Alternative Mechanism for White Adipose Tissue Lipolysis after Thermal Injury

Metabolic response to burn injury: a comprehensive bibliometric study

Objective

Burns lead to systemic changes manifested by systemic disturbances in water-electrolyte balance and systemic metabolic and inflammatory responses. The hypermetabolic response after a burn injury relies on metabolic, hormonal, and inflammatory dysregulation mechanisms. This study aimed to provide a comprehensive bibliometric analysis of the burn metabolism research field, identifying key trends, influential contributors, and emerging research hotspots to inform future investigative efforts. Ultimately, we conducted an extensive review of the literature, synthesizing the findings to clarify the present understanding within our field of study.

Methods

We obtained 8,823 scientific publications on burn injury and metabolism from the core Web of Science (WOS) database collection. In this work, biblioshiny was used to visualize and analyze the data, and VOSviewer was used to verify the results.

Results

From a total of 8,823 publications, we found a general upward trend in annual publications and citation frequency. According to Bradford's Law, 21 high-production journals were classified as core sources based on the number of publications, and the most productive journal was Burns. The most published countries and authors in this field were the United States and Herndon DN. The most local cited document in this field was the article titled "Catecholamines: Mediator of the Hypermetabolic Response to Thermal Injury" authored by Wilmore DW. The thematic map showed that studies on injury, thermal injury, and sepsis were relatively mature. In contrast, research on metabolism, stress, and responses, and research on mortality, resistance, and management were less well-developed but were essential for the field.

Conclusion

Research on burns and metabolism is increasing. Based on the bibliometric analysis, our study summarized the complex interplay between burn-induced systemic metabolic alterations and inflammatory responses, emphasizing the significance of hypermetabolism and its management. The role of propranolol, insulin, oxandrolone, and nutritional interventions in modulating the hypermetabolic state was discussed. Additionally, our study underscored the challenges of managing sepsis and drug-resistant infections in burn patients as an important future area of research.

The NLRP3 inflammasome in burns: a novel potential therapeutic target

Burns are an underestimated serious injury negatively impacting survivors physically, psychologically and economically, and thus are a considerable public health burden. Despite significant advancements in burn treatment, many burns still do not heal or develop serious complications/sequelae. The nucleotide-binding oligomerization domain-like receptors (NLRs) family pyrin domain-containing 3 (NLRP3) inflammasome is a critical regulator of wound healing, including burn wound healing. A better understanding of the pathophysiological mechanism underlying the healing of burn wounds may help find optimal therapeutic targets to promote the healing of burn wounds, reduce complications/sequelae following burn, and maximize the restoration of structure and function of burn skin. This review aimed to summarize current understanding of the roles and regulatory mechanisms of the NLRP3 inflammasome in burn wound healing, as well as the preclinical studies of the involvement of NLRP3 inhibitors in burn treatment, highlighting the potential application of NLRP3-targeted therapy in burn wounds.

Tolerance to Glucose and Lipid High Metabolic Reactions After Burns in an Obese Rat Model

The goal of this study was to determine what effect obese body weight and a burn injury can have on the metabolism of glucose and lipids in rats. We used a 3 * 3 factorial model design to provide basic glucose and lipid metabolic data characterizing the interaction between different weight and burn injury groups. Two hundred Sprague Dawley rats were categorized into three weight groups (normal, overweight, obese) and then further divided into control, second-degree, and third-degree burn groups. Our model compared interactions between weight and burn injury factors according to the above groups. Blood glucose and lipid metabolism indicators were monitored on the 1st, 3rd, 7th, and 14th days after burn injury occurred, and burned skin and blood samples were collected for testing. Compared with the normal weight group, the overweight group's fast blood glucose, fast insulin, and homeostasis model assessment of insulin resistance were higher (P < .05), and FBG in the obese group was higher than the normal weight group (P < .05). Burn injuries combined with obese body weight had an interactive effect on fast blood glucose, fast insulin, and homeostasis model assessment of insulin resistance after burn injury (P < .05). Burn injury combined with obese body weight had an interaction on low-density lipoprotein cholesterol on the 3rd day after burn injury (P < .05). Burn injury combined with obese weight had no interaction on triglyceride, total cholesterol, and high-density lipoprotein cholesterol (P > .05). Rats in the overweight and obese weight groups were observed to develop an adaptation and tolerance to a higher metabolic rate after burn injuries occurred.

