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

Functions of NOD-Like Receptors in Human Diseases

Nucleotide-binding and oligomerization domain NOD-like receptors (NLRs) are highly conserved cytosolic pattern recognition receptors that perform critical functions in surveying the intracellular environment for the presence of infection, noxious substances, and metabolic perturbations. Sensing of these danger signals by NLRs leads to their oligomerization into large macromolecular scaffolds and the rapid deployment of effector signaling cascades to restore homeostasis. While some NLRs operate by recruiting and activating inflammatory caspases into inflammasomes, others trigger inflammation via alternative routes including the nuclear factor-κB, mitogen-activated protein kinase, and regulatory factor pathways. The critical role of NLRs in development and physiology is demonstrated by their clear implications in human diseases. Mutations in the genes encoding NLRP3 or NLRP12 lead to hereditary periodic fever syndromes, while mutations in CARD15 that encodes NOD2 are linked to Crohn’s disease or Blau’s syndrome. Genome-wide association studies (GWASs) have identified a number of risk alleles encompassing NLR genes in a host of diseases including allergic rhinitis, multiple sclerosis, inflammatory bowel disease, asthma, multi-bacillary leprosy, vitiligo, early-onset menopause, and bone density loss in elderly women. Animal models have allowed the characterization of underlying effector mechanisms in a number of cases. In this review, we highlight the functions of NLRs in health and disease and discuss how the characterization of their molecular mechanisms provides new insights into therapeutic strategies for the management of inflammatory pathologies.

Long-term exposure to a high-fat diet results in the development of glucose intolerance and insulin resistance in interleukin-1 receptor I-deficient mice

Emerging evidence has demonstrated that saturated fatty acids prime pro-IL-1β production and inflammasome-mediated IL-1β activation is critical in obesity-associated insulin resistance (IR). Nonetheless, IL-1 receptor I-deficient (IL-1RI(-/-)) mice develop mature-onset obesity despite consuming a low-fat diet (LFD). With this apparent contradiction, the present study evaluated whether IL-1RI(-/-) mice were protected against long-term (6 mo) high-fat diet (HFD)-induced IR. Male wild-type and IL-1RI(-/-) mice were fed LFD or HFD for 3 or 6 mo, and glucose and insulin tolerance tests were performed. Adipose insulin sensitivity, cytokine profiles, and adipocyte morphology were assessed. The adipogenic potential of stromal vascular fraction was determined. Hepatic lipid accumulation and insulin sensitivity were characterized. IL-1RI(-/-) mice developed glucose intolerance and IR after 6 mo HFD compared with 3 mo HFD, coincident with enhanced weight gain, hyperinsulinemia, and hyperleptinemia. The aggravated IR phenotype was associated with loss of adipose functionality, switch from adipocyte hyperplasia to hypertrophy and hepatosteatosis. Induction of adipogenic genes was reduced in IL-1RI(-/-) preadipocytes after 6 mo HFD compared with 3 mo HFD. Obese LFD-IL-1RI(-/-) mice exhibited preserved metabolic health. IL-1RI(-/-) mice develop glucose intolerance and IR after 6 mo HFD intervention. While mature-onset obesity is evident in LFD-IL-1RI(-/-) mice, the additional metabolic insult of HFD was required to drive adipose inflammation and systemic IR. These findings indicate an important interaction between dietary fat and IL-1, relevant to optimal metabolic health.

Activation and regulation of the pattern recognition receptors in obesity-induced adipose tissue inflammation and insulin resistance

Obesity-associated chronic tissue inflammation is a key contributing factor to type 2 diabetes mellitus, and a number of studies have clearly demonstrated that the immune system and metabolism are highly integrated. Recent advances in deciphering the various immune cells and signaling networks that link the immune and metabolic systems have contributed to our understanding of the pathogenesis of obesity-associated inflammation. Other recent studies have suggested that pattern recognition receptors in the innate immune system recognize various kinds of endogenous and exogenous ligands, and have a crucial role in initiating or promoting obesity-associated chronic inflammation. Importantly, these mediators act on insulin target cells or on insulin-producing cells impairing insulin sensitivity and its secretion. Here, we discuss how various pattern recognition receptors in the immune system underlie the etiology of obesity-associated inflammation and insulin resistance, with a particular focus on the TLR (Toll-like receptor) family protein Radioprotective 105 (RP105)/myeloid differentiation protein-1 (MD-1).

Neuroendocrine and cardiac metabolic dysfunction and NLRP3 inflammasome activation in adipose tissue and pancreas following chronic spinal cord injury in the mouse

CVD (cardiovascular disease) represents a leading cause of mortality in chronic SCI (spinal cord injury). Several component risk factors are observed in SCI; however, the underlying mechanisms that contribute to these risks have not been defined. Central and peripheral chronic inflammation is associated with metabolic dysfunction and CVD, including adipokine regulation of neuroendocrine and cardiac function and inflammatory processes initiated by the innate immune response. We use female C57 Bl/6 mice to examine neuroendocrine, cardiac, adipose and pancreatic signaling related to inflammation and metabolic dysfunction in response to experimentally induced chronic SCI. Using immuno-histochemical, -precipitation, and -blotting analysis, we show decreased POMC (proopiomelanocortin) and increased NPY (neuropeptide-Y) expression in the hypothalamic ARC (arcuate nucleus) and PVN (paraventricular nucleus), 1-month post-SCI. Long-form leptin receptor (Ob-Rb), JAK2 (Janus kinase)/STAT3 (signal transducer and activator of transcription 3)/p38 and RhoA/ROCK (Rho-associated kinase) signaling is significantly increased in the heart tissue post-SCI, and we observe the formation and activation of the NLRP3 (NOD-like receptor family, pyrin domain containing 3) inflammasome in VAT (visceral adipose tissue) and pancreas post-SCI. These data demonstrate neuroendocrine signaling peptide alterations, associated with central inflammation and metabolic dysfunction post-SCI, and provide evidence for the peripheral activation of signaling mechanisms involved in cardiac, VAT and pancreatic inflammation and metabolic dysfunction post-SCI. Further understanding of biological mechanisms contributing to SCI-related inflammatory processes and metabolic dysfunction associated with CVD pathology may help to direct therapeutic and rehabilitation countermeasures.

