The inflammasome-mediated caspase-1 activation controls adipocyte differentiation and insulin sensitivity

Effects of gevokizumab on glycemia and inflammatory markers in type 2 diabetes

OBJECTIVE

Metabolic activation of the innate immune system governed by interleukin (IL)-1β contributes to β-cell failure in type 2 diabetes. Gevokizumab is a novel, human-engineered monoclonal anti-IL-1β antibody. We evaluated the safety and biological activity of gevokizumab in patients with type 2 diabetes.

RESEARCH DESIGN AND METHODS

In a placebo-controlled, dose-escalation study, a total of 98 patients were randomly assigned to placebo (17 subjects) or gevokizumab (81 subjects) at increasing doses and dosing schedules. The primary objective of the study was to evaluate the safety profile of gevokizumab in type 2 diabetes. The secondary objectives were to assess pharmacokinetics for different dose levels, routes of administration, and regimens and to assess biological activity.

RESULTS

The study drug was well tolerated with no serious adverse events. There was one hypoglycemic event whereupon concomitant insulin treatment had to be reduced. Clearance of gevokizumab was consistent with that for a human IgG(2), with a half-life of 22 days. In the combined intermediate-dose group (single doses of 0.03 and 0.1 mg/kg), the mean placebo-corrected decrease in glycated hemoglobin was 0.11, 0.44, and 0.85% after 1, 2 (P = 0.017), and 3 (P = 0.049) months, respectively, along with enhanced C-peptide secretion, increased insulin sensitivity, and a reduction in C-reactive protein and spontaneous and inducible cytokines.

CONCLUSIONS

This novel IL-1β-neutralizing antibody improved glycemia, possibly via restored insulin production and action, and reduced inflammation in patients with type 2 diabetes. This therapeutic agent may be able to be used on a once-every-month or longer schedule.

Treating inflammation by blocking interleukin-1 in a broad spectrum of diseases

Interleukin-1 (IL-1) is a highly active pro-inflammatory cytokine that lowers pain thresholds and damages tissues. Monotherapy blocking IL-1 activity in autoinflammatory syndromes results in a rapid and sustained reduction in disease severity, including reversal of inflammation-mediated loss of sight, hearing and organ function. This approach can therefore be effective in treating common conditions such as post-infarction heart failure, and trials targeting a broad spectrum of new indications are underway. So far, three IL-1-targeted agents have been approved: the IL-1 receptor antagonist anakinra, the soluble decoy receptor rilonacept and the neutralizing monoclonal anti-IL-1β antibody canakinumab. In addition, a monoclonal antibody directed against the IL-1 receptor and a neutralizing anti-IL-1α antibody are in clinical trials.

Immunological complications of obesity

Current approaches for the treatment of obesity, including diet and lifestyle changes, have not been successful in curtailing the obesity epidemic. The higher incidence of inflammation-associated chronic disease and greater susceptibility to infection in obese people represents a growing health threat. Improved understanding of the immunological processes that regulate obesity may yield new treatments for obesity-associated disorders.

ATP release from dying autophagic cells and their phagocytosis are crucial for inflammasome activation in macrophages

Pathogen-activated and damage-associated molecular patterns activate the inflammasome in macrophages. We report that mouse macrophages release IL-1β while co-incubated with pro-B (Ba/F3) cells dying, as a result of IL-3 withdrawal, by apoptosis with autophagy, but not when they are co-incubated with living, apoptotic, necrotic or necrostatin-1 treated cells. NALP3-deficient macrophages display reduced IL-1β secretion, which is also inhibited in macrophages deficient in caspase-1 or pre-treated with its inhibitor. This finding demonstrates that the inflammasome is activated during phagocytosis of dying autophagic cells. We show that activation of NALP3 depends on phagocytosis of dying cells, ATP release through pannexin-1 channels of dying autophagic cells, P₂X₇ purinergic receptor activation, and on consequent potassium efflux. Dying autophagic Ba/F3 cells injected intraperitoneally in mice recruit neutrophils and thereby induce acute inflammation. These findings demonstrate that NALP3 performs key upstream functions in inflammasome activation in mouse macrophages engulfing dying autophagic cells, and that these functions lead to pro-inflammatory responses.

Dietary saturated fatty acids prime the NLRP3 inflammasome via TLR4 in dendritic cells-implications for diet-induced insulin resistance

SCOPE

Inflammasome-mediated inflammation is a critical regulator of obesity-induced insulin resistance (IR). We hypothesized that saturated fatty acids (SFA) directly prime the NLRP3 inflammasome via TLR4 concurrent with IR. We focused on dendritic cells (DCs) (CD11c(+) CD11b(+) F4/80(-) ), which are recruited into obese adipose tissue following high-fat diet (HFD) challenge and are a key cell in inflammasome biology.

METHODS AND RESULTS

C57BL/6 mice were fed HFD for 16 weeks (45% kcal palm oil), glucose homeostasis was monitored by glucose and insulin tolerance tests. Stromal vascular fraction (SVF) cells were isolated from adipose and analyzed for CD11c(+) CD11b(+) F480(-) DC. Following coculture with bone marrow derived DC (BMDC) insulin-stimulated (3) H-glucose transport into adipocytes, IL-1β secretion and caspase-1 activation was monitored. BMDCs primed with LPS (100 ng/mL), linoleic acid (LA; 200 μM), or palmitic acid (PA; 200 μM) were used to monitor inflammasome activation. We demonstrated significant infiltration of DCs into adipose after HFD. HFD-derived DCs reduce adipocyte insulin sensitivity upon coculture co-incident with enhanced adipocyte caspase-1 activation/IL-1β secretion. HFD-derived DCs are skewed toward a pro-inflammatory phenotype with increased IL-1β secretion, IL-1R1, TLR4, and caspase-1 expression. Complementary in vitro experiments demonstrate that TLR4 is critical in propagating SFA-mediated inflammasome activation.

CONCLUSION

SFA represent metabolic triggers priming the inflammasome, promoting adipocyte inflammation/IR, suggesting direct effects of SFA on inflammasome activation via TLR4.