Multiorgan Metabolomics and Lipidomics Provide New Insights Into Fat Infiltration in the Liver, Muscle Wasting, and Liver-Muscle Crosstalk Following Burn Injury

Burn injury mediated hypermetabolic syndrome leads to increased mortality among severe burn victims, due to liver failure and muscle wasting. Metabolic changes may persist up to 2 years following the injury. Thus, understanding the underlying mechanisms of the pathology is crucially important to develop appropriate therapeutic approaches. We present detailed metabolomic and lipidomic analyses of the liver and muscle tissues in a rat model with a 30% body surface area burn injury located at the dorsal skin. Three hundred and thirty-eight of 1587 detected metabolites and lipids in the liver and 119 of 1504 in the muscle tissue exhibited statistically significant alterations. We observed excessive accumulation of triacylglycerols, decreased levels of S-adenosylmethionine, increased levels of glutamine and xenobiotics in the liver tissue. Additionally, the levels of gluconeogenesis, glycolysis, and tricarboxylic acid cycle metabolites are generally decreased in the liver. On the other hand, burn injury muscle tissue exhibits increased levels of acyl-carnitines, alpha-hydroxyisovalerate, ophthalmate, alpha-hydroxybutyrate, and decreased levels of reduced glutathione. The results of this preliminary study provide compelling observations that liver and muscle tissues undergo distinctly different changes during hypermetabolism, possibly reflecting liver-muscle crosstalk. The liver and muscle tissues might be exacerbating each other's metabolic pathologies, via excessive utilization of certain metabolites produced by each other.

Adipose tissue-liver crosstalk during pathologic changes caused by vinyl chloride metabolites in mice

Volatile organic compounds (VOCs), such as vinyl chloride (VC), can be directly toxic at high concentrations. However, we have shown that 'nontoxic' exposures to VC and its metabolite chloroethanol (CE) enhances experimental non-alcoholic fatty liver disease (NAFLD), suggesting an unpredicted interaction. Importantly, VOC exposure has been identified as a potential risk factor for the development of obesity and its sequelae in humans. As there is a known axis between adipose and hepatic tissue in NAFLD, the impact of CE on white adipose tissue (WAT) inflammation and lipolysis was investigated. Mice were administered CE (or vehicle) once, after 10 weeks of being fed high-fat or low-fat diet (LFD). CE significantly enhanced hepatic steatosis and inflammation caused by HFD. HFD significantly increased the size of epididymal fat pads, which was enhanced by CE. The relative size of adipocyte lipid droplets increased by HFD + CE, which was also correlated with increased expression of lipid-associated proteins (e.g., PLINs). CE also enhanced HFD-induced indices of WAT inflammation, and ER stress. Hepatic-derived circulating FGF21, a major modulator of WAT lipolysis, which is hypothesized to thereby regulate hepatic steatosis, was significantly increased by CE in animals fed HFD. Taken together these data support the hypothesis that environmental toxicant exposure can exacerbate the severity of NAFLD/NASH, involving the liver-adipose axis in this process. Specifically, CE enhances local inflammation and alters lipid metabolism and WAT-mediated hepatic steatosis due to changes in WAT lipolysis.

