.2 ± 1.9 vs. 4.8 ± 7.7 dl/l, p = 0.03). The significant decrease in serum adiponectin seen during the trial course within the placebo (10 ± 8.8 to 5.8 ± 4.9 mg/l, p = 0.03) and Collagen (11.8 ± 14 to 8.6 ± 11.7 mg/l, p = 0.03) groups was prevented in the Collagen. ω-3 group (p = 0.4). Circulating FGF21 decreased significantly within the Collagen (p = 0.005) and Collagen. ω-3 (p = 0.02) groups at the end of week three compared to the baseline.

CONCLUSIONS

Adding collagen hydrolysate as part of adjunctive therapy improved wound healing rate and quality. These findings as well as the efficacy of omega-3 FAs need to be further confirmed in larger populations. This study was registered with the Iranian Registry of Clinical Trials (IRCT20090901002394N42).

Unveiling the Role of the Proton Gateway, Uncoupling Proteins (UCPs), in Cancer Cachexia

Uncoupling proteins (UCPs) are identified as carriers of proton ions between the mitochondrial inner membrane and the mitochondrial matrix. ATP is mainly generated through oxidative phosphorylation in mitochondria. The proton gradient is generated across the inner mitochondrial membrane and the mitochondrial matrix, which facilitates a smooth transfer of electrons across ETC complexes. Until now, it was thought that the role of UCPs was to break the electron transport chain and thereby inhibit the synthesis of ATP. UCPs allow protons to pass from the inner mitochondrial membrane to the mitochondrial matrix and decrease the proton gradient across the membrane, which results in decreased ATP synthesis and increased production of heat by mitochondria. In recent years, the role of UCPs in other physiological processes has been deciphered. In this review, we first highlighted the different types of UCPs and their precise location across the body. Second, we summarized the role of UCPs in different diseases, mainly metabolic disorders such as obesity and diabetes, cardiovascular complications, cancer, wasting syndrome, neurodegenerative diseases, and kidney complications. Based on our findings, we conclude that UCPs play a major role in maintaining energy homeostasis, mitochondrial functions, ROS production, and apoptosis. Finally, our findings reveal that mitochondrial uncoupling by UCPs may treat many diseases, and extensive clinical studies are required to meet the unmet need of certain diseases.

Burns: Classification, Pathophysiology, and Treatment: A Review

Burns and their treatment are a significant medical problem. The loss of the physical barrier function of the skin opens the door to microbial invasion and can lead to infection. The repair process of the damage caused by the burn is impaired due to the enhanced loss of fluids and minerals through the burn wound, the onset of hypermetabolism with the concomitant disruption of nutrient supply, and derangements in the endocrine system. In addition, the initiated inflammatory and free radical processes drive the progression of oxidative stress, the inhibition of which largely depends on an adequate supply of antioxidants and minerals. Clinical experience and research provide more and more data to make the treatment of patients with thermal injury increasingly effective. The publication discusses disorders occurring in patients after thermal injury and the methods used at various stages of treatment.

Adipose Tissue Remodeling in Pathophysiology

Rather than serving as a mere onlooker, adipose tissue is a complex endocrine organ and active participant in disease initiation and progression. Disruptions of biological processes operating within adipose can disturb healthy systemic physiology, the sequelae of which include metabolic disorders such as obesity and type 2 diabetes. A burgeoning interest in the field of adipose research has allowed for the elucidation of regulatory networks underlying both adipose tissue function and dysfunction. Despite this progress, few diseases are treated by targeting maladaptation in the adipose, an oft-overlooked organ. In this review, we elaborate on the distinct subtypes of adipocytes, their developmental origins and secretory roles, and the dynamic interplay at work within the tissue itself. Central to this discussion is the relationship between adipose and disease states, including obesity, cachexia, and infectious diseases, as we aim to leverage our wealth of knowledge for the development of novel and targeted therapeutics.