The purinergic 2X7 receptor participates in renal inflammation and injury induced by high-fat diet: possible role of NLRP3 inflammasome activation

Renal disease associated with type 2 diabetes and the metabolic syndrome is characterized by a distinct inflammatory phenotype. The purinergic 2X7 receptor (P2X7 R) and the nucleotide-binding and oligomerization domain-like receptor containing a pyrin domain 3 (NLRP3) inflammasome have been separately shown to play a role in two models of non-metabolic chronic kidney disease. Moreover, the NLRP3 inflammasome has been implicated in chronic low-grade sterile inflammation characterizing metabolic disorders, though the mechanism(s) involved in inflammasome activation under these conditions are still unknown. We investigated the role of P2X7 R (through activation of the NLRP3 inflammasome) in renal inflammation and injury induced by a high-fat diet, an established model of the metabolic syndrome. On a high-fat diet, mice lacking P2X7 R developed attenuated renal functional and structural alterations as well as reduced inflammation, fibrosis, and oxidative/carbonyl stress, as compared with wild-type animals, in the absence of significant differences in metabolic parameters. This was associated with blunted up-regulation of the NLRP3 inflammasome components NLRP3, apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), pro-caspase 1, pro-interleukin (IL)-1β, and pro-IL-18, as well as reduced inflammasome activation, as evidenced by decreased formation of mature caspase 1, whereas mature IL-1β and IL-18 were not detected. Up-regulated expression of NLRP3 and pro-caspase 1, post-translational processing of pro-caspase-1, and release of IL-18 in response to lipopolysaccharide + 2'(3')-O-(4-benzoylbenzoyl)ATP were attenuated by P2X7 R silencing in cultured mouse podocytes. Protein and mRNA expression of P2X7 R, NLRP3, and ASC were also increased in kidneys from subjects with type 2 diabetes and the metabolic syndrome, showing histologically documented renal disease. These data provide evidence of a major role for the purinergic system, at least in part through activation of the NLRP3 inflammasome, in the process driving 'metabolic' renal inflammation and injury and identify P2X7 R and NLRP3 as novel therapeutic targets.

Fatty acid-induced mitochondrial uncoupling elicits inflammasome-independent IL-1α and sterile vascular inflammation in atherosclerosis

Chronic inflammation is a fundamental aspect of metabolic disorders such as obesity, diabetes and cardiovascular disease. Cholesterol crystals are metabolic signals that trigger sterile inflammation in atherosclerosis, presumably by activating inflammasomes for IL-1β production. We found here that atherogenesis was mediated by IL-1α and we identified fatty acids as potent inducers of IL-1α-driven vascular inflammation. Fatty acids selectively stimulated the release of IL-1α but not of IL-1β by uncoupling mitochondrial respiration. Fatty acid-induced mitochondrial uncoupling abrogated IL-1β secretion, which deviated the cholesterol crystal-elicited response toward selective production of IL-1α. Our findings delineate a previously unknown pathway for vascular immunopathology that links the cellular response to metabolic stress with innate inflammation, and suggest that IL-1α, not IL-1β, should be targeted in patients with cardiovascular disease.

Variants of NLRP3 gene are associated with insulin resistance in Chinese Han population with type-2 diabetes

AIMS

Nod like receptor pyrin domain containing 3 (NLRP3) is the best characterized member of nod like receptor family. Recent studies suggest that NLRP3 plays a crucial role in the pathogenesis of type-2 diabetes (T2DM), and variants in NLRP3 affect its mRNA stability and expression. Therefore, we hypothesize that the variants in NLRP3 gene may contribute to T2DM susceptibility. The aim of this study is to evaluate the association of NLRP3 SNPs with T2DM in Chinese Han patients.

METHODS

Two common variants in NLRP3 gene, rs10754558 and rs4612666, were detected using the polymerase chain reaction-restriction fragment length polymorphism procedure in 952 unrelated T2DM patients and 871 healthy controls. All participants were unrelated Chinese Hans.

RESULTS

The GG genotype and G allele frequencies of rs10754558 were significantly higher in T2DM patients than those in controls (for GG genotype, 19.6% vs. 14.5%, p=0.019; for G allele, 43.9% vs. 39.8%, p=0.013). The GG genotype of rs10754558 was significantly associated with higher LDL-C levels and more prone to insulin resistance, as evaluated by HOMA-IR or QUICK indexes.

CONCLUSIONS

The variant (rs10754558) in NLRP3 is related to insulin resistance and increased risk of T2DM in Chinese Han population.

Diet-induced obesity, adipose inflammation, and metabolic dysfunction correlating with PAR2 expression are attenuated by PAR2 antagonism

Excessive uptake of fatty acids and glucose by adipose tissue triggers adipocyte dysfunction and infiltration of immune cells. Altered metabolic homeostasis in adipose tissue promotes insulin resistance, type 2 diabetes, hypertension, and cardiovascular disease. Inflammatory and metabolic processes are mediated by certain proteolytic enzymes that share a common cellular target, protease-activated receptor 2 (PAR2). This study showed that human and rat obesity correlated in vivo with increased expression of PAR2 in adipose tissue, primarily in stromal vascular cells (SVCs) including macrophages. PAR2 was expressed more than other PARs on human macrophages and was increased by dietary fatty acids (palmitic, stearic, and myristic). A novel PAR2 antagonist, GB88 (5-isoxazoyl-Cha-Ile-spiroindene-1,4-piperidine), given orally at 10 mg/kg/d (wk 8-16) reduced body weight by ∼10% in obese rats fed a high-carbohydrate high-fat (HCHF) diet for 16 wk, and strongly attenuated adiposity, adipose tissue inflammation, infiltrated macrophages and mast cells, insulin resistance, and cardiac fibrosis and remodeling; while reversing liver and pancreatic dysfunction and normalizing secretion of PAR2-directed glucose-stimulated insulin secretion in MIN6 β cells. In summary, PAR2 is a new biomarker for obesity, and its expression is stimulated by dietary fatty acids; PAR2 is a substantial contributor to inflammatory and metabolic dysfunction; and a PAR2 antagonist inhibits diet-induced obesity and inflammatory, metabolic, and cardiovascular dysfunction.

Activation of the Nlrp3 inflammasome in infiltrating macrophages by endocannabinoids mediates beta cell loss in type 2 diabetes

Type 2 diabetes mellitus (T2DM) progresses from compensated insulin resistance to beta cell failure resulting in uncompensated hyperglycemia, a process replicated in the Zucker diabetic fatty (ZDF) rat. The Nlrp3 inflammasome has been implicated in obesity-induced insulin resistance and beta cell failure. Endocannabinoids contribute to insulin resistance through activation of peripheral CB1 receptors (CB₁Rs) and also promote beta cell failure. Here we show that beta cell failure in adult ZDF rats is not associated with CB₁R signaling in beta cells, but rather in M1 macrophages infiltrating into pancreatic islets, and that this leads to activation of the Nlrp3-ASC inflammasome in the macrophages. These effects are replicated in vitro by incubating wild-type human or rodent macrophages, but not macrophages from CB₁R-deficient (Cnr1(-/-)) or Nlrp3(-/-) mice, with the endocannabinoid anandamide. Peripheral CB₁R blockade, in vivo depletion of macrophages or macrophage-specific knockdown of CB₁R reverses or prevents these changes and restores normoglycemia and glucose-induced insulin secretion. These findings implicate endocannabinoids and inflammasome activation in beta cell failure and identify macrophage-expressed CB₁R as a therapeutic target in T2DM.

Treatment implications of the altered cytokine-insulin axis in neurodegenerative disease

The disappointments of a series of large anti-amyloid trials have brought home the point that until the driving force behind Alzheimer's disease, and the way it causes harm, are firmly established and accepted, researchers will remain ill-equipped to find a way to treat patients successfully. The origin of inflammation in neurodegenerative diseases is still an open question. We champion and expand the argument that a shift in intracellular location of α-synuclein, thereby moving a key methylation enzyme from the nucleus, provides global hypomethylation of patients' cerebral DNA that, through being sensed by TLR9, initiates production of the cytokines that drive these cerebral inflammatory states. After providing a background on the relevant inflammatory cytokines, this commentary then discusses many of the known alternatives to the primary amyloid argument of the pathogenesis of Alzheimer's disease, and the treatment approaches they provide. A key point to appreciate is the weight of evidence that inflammatory cytokines, largely through increasing insulin resistance and thereby reducing the strength of the ubiquitously important signaling mediated by insulin, bring together most of these treatments under development for neurodegenerative disease under the one roof. Moreover, the principles involved apply to a wide range of inflammatory diseases on both sides of the blood brain barrier.