Biochemical regulation of the inflammasome

The extensively studied cytokine IL-1β is an important mediator of the inflammatory response. However, dysregulated release of IL-1β can be detrimental and is attributed to the progression and pathogenesis of multiple inflammatory diseases including, rhuematoid arthritis (RA), atherosclerosis, type 2 diabetes (T2D), Alzheimers disease and gout. IL-1β is encoded as a pro-protein. A multi-protein molecular scaffold termed the "Inflammasome" is responsible for the tightly controlled and coordinated processing of pro-IL-1β. The activation of several NLR (nucleotide-binding oligomerization domain (NOD)-like receptor) family members and PYHIN (pyrin and HIN domain) proteins can drive the formation of inflammasomes. However, the exact biochemical mechanisms governing their activation have been the subject of much research. Different inflammasomes have been demonstrated to respond to the same pathogen inducing a cooperative immune response accountable for the clearance of infection. Here, we review current knowledge surrounding the biochemical regulation of the NLRP1, NLRP3, NLRC4, AIM2 and IFI16 inflammasomes.

The ATP-P2X7 signaling axis is dispensable for obesity-associated inflammasome activation in adipose tissue

Inflammasome activation in adipose tissue has been implicated in obesity-associated insulin resistance and type 2 diabetes. However, when and how inflammasome is activated in adipose tissue remains speculative. Here we test the hypothesis that extracellular ATP, a potent stimulus of inflammasome in macrophages via purinergic receptor P2X, ligand-gated ion channel, 7 (P2X(7)), may play a role in inflammasome activation in adipose tissue in obesity. Our data show that inflammasome is activated in adipose tissue upon 8-week feeding of 60% high-fat diet (HFD), coinciding with the onset of hyperglycemia and hyperinsulinemia as well as the induction of P2X(7) in adipose tissue. Unexpectedly, P2X(7)-deficient animals on HFD exhibit no changes in metabolic phenotypes, inflammatory responses, or inflammasome activation when compared with the wild-type controls. Similar observations have been obtained in hematopoietic cell-specific P2X(7)-deficient animals generated by bone marrow transplantation. Thus, we conclude that inflammasome activation in adipose tissue in obesity coincides with the onset of hyperglycemia and hyperinsulinemia but, unexpectedly, is not mediated by the ATP-P2X(7) signaling axis. The nature of the inflammasome-activating danger signal(s) in adipose tissue in obesity remains to be characterized.

Caspase-1 inhibitors from an extremophilic fungus that target specific leukemia cell lines

Berkeley Pit Lake, Butte, Montana, is a 540 m deep abandoned open-pit copper mine filled with over 140 billion liters of acidic, metal-sulfate-contaminated water. This harsh environment has yielded several microorganisms that produce interesting biologically active compounds. Several polyketide metabolites including the new berkazaphilones A (1) and B (2) and octadienoic acid derivatives berkedienoic acid (13) and berkedienolactone (15), as well as previously reported azaphilone 4, vermistatin (6), dihydrovermistatin (7), penisimplicissin (8), aldehyde 9, and methylparaconic acid (11), were isolated from a culture broth of Penicillium rubrum taken from a depth of 270 m. The structures of these compounds were deduced by interpretation of spectroscopic data. The compounds were isolated either for their inhibition of the signal transduction enzyme caspase-1 or because of their structural similarity to these inhibitors. Selected compounds were further evaluated for their ability to inhibit interleukin-1β production by inflammasomes in induced THP-1 cells. Berkazaphilones B (2) and C (4) and vermistatin analogue penisimplicissin (8) exhibited selective activity against leukemia cancer cell lines in the National Cancer Institute 60 human cell line assay.

A role for the NLRP3 inflammasome in metabolic diseases--did Warburg miss inflammation?

The inflammasome is a protein complex that comprises an intracellular sensor (typically a Nod-like receptor), the precursor procaspase-1 and the adaptor ASC. Inflammasome activation leads to the maturation of caspase-1 and the processing of its substrates, interleukin 1β (IL-1β) and IL-18. Although initially the inflammasome was described as a complex that affects infection and inflammation, subsequent evidence has suggested that inflammasome activation influences many metabolic disorders, including atherosclerosis, type 2 diabetes, gout and obesity. Another feature of inflammation in general and the inflammasome specifically is that the activation process has a profound effect on aerobic glycolysis (the 'Warburg effect'). Here we explore how the Warburg effect might be linked to inflammation and inflammasome activation.

Inflammasomes: far beyond inflammation

Nearly a decade ago, the concept of inflammasomes was introduced. Since then, the biochemical characterization of the inflammasomes has led to a richer understanding of innate immune responses in the context of infection and sterile inflammation. This has provided the rationale for successful clinical therapies for a spectrum of hereditary periodic fever syndromes and potentially for some metabolic pathologies.

Mechanisms of inflammatory responses in obese adipose tissue

The fields of immunology and metabolism are rapidly converging on adipose tissue. During obesity, many immune cells infiltrate or populate in adipose tissue and promote a low-grade chronic inflammation. Studies to date have suggested that perturbation of inflammation is critically linked to nutrient metabolic pathways and to obesity-associated complications such as insulin resistance and type 2 diabetes. Despite these advances, however, many open questions remain including how inflammatory responses are initiated and maintained, how nutrients impact the function of various immune populations, and how inflammatory responses affect systemic insulin sensitivity. Here we review recent studies on the roles of various immune cells at different phases of obesity and discuss molecular mechanisms underlying obesity-associated inflammation. Better understanding of the events occurring in adipose tissue will provide insights into the pathophysiological role of inflammation in obesity and shed light on the pathogenesis of obesity-associated metabolic syndrome.

The cellular and signaling networks linking the immune system and metabolism in disease

It is now recognized that obesity is driving the type 2 diabetes epidemic in Western countries. Obesity-associated chronic tissue inflammation is a key contributing factor to type 2 diabetes and cardiovascular disease, and a number of studies have clearly demonstrated that the immune system and metabolism are highly integrated. Recent advances in deciphering the various cellular and signaling networks that participate in linking the immune and metabolic systems together have contributed to understanding of the pathogenesis of metabolic diseases and may also inform new therapeutic strategies based on immunomodulation. Here we discuss how these various networks underlie the etiology of the inflammatory component of insulin resistance, with a particular focus on the central roles of macrophages in adipose tissue and liver.