Astragalus polysaccharide prevents heart failure-induced cachexia by alleviating excessive adipose expenditure in white and brown adipose tissue

BACKGROUND

Astragalus polysaccharide (APS) is a key active ingredient isolated from Astragalus membranaceus that has been reported to be a potential treatment for obesity and diabetes by regulating lipid metabolism and adipogenesis, alleviating inflammation, and improving insulin resistance. However, whether APS regulates lipid metabolism in the context of cachexia remains unclear. Therefore, this study analysed the effects of APS on lipid metabolism and adipose expenditure in a heart failure (HF)-induced cardiac cachexia rat model.  METHODS: A salt-sensitive hypertension-induced cardiac cachexia rat model was used in the present study. Cardiac function was detected by echocardiography. The histological features and fat droplets in fat tissue and liver were observed by H&E staining and Oil O Red staining. Immunohistochemical staining, Western blotting and RT‒qPCR were used to detect markers of lipolysis and adipose browning in white adipose tissue (WAT) and thermogenesis in brown adipose tissue (BAT). Additionally, sympathetic nerve activity and inflammation in adipose tissue were detected.

RESULTS

Rats with HF exhibited decreased cardiac function and reduced adipose accumulation as well as adipocyte atrophy. In contrast, administration of APS not only improved cardiac function and increased adipose weight but also prevented adipose atrophy and FFA efflux in HF-induced cachexia. Moreover, APS inhibited HF-induced lipolysis and browning of white adipocytes since the expression levels of lipid droplet enzymes, including HSL and perilipin, and beige adipocyte markers, including UCP-1, Cd137 and Zic-1, were suppressed after administration of APS. In BAT, treatment with APS inhibited PKA-p38 MAPK signalling, and these effects were accompanied by decreased thermogenesis reflected by decreased expression of UCP-1, PPAR-γ and PGC-1α and reduced FFA β-oxidation in mitochondria reflected by decreased Cd36, Fatp-1 and Cpt1. Moreover, sympathetic nerve activity and interleukin-6 levels were abnormally elevated in HF rats, and astragalus polysaccharide could inhibit their activity.

CONCLUSION

APS prevented lipolysis and adipose browning in WAT and decreased BAT thermogenesis. These effects may be related to suppressed sympathetic activity and inflammation. This study provides a potential approach to treat HF-induced cardiac cachexia.

The sympathetic nervous system in the 21st century: Neuroimmune interactions in metabolic homeostasis and obesity

The sympathetic nervous system maintains metabolic homeostasis by orchestrating the activity of organs such as the pancreas, liver, and white and brown adipose tissues. From the first renderings by Thomas Willis to contemporary techniques for visualization, tracing, and functional probing of axonal arborizations within organs, our understanding of the sympathetic nervous system has started to grow beyond classical models. In the present review, we outline the evolution of these findings and provide updated neuroanatomical maps of sympathetic innervation. We offer an autonomic framework for the neuroendocrine loop of leptin action, and we discuss the role of immune cells in regulating sympathetic terminals and metabolism. We highlight potential anti-obesity therapeutic approaches that emerge from the modern appreciation of SNS as a neural network vis a vis the historical fear of sympathomimetic pharmacology, while shifting focus from post- to pre-synaptic targeting. Finally, we critically appraise the field and where it needs to go.

Browning of the white adipose tissue regulation: new insights into nutritional and metabolic relevance in health and diseases