Connecting type 1 and type 2 diabetes through innate immunity

The escalating epidemic of obesity has driven the prevalence of both type 1 and 2 diabetes mellitus to historically high levels. Chronic low-grade inflammation, which is present in both type 1 and type 2 diabetics, contributes to the pathogenesis of insulin resistance. The accumulation of activated innate immune cells in metabolic tissues results in release of inflammatory mediators, in particular, IL-1β and TNFα, which promote systemic insulin resistance and β-cell damage. In this article, we discuss the central role of innate immunity and, in particular, the macrophage in insulin sensitivity and resistance, β-cell damage, and autoimmune insulitis. We conclude with a discussion of the therapeutic implications of this integrated understanding of diabetic pathology.

How biologics targeting the IL-1 system are being considered for the treatment of type 2 diabetes

Metabolic diseases are associated with activation of the innate immune system in various tissues and characterized by elevated inflammatory factors and the presence of immune cells. Type 2 diabetes develops when islet beta cells are deficient in producing sufficient insulin to overcome peripheral insulin resistance. Intra-islet IL-1β activity diminishes beta cell function and survival and governs islet inflammation. Targeting the IL-1 system with the IL-1 receptor antagonist IL1Ra improved insulin secretion, glycaemia and reduced systemic inflammation in a proof of concept study with patients with type 2 diabetes. Currently, long lasting and specific IL-1β blocking antibodies are being evaluated in clinical trials and this may lead to a novel cytokine-based treatment for type 2 diabetes.

The immune system as a sensor of the metabolic state

Mammals possess a remarkable ability to maintain and defend a constant internal milieu against diverse environmental threats. Unsurprisingly, the two systems tasked with these duties, metabolism and immunity, have evolved to share a common modular architecture that allows extensive bidirectional communication and coordination. Indeed, recent observations have highlighted numerous functionally critical immune regulatory modules located within diverse metabolic circuits. In this review, we discuss the architectural commonality between immunity and metabolism and highlight how these two primordially disparate systems leverage shared regulatory axes to coordinate metabolic physiology under conditions of normality and chronic overnutrition. Such an integrated perspective both advances our understanding of basic physiology and highlights potential opportunities for therapeutic intervention in metabolic dysfunction.

Obese adipocytes show ultrastructural features of stressed cells and die of pyroptosis

We previously suggested that, in obese animals and humans, white adipose tissue inflammation results from the death of hypertrophic adipocytes; these are then cleared by macrophages, giving rise to distinctive structures we denominated crown-like structures. Here we present evidence that subcutaneous and visceral hypertrophic adipocytes of leptin-deficient (ob/ob and db/db) obese mice exhibit ultrastructural abnormalities (including calcium accumulation and cholesterol crystals), many of which are more common in hyperglycemic db/db versus normoglycemic ob/ob mice and in visceral versus subcutaneous depots. Degenerating adipocytes whose intracellular content disperses in the extracellular space were also noted in obese mice; in addition, increased anti-reactive oxygen species enzyme expression in obese fat pads, documented by RT-PCR and immunohistochemistry, suggests that ultrastructural changes are accompanied by oxidative stress. RT-PCR showed NLRP3 inflammasome activation in the fat pads of both leptin-deficient and high-fat diet obese mice, in which formation of active caspase-1 was documented by immunohistochemistry in the cytoplasm of several hypertrophic adipocytes. Notably, caspase-1 was not detected in FAT-ATTAC transgenic mice, where adipocytes die of apoptosis. Thus, white adipocyte overexpansion induces a stress state that ultimately leads to death. NLRP3-dependent caspase-1 activation in hypertrophic adipocytes likely induces obese adipocyte death by pyroptosis, a proinflammatory programmed cell death.

NLRP7 and related inflammasome activating pattern recognition receptors and their function in host defense and disease

Host defense requires the maturation and release of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18 and the induction of pyroptotic cell death, which depends on the activation of inflammatory Caspases within inflammasomes by innate immune cells. Several cytosolic pattern recognition receptors (PRRs) have been implicated in this process in response to infectious and sterile agonists. Here we summarize the current knowledge on inflammasome-organizing PRRs, emphasizing the recently described NLRP7, and their implications in human disease.

The NLRP3 Inflammasome as a novel player of the intercellular crosstalk in metabolic disorders

The combination of obesity and type 2 diabetes is a serious health problem, which is projected to afflict 300 million people worldwide by 2020. Both clinical and translational laboratory studies have demonstrated that chronic inflammation is associated with obesity and obesity-related conditions such as insulin resistance. However, the precise etiopathogenetic mechanisms linking obesity to diabetes remain to be elucidated, and the pathways that mediate this phenomenon are not fully characterized. One of the most recently identified signaling pathways, whose activation seems to affect many metabolic disorders, is the “inflammasome,” a multiprotein complex composed of NLRP3 (nucleotide-binding domain and leucine-rich repeat protein 3), ASC (apoptosis-associated speck-like protein containing a CARD), and procaspase-1. NLRP3 inflammasome activation leads to the processing and secretion of the proinflammatory cytokines interleukin- (IL-) 1β and IL-18. The goal of this paper is to review new insights on the effects of the NLRP3 inflammasome activation in the complex mechanisms of crosstalk between different organs, for a better understanding of the role of chronic inflammation in metabolic disease pathogenesis. We will provide here a perspective on the current research on NLRP3 inflammasome, which may represent an innovative therapeutic target to reverse the detrimental metabolic consequences of the metabolic inflammation.

Lack of oncostatin M receptor β leads to adipose tissue inflammation and insulin resistance by switching macrophage phenotype

Oncostatin M (OSM), a member of the IL-6 family of cytokines, plays important roles in a variety of biological functions, including inflammatory responses. However, the roles of OSM in metabolic diseases are unknown. We herein analyzed the metabolic parameters of OSM receptor β subunit-deficient (OSMRβ(-/-)) mice under normal diet conditions. At 32 weeks of age, OSMRβ(-/-) mice exhibited mature-onset obesity, severer hepatic steatosis, and insulin resistance. Surprisingly, insulin resistance without obesity was observed in OSMRβ(-/-) mice at 16 weeks of age, suggesting that insulin resistance precedes obesity in OSMRβ(-/-) mice. Both OSM and OSMRβ were expressed strongly in the adipose tissue and little in some other metabolic organs, including the liver and skeletal muscle. In addition, OSMRβ is mainly expressed in the adipose tissue macrophages (ATMs) but not in adipocytes. In OSMRβ(-/-) mice, the ATMs were polarized to M1 phenotypes with the augmentation of adipose tissue inflammation. Treatment of OSMRβ(-/-) mice with an anti-inflammatory agent, sodium salicylate, improved insulin resistance. In addition, the stimulation of a macrophage cell line, RAW264.7, and peritoneal exudate macrophages with OSM resulted in the increased expression of M2 markers, IL-10, arginase-1, and CD206. Furthermore, treatment of C57BL/6J mice with OSM increased insulin sensitivity and polarized the phenotypes of ATMs to M2. Thus, OSM suppresses the development of insulin resistance at least in part through the polarization of the macrophage phenotypes to M2, and OSMRβ(-/-) mice provide a unique mouse model of metabolic diseases.