Inhibition of Lipolysis With Acipimox Attenuates Postburn White Adipose Tissue Browning and Hepatic Fat Infiltration

Extensive burn injuries promote an increase in the lipolysis of white adipose tissue (WAT), a complication that enhances postburn hypermetabolism contributing to hyperlipidemia and hepatic steatosis. The systemic increase of free fatty acids (FFAs) due to burn-induced lipolysis and subsequent organ fatty infiltration may culminate in multiple organ dysfunction and, ultimately, death. Thus, reducing WAT lipolysis to diminish the mobilization of FFAs may render an effective means to improve outcomes postburn. Here, we investigated the metabolic effects of Acipimox, a clinically approved drug that suppresses lipolysis via inhibition of hormone-sensitive lipase (HSL). Using a murine model of thermal injury, we show that specific inhibition of HSL with Acipimox effectively suppresses burn-induced lipolysis in the inguinal WAT leading to lower levels of circulating FFAs at 7 days postburn (P < 0.05). The FFA substrate shortage indirectly repressed the thermogenic activation of adipose tissue after injury, reflected by the decrease in protein expression of key browning markers, UCP-1 (P < 0.001) and PGC-1α (P < 0.01). Importantly, reduction of FFA mobilization by Acipimox significantly decreased liver weight and intracellular fat accumulation (P < 0.05), suggesting that it may also improve organ function postburn. Our data validate the pharmacological inhibition of lipolysis as a potentially powerful therapeutic strategy to counteract the detrimental metabolic effects induced by burn.

Metformin alleviates muscle wasting post-thermal injury by increasing Pax7-positive muscle progenitor cells

BACKGROUND

Profound skeletal muscle wasting and weakness is common after severe burn and persists for years after injury contributing to morbidity and mortality of burn patients. Currently, no ideal treatment exists to inhibit muscle catabolism. Metformin is an anti-diabetic agent that manages hyperglycemia but has also been shown to have a beneficial effect on stem cells after injury. We hypothesize that metformin administration will increase protein synthesis in the skeletal muscle by increasing the proliferation of muscle progenitor cells, thus mitigating muscle atrophy post-burn injury.

METHODS

To determine whether metformin can attenuate muscle catabolism following burn injury, we utilized a 30% total burn surface area (TBSA) full-thickness scald burn in mice and compared burn injuries with and without metformin treatment. We examined the gastrocnemius muscle at 7 and 14 days post-burn injury.

RESULTS

At 7 days, burn injury significantly reduced myofiber cross-sectional area (CSA) compared to sham, p < 0.05. Metformin treatment significantly attenuated muscle catabolism and preserved muscle CSA at the sham size. To investigate metformin's effect on satellite cells (muscle progenitors), we examined changes in Pax7, a transcription factor regulating the proliferation of muscle progenitors. Burned animals treated with metformin had a significant increase in Pax7 protein level and the number of Pax7-positive cells at 7 days post-burn, p < 0.05. Moreover, through BrdU proliferation assay, we show that metformin treatment increased the proliferation of satellite cells at 7 days post-burn injury, p < 0.05.

CONCLUSION

In summary, metformin's various metabolic effects and its modulation of stem cells make it an attractive alternative to mitigate burn-induced muscle wasting while also managing hyperglycemia.

Increased proliferation of hepatic periportal ductal progenitor cells contributes to persistent hypermetabolism after trauma

Prolonged and persistent hypermetabolism and excessive inflammatory response after severe trauma is detrimental and associated with poor outcome. The predisposing pathology or signals mediating this complex response are essentially unknown. As the liver is the central organ mediating the systemic metabolic responses and considering that adult hepatic stem cells are on top of the hierarchy of cell differentiation and may pass epigenetic information to their progeny, we asked whether liver progenitor cells are activated, signal hypermetabolism upon post-traumatic cellular stress responses, and pass this to differentiated progeny. We generated Sox9CreERT2 : ROSA26 EYFP mice to lineage-trace the periportal ductal progenitor cells (PDPCs) and verify the fate of these cells post-burn. We observed increased proliferation of PDPCs and their progeny peaking around two weeks post-burn, concomitant with the hepatomegaly and the cellular stress responses. We then sorted out PDPCs, PDPC-derived hepatocytes and mature hepatocytes, compared their transcriptome and showed that PDPCs and their progeny present a significant up-regulation in signalling pathways associated with inflammation and metabolic activation, contributing to persistent hypermetabolic and hyper-inflammatory state. Furthermore, concomitant down-regulation of LXR signalling in PDPCs and their progeny implicates the therapeutic potential of early and short-term administration of LXR agonists in ameliorating such persistent hypermetabolism.