Adipose tissues are dynamic tissues that play crucial physiological roles in maintaining health and homeostasis. Although white adipose tissue and brown adipose tissue are currently considered key endocrine organs, they differ functionally and morphologically. The existence of the beige or brite adipocytes, cells displaying intermediary characteristics between white and brown adipocytes, illustrates the plastic nature of the adipose tissue. These cells are generated through white adipose tissue browning, a process associated with augmented non-shivering thermogenesis and metabolic capacity. This process involves the upregulation of the uncoupling protein 1, a molecule that uncouples the respiratory chain from Adenosine triphosphate synthesis, producing heat. β-3 adrenergic receptor system is one important mediator of white adipose tissue browning, during cold exposure. Surprisingly, hyperthermia may also induce beige activation and white adipose tissue beiging. Physical exercising copes with increased levels of specific molecules, including Beta-Aminoisobutyric acid, irisin, and Fibroblast growth factor 21 (FGF21), which induce adipose tissue browning. FGF21 is a stress-responsive hormone that interacts with beta-klotho. The central roles played by hormones in the browning process highlight the relevance of the individual lifestyle, including circadian rhythm and diet. Circadian rhythm involves the sleep-wake cycle and is regulated by melatonin, a hormone associated with UCP1 level upregulation. In contrast to the pro-inflammatory and adipose tissue disrupting effects of the western diet, specific food items, including capsaicin and n-3 polyunsaturated fatty acids, and dietary interventions such as calorie restriction and intermittent fasting, favor white adipose tissue browning and metabolic efficiency. The intestinal microbiome has also been pictured as a key factor in regulating white tissue browning, as it modulates bile acid levels, important molecules for the thermogenic program activation. During embryogenesis, in which adipose tissue formation is affected by Bone morphogenetic proteins that regulate gene expression, the stimuli herein discussed influence an orchestra of gene expression regulators, including a plethora of transcription factors, and chromatin remodeling enzymes, and non-coding RNAs. Considering the detrimental effects of adipose tissue browning and the disparities between adipose tissue characteristics in mice and humans, further efforts will benefit a better understanding of adipose tissue plasticity biology and its applicability to managing the overwhelming burden of several chronic diseases.

Enhancing the accuracy of surgical wound excision following burns trauma via application of Rapid Evaporative IonisationMass Spectrometry (REIMS)

BACKGROUND

Surgical wound excision is a necessary procedure for burn patients that require the removal of eschar. The extent of excision is currently guided by clinical judgement, with excessinto healthy tissue potentially leading to excessive scar, or inadequate debridement increasing risk of infection. Thus, an objective real-time measure to facilitate accurate excision could support clinical judgement and improve this surgical procedure. This study was designed to investigate the potential use of Rapid evaporative ionisation mass spectrometry (REIMS) as a tool to support data-driven objective tissue excision.

METHODS

Data were acquired using a multi-platform approach that consisted of both Rapid Evaporative Ionisation Mass Spectrometry (REIMS) performed on intact skin, and comprehensive liquid chromatography-mass spectrometry (LC-MS/MS) lipidomics performed on homogenised skin tissue extracts. Data were analysed using principal components analysis (PCA) and multivariate orthogonal projections to latent squares discriminant analysis (OPLS-DA) and logistic regression to determine the predictability of the models.

RESULTS

PCA and OPLS-DA models of the REIMS and LC-MS/MS lipidomics data reported separation of excised and healthy tissue. Molecular fingerprints generated from REIMS analysis of healthy skin tissue revealed a high degree of heterogeneity, however, intra-individual variance was smaller than inter-individual variance. Both platforms indicated high levels of skin classification accuracy. In addition, OPLS-DA of the LC-MS/MS lipidomic data revealed significant differences in specific lipid classes between healthy control and excised skin samples; including lower free fatty acids (FFA), monoacylglycerols (MAG), lysophosphatidylglycerol (LPG) and lysophosphatidylethanolamines (LPE) in excised tissue and higher lactosylceramides (LCER) and cholesterol esters (CE) compared to healthy control tissue.

CONCLUSIONS

Having established the heterogeneity in the biochemical composition of healthy skin using REIMS and LC-MS/MS, our data show that REIMS has the potential to distinguish between excied and healthy skin tissue samples. This pilot study suggests that REIMS may be an effective tool to support accurate tissue excision during burn surgery.