Innate immune receptors: key regulators of metabolic disease progression

The study of the intersection of immunology and metabolism is a growing field fueled by the increased prevalence of obesity-associated pathologies. Importantly, the capacity of the innate immune system to sense metabolic stress induced by nutritional surplus has been linked with the progression of obesity, insulin resistance, type 2 diabetes mellitus, nonalcoholic fatty liver disease, and atherosclerosis. Moreover, it is clear that the innate immune system regulates the composition of the intestinal microbiota, which impacts multiple host metabolic processes. Here we review recent studies in this emerging field with an emphasis on how innate immune receptors determine metabolic disease progression.

Selective Kv1.3 channel blocker as therapeutic for obesity and insulin resistance

Obesity is an epidemic, calling for innovative and reliable pharmacological strategies. Here, we show that ShK-186, a selective and potent blocker of the voltage-gated Kv1.3 channel, counteracts the negative effects of increased caloric intake in mice fed a diet rich in fat and fructose. ShK-186 reduced weight gain, adiposity, and fatty liver; decreased blood levels of cholesterol, sugar, HbA1c, insulin, and leptin; and enhanced peripheral insulin sensitivity. These changes mimic the effects of Kv1.3 gene deletion. ShK-186 did not alter weight gain in mice on a chow diet, suggesting that the obesity-inducing diet enhances sensitivity to Kv1.3 blockade. Several mechanisms may contribute to the therapeutic benefits of ShK-186. ShK-186 therapy activated brown adipose tissue as evidenced by a doubling of glucose uptake, and increased β-oxidation of fatty acids, glycolysis, fatty acid synthesis, and uncoupling protein 1 expression. Activation of brown adipose tissue manifested as augmented oxygen consumption and energy expenditure, with no change in caloric intake, locomotor activity, or thyroid hormone levels. The obesity diet induced Kv1.3 expression in the liver, and ShK-186 caused profound alterations in energy and lipid metabolism in the liver. This action on the liver may underlie the differential effectiveness of ShK-186 in mice fed a chow vs. an obesity diet. Our results highlight the potential use of Kv1.3 blockers for the treatment of obesity and insulin resistance.

Type 2 diabetes mellitus: a metabolic autoinflammatory disease

The recent molecular, biologic, and genetic understanding of the inflammasome has revolutionized the diagnosis of and therapy for the phenotypically heterogeneous group of rare oligogenic disorders, now recognized to have autoinflammatory origin. This article reviews the importance of inflammasome activation in the central and peripheral mechanisms underlying a common, multifactorial, lifestyle-related, and polygenetic disease (type 2 diabetes mellitus), and conceptualizes the notion that this health challenge should now be recognized to have an autoinflammatory cause. It is hoped that targeting these mechanisms will enable the introduction of novel therapies that attack the basic pathogenetic mechanisms of type 2 diabetes mellitus rather than the epiphenomena that are its consequences.

Mechanisms of obesity-induced inflammation and insulin resistance: insights into the emerging role of nutritional strategies

Obesity and associated chronic inflammation initiate a state of insulin resistance (IR). The secretion of chemoattractants such as MCP-1 and MIF and of cytokines IL-6, TNF-α, and IL-1β, draw immune cells including dendritic cells, T cells, and macrophages into adipose tissue (AT). Dysfunctional AT lipid metabolism leads to increased circulating free fatty acids, initiating inflammatory signaling cascades in the population of infiltrating cells. A feedback loop of pro-inflammatory cytokines exacerbates this pathological state, driving further immune cell infiltration and cytokine secretion and disrupts the insulin signaling cascade. Disruption of normal AT function is causative of defects in hepatic and skeletal muscle glucose homeostasis, resulting in systemic IR and ultimately the development of type 2 diabetes. Pharmaceutical strategies that target the inflammatory milieu may have some potential; however there are a number of safety concerns surrounding such pharmaceutical approaches. Nutritional anti-inflammatory interventions could offer a more suitable long-term alternative; whilst they may be less potent than some pharmaceutical anti-inflammatory agents, this may be advantageous for long-term therapy. This review will investigate obese AT biology, initiation of the inflammatory, and insulin resistant environment; and the mechanisms through which dietary anti-inflammatory components/functional nutrients may be beneficial.

Metabolic inflammation: role of cytokines in the crosstalk between adipose tissue and liver

The innate immune system and its major mediators, i.e., cytokines, are increasingly recognized as being of crucial importance in metabolic inflammation as observed in morbid obesity and type 2 diabetes (T2D). Morbid obesity is commonly associated with adipose tissue inflammation. Adipose tissue inflammation is characterized by an increased expression of various pro-inflammatory cytokines such as tumor necrosis factor-alpha, interleukin-1 and -6, and by a rather heterogenous cellular infiltrate including monocytes/macrophages, neutrophils, B lymphocytes, T lymphocytes, and others. It has been demonstrated that in patients with severe obesity and fatty liver disease, expression of these pro-inflammatory cytokines in adipose tissue is 100-1000 times higher compared with that in the liver. Therefore, the adipose tissue can be considered in the state of severe obesity as the "cytokine factory" of the body. Rapid weight loss almost entirely eliminates pro-inflammatory cytokines in the adipose tissue, and therefore provides a very potent anti-inflammatory strategy. In conclusion, there is increasing evidence that peripheral tissues such as the adipose tissue may affect disease processes in target organs such as the liver, pancreas, heart, or blood vessels, and may therefore significantly contribute to chronic inflammation as observed in obesity and T2D.

New pathways to control inflammatory responses in adipose tissue

Obesity is characterized by the presence of chronic inflammation in adipose tissue, particularly in the visceral compartment, that has been causally linked to development of obesity-associated comorbidities. This link can be either direct or indirect, through induction of insulin resistance. This review summarizes recent evidence on potential pharmacological targets of adipose tissue inflammation, with emphasis on mediators that are being studied for intervention in chronic inflammatory diseases and are therefore viable therapeutical candidates. Specifically, we discuss evidence on the role of the inflammasome and its downstream products as a potential target for anti-inflammatory strategies as well as T regulatory (Treg) cells and mediators involved in the resolution phase of inflammation such as resolvins, protectins, annexin A1 (ANXA1) and galectins as potential targets for novel agonist therapies.