A Single-Blind Study Evaluating the Efficacy of Gold Nanoparticle Photothermal-Assisted Liposuction in an Ex Vivo Human Tissue Model

BACKGROUND

Liposuction is one of the most performed cosmetic surgery procedures. In a previously reported study, gold-nanoparticle (GNP) laser-assisted liposuction (NanoLipo) was shown to improve procedure parameters and outcomes in a porcine model.

OBJECTIVES

An ex vivo human liposuction model was developed to assess the ease, efficacy, and outcomes of NanoLipo, and to further explore its mechanism of action in facilitating liposuction.

METHODS

NanoLipo was compared to a control without GNPs in sets of fresh, nonperfused, anatomically symmetric, matched tissue specimens from 12 patients. A subset of three experiments was performed under single-blinded conditions. Intraoperative assessments included lipoaspirate volume, percentage of free oil, ease of removal, and temperature rise. Specimens were palpated, visualized for evenness, and graded with and without skin. Postoperative assessment included viability staining of the lipoaspirate and remaining tissues. Microcomputed tomography was used to assess the distribution of infused GNPs within the tissues.

RESULTS

NanoLipo consistently removed more adipose tissue with more liberated triglycerides compared to control. NanoLipo specimens were smoother, thinner, and had fewer and smaller irregularities. Infused solutions preferentially distributed between fibrous membranes and fat pearls. After NanoLipo, selective structural-tissue disruptions, indicated by loss of metabolic activity, were observed. Thus, NanoLipo likely creates a bimodal mechanism of action whereby fat lobules are dislodged from surrounding fibro-connective tissue, while lipolysis is simultaneously induced.

CONCLUSIONS

NanoLipo showed many advantages compared to control under blinded and nonblinded conditions. This technology may be promising in facilitating fat removal.

LEVEL OF EVIDENCE 5

Hepatic Amiodarone Lipotoxicity Is Ameliorated by Genetic and Pharmacological Inhibition of Endoplasmatic Reticulum Stress

Amiodarone is a commonly used antiarrhythmic drug and can cause liver steatosis. We investigated the role of endoplasmic reticulum (ER) stress/unfolded protein response in the pathogenesis of amiodarone-induced steatosis. Amiodarone-induced liver injury was obtained by 1 intraperitoneal injection to wild-type (WT) or C/EBP homologous protein knock-out mice (Ddit3-/-). Amiodarone directly reduced intracellular ATP and Ca2+ in hepatocytes invitro, inducing ER stress and lipid accumulation. In vivo, amiodarone-driven liver damage and lipid accumulation was accompanied by activation of ER stress/unfolded protein response, as demonstrated by up-regulation of genes encoding key ER stress mediators and by phosphorylation of eIF2α. In contrast to WT mice, Ddit3-/- mice were protected from amiodarone-induced ER stress and lipid accumulation. Importantly, amiodarone-induced lipid accumulation was not mediated by de novo hepatic lipogenesis, increased adipose tissue lipolysis or increased hepatic uptake of triglycerides or free fatty acids. Rather, amiodarone strongly increased hepatic mRNA expression of lipid droplet proteins, particularly Cidea and Cidec, in WT, but less so in Ddit3-/- mice, suggesting a link between ER stress and increased triglyceride storage. Moreover, while insulin attenuated amiodarone-induced phosphorylation of hormone sensitive lipase (HSL) in WT, it did not affect pHSL in Ddit3-/-, indicating increased lipolysis and therefore reduced lipid accumulation in these mice. Finally, ER stress attenuation using 2 different pharmacological chaperones reduced lipid accumulation, accompanied by reduced mRNA expression of Cidec. In conclusion, amiodarone-induced ER stress drives liver steatosis and may be considered for therapeutic targeting.