Angiotensin II Promotes White Adipose Tissue Browning and Lipolysis in Mice

Emerging evidence has revealed that all components of the renin-angiotensin system (RAS) are present in adipose tissue. Angiotensin II (Ang II), the major bioactive component of the RAS, has been recognized as an adipokine involved in regulating energy homeostasis. However, the precise role of Ang II in white adipose tissue (WAT) remodeling remains to be elucidated. In this present study, C57BL/C male mice were continuously infused with different doses of Ang II (1.44 mg/kg/d or 2.5 mg/kg/d) or saline for 2 weeks and treated with or without the Ang II type 1 receptor blocker valsartan. H&E staining and immunohistochemistry were conducted to investigate the white-to-brown fat conversion. The level of serum total cholesterol (TC), triglyceride (TG), low-density lipoprotein cholesterol (LDL-C), and high-density lipoprotein cholesterol (HDL-C) was measured. RNA sequencing was employed to explore the differentially expressed genes and their enriched pathways between control and Ang II groups. Our results showed that Ang II substantially resulted in loss of body weight and fat mass. Most importantly, Ang II treatment induced WAT browning in mice, which was partially attenuated by valsartan treatment. Furthermore, Ang II perturbed the serum lipid profiles. Ang II treatment elevated serum levels of TC, TG, LDL-C, and HDL-C in mice. Mechanistically, thermogenesis, cell respiration, and lipid metabolism-associated mRNAs showed significantly increased expression profiling in Ang II-treated WATs compared with control WATs. Moreover, we found that Ang II treatment enhanced AMPK phosphorylation in adipocytes. Therefore, Ang II promotes WAT browning and lipolysis via activating the AMPK signaling pathway.

The involvement of the adrenergic nervous system in activating human brown adipose tissue and browning

Obesity is a chronic condition of multifactorial etiology characterized by excessive body fat due to a calorie intake higher than energy expenditure. Given the intrinsic limitations of surgical interventions and the difficulties associated with lifestyle changes, pharmacological manipulation is currently one of the main therapies for metabolic diseases. Approaches aiming to promote energy expenditure through induction of thermogenesis have been explored and, in this context, brown adipose tissue (BAT) activation and browning have been shown to be promising strategies. Although such processes are physiologically stimulated by the sympathetic nervous system, not all situations that are known to increase adrenergic signaling promote a concomitant increase in BAT activation or browning in humans. Thus, a better understanding of factors involved in the thermogenesis attributed to these tissues is needed to enable the development of future therapies against obesity. Herein we carry out a critical review of original articles in humans under conditions previously known to trigger adrenergic responses-namely, cold, catecholamine-secreting tumor (pheochromocytoma and paraganglioma), burn injury, and adrenergic agonists-and discuss which of them are associated with increased BAT activation and browning. BAT is clearly stimulated in individuals exposed to cold or treated with high doses of the β3-adrenergic agonist mirabegron, whereas browning is certainly induced in patients after burn injury or with pheochromocytoma, as well as in individuals treated with β3-adrenergic agonist mirabegron for at least 10 weeks. Given the potential effect of increasing energy expenditure, adrenergic stimuli are promising strategies in the treatment of metabolic diseases.

Beige Adipocyte as the Flame of White Adipose Tissue: Regulation of Browning and Impact of Obesity

Beige adipocyte, the third and relatively new type of adipocyte, can emerge in white adipose tissue (WAT) under thermogenic stimulations that is termed as browning of WAT. Recent studies suggest that browning of WAT deserves more attention and therapies targeting browning of WAT can be helpful for reducing obesity. Beyond the major inducers of browning, namely cold and β 3-adrenergic stimulation, beige adipocytes are affected by several factors, and excess adiposity per se may also influence the browning process. The objective of the present review is to provide an overview of recent clinical and preclinical studies on the hormonal and nonhormonal factors that affect the browning of WAT. This review further focuses on the role of obesity per se on browning process.

