NLRP3 Inflammasome Modulates Post-Burn Lipolysis and Hepatic Fat Infiltration via Fatty Acid Synthase

Role of pattern recognition receptors in the development of MASLD and potential therapeutic applications

Metabolic dysfunction-associated steatotic liver disease (MASLD) has become one of the most prevalent liver diseases worldwide, and its occurrence is strongly associated with obesity, insulin resistance (IR), genetics, and metabolic stress. Ranging from simple fatty liver to metabolic dysfunction-associated steatohepatitis (MASH), even to severe complications such as liver fibrosis and advanced cirrhosis or hepatocellular carcinoma, the underlying mechanisms of MASLD progression are complex and involve multiple cellular mediators and related signaling pathways. Pattern recognition receptors (PRRs) from the innate immune system, including Toll-like receptors (TLRs), C-type lectin receptors (CLRs), NOD-like receptors (NLRs), RIG-like receptors (RLRs), and DNA receptors, have been demonstrated to potentially contribute to the pathogenesis for MASLD. Their signaling pathways can induce inflammation, mediate oxidative stress, and affect the gut microbiota balance, ultimately resulting in hepatic steatosis, inflammatory injury and fibrosis. Here we review the available literature regarding the involvement of PRR-associated signals in the pathogenic and clinical features of MASLD, in vitro and in animal models of MASLD. We also discuss the emerging targets from PRRs for drug developments that involved agent therapies intended to arrest or reverse disease progression, thus enabling the refinement of therapeutic targets that can accelerate drug development.

Role of epidural fat in the local milieu: what we know and what we don't

PURPOSE

Traditionally, the epidural fat (EF) is known as a physical buffer for the dural sac against the force and a lubricant facilitating the relative motion of the latter on the osseous spine. Along with the development of the studies on EF, controversies still exist on vital questions, such as the underlying mechanism of the spinal epidural lipomatosis. Meanwhile, the scattered and fragmented researches hinder the global insight into the seemingly dispensable tissue.

METHODS

Herein, we reviewed literature on the EF and its derivatives to elucidate the dynamic change and complex function of EF in the local milieu, especially at the pathophysiological conditions. We start with an introduction to EF and the current pathogenic landscape, emphasizing the interlink between the EF and adjacent structures. We generally categorize the major pathological changes of the EF into hypertrophy, atrophy, and inflammation.

RESULTS AND CONCLUSIONS

It is acknowledged that not only the EF (or its cellular components) may be influenced by various endogenic/exogenic and focal/systematic stimuli, but the adjacent structures can also in turn be affected by the EF, which may be a hidden pathogenic clue for specific spinal disease. Meanwhile, the unrevealed sections, which are also the directions the future research, are proposed according to the objective result and rational inference. Further effort should be taken to reveal the underlying mechanism and develop novel therapeutic pathways for the relevant diseases.

NLRP3 Deficiency in Nonimmune Cells Averts Obesity-Induced Fatty Liver Disease

Obesity predisposes to metabolic dysfunction-associated fatty liver disease (MAFLD), cardiovascular disease, and type 2 diabetes. Accumulating evidence suggests a complex role of NLR family pyrin domain containing 3 (NLRP3) inflammasome function in multiple manifestations of the metabolic syndrome, with contradictory results. Its broad expression and pleiotropic functions during obesity led us to investigate the contribution of its expression in nonimmune versus immune cells to the development of obesity and MAFLD. Bone marrow chimerism was used to target NLRP3 deficiency to immune (ImmuneΔNlrp3) versus nonimmune (NonimmuneΔNlrp3) cells. Irradiated WT mice reconstituted with WT bone marrow served as controls. Mice were fed a 60% high-fat diet for 16 weeks. NonimmuneΔNlrp3 mice gained less weight and displayed reduced liver and epididymal white adipose tissue (epiWAT) mass. They also exhibited reduced adipocyte hypertrophy and increased epiWAT adipogenesis and lipolysis. Notable was the diminished hepatic steatosis in NonimmuneΔNlrp3 livers, which persisted even following equilibration of their body weight to that of the control. This was accompanied by a decline in liver triglycerides and in expression of transcriptional modules involved with lipid uptake, storage, and de novo lipogenesis. Thermogenic pathways in brown adipose tissue were comparable to control mice, but an elevation was observed in the genes encoding for lipid transporters and fatty acid oxidation. In contrast, deletion of NLRP3 in the immune cell compartment had limited effects on obesity and hepatic steatosis. Collectively, our results outline a prominent role for NLRP3 in nonimmune cells in facilitating MAFLD during constant energy surplus.