Obesity development in caspase-1-deficient mice

Caspase-1 is a member of the intracellular cysteine protease family that mediates inflammation through the activation of the cytokines interleukin-1β (IL-1β) and interleukin-18 (IL-18). As mice lacking IL-18 become obese and insulin resistant, and both IL-18 and IL-1β have a role in overall energy balance, we sought to determine whether caspase-1 deficiency also causes obesity. Male and female caspase-1-deficient (caspase-1-/-) and control (wild-type (WT)) mice were fed either a high-fat (HF, 45% of kcal) or a low-fat (LF, 10% of kcal) synthetic diet starting at 6 weeks of age. Caspase-1-/- mice maintained lower but detectable levels of IL-18 compared with WT mice. Plasma IL-1β levels were below the detection limit for both KO and WT mice. Male caspase-1-/- mice gained extra fat mass by 16 weeks on the HF diet, but not until 40 weeks on the LF diet. Female capase-1-/- mice gained more fat by 28 weeks but only on the HF diet. These data indicate that caspase-1-/- mice develop obesity with an age and sex-dependent differences, and only male mice display obesity on LF diet. Overall, this study suggests that the lower level of IL-18 in caspase-1-/- mice might be causing obesity development similarly to IL-18-deficient mice.

Inflammasome in intestinal inflammation and cancer

The activation of specific cytosolic pathogen recognition receptors, the nucleotide-binding-oligomerization-domain- (NOD-) like receptors (NLRs), leads to the assembly of the inflammasome, a multimeric complex platform that activates caspase-1. The caspase-1 pathway leads to the upregulation of important cytokines from the interleukin (IL)-1 family, IL-1β, and IL-18, with subsequent activation of the innate immune response. In this review, we discuss the molecular structure, the mechanisms behind the inflammasome activation, and its possible role in the pathogenesis of inflammatory bowel diseases and intestinal cancer. Here, we show that the available data points towards the importance of the inflammasome in the innate intestinal immune response, being the complex involved in the maintenance of intestinal homeostasis, correct intestinal barrier function and efficient elimination of invading pathogens.

Mechanisms of disease: inflammasome activation and the development of type 2 diabetes

Over the recent past, the importance of aberrant immune cell activation as one of the contributing mechanisms to the development of insulin-resistance and type 2 diabetes (T2D) has been recognized. Among the panoply of pro-inflammatory cytokines that are linked to chronic metabolic diseases, new data suggests that interleukin-1β (IL-1β) may play an important role in initiating and sustaining inflammation-induced organ dysfunction in T2D. Therefore, factors that control secretion of bioactive IL-1β have therapeutic implications. In this regard, the identification of multiprotein scaffolding complexes, "inflammasomes," has been a great advance in our understanding of this process. The secretion of bioactive IL-1β is predominantly controlled by activation of caspase-1 through assembly of a multiprotein scaffold, "inflammasome" that is composed of NLRP3 (nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing-3) ASC (apoptosis associated speck-like protein containing a CARD) and procaspase-1. The NLRP3 inflammasome appears to be an important sensor of metabolic dysregulation and controls obesity-associated insulin resistance and pancreatic beta cell dysfunction. Initial clinical "proof of concept" studies suggest that blocking IL-1β may favorably modulate factors related to development and treatment of T2D. However, this potential therapeutic approach remains to be fully substantiated through phase-II clinical studies. Here, we outline the new immunological mechanisms that link metabolic dysfunction to the emergence of chronic inflammation and discuss the opportunities and challenges of future therapeutic approaches to dampen NLRP3 inflammasome activation or IL-1β signaling for controlling type 2 diabetes.

Role of caspase-1 in regulation of triglyceride metabolism

Caspase-1 is a cysteine protease that can be activated by both endogenous and exogenous inflammatory stimuli and has been shown to have important functions in processes as diverse as proteolytic activation of cytokines, cell death, and membrane repair. Caspase-1-dependent production of the inflammatory cytokines IL-1 and IL-18 has also been implicated in the regulation of appetite, body weight, glucose homeostasis, and lipid metabolism. Consistent with the emerging views of caspase-1 in metabolic regulation, we find that caspase-1-deficient mice have dramatically accelerated triglyceride clearance, without alteration in lipid production or absorption, and resultant decrease in steady-state circulating triglyceride and fatty acid levels. Surprisingly, this effect is independent of IL-1-family signaling, supporting the concept that caspase-1 influences lipid metabolism through multiple mechanisms, not limited to cytokines.

NLRP3 inflammasomes show high expression in aorta of patients with atherosclerosis

OBJECTIVES

The aim of the present study was to investigate the expression of NLRP3 in aorta of patients with coronary atherosclerosis and to explore the association between aortic expression levels of NLRP3 and atherosclerotic risk factors.

METHODS

We collected small pieces of ascending aorta from 36 patients undergoing coronary artery bypass graft (CABG) surgery, and the arterial tissues from 10 subjects without atherosclerosis through the kidney donation program were taken as control. The expression of NLRP3 of the research and control group was determined by immunohistochemistry. Gensini score was used to evaluate the severity of coronary atherosclerosis.

RESULTS

NLRP3 was strongly expressed in aorta of CABG patients. The aortic NLRP3 expression was elevated in patients with hypertension or diabetes, and smokers. The NLRP3 expression in aorta was positively correlated with total cholesterol, low density lipoprotein cholesterol, and lipoprotein(a) (P<0.05); but negatively correlated with high density lipoprotein cholesterol (P<0.05). Spearman correlation revealed that aortic NLRP3 expression had significant correlation with Gensini coronary severity scores (P<0.05).

CONCLUSIONS

NLRP3 was overexpressed in aorta of patients with coronary atherosclerosis and the aortic NLRP3 expression is correlated with the severity of coronary artery disease and the atherosclerotic risk factors.

Inflammation links excess fat to insulin resistance: the role of the interleukin-1 family

A growing body of evidence suggests that cytokines of the interleukin-1 (IL-1) family, particularly IL-1β but also IL-1Ra and IL-18, are involved in obesity-associated inflammation. IL-1β is produced via cleavage of pro-IL-1β by caspase-1, which in turn is activated by a multiprotein complex called the inflammasome. The components of the NLRP3 inflammasome are involved in sensing obesity-associated danger signals, both in mice and in human (obese) subjects, with caspase-1 seemingly the most crucial regulator. Autophagy is upregulated in obesity and may function as a mechanism to control IL-1β gene expression in adipose tissue to mitigate chronic inflammation. All these mechanisms are operative in human adipose tissue and appear to be more pronounced in human visceral compared to subcutaneous tissue. In animal studies, blocking caspase-1 activity results in decreased weight gain, decreased inflammation, and improved insulin sensitivity. Human intervention studies with IL-1Ra (anakinra) have reported beneficial effects in patients with diabetes, yet without significant changes in insulin sensitivity. Clearly, the IL-1 family of cytokines, especially IL-1β, plays an important role in obesity-associated inflammation and insulin resistance and may represent a therapeutic target to reverse the detrimental metabolic consequences of obesity.