Aging Impairs the Cellular Interplay between Myeloid Cells and Mesenchymal Cells during Skin Healing in Mice

Impaired wound healing is a major issue in the elderly population and is associated with substantial health and economic burden, which is exponentially increasing with the growing aging population. While the underlying pathobiology of disturbed skin healing by aging is linked to several genetic and epigenetic factors, little is known about the cell-cell interaction during the wound healing process in aged individuals, particularly the mesenchymal stem cell (MSCs)-macrophages axis. In this study, by using a thermal injury animal model in which we compared the wound healing process of adult and young mice, we found that the insufficient pool of MSCs in adult animals are deficient in migrating to the wound bed and instead are restricted to the wound edge. We identified a deficiency of a CD90-positive MSC subpopulation in the wounds of adult animals, which is positively correlated with the number of F4/80+ macrophages. In vitro, we found that CD90+ cells preferentially adhere to the myeloid cells forming doublet cells. Thus, our findings highlight that in adult mice subjected to a thermal injury, impaired wound healing is likely mediated by a disturbed cellular interplay between myeloid cells and mesenchymal cells.

Local hyperthermia therapy induces browning of white fat and treats obesity

Beige fat plays key roles in the regulation of systemic energy homeostasis; however, detailed mechanisms and safe strategy for its activation remain elusive. In this study, we discovered that local hyperthermia therapy (LHT) targeting beige fat promoted its activation in humans and mice. LHT achieved using a hydrogel-based photothermal therapy activated beige fat, preventing and treating obesity in mice without adverse effects. HSF1 is required for the effects since HSF1 deficiency blunted the metabolic benefits of LHT. HSF1 regulates Hnrnpa2b1 (A2b1) transcription, leading to increased mRNA stability of key metabolic genes. Importantly, analysis of human association studies followed by functional analysis revealed that the HSF1 gain-of-function variant p.P365T is associated with improved metabolic performance in humans and increased A2b1 transcription in mice and cells. Overall, we demonstrate that LHT offers a promising strategy against obesity by inducing beige fat activation via HSF1-A2B1 transcriptional axis.

Integrating adipocyte insulin signaling and metabolism in the multi-omics era

Insulin stimulates glucose uptake into adipocytes via mTORC2/AKT signaling and GLUT4 translocation and directs glucose carbons into glycolysis, glycerol for TAG synthesis, and de novo lipogenesis. Adipocyte insulin resistance is an early indicator of type 2 diabetes in obesity, a worldwide health crisis. Thus, understanding the interplay between insulin signaling and central carbon metabolism pathways that maintains adipocyte function, blood glucose levels, and metabolic homeostasis is critical. While classically viewed through the lens of individual enzyme-substrate interactions, advances in mass spectrometry are beginning to illuminate adipocyte signaling and metabolic networks on an unprecedented scale, yet this is just the tip of the iceberg. Here, we review how 'omics approaches help to elucidate adipocyte insulin action in cellular time and space.

An immune-sympathetic neuron communication axis guides adipose tissue browning in cancer-associated cachexia

Cancer-associated cachexia (CAC) is a hypermetabolic syndrome characterized by unintended weight loss due to the atrophy of adipose tissue and skeletal muscle. A phenotypic switch from white to beige adipocytes, a phenomenon called browning, accelerates CAC by increasing the dissipation of energy as heat. Addressing the mechanisms of white adipose tissue (WAT) browning in CAC, we now show that cachexigenic tumors activate type 2 immunity in cachectic WAT, generating a neuroprotective environment that increases peripheral sympathetic activity. Increased sympathetic activation, in turn, results in increased neuronal catecholamine synthesis and secretion, β-adrenergic activation of adipocytes, and induction of WAT browning. Two genetic mouse models validated this progression of events. 1) Interleukin-4 receptor deficiency impeded the alternative activation of macrophages, reduced sympathetic activity, and restrained WAT browning, and 2) reduced catecholamine synthesis in peripheral dopamine β-hydroxylase (DBH)-deficient mice prevented cancer-induced WAT browning and adipose atrophy. Targeting the intraadipose macrophage-sympathetic neuron cross-talk represents a promising therapeutic approach to ameliorate cachexia in cancer patients.