Oxidative stress and inflammation interactions in human obesity

Obesity is often characterized by increased oxidative stress and exacerbated inflammatory outcomes accompanying infiltration of immune cells in adipocytes. The oxidative stress machinery and inflammatory signaling are not only interrelated, but their impairment can lead to an inhibition of insulin responses as well as a higher risk of cardiovascular diseases and associated features. Mitochondria, in addition to energy transformation, play a role in apoptosis, cellular proliferation, as well as in the cellular redox state control. Under certain circumstances, protons are able to re-enter the mitochondrial matrix via different uncoupling proteins, disturbing free radical production by mitochondria. Disorders of the mitochondrial electron transport chain, over-generation of reactive oxygen species, and lipoperoxides or alterations in antioxidant defenses have been reported in situations of obesity and type-2 diabetes. On the other hand, obesity has been linked to a low grade pro-inflammatory state, in which impairments in the oxidative stress and antioxidant mechanism could be involved. The current scientific evidence highlights the need of investigating the interplay between oxidative stress and inflammation with obesity/diabetes onset as well as the interactions of such factors either as a cause or consequence of obesity. The signaling mediated by the activation of inflammatory markers or nuclear factor kappa β and other transcription factors as central regulators of inflammation are key issues to understanding oxidative stress responses in obesity. This review aims at summarizing the main mechanisms and interplay factors between oxidative stress and inflammation in human obesity according to the last 10 years of research in the field.

Inflammasome activation in obesity-related inflammatory diseases and autoimmunity

The inflammasome is a highly regulated protein complex that triggers caspase-1 activation and subsequent secretion of IL-1β and IL-18. Recognition of microbial components and danger signals by NOD-like receptor (NLR) family members in the cytosol promotes inflammasome activation and downstream inflammatory cytokine production. Pathogen recognition by NLRs and downstream release of inflammasome-derived cytokines are important in host defense against numerous infections. Recent studies have also identified a unique role for inflammasome regulation in the induction and pathogenesis of multiple autoimmune and inflammatory disorders. We now know that obesity-related factors and endogenous markers of cellular stress can lead to unchecked activation of the inflammasome and provoke inflammation and subsequent destruction of vital organs. This review will highlight recent findings that link inflammasome signaling to the progression of autoinflammatory and autoimmune diseases. We will focus on the contribution of inflammasome activation to the pathogenesis of autoinflammatory and autoimmune diseases that are of major significance to human health including type 2 diabetes, atherosclerosis, multiple sclerosis, and type 1 diabetes.

Role and function of macrophages in the metabolic syndrome

Macrophages are key innate immune effector cells best known for their role as professional phagocytes, which also include neutrophils and dendritic cells. Recent evidence indicates that macrophages are also key players in metabolic homoeostasis. Macrophages can be found in many tissues, where they respond to metabolic cues and produce pro- and/or anti-inflammatory mediators to modulate metabolite programmes. Certain metabolites, such as fatty acids, ceramides and cholesterol crystals, elicit inflammatory responses through pathogen-sensing signalling pathways, implicating a maladaptation of macrophages and the innate immune system to elevated metabolic stress associated with overnutrition in modern societies. The outcome of this maladaptation is a feedforward inflammatory response leading to a state of unresolved inflammation and a collection of metabolic pathologies, including insulin resistance, fatty liver, atherosclerosis and dyslipidaemia. The present review summarizes what is known about the contributions of macrophages to metabolic diseases and the signalling pathways that are involved in metabolic stress-induced macrophage activation. Understanding the role of macrophages in these processes will help us to develop therapies against detrimental effects of the metabolic syndrome.

Expression of NLRP3 inflammasome and T cell population markers in adipose tissue are associated with insulin resistance and impaired glucose metabolism in humans

Recent studies in rodents indicate that the nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing-3 (NLRP3) inflammasome and a proinflammatory shift in the T cell population in adipose tissue (AT) contribute to AT inflammation and insulin resistance. We investigated: (1) the interplay between the NLRP3 inflammasome and T cell populations in abdominal subcutaneous AT in obese and lean humans in relation to AT inflammatory processes, and (2) involvement of the NLRP3 inflammasome and T cell populations in insulin resistance. Abdominal subcutaneous AT biopsies were collected in 10 obese men with impaired glucose tolerance and 9 lean normal glucose tolerant age-matched controls. AT gene expression of NLRP3 inflammasome-related genes and markers of T cell populations, chemoattraction, macrophage infiltration and other aspects of inflammation were examined. Furthermore, we examined systemic adaptive immune activation and insulin sensitivity (hyperinsulinemic-euglycemic clamp). CASPASE-1 mRNA and the proportion of T(h)1 transcripts (TBX21/CD3ɛ) were significantly higher in AT from obese compared with lean subjects. CASPASE-1 expression and a relative increase in T(h)1 transcripts in AT were strongly associated with insulin resistance and impairments in glucose homeostasis. Gene expression of NLRP3, CASPASE-1, CD3ɛ (pan T cells), TBX21 (T(h)1 cells) and RORC (T(h)17 cells) was positively, whereas GATA3 (T(h)2 cells) was inversely correlated with AT inflammation. Our data suggest that NLRP3 inflammasome activation and a T(h)1 shift in the T cell population in AT of obese subjects is related to insulin resistance and impaired glucose metabolism, which may be explained by AT inflammatory processes.

Inflammasomes in health and disease

Inflammasomes are a group of protein complexes built around several proteins, including NLRP3, NLRC4, AIM2 and NLRP6. Recognition of a diverse range of microbial, stress and damage signals by inflammasomes results in direct activation of caspase-1, which subsequently induces secretion of potent pro-inflammatory cytokines and a form of cell death called pyroptosis. Inflammasome-mediated processes are important during microbial infections and also in regulating both metabolic processes and mucosal immune responses. We review the functions of the different inflammasome complexes and discuss how aberrations in them are implicated in the pathogenesis of human diseases.

Obesity and mortality risk among younger dialysis patients

BACKGROUND AND OBJECTIVES

Many studies show that obesity in dialysis patients is not strongly associated with mortality but not whether this modest association is constant over age. This study investigated the extent to which the relation of body mass index (BMI) and mortality differs between younger and older dialysis patients.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Adult dialysis patients were prospectively followed from their first dialysis treatment for 7 years or until death or transplantation. Patients were stratified by age (<65 or ≥65 years) and baseline BMI (<20, 20-24 [reference], 25-29, and ≥30 kg/m(2)).