NLRP3 knockout enhances immune infiltration and inflammatory responses and improves survival in a burn sepsis model

Although sepsis in burn patients is a major contributor to mortality, treatments are not always effective and underlying mechanisms have yet to be completely elucidated. NLRP3 inflammasome orchestrates burn-induced, inflammatory-driven pathophysiologic processes. Here, we determined the mechanism of NLRP3 inflammasome activation on bacterial clearance and mortality in burn sepsis. We obtained tissue and blood from 30 wild-type and 30 Nlrp3^-/- mice. Mice were subjected to a two-hit model of 25-30% TBSA scald burn followed by Pseudomonas aeruginosa wound infection 72 hours after injury. We also obtained tissue from 34 adult burn patients (≥18 years of age) with early (0-11 days post-burn) and later (≥12 days post-burn) surgical time-points and ten healthy controls. Murine studies indicated that Nlrp3^-/- had 30% improved survival and bacterial clearance at the site of injury and is systemically relative to burn sepsis wild type. Greater macrophage and neutrophil infiltration occurred acutely after infection (12 hours) to the site of injury and adipose tissue. This was followed by increased macrophage and neutrophil infiltration to lymphoid organs and liver beyond the acute phase (24 and 72 hours). Interestingly, Nlrp3 ablation increased acute systemic inflammation (IL-6, TNF-α, IL-1β). Septic burn patients had persistently increased adipose NLRP3 by-product expression beyond the acute phase that was more pronounced in late-onset sepsis. Our findings suggest that Nlrp3 genetic ablation enhanced acute tissue-specific inflammatory responsiveness. Likely, this occurs by paradoxically increasing acute immune infiltration and inflammation with a non-persistent response. Clinically, persistent NLRP3-mediated inflammation occurs in septic versus normal burn patients and potentially detrimentally impacts patient outcomes.

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.

Adipose-specific ATGL ablation reduces burn injury-induced metabolic derangements in mice

Hypermetabolism following severe burn injuries is associated with adipocyte dysfunction, elevated beige adipocyte formation, and increased energy expenditure. The resulting catabolism of adipose leads to detrimental sequelae such as fatty liver, increased risk of infections, sepsis, and even death. While the phenomenon of pathological white adipose tissue (WAT) browning is well-documented in cachexia and burn models, the molecular mechanisms are essentially unknown. Here, we report that adipose triglyceride lipase (ATGL) plays a central role in burn-induced WAT dysfunction and systemic outcomes. Targeting adipose-specific ATGL in a murine (AKO) model resulted in diminished browning, decreased circulating fatty acids, and mitigation of burn-induced hepatomegaly. To assess the clinical applicability of targeting ATGL, we demonstrate that the selective ATGL inhibitor atglistatin mimics the AKO results, suggesting a path forward for improving patient outcomes.

Burn-induced hypermetabolism and skeletal muscle dysfunction

Critical illnesses, including sepsis, cancer cachexia, and burn injury, invoke a milieu of systemic metabolic and inflammatory derangements that ultimately results in increased energy expenditure leading to fat and lean mass catabolism. Burn injuries present a unique clinical challenge given the magnitude and duration of the hypermetabolic response compared with other forms of critical illness, which drastically increase the risk of morbidity and mortality. Skeletal muscle metabolism is particularly altered as a consequence of burn-induced hypermetabolism, as it primarily provides a main source of fuel in support of wound healing. Interestingly, muscle catabolism is sustained long after the wound has healed, indicating that additional mechanisms beyond wound healing are involved. In this review, we discuss the distinctive pathophysiological response to burn injury with a focus on skeletal muscle function and metabolism. We first examine the diverse consequences on skeletal muscle dysfunction between thermal, electrical, and chemical burns. We then provide a comprehensive overview of the known mechanisms underlying skeletal muscle dysfunction that may be attributed to hypermetabolism. Finally, we review the most promising current treatment options to mitigate muscle catabolism, and by extension improve morbidity and mortality, and end with future directions that have the potential to significantly improve patient care.