The inflammation highway: metabolism accelerates inflammatory traffic in obesity

As humans evolved, perhaps the two strongest selection determinants of survival were a robust immune response able to clear bacterial, viral, and parasitic infection and an ability to efficiently store nutrients to survive times when food sources were scarce. These traits are not mutually exclusive. It is now apparent that critical proteins necessary for regulating energy metabolism, such as peroxisome proliferator-activated receptors, Toll-like receptors, and fatty acid-binding proteins, also act as links between nutrient metabolism and inflammatory pathway activation in immune cells. Obesity in humans is a symptom of energy imbalance: the scale has been tipped such that energy intake exceeds energy output and may be a result, in part, of evolutionary selection toward a phenotype characterized by efficient energy storage. As discussed in this review, obesity is a state of low-grade, chronic inflammation that promotes the development of insulin resistance and diabetes. Ironically, the formation of systemic and/or local, tissue-specific insulin resistance upon inflammatory cell activation may actually be a protective mechanism that co-evolved to repartition energy sources within the body during times of stress during infection. However, the point has been reached where a once beneficial adaptive trait has become detrimental to the health of the individual and an immense public health and economic burden. This article reviews the complex relationship between obesity, insulin resistance/diabetes, and inflammation, and although the liver, brain, pancreas, muscle, and other tissues are relevant, we focus specifically on how the obese adipose microenvironment can promote immune cell influx and sustain damaging inflammation that can lead to the onset of insulin resistance and diabetes. Finally, we address how substrate metabolism may regulate the immune response and discuss how fuel uptake and metabolism may be a targetable approach to limit or abrogate obesity-induced inflammation.

Redox-inflammatory synergy in the metabolic syndrome

Metabolic syndrome (MetS) comprises interrelated disease states including obesity, insulin resistance and type 2 diabetes (T2DM), dyslipidemia, and hypertension. Essential to normal physiological function, and yet massively damaging in excess, oxidative stress and inflammation are pivotal common threads among the pathologies of MetS. Increasing evidence indicates that redox and inflammatory dysregulation parallels the syndrome's physiological, biochemical, and anthropometric features, leading many to consider the pro-oxidative, pro-inflammatory milieu an unofficial criterion in itself. Left unchecked, cross-promotion of oxidative stress and inflammation creates a feed-forward cycle that can initiate and advance disease progression. Such redox-inflammatory integration is evident in the pathogenesis of obesity, insulin resistance and T2DM, atherogenic dyslipidemia, and hypertension, and is thus hypothesized to be the "common soil" from which they develop. The present review highlights the synergistic contributions of redox-inflammatory processes to each of the components of the MetS.

TRAIL (TNF-related apoptosis-inducing ligand) regulates adipocyte metabolism by caspase-mediated cleavage of PPARgamma

Tumor necrosis factor α (TNFα) and other members of the TNF family affect adipose tissue metabolism and contribute to the obesity-related inflammation of adipose tissue. Here, we sought to identify the effects of TRAIL (TNF-related apoptosis-inducing ligand) on fat cell biology. TRAIL-receptor 2 (TRAIL-R2) and its mouse homolog DR5 were regulated upon acute and chronic energy imbalance in murine and human adipose tissue. TRAIL inhibited insulin-stimulated glucose uptake and de novo lipogenesis in human adipocytes. Interestingly, TRAIL did not interfere with the phosphorylation of insulin-stimulated kinases such as Akt or Erk and did not activate the NF-κB pathway. Instead, TRAIL activated cleavage of caspase-8 and caspase-3. The subsequent cleavage of PPARγ led to its inactivation and resulted in reduced expression of lipogenic genes, such as Glut-4, FASN, and ACC. Taken together, we discovered a so far unknown function of the death ligand TRAIL in regulating adipocyte metabolism. Our results imply that TRAIL/TRAIL-R system might provide a new target for the prevention and treatment of obesity and its co-morbidities.

The immune system's involvement in obesity-driven type 2 diabetes

Type 2 diabetes is now a worldwide epidemic, strongly correlated with an elevated incidence of obesity. Obesity-associated adipose tissue inflammation is a major cause of the decreased insulin sensitivity seen in type 2 diabetes. Recent studies have shed light on the cross-talk between the immune system and organismal metabolism. This review discusses the connection between inflammation in adipose tissue and systemic insulin resistance, focusing on the roles of innate and adaptive immune cell subsets in the pathogenesis of this metabolic disease.

Interleukin-1β regulates fat-liver crosstalk in obesity by auto-paracrine modulation of adipose tissue inflammation and expandability

The inflammasome has been recently implicated in obesity-associated dys-metabolism. However, of its products, the specific role of IL-1β was clinically demonstrated to mediate only the pancreatic beta-cell demise, and in mice mainly the intra-hepatic manifestations of obesity. Yet, it remains largely unknown if IL-1β, a cytokine believed to mainly function locally, could regulate dysfunctional inter-organ crosstalk in obesity. Here we show that High-fat-fed (HFF) mice exhibited a preferential increase of IL-1β in portal compared to systemic blood. Moreover, portally-drained mesenteric fat transplantation from IL-1βKO donors resulted in lower pyruvate-glucose flux compared to mice receiving wild-type (WT) transplant. These results raised a putative endocrine function for visceral fat-derived IL-1β in regulating hepatic gluconeogenic flux. IL-1βKO mice on HFF exhibited only a minor or no increase in adipose expression of pro-inflammatory genes (including macrophage M1 markers), Mac2-positive crown-like structures and CD11b-F4/80-double-positive macrophages, all of which were markedly increased in WT-HFF mice. Further consistent with autocrine/paracrine functions of IL-1β within adipose tissue, adipose tissue macrophage lipid content was increased in WT-HFF mice, but significantly less in IL-1βKO mice. Ex-vivo, adipose explants co-cultured with primary hepatocytes from WT or IL-1-receptor (IL-1RI)-KO mice suggested only a minor direct effect of adipose-derived IL-1β on hepatocyte insulin resistance. Importantly, although IL-1βKOs gained weight similarly to WT-HFF, they had larger fat depots with similar degree of adipocyte hypertrophy. Furthermore, adipogenesis genes and markers (pparg, cepba, fabp4, glut4) that were decreased by HFF in WT, were paradoxically elevated in IL-1βKO-HFF mice. These local alterations in adipose tissue inflammation and expansion correlated with a lower liver size, less hepatic steatosis, and preserved insulin sensitivity. Collectively, we demonstrate that by promoting adipose inflammation and limiting fat tissue expandability, IL-1β supports ectopic fat accumulation in hepatocytes and adipose-tissue macrophages, contributing to impaired fat-liver crosstalk in nutritional obesity.

Recent advances in obesity-induced inflammation and insulin resistance

It has been demonstrated in rodents and humans that chronic inflammation characterized by macrophage infiltration occurs mainly in adipose tissue or liver during obesity, in which activation of immune cells is closely associated with insulin sensitivity. Macrophages can be classified as classically activated (M1) macrophages that support microbicidal activity or alternatively activated (M2) macrophages that support allergic and antiparasitic responses. In the context of insulin action, M2 macrophages sustain insulin sensitivity by secreting IL-4 and IL-10, while M1 macrophages induce insulin resistance through the secretion of proinflammatory cytokines, such as TNFα. Polarization of M1/M2 is controlled by various dynamic functions of other immune cells. It has been demonstrated that, in a lean state, TH2 cells, Treg cells, natural killer T cells, or eosinophils contribute to the M2 activation of macrophages by secreting IL-4 or IL-10. In contrast, obesity causes alteration of the constituent immune cells, in which TH1 cells, B cells, neutrophils, or mast cells induce M1 activation of macrophages by the elevated secretion of TNFα and IFNγ. Increased secretion of TNFα and free fatty acids from hypertrophied adipocytes also contributes to the M1 activation of macrophages. Since obesity-induced insulin resistance is established by macrophage infiltration and the activation of immune cells inside tissues, identification of the factors that regulate accumulation and the intracellular signaling cascades that define polarization of M1/M2 would be indispensable. Regulation of these factors would lead to the pharmacological inhibition of obesity-induced insulin resistance. In this review, we introduce molecular mechanisms relevant to the pathophysiology and review the most recent studies of clinical applications targeting chronic inflammation.