RESULTS

The study sample included 984 patients younger than 65 years and 765 patients 65 years or older; cumulative survival proportions at end of follow-up were 50% and 16%. Age-standardized mortality rate was 1.7 times higher in obese younger patients than those with normal BMI, corresponding to an excess rate of 5.2 deaths/100 patient-years. Mortality rates were almost equal between obese older patients and those with normal BMI. Excess rates of younger and older patients with low compared with normal BMI were 8.7 and 1.1 deaths/100 patient-years. After adjustment for age, sex, smoking, comorbidity, and treatment modality, hazard ratios by increasing BMI were 2.00, 1, 0.95, and 1.57 for younger patients and 1.07, 1, 0.88, and 0.91 for older patients, implying that obesity is a 1.7-fold (95% confidence interval, 1.1- to 2.9-fold) stronger risk factor in younger than older patients.

CONCLUSIONS

In contrast to older dialysis patients, younger patients with low or very high BMI had a substantially elevated risk for death.

The inflammasome puts obesity in the danger zone

Obesity-induced inflammation is an important contributor to the induction of insulin resistance. Recently, the cytokine interleukin-1β (IL-1β) has emerged as a prominent instigator of the proinflammatory response in obesity. Several studies over the last year have subsequently deciphered the molecular mechanisms responsible for IL-1β activation in adipose tissue, liver, and macrophages and demonstrated a central role of the processing enzyme caspase-1 and of the protein complex leading to its activation called the inflammasome. These data suggest that activation of the inflammasome represents a crucial step in the road from obesity to insulin resistance and type 2 diabetes.

Adipose tissue inflammation and ectopic lipid accumulation

Obesity may be viewed as a chronic low-grade inflammatory disease as well as a metabolic disease. Indeed, unbalanced production of pro- and anti-inflammatory adipocytokines critically contributes to the obesity-induced insulin resistance. In addition to lipid-laden mature adipocytes, adipose tissue is composed of various stromal cells such as preadipocytes, endothelial cells, fibroblasts, and immune cells that may be involved in adipose tissue functions. Accumulating evidence has suggested that adipocytes and stromal cells in adipose tissue change dramatically in number and cell type during the course of obesity, which is referred to as "adipose tissue remodeling." Among stromal cells, infiltration of macrophages in obese adipose tissue precedes the development of insulin resistance in animal models, suggesting that they are crucial for adipose tissue inflammation. We have provided evidence suggesting that a paracrine loop involving saturated fatty acids and tumor necrosis factor-α derived from adipocytes and macrophages, respectively, aggravates obesity-induced adipose tissue inflammation. On the other hand, storing excessive energy as triglyceride is also a fundamental function of adipose tissue. Recent evidence suggests that reduced lipid storage in obese adipose tissue contributes to ectopic lipid accumulation in non-adipose tissues such as the liver, skeletal muscle, and pancreas, where lipotoxicity impairs their metabolic functions. Notably, chronic inflammation is capable of inducing insulin resistance, lipolysis, and interstitial fibrosis in adipose tissue, all of which may reduce the lipid-storing function. Understanding the molecular mechanism underlying adipose tissue remodeling may lead to the identification of novel therapeutic strategies to prevent or treat obesity-induced adipose tissue inflammation.

Implication of inflammatory signaling pathways in obesity-induced insulin resistance

Obesity is characterized by the development of a low-grade chronic inflammatory state in different metabolic tissues including adipose tissue and liver. This inflammation develops in response to an excess of nutrient flux and is now recognized as an important link between obesity and insulin resistance. Several dietary factors like saturated fatty acids and glucose as well as changes in gut microbiota have been proposed as triggers of this metabolic inflammation through the activation of pattern-recognition receptors (PRRs), including Toll-like receptors (TLR), inflammasome, and nucleotide oligomerization domain (NOD). The consequences are the production of pro-inflammatory cytokines and the recruitment of immune cells such as macrophages and T lymphocytes in metabolic tissues. Inflammatory cytokines activate several kinases like IKKβ, mTOR/S6 kinase, and MAP kinases as well as SOCS proteins that interfere with insulin signaling and action in adipocytes and hepatocytes. In this review, we summarize recent studies demonstrating that PRRs and stress kinases are important integrators of metabolic and inflammatory stress signals in metabolic tissues leading to peripheral and central insulin resistance and metabolic dysfunction. We discuss recent data obtained with genetically modified mice and pharmacological approaches suggesting that these inflammatory pathways are potential novel pharmacological targets for the management of obesity-associated insulin resistance.

Sensing damage by the NLRP3 inflammasome

The NLRP3 inflammasome is activated in response to a variety of signals that are indicative of damage to the host including tissue damage, metabolic stress, and infection. Upon activation, the NLRP3 inflammasome serves as a platform for activation of the cysteine protease caspase-1, which leads to the processing and secretion of the proinflammatory cytokines interleukin-1β (IL-1β) and IL-18. Dysregulated NLRP3 inflammasome activation is associated with both heritable and acquired inflammatory diseases. Here, we review new insights into the mechanism of NLRP3 inflammasome activation and its role in disease pathogenesis.