Damage-Associated Molecular Patterns and the Systemic Immune Consequences of Severe Thermal Injury

Thermal injury is often associated with a proinflammatory state resulting in serious complications. After a burn, the innate immune system is activated with subsequent immune cell infiltration and cytokine production. Although the innate immune response is typically beneficial, an excessive activation leads to cytokine storms, multiple organ failure, and even death. This overwhelming immune response is regulated by damage-associated molecular patterns (DAMPs). DAMPs are endogenous molecules that are actively secreted by immune cells or passively released by dead or dying cells that can bind to pathogen recognition receptors in immune and nonimmune cells. Recent studies involving animal models along with human studies have drawn great attention to the possible pathological role of DAMPs as an immune consequence of thermal injury. In this review, we outline DAMPs and their function in thermal injury, shedding light on the mechanism of sterile inflammation during tissue injury and identifying new immune targets for treating thermal injury.

Adipose Tissue Metabolic Function and Dysfunction: Impact of Burn Injury

For decades, adipose tissue had been considered as merely a storage depot and cushion to protect organs against trauma and injury. However, in recent years, a number of impactful studies have pinpointed the adipose tissue as an endocrine organ mediating systemic dysfunction in not only metabolic disorders such as obesity, but also in the stages following traumatic events such as severe burns. For instance, thermal injury induces a chronic β-adrenergic response associated with drastic increases in adipose lipolysis, macrophage infiltration and IL-6 mediated browning of white adipose tissue (WAT). The downstream consequences of these physiological changes to adipose, such as hepatomegaly and muscle wasting, are only now coming to light and suggest that WAT is both a culprit in and initiator of metabolic disorders after burn injury. To that effect, the aim of this review is to chronicle and critically analyze the scientific advances made in the study of adipose tissue with regards to its role in orchestrating the hypermetabolic response and detrimental effects of burn injury. The topics covered include the magnitude of the lipolytic response following thermal trauma and how WAT browning and inflammation perpetuate this cycle as well as how WAT physiology impacts insulin resistance and hyperglycemia post-burn. To conclude, we discuss how these findings can be translated from bench to bedside in the form of therapeutic interventions which target physiological changes to WAT to restore systemic homeostasis following a severe burn.

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.

NLRP3 Inflammasome in Inflammation and Metabolism: Identifying Novel Roles in Postburn Adipose Dysfunction

Inflammasomes are multiprotein complexes that respond to pathogen or host associated damage markers, leading to caspase-1 maturation and processing of pro-inflammatory cytokines. Initially, inflammasomes were implicated primarily in inflammatory and infectious conditions. However, increasing evidence demonstrates broader roles beyond inflammation, including regulation of adipose tissue metabolism after burns. Here, we conducted a search for articles on PubMed, Web of Science, Embase, Scopus, and UpToDate with applied search strategies including a combination of "burns," "trauma," "(NLRP3) inflammasome," "metabolic conditions," "white adipose tissue," "macrophages," "browning," and "lipolysis" and included papers from 2000 to 2020. We discuss unexpected roles for NLRP3, the most characterized inflammasome to date, as a key metabolic driver in a variety of conditions. In particular, we highlight the function of NLRP3 inflammasome in burn trauma, which is characterized by both hyperinflammation and hypermetabolism. We identify a critical part for NLRP3 activation in macrophage dynamics and delineate a novel role in postburn white adipose tissue remodeling, a pathological response associated with hypermetabolism and poor clinical outcomes. Mechanistically, how inflammation and inflammasome activation is linked to postburn hypermetabolism is a novel concept to contemplate, and herein we provide evidence of an immunometabolic crosstalk between adipocytes and infiltrating macrophages.

Covalent modification of nephrilin peptide with valproic acid increases its efficacy as a therapeutic in burn trauma

Introduction:

Nephrilin peptide, a designed inhibitor of Rictor complex, modulates systemic responses to trauma, alleviating clinically relevant variables in a rat scald model and sepsis mortality in a mouse model. This study explores the possibility that chemical conjugation of small molecules to the aminoterminus of nephrilin can modify its biological activity in the rat scald model.