Accumulation of myeloid lineage cells is mapping out liver fibrosis post injury: a targetable lesion using Ketanserin

Liver fibrosis is problematic after persistent injury. However, little is known about its response to an acute insult. Accumulation of myeloid lineage cells contributes into the promotion and resolution of inflammation and fibrosis. Using Cre-transgenic mice that specifically mark myeloid lineage cells with EYFP and burn as a model of acute systemic injury, we investigated the role of myeloid lineage cells in the liver after acute injury. Our data show that thermal injury in mice (30% total body surface area) induces fibrosis predominantly around portal venules whereas myeloid cells are enriched throughout the liver. The fibrosis peaks around 1-2 weeks post injury and resolves by week 3. Ablating myeloid cells led to lower fibrosis. Through FACS sorting, we isolated myeloid lineage cells (EYFP +ve cells) from injured animals and from the control uninjured animals and subjected the extracted RNA from these cells to microarray analysis. Microarray analysis revealed an inflammatory signature for EYFP +ve cells isolated from injured animals in comparison with control cells. Moreover, it showed modulation of components of the serotonin (5-HT) pathway in myeloid cells. Antagonizing the 5HT2A/2C receptor decreased fibrosis in thermally injured mice by skewing macrophages away from their pro-fibrotic phenotype. Macrophages conditioned with Ketanserin showed a lower pro-fibrotic phenotype in a co-culture system with mesenchymal cells. There is a spatiotemporal pattern in liver fibrosis post-thermal injury, which is associated with the influx of myeloid cells. Treating mice with a 5HT2A/2C receptor antagonist promotes an anti-fibrotic effect, through modulating the phenotype of macrophages.

Hepatic steatosis associated with decreased β-oxidation and mitochondrial function contributes to cell damage in obese mice after thermal injury

Severely burned patients who are morbidly obese have poor clinical outcomes with aggravated metabolic consequences, a higher incidence of multiple organ dysfunction/failure, and significantly increased morbidity and mortality. The underlying mechanisms of these adverse outcomes are essentially unknown. Since the liver is one of the central metabolic organs, we hypothesized that thermal injury in obese patients leads to substantially increased lipolysis, hepatic fat infiltration, resulting in profound hepatic cellular and organellar alterations, consequently causing liver damage and severely augmented metabolic dysfunction. We tested this hypothesis using an obese mouse model subjected to a 20% total body surface area burn injury. C57BL/6 mice were randomly divided into low-fat diet (LFD) and high-fat diet (HFD) sham and burn groups (n = 6 per group) and fed for 16 weeks. 7 days after the thermal injury portal and cardiac blood were taken separately and liver tissue was collected for western blotting and immunohistochemical analysis. Gross examination of the liver showed apparent lipid infiltration in HFD fed and burned mice. We confirmed that augmented ER stress and inhibition of Akt-mTOR signaling dysregulated calcium homeostasis, contributed to the decrease of ER-mitochondria contact, and reduced mitochondrial β-oxidation in HFD fed and burned mice, leading to profound hepatic fat infiltration and substantial liver damage, hence increased morbidity and mortality. We conclude that obesity contributes to hepatic fat infiltration by suppressing β-oxidation, inducing cell damage and subsequent organ dysfunction after injury.

The response of muscle progenitor cells to cutaneous thermal injury

BACKGROUND

Severe burn results in a systemic response that leads to significant muscle wasting. It is believed that this rapid loss in muscle mass occurs due to increased protein degradation combined with reduced protein synthesis. Alterations in the microenvironment of muscle progenitor cells may partially account for this pathology. The aim of this study was to ascertain the response of muscle progenitor cells following thermal injury in mice and to enlighten the cellular cascades that contribute to the muscle wasting.

METHODS

C57BL/6 mice received a 20% total body surface area (TBSA) thermal injury. Gastrocnemius muscle was harvested at days 2, 7, and 14 following injury for protein and histological analysis.