Genetic dissection of the functions of the melanocortin-3 receptor, a seven-transmembrane G-protein-coupled receptor, suggests roles for central and peripheral receptors in energy homeostasis

The melanocortin-3 receptor (MC3R) gene is pleiotropic, influencing body composition, natriuresis, immune function, and entrainment of circadian rhythms to nutrient intake. MC3Rs are expressed in hypothalamic and limbic regions of the brain and in peripheral tissues. To investigate the roles of central MC3Rs, we inserted a "lox-stop-lox" (LoxTB) 5' of the translation initiation codon of the mouse Mc3r gene and reactivated transcription using neuron-specific Cre transgenic mice. As predicted based on earlier observations of Mc3r knock-out mice, Mc3r(TB/TB) mice displayed reduced lean mass, increased fat mass, and accelerated diet-induced obesity. Surprisingly, rescuing Mc3r expression in the nervous system using the Nestin-Cre transgene only partially rescued obesity in chow-fed conditions and had no impact on the accelerated diet-induced obesity phenotype. The ventromedial hypothalamus (VMH), a critical node in the neural networks regulating feeding-related behaviors and metabolic homeostasis, exhibits dense Mc3r expression relative to other brain regions. To target VMH MC3R expression, we used the steroidogenic factor-1 Cre transgenic mouse. Although restoring VMH MC3R signaling also had a modest impact on obesity, marked improvements in metabolic homeostasis were observed. VMH MC3R signaling was not sufficient to rescue the lean mass phenotype or the regulation of behaviors anticipating food anticipation. These results suggest that actions of MC3Rs impacting on energy homeostasis involve both central and peripheral sites of action. The impact of central MC3Rs on behavior and metabolism involves divergent pathways; VMH MC3R signaling improves metabolic homeostasis but does not significantly impact on the expression of behaviors anticipating nutrient availability.

The inflammasome: in memory of Dr. Jurg Tschopp

A decade ago, Jurg Tschopp introduced the concept of the inflammasome. This exciting discovery of a macromolecular complex that senses 'danger' and initiates the inflammatory response contributed to a renaissance in the fields of innate immunity and cell death. Jurg led the biochemical characterization of the inflammasome complex and demonstrated that spontaneous hyperactivation of this interleukin (IL)-1β processing machinery is the molecular basis of a spectrum of hereditary periodic fever syndromes, caused by mutated forms of the inflammasome scaffolding receptor, NLRP3. The identification of the underlying mechanism in these disorders has led to their now successful therapy, with the use of the IL-1 receptor antagonist in the clinic. Jurg's pioneering work has subsequently defined a number of inflammasome agonists ranging from microbial molecules expressed during infection, to triggers of sterile inflammation, most notably gout-associated uric acid crystals, asbestos, silica and nanoparticles. More recently, Jurg introduced the critical new concept of the metabolic inflammasome, which senses metabolic stress and contributes to the onset of the metabolic syndrome associated with obesity and type 2 diabetes. Jurg was an outstanding and skillful biochemist, an elegant and rigorous researcher often far ahead of his peers. He was a truly amiable person, fair, generous and inspiring, and will be most remembered for his infectious enthusiasm. We write this review article on the inflammasome in his honor and dedicate it to his memory.

Nucleotide-binding oligomerization domain-like receptors and inflammasomes in the pathogenesis of non-microbial inflammation and diseases

The nucleotide-binding oligomerization domain (NOD)-like receptor (NLR) or nucleotide-binding domain leucine-rich repeat-containing family of genes plays an important role in the development of innate immune responses. Some family members are known to form multiprotein complexes known as inflammasomes that regulate the processing and secretion of proinflammatory mediators, such as interleukin-1β and interleukin-18. Activity of the inflammasome is triggered not only by microbial infection, but also by a wide range of both exogenous and endogenous noninfectious stimuli. Consequently, the dysregulation of inflammasome activity is associated with numerous proinflammatory, non-microbial human diseases. The discovery of NLRP3 gene mutations in autoinflammatory diseases such as Muckle-Wells syndrome has led to the association of NLRs in the pathogenesis of many non-microbial diseases that include arthritis, neurodegenerative disorders, metabolic disorders (obesity and diabetes), cardiovascular disease (atherosclerosis, myocardial infarction), inflammatory bowel disease, kidney disease and hypersensitivity dermatitis. A number of NLRs are also associated with human disease in the absence of inflammasome activity, suggesting additional roles for NLRs in the regulation of inflammation and disease. This review serves to provide a summary of NLR-associated diseases and, where possible, the mechanisms behind the associations.

Macrophage-mediated inflammation in metabolic disease

Metabolism and immunity are two fundamental systems of metazoans. The presence of immune cells, such as macrophages, in metabolic tissues suggests dynamic, ongoing crosstalk between these two regulatory systems. Here, we discuss how changes in the recruitment and activation of macrophages contribute to metabolic homeostasis. In particular, we focus our discussion on the pathogenic and protective functions of classically and alternatively activated macrophages, respectively, in experimental models of obesity and metabolic disease.

Innate immune recognition receptors and damage-associated molecular patterns in plaque inflammation

PURPOSE OF REVIEW

To highlight critical advances achieved over the last year in the study of endogenous proatherogenic danger signals and corresponding molecular mechanism of innate immune signalling in atherosclerosis.

RECENT FINDINGS

The identity and signalling mechanisms of LDL-derived inflammatory components are central in understanding the pathogenic role of modified LDL in the development of atherosclerosis. Studies in the preceding years have revealed LDL-derived phospholipids and cholesterol crystals as endogenous danger signals. These danger signals trigger Toll-like receptors and nucleotide-binding oligomerization domain-like receptors inflammasome respectively, thereby instigating inflammatory responses and promoting disease progression.

SUMMARY

Recent understandings of the causal role of LDL in atherosclerosis provide a new perspective on modified LDL-derived danger signals. These insights suggest dysregulated Toll-like receptor and nucleotide-binding oligomerization domain inflammasome signalling as an important mechanism underlying atherogenesis.

The inflammasome and caspase-1 activation: a new mechanism underlying increased inflammatory activity in human visceral adipose tissue