Methods:

One of four molecules (valproic acid, decanoic acid, fenofibric acid and ibuprofen) was chemically attached to the amino terminus of nephrilin during synthesis. Animals were treated with each modified nephrilin by subcutaneous bolus injection on days 1–7 post-burn.

Results:

Compared to nephrilin, valproic acid-modified nephrilin showed significantly (all p < 0.05) improved systemic effects on kidney function (creatinine 0.17 ± 0.03 vs 0.31 ± 0.09 mg/dL), glycemic control (AUC 57.5 ± 40 vs 136.4 ± 69.2 mg.dL.hr), inflammation (IL-6 24 ± 9 vs 39 ± 8 pg/ml), pathological angiogenesis (1.46 ± 0.87 vs 6.53 ± 3.16 pct pixels) and weight gain (3.74 ± 0.31 vs 2.99 ± 0.53 slope), all variables previously shown to bear upon clinically relevant burn injury outcomes.

Conclusion:

Modification of nephrilin with valproic acid increases the efficacy of nephrilin peptide in burns.

NLRP3 inflammasome activity is required for wound healing after burns

Survival of burn patients is contingent on effective wound healing, a complex process that requires coordinated responses of myeloid cells and inflammatory pathways. NLRP3, which serves as a platform for secretion of proinflammatory cytokines, is implicated as a central regulator of wound healing. However, its role during the acute dermal and epidermal regeneration in the context of burns is unknown. Wild-type (WT) and NLRP3^-/- mice were exposed to a 30% TBSA scald burn. Gene expression was conducted via real-time polymerase chain reaction. Trichrome staining was used to assess collagen deposition and granulation tissue formation. F4/80 immunostaining compared macrophage infiltration. Flow cytometric analysis was used to characterize skin macrophage distribution and profile. NLRP3, IL1β and IL18 expression was upregulated in skin after burn, and these changes were nonexistent in NLRP3^-/-. NLRP3^-/- had decreased expression of proinflammatory cytokines, chemokines, inflammatory markers, and growth factors at 3 days (P < 0.05). NLRP3^-/- burn skin demonstrated significantly less macrophage infiltration and higher expression of anti-inflammatory markers Arg1 and Fizz1 (P < 0.05) compared to WT. Trichrome staining showed decreased collagen deposition compared to WT. We show that NLRP3 is protective in burn wound healing, primarily through production of inflammatory mediators, macrophage recruitment, and polarization to a proinflammatory phenotype. Our findings highlight a central role of NLRP3 in wound healing through regulation of inflammation and macrophage polarization after burns.

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.

Positive effects of ferric iron on the systemic efficacy of nephrilin peptide in burn trauma

INTRODUCTION

Nephrilin peptide is a designed inhibitor of Rictor complex (also known as mTORC2), an evolutionarily conserved assembly believed to modulate responses to cellular stress. We previously demonstrated the ability of nephrilin peptide to suppress neuroinflammation, loss of body mass, glycaemic control and kidney function in a rat scald model, as well as sepsis mortality in a mouse model. The present study explores the effect of nephrilin plus iron formulations on clinically relevant outcomes in the rat scald model.

METHODS

Animals were treated with nephrilin by subcutaneous bolus injection on post-burn days 1-7. Equimolar ferric iron in the formulation improved the positive systemic effects of nephrilin on kidney function, glycaemic control, oxidative stress, early hyperinflammation, late inflammasome activation, hyperangiogenesis and body mass, all variables previously shown to bear upon clinically relevant burn injury outcomes. The sparing effects of nephrilin-iron were demonstrated in both sexes.

DISCUSSION

Surprisingly, optimum daily treatment doses were in the range of 2-4 mg/kg, while 8 mg/kg was less effective, suggesting the possibility of marginal pro-oxidant effects from the 'free' iron fraction. Thus, although ferric iron in the nephrilin formulation is clearly helpful, care must be exercised to select an optimum treatment dose.

CONCLUSION

Iron increases the efficacy of nephrilin peptide in burns.