Quantification of adipose tissue leukocytosis in obesity

The infiltration of immune cell subsets in adipose tissue termed "adipose tissue leukocytosis" is a critical event in the development of chronic inflammation and obesity-associated comorbidities. Given that a significant proportion of cells in adipose tissue of obese patients are of hematopoietic lineage, the distinct adipose depots represent an uncharacterized immunological organ that can impact metabolic functions. Here, we describe approaches to characterize and isolate leukocytes from the complex adipose tissue microenvironment, to aid mechanistic studies to better understand the role of specific pattern recognition receptors (PRRs) such as inflammasomes in adipose-immune cross talk.

Upregulated NLRP3 inflammasome activation in patients with type 2 diabetes

Despite the recent attention focused on the roles of the nucleotide binding and oligomerization domain-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome in the pathogenesis of type 2 diabetes, little is known about the ex vivo profile of inflammasome activation in type 2 diabetic patients. In this study, we investigated patterns of NLRP3 inflammasome activation in monocyte-derived macrophages (MDMs) from drug-naïve patients with newly diagnosed type 2 diabetes. Type 2 diabetic subjects had significantly increased mRNA and protein expression of NLRP3, apoptosis-associated speck-like protein containing a CARD (ASC), and proinflammatory cytokines in MDMs cultured with autologous sera compared with healthy controls. Upregulated interleukin (IL)-1β maturation, IL-18 secretion, and caspase-1 cleavage were observed in MDMs from type 2 diabetic patients after stimulation with various danger molecules (ATP, high-mobility group protein B1, free fatty acids, islet amyloid polypeptide, and monosodium uric acid crystals). Mitochondrial reactive oxygen species and NLRP3 were required for IL-1β synthesis in MDMs. Finally, 2 months of therapy with the antidiabetic drug metformin significantly inhibited the maturation of IL-1β in MDMs from patients with type 2 diabetes through AMP-activated protein kinase (AMPK) activation. Taken together, these data suggest that NLRP3 inflammasome activation is elevated in myeloid cells from type 2 diabetic patients and that antidiabetic treatment with metformin contributes to modulation of inflammasome activation in type 2 diabetes.

TRPM2 links oxidative stress to NLRP3 inflammasome activation

Exposure to particulate crystals can induce oxidative stress in phagocytes, which triggers NLRP3 inflammasome-mediated interleukin-1β secretion to initiate undesirable inflammatory responses that are associated with both autoinflammatory and metabolic diseases. Although mitochondrial reactive oxygen species have a central role in NLRP3 inflammasome activation, how reactive oxygen species signal assembly of the NLRP3 inflammasome remains elusive. Here, we identify liposomes as novel activators of the NLRP3 inflammasome and further demonstrate that liposome-induced inflammasome activation also requires mitochondrial reactive oxygen species. Moreover, we find that stimulation with liposomes/crystals induced reactive oxygen species-dependent calcium influx via the TRPM2 channel and that macrophages deficient in TRPM2 display drastically impaired NLRP3 inflammasome activation and interleukin-1β secretion. Consistently, Trpm2(-/-) mice are resistant to crystal-/liposome-induced interleukin-1β-mediated peritonitis in vivo. Together, these results identify TRPM2 as a key factor that links oxidative stress to the NLRP3 inflammasome activation. Therefore, targeting TRPM2 may be effective for the treatment of NLRP3 inflammasome-associated inflammatory disorders.

Caspase-1 deficiency in mice reduces intestinal triglyceride absorption and hepatic triglyceride secretion

Caspase-1 is known to activate the proinflammatory cytokines IL-1β and IL-18. Additionally, it can cleave other substrates, including proteins involved in metabolism. Recently, we showed that caspase-1 deficiency in mice strongly reduces high-fat diet-induced weight gain, at least partly caused by an increased energy production. Increased feces secretion by caspase-1-deficient mice suggests that lipid malabsorption possibly further reduces adipose tissue mass. In this study we investigated whether caspase-1 plays a role in triglyceride-(TG)-rich lipoprotein metabolism using caspase-1-deficient and wild-type mice. Caspase-1 deficiency reduced the postprandial TG response to an oral lipid load, whereas TG-derived fatty acid (FA) uptake by peripheral tissues was not affected, demonstrated by unaltered kinetics of [(3)H]TG-labeled very low-density lipoprotein (VLDL)-like emulsion particles. An oral gavage of [(3)H]TG-containing olive oil revealed that caspase-1 deficiency reduced TG absorption and subsequent uptake of TG-derived FA in liver, muscle, and adipose tissue. Similarly, despite an elevated hepatic TG content, caspase-1 deficiency reduced hepatic VLDL-TG production. Intestinal and hepatic gene expression analysis revealed that caspase-1 deficiency did not affect FA oxidation or FA uptake but rather reduced intracellular FA transport, thereby limiting lipid availability for the assembly and secretion of TG-rich lipoproteins. The current study reveals a novel function for caspase-1, or caspase-1-cleaved substrates, in controlling intestinal TG absorption and hepatic TG secretion.

Innate immune activation in obesity

The innate immune system is a prewired set of cellular and humoral components that has developed to sense perturbations in normal physiology and trigger responses to restore the system back to baseline. It is now understood that many of these components can also sense the physiologic changes that occur with obesity and be activated. While the exact reasons for this chronic immune response to obesity are unclear, there is strong evidence to suggest that innate inflammatory systems link obesity and disease. Based on this, anti-inflammatory therapies for diseases like type 2 diabetes and metabolic syndrome may form the core of future treatment plans. This review will highlight the components involved in the innate immune response and discuss the evidence that they contribute to the pathogenesis of obesity-associated diseases.

IL-1 family cytokines trigger sterile inflammatory disease

Inflammation plays vital roles in protective responses against pathogens and tissue repair, however, improper resolution of inflammatory networks is centrally involved in the pathogenesis of many acute and chronic diseases. Extensive advances have been made in recent years to define the inflammatory processes that are required for pathogen clearance, however, in comparison, less is known about the regulation of inflammation in sterile settings. Over the past decade non-communicable chronic diseases that are potentiated by sterile inflammation have replaced infectious diseases as the major threat to global human health. Thus, improved understanding of the sterile inflammatory process has emerged as one of the most important areas of biomedical investigation during our time. In this review we highlight the central role that interleukin-1 family cytokines play in sterile inflammatory diseases.