The immune competent abdominal adipose tissue, either stored viscerally [visceral adipose tissue (VAT)] or sc [sc adipose tissue (SAT)], has been identified as a source of IL-1β and IL-18. To become active, the proforms of these cytokines require processing by caspase-1, which itself is mediated by the inflammasome. In this descriptive study, we investigate the expression of inflammasome components and caspase-1 in human fat and determine whether caspase-1 activity contributes to the enhanced inflammatory status of VAT. Paired SAT and VAT biopsies from 10 overweight subjects (body mass index, 25-28 kg/m(2)) were used to study the cellular composition and the intrinsic inflammatory capacity of both adipose tissue depots. The percentage of CD8(+) T cells within the lymphocyte fraction was significantly higher in VAT compared with SAT (41.6 vs. 30.4%; P < 0.05). Adipose tissue cultures showed a higher release of IL-1β (10-fold; P < 0.05), IL-18 (3-fold; P < 0.05), and IL-6 and IL-8 (3-fold, P < 0.05; and 4-fold, P < 0.05, respectively) from VAT compared with SAT that was significantly reduced by inhibiting caspase-1 activity. In addition, caspase-1 activity was 3-fold (P < 0.05) higher in VAT compared with SAT, together with an increase in the protein levels of the inflammasome members apoptosis-associated speck-like protein containing a C-terminal caspase-recruitment domain (2-fold; P < 0.05) and nucleotide-binding oligomerization domain-like receptor pyrin domain containing 3 (2-fold; nonsignificant). Finally, caspase-1 activity levels were positively correlated with the percentage of CD8(+) T cells present in adipose tissue. Our results show that caspase-1 and nucleotide-binding oligomerization domain-like receptor pyrin domain containing 3 inflammasome members are abundantly present in human VAT. The increased intrinsic caspase-1 activity in VAT represents a novel and specific inflammatory pathway that may determine the proinflammatory character of this specific depot.

NOD1 activators link innate immunity to insulin resistance

OBJECTIVE

Insulin resistance associates with chronic inflammation, and participatory elements of the immune system are emerging. We hypothesized that bacterial elements acting on distinct intracellular pattern recognition receptors of the innate immune system, such as bacterial peptidoglycan (PGN) acting on nucleotide oligomerization domain (NOD) proteins, contribute to insulin resistance.

RESEARCH DESIGN AND METHODS

Metabolic and inflammatory properties were assessed in wild-type (WT) and NOD1/2(-/-) double knockout mice fed a high-fat diet (HFD) for 16 weeks. Insulin resistance was measured by hyperinsulinemic euglycemic clamps in mice injected with mimetics of meso-diaminopimelic acid-containing PGN or the minimal bioactive PGN motif, which activate NOD1 and NOD2, respectively. Systemic and tissue-specific inflammation was assessed using enzyme-linked immunosorbent assays in NOD ligand-injected mice. Cytokine secretion, glucose uptake, and insulin signaling were assessed in adipocytes and primary hepatocytes exposed to NOD ligands in vitro.

RESULTS

NOD1/2(-/-) mice were protected from HFD-induced inflammation, lipid accumulation, and peripheral insulin intolerance. Conversely, direct activation of NOD1 protein caused insulin resistance. NOD1 ligands induced peripheral and hepatic insulin resistance within 6 h in WT, but not NOD1(-/-), mice. NOD2 ligands only modestly reduced peripheral glucose disposal. NOD1 ligand elicited minor changes in circulating proinflammatory mediators, yet caused adipose tissue inflammation and insulin resistance of muscle AS160 and liver FOXO1. Ex vivo, NOD1 ligand caused proinflammatory cytokine secretion and impaired insulin-stimulated glucose uptake directly in adipocytes. NOD1 ligand also caused inflammation and insulin resistance directly in primary hepatocytes from WT, but not NOD1(-/-), mice.

CONCLUSIONS

We identify NOD proteins as innate immune components that are involved in diet-induced inflammation and insulin intolerance. Acute activation of NOD proteins by mimetics of bacterial PGNs causes whole-body insulin resistance, bolstering the concept that innate immune responses to distinctive bacterial cues directly lead to insulin resistance. Hence, NOD1 is a plausible, new link between innate immunity and metabolism.

Inflammasome is a central player in the induction of obesity and insulin resistance

Inflammation plays a key role in the pathogenesis of obesity. Chronic overfeeding leads to macrophage infiltration in the adipose tissue, resulting in proinflammatory cytokine production. Both microbial and endogenous danger signals trigger assembly of the intracellular innate immune sensor Nlrp3, resulting in caspase-1 activation and production of proinflammatory cytokines IL-1β and IL-18. Here, we showed that mice deficient in Nlrp3, apoptosis-associated speck-like protein, and caspase-1 were resistant to the development of high-fat diet-induced obesity, which correlated with protection from obesity-induced insulin resistance. Furthermore, hepatic triglyceride content, adipocyte size, and macrophage infiltration in adipose tissue were all reduced in mice deficient in inflammasome components. Monocyte chemoattractant protein (MCP)-1 is a key molecule that mediates macrophage infiltration. Indeed, defective inflammasome activation was associated with reduced MCP-1 production in adipose tissue. Furthermore, plasma leptin and resistin that affect energy use and insulin sensitivity were also changed by inflammasome-deficiency. Detailed metabolic and molecular phenotyping demonstrated that the inflammasome controls energy expenditure and adipogenic gene expression during chronic overfeeding. These findings reveal a critical function of the inflammasome in obesity and insulin resistance, and suggest inhibition of the inflammasome as a potential therapeutic strategy.

Kupffer cell depletion prevents but has no therapeutic effect on metabolic and inflammatory changes induced by a high-fat diet

We aimed to evaluate activation of macrophages in insulin-sensitive tissues (liver, adipose tissue, and muscles) under high-fat diet (HFD) and elucidate the role of Kupffer cells (KC) in HFD-induced insulin resistance. Tissue macrophage populations, insulin signaling, and sensitivity were evaluated in mice fed a HFD for 4 or 16 wk. Selective KC depletion was obtained by intravenous injections of liposome-encapsulated clodronate. Mice fed a HFD for 4 to 16 wk have hepatic and peripheral insulin resistance together with macrophage recruitment in the adipose tissue but not in the liver. Depletion of KC for the last 10 d of the 16 wk experiment fails to improve insulin sensitivity compared to PBS-treated animals. In contrast, preventive KC depletion prior to and during the 4 wk HFD attenuates the development of obesity, adiposity, adipose tissue inflammation (P<0.01 vs. PBS group), and insulin resistance (P<0.01). Interestingly, in mice fed a normal diet, prolonged KC depletion ameliorates insulin sensitivity and decreases adiposity without altering physiological body weight gain or food intake. Preventive and prolonged KC depletion ameliorates insulin sensitivity and prevents adipose tissue inflammation, suggesting a communication between the liver and the adipose tissue in the development of HFD-induced metabolic alterations.