NLRP3 inflammasome mediates white adipose tissue browning after burn

A hallmark after burn is the stress and inflammatory-induced hypermetabolic response. Recently, we and others found that browning of white adipose tissue (WAT) is a critical component of this complex detrimental response. Although browning and inflammation have been independently delineated to occur after injury, their interaction is currently not well defined. One of the master regulators of inflammation and adipose tissue remodeling after burns is nucleotide-binding and oligomerization domain, leucine rich repeat and pyrin domain containing 3 (NLRP3) inflammasome. The aim of this this study was to determine whether NLRP3 modulates and activates WAT browning after burn. To obtain molecular and mechanistic insights, we used an NLRP3 knockout (NLRP3^-/-) murine burn model. We demonstrated that genetic deletion of NLRP3 promoted persistent and augmented browning in adipocytes, evidenced by increased gene expression of peroxisome proliferator-activated receptor γ and CIDEA at 3 days (5.74 vs. 0.29, P < 0.05; 26.0 vs. 0.71, P < 0.05) and uncoupling protein 1 (UCP1) and PGC1α at 7 days (7,406 vs. 3,894, P < 0.05; 20.6 vs. 2.52, P < 0.01) and enhanced UCP1 staining and multilocularity. Additionally, the main regulator of postburn WAT browning, IL-6, was elevated in the plasma acutely after burn in NLRP3^-/- compared with wild-type counterparts (478.9 vs. 67.1 pg/mL, P < 0.05 at 3 days). These results suggest that NLRP3 has antibrowning effects and that blocking NLRP3 increases thermogenesis and augments browning via increased levels of IL-6. Our findings provide insights into targeting innate inflammatory systems for regulation of adaptive thermogenesis, a critical response after burns and other hypermetabolic conditions.

Aggregated and Hyperstable Damage-Associated Molecular Patterns Are Released During ER Stress to Modulate Immune Function

Chronic ER stress occurs when protein misfolding in the Endoplasmic reticulum (ER) lumen remains unresolved despite activation of the unfolded protein response. We have shown that traumatic injury such as a severe burn leads to chronic ER stress in vivo leading to systemic inflammation which can last for more than a year. The mechanisms linking chronic ER stress to systemic inflammatory responses are not clear. Here we show that induction of chronic ER stress leads to the release of known and novel damage-associated molecular patterns (DAMPs). The secreted DAMPs are aggregated and markedly protease resistant. ER stress-derived DAMPs activate dendritic cells (DCs) which are then capable of polarizing naïve T cells. Our findings indicate that induction of chronic ER stress may lead to the release of hyperstable DAMPs into the circulation resulting in persistent systemic inflammation and adverse outcomes.

Inhibition of NLRP3 inflammasome in tumor microenvironment leads to suppression of metastatic potential of cancer cells

Tumor microenvironment favors tumor cells to promote their growth and metastasis such as migration, invasion, and angiogenesis. IL-1β, one of the inflammatory cytokines released from myeloid cells in tumor microenvironment, plays an important role in development and progress of tumor. The activation of inflammasome is a critical step to secrete mature IL-1β through stepwise reactions to activate capspase-1. Therefore, we investigated whether the inhibition of NACHT, LRR and PYD domains-containing protein 3 (NLRP3) inflammasome in macrophages regulated the metastatic potential of tumor cells. NLRP3 inflammasome was activated by ATP in bone marrow-derived primary mouse macrophages. The metastatic potential of mouse melanoma cell line (B16F10) was determined by migration and invasion assays with transwell system. ATP-treated wild-type macrophages increased the migration and invasion of melanoma cells. However, NLRP3- or caspase-1-knockout macrophages exhibited greatly diminished ability to promote the migration and invasion of melanoma cells. In addition, treatment with celastrol, an inhibitor of NLRP3 inflammasome, reduced the potency of macrophages to stimulate migration and invasion of melanoma cells. The results demonstrate that inhibition of the NLRP3 inflammasome in macrophages by genetic deficiency or a pharmacological inhibitor is linked to suppression of the metastatic potential of tumor cells. The results would provide a novel anti-cancer strategy to modulate tumor microenvironment by suppressing NLRP3 inflammasome and consequently reducing IL-1β production.