Lytic cocktail: An effective method to alleviate severe burn induced hyper-metabolism through regulating white adipose tissue browning

Background

Browning of white adipose tissue is associated with elevated resting metabolic rates and is considered to be one of the indispensable causes of hypermetabolism in burn patients. Hypermetabolism means increased resting energy expenditure, raised body temperature and acute-phase proteins. Persistently elevated levels of circulating stress hormones have been reported to induce browning of subcutaneous white adipose tissue. The lytic cocktail is a combination of medicines pethidine, chlorpromazine, and promethazine that has been used clinically in sedation for the management of patients. As reported this sedative treatment can reduce the expression of catecholamines in major burn rats. Thus, in this paper we focused on the effects of lytic cocktail in the regulation of white adipose tissue browning and hypermetabolism and we further investigated the underlying mechanism.

Methods

A 30% total body surface area (TBSA) Ⅲ degree scald rat model was used for this study. The rats were randomly divided into a sham scald group, a scalding with immediate resuscitation group, and a group of scalding with immediate resuscitation and lytic cocktail treatment. The levels of norepinephrine and epinephrine in plasma were dynamically detected. Changes of the rat body weight and food intake were recorded and compared as indexes of metabolism responses after post-scalding. For the study of white adipose tissue browning, inguinal adipose tissue was used. Metabolic changes, while indicatives of white fat browning were measured by PET/CT. The expression of white adipose browning related proteins and the changes of mitochondria number were used to assess browning of inguinal adipose.

Results

The level of plasma catecholamines norepinephrine and epinephrine in the lytic cocktail-treated group was significantly lower than the other two groups. Morphology and PET/CT showed that the inguinal white adipose browning was inhibited in the lytic cocktail treated group, whereas scalding with immediate resuscitation group showed browning of white adipose. The number of mitochondria, the expressions of white adipose browning related proteins in the lytic cocktail group were also significantly lower than that of the group of scalding with immediate resuscitation.

Conclusion

By reducing expression of heat-related proteins, the application of lytic cocktail medicines inhibits the white adipose tissue browning, which suppresses hypermetabolism in scalded rats. The mechanism might be related to decreased expression levels of stress hormones induced by lytic cocktail. This research suggests that lytic cocktails may be an effective treatment for hypermetabolism after severe burn injury.

Current understanding of thermo(dys)regulation in severe burn injury and the pathophysiological influence of hypermetabolism, adrenergic stress and hypothalamic regulation—a systematic review

Background

In this systematic review, we summarize the aetiology as well as the current knowledge regarding thermo(dys)regulation and hypothermia after severe burn trauma and aim to present key concepts of pathophysiology and treatment options. Severe burn injuries with >20% total body surface area (TBSA) affected commonly leave the patient requiring several surgical procedures, prolonged hospital stays and cause substantial changes to body composition and metabolism in the acute and long-term phase. Particularly in severely burned patients, the loss of intact skin and the dysregulation of peripheral and central thermoregulatory processes may lead to substantial complications.

Methods

A systematic and protocol-based search for suitable publications was conducted following the PRISMA guidelines. Articles were screened and included if deemed eligible. This encompasses animal-based in vivo studies as well as clinical studies examining the control-loops of thermoregulation and metabolic stability within burn patients

Results

Both experimental animal studies and clinical studies examining thermoregulation and metabolic functions within burn patients have produced a general understanding of core concepts which are, nonetheless, lacking in detail. We describe the wide range of pathophysiological alterations observed after severe burn trauma and highlight the association between thermoregulation and hypermetabolism as well as the interactions between nearly all organ systems. Lastly, the current clinical standards of mitigating the negative effects of thermodysregulation and hypothermia are summarized, as a comprehensive understanding and implementation of the key concepts is critical for patient survival and long-term well-being.