RESULTS

We observed a decrease in myofiber cross-sectional area at 2 days post-burn. This muscle atrophy was compensated for by an increase in myofiber cross-sectional area at 7 and 14 days post-burn. Myeloperoxidase (MPO)-positive cells (neutrophils) increased significantly at 2 days. Moreover, through Western blot analysis of two key mediators of the proteolytic pathway, we show there is an increase in Murf1 and NF-κB 2 days post-burn. MPO-positive cells were also positive for NF-κB, suggesting that neutrophils attain NF-κB activity in the muscle. Unlike inflammatory and proteolytic pathways, the number of Pax7-positive muscle progenitor cells decreased significantly 2 days post-burn. This was followed by a recovery in the number of Pax7-positive cells at 7 and 14 days, suggesting proliferation of muscle progenitors that accompanied regrowth.

CONCLUSION

Our data show a biphasic response in the muscles of mice exposed to burn injury, with phenotypic characteristics of muscle atrophy at 2 days while compensation was observed later with a change in Pax7-positive muscle progenitor cells. Targeting muscle progenitors may be of therapeutic benefit in muscle wasting observed after burn injury.

Adipose tissue: between the extremes

Adipose tissue represents a critical component in healthy energy homeostasis. It fulfills important roles in whole-body lipid handling, serves as the body's major energy storage compartment and insulation barrier, and secretes numerous endocrine mediators such as adipokines or lipokines. As a consequence, dysfunction of these processes in adipose tissue compartments is tightly linked to severe metabolic disorders, including obesity, metabolic syndrome, lipodystrophy, and cachexia. While numerous studies have addressed causes and consequences of obesity-related adipose tissue hypertrophy and hyperplasia for health, critical pathways and mechanisms in (involuntary) adipose tissue loss as well as its systemic metabolic consequences are far less understood. In this review, we discuss the current understanding of conditions of adipose tissue wasting and review microenvironmental determinants of adipocyte (dys)function in related pathophysiologies.

An AMP-activated protein kinase-stabilizing peptide ameliorates adipose tissue wasting in cancer cachexia in mice

Cachexia represents a fatal energy-wasting syndrome in a large number of patients with cancer that mostly results in a pathological loss of skeletal muscle and adipose tissue. Here we show that tumor cell exposure and tumor growth in mice triggered a futile energy-wasting cycle in cultured white adipocytes and white adipose tissue (WAT), respectively. Although uncoupling protein 1 (Ucp1)-dependent thermogenesis was dispensable for tumor-induced body wasting, WAT from cachectic mice and tumor-cell-supernatant-treated adipocytes were consistently characterized by the simultaneous induction of both lipolytic and lipogenic pathways. Paradoxically, this was accompanied by an inactivated AMP-activated protein kinase (Ampk), which is normally activated in peripheral tissues during states of low cellular energy. Ampk inactivation correlated with its degradation and with upregulation of the Ampk-interacting protein Cidea. Therefore, we developed an Ampk-stabilizing peptide, ACIP, which was able to ameliorate WAT wasting in vitro and in vivo by shielding the Cidea-targeted interaction surface on Ampk. Thus, our data establish the Ucp1-independent remodeling of adipocyte lipid homeostasis as a key event in tumor-induced WAT wasting, and we propose the ACIP-dependent preservation of Ampk integrity in the WAT as a concept in future therapies for cachexia.

White Adipose Tissue Browning: A Double-edged Sword

The study of white adipose tissue (WAT) 'browning' has become a 'hot topic' in various acute and chronic metabolic conditions, based on the idea that WAT browning might be able to facilitate weight loss and improve metabolic health. However, this view cannot be translated into all areas of medicine. Recent studies identified effects of browning associated with adverse outcomes, and as more studies are being conducted, a very different picture has emerged about WAT browning and its detrimental effect in acute and chronic hypermetabolic conditions. Therefore, the notion that browning is supposedly beneficial may be inadequate. In this review we analyze how and why browning in chronic hypermetabolic associated diseases can be detrimental and lead to adverse outcomes.