Purinergic regulation of high-glucose-induced caspase-1 activation in the rat retinal Müller cell line rMC-1

Chronic activation of proinflammatory caspase-1 in the retinas of diabetic animals and patients in vivo and retinal Müller cells in vitro is well documented. In this study we characterized how elevated glucose and extracellular purines contribute to the activation of caspase-1 in a cultured rat Müller cell (rMC-1) model. The ability of high glucose (25 mM, 24 h) to activate caspase-1 was attenuated by either apyrase, which metabolizes extracellular ATP to AMP, or adenosine deaminase (ADA), which metabolizes extracellular adenosine to inosine. This suggested that autocrine stimulation of ATP-sensing P2 receptors and adenosine-sensing P1 receptors may in part mediate the response to high glucose. Exogenous ATP, 5'-N-ethylcarboxamido-adenosine (NECA), a nonselective P1 receptor agonist, or forskolin (FSK) increased caspase-1 activity in rMC-1 cells cultured in control glucose (5 mM) medium. Accumulation of active caspase-1 was also increased by dipyridamole, which suppresses adenosine reuptake. High-glucose stimulation of caspase-1 was attenuated by suramin, a nonselective P2 antagonist, or A2 adenosine receptor antagonists, but not by antagonism of P2X7 ATP-gated ion channel receptors. Although high glucose increased P2X7 mRNA, neither P2X7 protein nor function was detected in rMC-1 cells. The increased caspase-1 activity stimulated by high glucose, FSK, NECA, or ATP was correlated with increased gene expression of caspase-1 and thioredoxin-interacting-protein (TXNIP). These findings support a novel role for autocrine P1 and P2 purinergic receptors coupled to cAMP signaling cascades and transcriptional induction of caspase-1 in mediating the high-glucose-induced activation of caspase-1 and secretion of IL-1β in a cell culture model of nonhematopoietic retinal Müller cells.

Intestinal mucosal adherence and translocation of commensal bacteria at the early onset of type 2 diabetes: molecular mechanisms and probiotic treatment

A fat-enriched diet modifies intestinal microbiota and initiates a low-grade inflammation, insulin resistance and type-2 diabetes. Here, we demonstrate that before the onset of diabetes, after only one week of a high-fat diet (HFD), live commensal intestinal bacteria are present in large numbers in the adipose tissue and the blood where they can induce inflammation. This translocation is prevented in mice lacking the microbial pattern recognition receptors Nod1 or CD14, but overtly increased in Myd88 knockout and ob/ob mouse. This ‘metabolic bacteremia’ is characterized by an increased co-localization with dendritic cells from the intestinal lamina propria and by an augmented intestinal mucosal adherence of non-pathogenic Escherichia coli. The bacterial translocation process from intestine towards tissue can be reversed by six weeks of treatment with the probiotic strain Bifidobacterium animalis subsp. lactis 420, which improves the animals' overall inflammatory and metabolic status. Altogether, these data demonstrate that the early onset of HFD-induced hyperglycemia is characterized by an increased bacterial translocation from intestine towards tissues, fuelling a continuous metabolic bacteremia, which could represent new therapeutic targets.

The NLRP3 inflammasome in health and disease: the good, the bad and the ugly

While interleukin (IL)-1β plays an important role in combating the invading pathogen as part of the innate immune response, its dysregulation is responsible for a number of autoinflammatory disorders. Large IL-1β activating platforms, known as inflammasomes, can assemble in response to the detection of endogenous host and pathogen-associated danger molecules. Formation of these protein complexes results in the autocatalysis and activation of caspase-1, which processes precursor IL-1β into its secreted biologically active form. Inflammasome and IL-1β activity is required to efficiently control viral, bacterial and fungal pathogen infections. Conversely, excess IL-1β activity contributes to human disease, and its inhibition has proved therapeutically beneficial in the treatment of a spectrum of serious, yet relatively rare, heritable inflammasomopathies. Recently, inflammasome function has been implicated in more common human conditions, such as gout, type II diabetes and cancer. This raises the possibility that anti-IL-1 therapeutics may have broader applications than anticipated previously, and may be utilized across diverse disease states that are linked insidiously through unwanted or heightened inflammasome activity.

Adipose tissue remodeling as homeostatic inflammation

Evidence has accumulated indicating that obesity is associated with a state of chronic, low-grade inflammation. Obese adipose tissue is characterized by dynamic changes in cellular composition and function, which may be referred to as “adipose tissue remodeling”. Among stromal cells in the adipose tissue, infiltrated macrophages play an important role in adipose tissue inflammation and systemic insulin resistance. We have demonstrated that a paracrine loop involving saturated fatty acids and tumor necrosis factor-α derived from adipocytes and macrophages, respectively, aggravates obesity-induced adipose tissue inflammation. Notably, saturated fatty acids, which are released from hypertrophied adipocytes via the macrophage-induced lipolysis, serve as a naturally occurring ligand for Toll-like receptor 4 complex, thereby activating macrophages. Such a sustained interaction between endogenous ligands derived from parenchymal cells and pathogen sensors expressed in stromal immune cells should lead to chronic inflammatory responses ranging from the basal homeostatic state to diseased tissue remodeling, which may be referred to as “homeostatic inflammation”. We, therefore, postulate that adipose tissue remodeling may represent a prototypic example of homeostatic inflammation. Understanding the molecular mechanism underlying homeostatic inflammation may lead to the identification of novel therapeutic strategies to prevent or treat obesity-related complications.

NLRP3 inflammasomes link inflammation and metabolic disease

A strong link between inflammation and metabolism is becoming increasingly evident. A number of recent landmark studies have implicated the activation of the NLRP3 inflammasome, an interleukin-1β family cytokine-activating protein complex, in a variety of metabolic diseases including obesity, atherosclerosis and type 2 diabetes. Here, we review these new developments and discuss their implications for a better understanding of inflammation in metabolic disease, and the prospects of targeting the NLRP3 inflammasome for therapeutic intervention.

Adipose tissue macrophages: phenotypic plasticity and diversity in lean and obese states

PURPOSE OF REVIEW

Proinflammatory adipose tissue macrophages (ATMs) contribute to obesity-associated disease morbidity. We will provide an update of the current state of knowledge regarding the phenotypic and functional diversity of ATMs in lean and obese mice and humans.