Regulation of stem cell differentiation in adipose tissue by chronic inflammation

1. Recent studies suggest that a local hypoxic response leads to chronic inflammation in the adipose tissue of obese individuals. The adipose tissue hypoxia may reflect a compensatory failure in the local vasculature system in response to obesity. 2. Studies suggest that inflammation stimulates angiogenesis and inhibits adipocyte activities in a feedback manner within the obese adipose tissue. Adipose-derived stem cells (ASC) are able to differentiate into multiple lineages of progenitor cells for adipocytes, endothelial cells, fibroblasts and pericytes. Differentiation of ASC into those progenitors is regulated by the adipose tissue microenvironment. 3. As a major factor in the microenvironment, inflammation may favour ASC differentiation into endothelial cells through the induction of angiogenic factors. At the same time, inflammation inhibits ASC differentiation into adipocytes by suppressing peroxisome proliferator-activated receptor γ activity and the insulin signalling pathway. In this context, inflammation may serve as a signal mediating the competition between adipocytes and endothelial cells for the limited source of ASC. 4. It is a new concept that inflammation mediates signals in the competition between adipocytes and endothelial cells for the limited ASC in obesity. There is a lot of evidence that inflammation promotes endothelial cell differentiation. However, this activity of inflammation remains to be established in adipose tissue. The present article reviews the literature in support of this conclusion.

Superimposition of postnatal calorie restriction protects the aging male intrauterine growth- restricted offspring from metabolic maladaptations

Intrauterine growth restriction (IUGR) results in dysregulated glucose homeostasis and adiposity in the adult. We hypothesized that with aging, these perturbations will wane, and superimposition of postnatal growth restriction (PNGR) on IUGR [intrauterine and postnatal growth restriction (IPGR)] will reverse the residual IUGR phenotype. We therefore undertook hyperinsulinemic-euglycemic clamp, energy balance, and physical activity studies during fed, fasted, and refed states, in light and dark cycles, on postweaned chow diet-fed more than 17-month aging male IUGR, PNGR, and IPGR vs. control (CON) rat offspring. Hyperinsulinemic-euglycemic clamp revealed similar whole-body insulin sensitivity and physical activity in the nonobese IUGR vs. CON, despite reduced heat production and energy expenditure. Compared with CON and IUGR, IPGR mimicking PNGR was lean and growth restricted with increased physical activity, O(2) consumption (VO(2)), energy intake, and expenditure. Although insulin sensitivity was no different in IPGR and PNGR, skeletal muscle insulin-induced glucose uptake was enhanced. This presentation proved protective against the chronologically earlier (5.5 months) development of obesity and dysregulated energy homeostasis after 19 wk on a postweaned high-fat diet. This protective role of PNGR on the metabolic IUGR phenotype needs future fine tuning aimed at minimizing unintended consequences.

Mitochondrial dysfunction and oxidative stress activate inflammasomes: impact on the aging process and age-related diseases

Oxidative stress and low-grade inflammation are the hallmarks of the aging process and are even more enhanced in many age-related degenerative diseases. Mitochondrial dysfunction and oxidative stress can provoke and potentiate inflammatory responses, but the mechanism has remained elusive. Recent studies indicate that oxidative stress can induce the assembly of multiprotein inflammatory complexes called the inflammasomes. Nod-like receptor protein 3 (NLRP3) is the major immune sensor for cellular stress signals, e.g., reactive oxygen species, ceramides, and cathepsin B. NLRP3 activation triggers the caspase-1-mediated maturation of the precursors of IL-1β and IL-18 cytokines. During aging, the autophagic clearance of mitochondria declines and dysfunctional mitochondria provoke chronic oxidative stress, which disturbs the cellular redox balance. Moreover, increased NF-κB signaling observed during aging could potentiate the expression of NLRP3 and cytokine proforms enhancing the priming of NLRP3 inflammasomes. Recent studies have demonstrated that NLRP3 activation is associated with several age-related diseases, e.g., the metabolic syndrome. We will review here the emerging field of inflammasomes in the appearance of the proinflammatory phenotype during the aging process and in age-related diseases.

IL1β down-regulation of sex hormone-binding globulin production by decreasing HNF-4α via MEK-1/2 and JNK MAPK pathways

Patients suffering from low-grade chronic inflammatory diseases, such as rheumatoid arthritis, osteoarthritis, diabetes, and obesity, have low plasma sex hormone-binding globulin (SHBG) levels. These diseases are characterized among other features by high plasma IL1β levels. The aim of the present study is to explore whether IL1β could regulate hepatic SHBG production to account for low SHBG levels in these diseases. We provide evidence that daily IL1β treatment reduces SHBG production in HepG2 cells by the down-regulation of HNF-4A via the MAPK kinase (MEK)-1/2 and c-Jun N-terminal kinase (JNK) MAPK signaling pathways through the activation c-Jun transcription factors. The human SHBG promoter sequence contains two putative activator protein 1 (AP1) binding sites recognized by c-Jun transcription factors, but they are not necessary for the IL1β-induced down-regulation of SHBG promoter activity in luciferase reporter gene assays. Daily treatment with IL1β reduces hepatic nuclear factor (HNF)-4α mRNA and protein levels via the MEK-1/2 and JNK MAPK signaling pathways. Moreover, IL1β rapidly decreased HNF-4α mRNA and protein levels while increased phospho-c-Jun protein levels after the treatment. Finally, daily IL1β treatment of human SHBG transgenic mice reduced plasma SHBG and SHBG mRNA levels. Moreover, IL1β treatment also reduced HNF-4α mRNA and protein levels while increased hepatic phospho-c-Jun protein levels. Our results show that IL1β reduces hepatic SHBG production by decreasing HNF-4α via MEK-1/2 and JNK MAPK pathways. In addition, our findings suggest that IL1β could be involved the low plasma SHBG levels reported in chronic low-grade inflammatory diseases.

High-fat diet triggers inflammation-induced cleavage of SIRT1 in adipose tissue to promote metabolic dysfunction

Adipose tissue plays an important role in storing excess nutrients and preventing ectopic lipid accumulation in other organs. Obesity leads to excess lipid storage in adipocytes, resulting in the generation of stress signals and the derangement of metabolic functions. SIRT1 is an important regulatory sensor of nutrient availability in many metabolic tissues. Here we report that SIRT1 functions in adipose tissue to protect from inflammation and obesity under normal feeding conditions, and to forestall the progression to metabolic dysfunction under dietary stress and aging. Genetic ablation of SIRT1 in adipose tissue leads to gene expression changes that highly overlap with changes induced by high-fat diet in wild-type mice, suggesting that dietary stress signals inhibit the activity of SIRT1. Indeed, we show that high-fat diet induces the cleavage of SIRT1 protein in adipose tissue by the inflammation-activated caspase-1, providing a link between dietary stress and predisposition to metabolic dysfunction.

Adipose tissue-resident immune cells: key players in immunometabolism

Adipose tissue (AT) plays a pivotal role in whole-body lipid and glucose homeostasis. AT exerts metabolic control through various immunological mechanisms that instigated a new research field termed immunometabolism. Here, we review AT-resident immune cells and their role as key players in immunometabolism. In lean subjects, AT-resident immune cells have housekeeping functions ranging from apoptotic cell clearance to extracellular matrix remodeling and angiogenesis. However, obesity provides bacterial and metabolic danger signals that mimic bacterial infection, and drives a shift in immune-cell phenotypes and numbers, classified as a prototypic T helper 1 (Th1) inflammatory response. The resulting AT inflammation and insulin resistance link obesity to its metabolic sequel, and suggests that targeted immunomodulatory interventions may be beneficial for obese patients.