Conclusions

The available in vivo animal models have provided many insights into the interwoven pathophysiology of severe burn injury, especially concerning thermoregulation. We offer an outlook on concepts of altered central thermoregulation from non-burn research as potential areas of future research interest and aim to provide an overview of the clinical implications of temperature management in burn patients.

Small animal models of thermal injury

Burns are a severe form of trauma that account for 1.1 million cases necessitating medical attention and 4500 mortalities annually in the United States alone. Importantly, the initial trauma is succeeded by extensive, prolonged physiological alterations that detrimentally impact multiple organ systems. Given the complexity of post-burn pathophysiology, in vitro experiments are insufficient to model thermal injuries. Therefore, compatible animal burn models are essential for studying burn-related phenomena. In this chapter, we discuss commonly employed small animal burn models and their comparability and applicability to human studies. In particular, we compare post-burn wound healing between the species as well as relevant hypermetabolic and inflammatory characteristics, providing a better understanding of the pros and cons of utilizing a small animal surrogate for human burns. We further provide an overview of the rodent scald burn model methodology as well as a comparison between elderly, aged and young animals, providing a guide for tailoring animal model choice based on the relevant research question.

Brown and beige adipose tissue: a novel therapeutic strategy for obesity and type 2 diabetes mellitus

Mammalian adipose tissue can be divided into two major types, namely, white adipose tissue (WAT) and brown adipose tissue (BAT). According to classical view, the main function of WAT is to store excess energy in the form of triglycerides, while BAT is a thermogenic tissue that acts a pivotal part in maintaining the core body temperature. White adipocytes display high plasticity and can transdifferentiate into beige adipocytes which have many similar morphological and functional properties with brown adipocytes under the stimulations of exercise, cold exposure and other factors. This phenomenon is also known as 'browning of WAT'. In addition to transdifferentiation, beige adipocytes can also come from de novo differentiation from tissue-resident progenitors. Activating BAT and inducing browning of WAT can accelerate the intake of glycolipids and reduce the insulin secretion requirement, which may be a new strategy to improve glycolipids metabolism and insulin resistance of obese and type 2 diabetes mellitus (T2DM) patients. This review mainly discusses the significance of brown and beige adipose tissues in the treatment of obesity and T2DM, and focuses on the effect of the browning agent on obesity and T2DM, which provides a brand-new theoretical reference for the prevention and treatment of obesity and T2DM.

Thermal Stress Induces Long-Term Remodeling of Adipose Tissue and Is Associated with Systemic Dysfunction

ABSTRACT

Severe burns are characterized by the magnitude and duration of the hypermetabolic response thereafter, and demarcated by the loss of lean body mass and catabolism of fat stores. The aim of the present study was to delineate the temporal and location-specific physiological changes to adipose depots and downstream consequences post-burn in a murine model of thermal injury. C57BL/6 mice were subjected to a 30% total body surface area burn and body mass, food intake, and tissue mass were monitored for various time points up until 60 days postinjury. Mitochondrial respirometry was performed using a Seahorse XF96 analyzer. Lipolytic markers and browning markers were analyzed via Western blotting and histology. A severe burn results in a futile cycle of lipolysis and white adipose tissue (WAT) browning, the sequelae of which include fat catabolism, hepatomegaly, and loss of body mass despite increased food intake. A dynamic remodeling of epididymal WAT was observed with acute and chronic increases in lipolysis. Moreover, we demonstrate that pathological browning of inguinal WAT persists up to 60 days post-burn, highlighting the magnitude of the β-adrenergic response to thermal injury. Our data suggests that adipose depots have a heterogeneous response to burns and that therapeutic interventions targeting these physiological changes can improve outcomes. These data may also have implications for treating catabolic conditions such as cancer cachexia as well as developing treatments for obesity and type II diabetes.