RECENT FINDINGS

The phenotypic diversity of ATMs is now known to include more than two types requiring an expansion of the simple concept of an M2 to M1 shift with obesity. Potential functions for ATMs now include the regulation of fibrosis and response to acute lipolysis in states of caloric restriction. Novel pathways that can potentiate ATM action have been identified, which include inflammasome activation and the response to lipodystrophic adipose tissue. Studies provide a new appreciation for the ability of ATMs to respond dynamically to the adipose tissue microenvironment.

SUMMARY

ATMs play a key role in shaping the inflammatory milieu within adipose tissue, and it is now apparent that ATM heterogeneity is acutely shaped by the adipose tissue environment. To account for the new findings, we propose a new nomenclature for ATM subtypes that takes into account their diversity.

A clinical perspective of IL-1β as the gatekeeper of inflammation

An expanding spectrum of acute and chronic non-infectious inflammatory diseases is uniquely responsive to IL-1β neutralization. IL-1β-mediated diseases are often called "auto-inflammatory" and the dominant finding is the release of the active form of IL-1β driven by endogenous molecules acting on the monocyte/macrophage. IL-1β activity is tightly controlled and requires the conversion of the primary transcript, the inactive IL-1β precursor, to the active cytokine by limited proteolysis. Limited proteolysis can take place extracellularly by serine proteases, released in particular by infiltrating neutrophils or intracellularly by the cysteine protease caspase-1. Therefore, blocking IL-1β resolves inflammation regardless of how the cytokine is released from the cell or how the precursor is cleaved. Endogenous stimulants such as oxidized fatty acids and lipoproteins, high glucose concentrations, uric acid crystals, activated complement, contents of necrotic cells, and cytokines, particularly IL-1 itself, induce the synthesis of the inactive IL-1β precursor, which awaits processing to the active form. Although bursts of IL-1β precipitate acute attacks of systemic or local inflammation, IL-1β also contributes to several chronic diseases. For example, ischemic injury, such as myocardial infarction or stroke, causes acute and extensive damage, and slowly progressive inflammatory processes take place in atherosclerosis, type 2 diabetes, osteoarthritis and smoldering myeloma. Evidence for the involvement of IL-1β and the clinical results of reducing IL-1β activity in this broad spectrum of inflammatory diseases are the focus of this review.

Mitochondria: Sovereign of inflammation?

NLRP3 inflammasome-dependent inflammatory responses are triggered by a variety of signals of host danger, including infection, tissue damage and metabolic dysregulation. How these diverse activators cause inflammasome activation is poorly understood. Recent data suggest that the mitochondria integrate these distinct signals and relay this information to the NLRP3 inflammasome. Dysfunctional mitochondria generate ROS, which is required for inflammasome activation. On the contrary, the NLRP3 inflammasome is negatively regulated by autophagy, which is a catabolic process that removes damaged or otherwise dysfunctional organelles, including mitochondria. In addition to the processing and secretion of pro-inflammatory cytokines such as IL-1β, NLRP3 inflammasome activation also influences cellular metabolic pathways such as glycolysis and lipogenesis. Mapping the connections between mitochondria, metabolism and inflammation is of great interest, as malfunctioning of this network is associated with many chronic inflammatory diseases.

Lack of interleukin-1 receptor I (IL-1RI) protects mice from high-fat diet-induced adipose tissue inflammation coincident with improved glucose homeostasis

OBJECTIVE

High-fat diet (HFD)-induced adipose tissue inflammation is a critical feature of diet-induced insulin resistance (IR); however, the contribution of interleukin-1 receptor I (IL-1RI)-mediated signals to this phenotype has not been defined. We hypothesized that lack of IL-1RI may ameliorate HFD-induced IR by attenuating adipose tissue inflammation.

RESEARCH DESIGN AND METHODS

Glucose homeostasis was monitored in chow- and HFD-fed wild-type (WT) and IL-1RI(-/-) mice by glucose tolerance and insulin tolerance tests. Macrophage recruitment and cytokine signature of adipose tissue macrophages was evaluated. Insulin sensitivity and cytokine secretion from adipose explants was quantified. Cytokine secretion and adipocyte insulin sensitivity was measured in cocultures of WT or IL-1RI(-/-) macrophages with 3T3L1 adipocytes. Synergistic effects of IL-1β with tumor necrosis factor (TNF)-α on inflammation was monitored in WT and IL-1RI(-/-) bone-marrow macrophages and adipose explants.

RESULTS

Lean and obese IL-1RI(-/-) animals exhibited enhanced glucose homeostasis by glucose tolerance test and insulin tolerance test. M1/M2 macrophage number in adipose tissue was comparable between genotypes; however, TNF-α and IL-6 secretion was lower from IL-1RI(-/-) adipose tissue macrophages. IL-1RI(-/-) adipose exhibited enhanced insulin sensitivity, elevated pAKT, lower cytokine secretion, and attenuated induction of phosphorylated signal transducer and activator of transcription 3 and suppressor of cytokine signaling molecule 3 after HFD. Coculture of WT, but not IL-1RI(-/-) macrophages, with 3T3L1 adipocytes enhanced IL-6 and TNF-α secretion, reduced adiponectin secretion, and impaired adipocyte insulin sensitivity. TNF-α and IL-1β potently synergized to enhance inflammation in WT macrophages and adipose, an effect lost in the absence of IL-1RI.

CONCLUSIONS

Lack of IL-1RI protects against HFD-induced IR coincident with reduced local adipose tissue inflammation, despite equivalent immune cell recruitment.

The emerging role of metabolic regulation in the functioning of Toll-like receptors and the NOD-like receptor Nlrp3

While it has long been suspected that inflammation participates in the pathogenesis of metabolic disorders such as the insulin resistance that occurs in type 2 diabetes, recent work suggests that this is not the only important interaction between metabolism and inflammation. Inroads into the understanding of the relationship between metabolic pathways and inflammation are indicating that signaling by innate immune receptors such as TLR4 and Nlrp3 regulate metabolism. TLRs have been shown to promote glycolysis, whilst Nlrp3-mediated production of IL-1β causes insulin resistance. A key role for the hypoxia-sensing transcription factor HIF1α in the functioning of macrophages activated by TLRs has also recently emerged. This review will assess recent evidence for these complex interactions and speculate on their importance for innate immunity and inflammation.