Palmitate- and lipopolysaccharide-activated macrophages evoke contrasting insulin responses in muscle cells

Effect of Short-Chain Fatty Acids on Inflammatory and Metabolic Function in an Obese Skeletal Muscle Cell Culture Model

The fermentation of non-digestible carbohydrates produces short-chain fatty acids (SCFAs), which have been shown to impact both skeletal muscle metabolic and inflammatory function; however, their effects within the obese skeletal muscle microenvironment are unknown. In this study, we developed a skeletal muscle in vitro model to mimic the critical features of the obese skeletal muscle microenvironment using L6 myotubes co-treated with 10 ng/mL lipopolysaccharide (LPS) and 500 µM palmitic acid (PA) for 24 h ± individual SCFAs, namely acetate, propionate and butyrate at 0.5 mM and 2.5 mM. At the lower SCFA concentration (0.5 mM), all three SCFA reduced the secreted protein level of RANTES, and only butyrate reduced IL-6 protein secretion and the intracellular protein levels of activated (i.e., ratio of phosphorylated-total) NFκB p65 and STAT3 (p < 0.05). Conversely, at the higher SCFA concentration (2.5 mM), individual SCFAs exerted different effects on inflammatory mediator secretion. Specifically, butyrate reduced IL-6, MCP-1 and RANTES secretion, propionate reduced IL-6 and RANTES, and acetate only reduced RANTES secretion (p < 0.05). All three SCFAs reduced intracellular protein levels of activated NFκB p65 and STAT3 (p < 0.05). Importantly, only the 2.5 mM SCFA concentration resulted in all three SCFAs increasing insulin-stimulated glucose uptake compared to control L6 myotube cultures (p < 0.05). Therefore, SCFAs exert differential effects on inflammatory mediator secretion in a cell culture model, recapitulating the obese skeletal muscle microenvironment; however, all three SCFAs exerted a beneficial metabolic effect only at a higher concentration via increasing insulin-stimulated glucose uptake, collectively exerting differing degrees of a beneficial effect on obesity-associated skeletal muscle dysfunction.

Mechanism of immune attack in the progression of obesity-related type 2 diabetes

Obesity and overweight are widespread issues in adults, children, and adolescents globally, and have caused a noticeable rise in obesity-related complications such as type 2 diabetes mellitus (T2DM). Chronic low-grade inflammation is an important promotor of the pathogenesis of obesity-related T2DM. This proinflammatory activation occurs in multiple organs and tissues. Immune cell-mediated systemic attack is considered to contribute strongly to impaired insulin secretion, insulin resistance, and other metabolic disorders. This review focused on highlighting recent advances and underlying mechanisms of immune cell infiltration and inflammatory responses in the gut, islet, and insulin-targeting organs (adipose tissue, liver, skeletal muscle) in obesity-related T2DM. There is current evidence that both the innate and adaptive immune systems contribute to the development of obesity and T2DM.

Differential Effects of Short-Chain Fatty Acids on L6 Myotube Inflammatory Mediator Production in Response to Lipopolysaccharide- or Palmitic Acid-Stimulation

Short-chain fatty acids (SCFA) produced from dietary non-digestible carbohydrate fermentation have metabolic effects in skeletal muscle; however, their effect on inflammatory mediator production is unknown. In this study, L6 myotubes were cultured with individual SCFA (acetate, propionate, and butyrate) at 0.5 mM and 2.5 mM ± 10 ng/mL lipopolysaccharide (LPS) or ± 500 µM palmitic acid (PA) for 24 h. In response to LPS, only butyrate had an effect at the lower concentration (0.5 mM), whereas at the higher concentration (2.5 mM) both propionate and butyrate reduced MCP-1, MIP-1α, and RANTES secretion (p < 0.05), and only butyrate reduced IL-6 secretion and intracellular protein levels of phospho-STAT3 (p < 0.05). In response to PA, 0.5 mM butyrate reduced protein expression of phospho-NFκB p65 and the secretion of IL-6, MIP-1α, and MCP-1, whereas all three SCFA reduced RANTES secretion (p < 0.05). At the 2.5 mM SCFA concentration combined with PA stimulation, all three SCFA reduced intracellular protein expression of phospho-NFκB p65 and phospho-STAT3 and secreted protein levels of MCP-1, IL-6, and RANTES, whereas only butyrate reduced secretion of MIP-1α (p < 0.05). Thus, SCFA exhibit differential effects on inflammatory mediator expression in response to LPS and PA stimulation, which has implications for their individual impacts on inflammation-mediated skeletal muscle dysfunction.

Molecular Mechanism of Lipotoxicity as an Interesting Aspect in the Development of Pathological States-Current View of Knowledge

Free fatty acids (FFAs) play numerous vital roles in the organism, such as contribution to energy generation and reserve, serving as an essential component of the cell membrane, or as ligands for nuclear receptors. However, the disturbance in fatty acid homeostasis, such as inefficient metabolism or intensified release from the site of storage, may result in increased serum FFA levels and eventually result in ectopic fat deposition, which is unfavorable for the organism. The cells are adjusted for the accumulation of FFA to a limited extent and so prolonged exposure to elevated FFA levels results in deleterious effects referred to as lipotoxicity. Lipotoxicity contributes to the development of diseases such as insulin resistance, diabetes, cardiovascular diseases, metabolic syndrome, and inflammation. The nonobvious organs recognized as the main lipotoxic goal of action are the pancreas, liver, skeletal muscles, cardiac muscle, and kidneys. However, lipotoxic effects to a significant extent are not organ-specific but affect fundamental cellular processes occurring in most cells. Therefore, the wider perception of cellular lipotoxic mechanisms and their interrelation may be beneficial for a better understanding of various diseases’ pathogenesis and seeking new pharmacological treatment approaches.

Association of Inflammatory Metabolic Activity of Psoas Muscle and Acute Myocardial Infarction: A Preliminary Observational Study with 18F-FDG PET/CT

Inflamed skeletal muscle promotes chronic inflammation in atherosclerotic plaques, thereby contributing to the increased risk of coronary artery disease (CAD). In this study, we evaluated the metabolic activity of psoas muscle, using ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT), and its association with carotid artery inflammation and acute myocardial infarction (AMI). In total, 90 participants (32 AMI, 33 chronic stable angina (CSA), and 25 control) were enrolled in this prospective study. Metabolic activity of skeletal muscle (SM) was measured by using maximum standardized uptake value (SUVmax) of psoas muscle, and corresponding psoas muscle area (SM area) was also measured. Carotid artery inflammation was evaluated by using the target-to background ratio (TBR) of carotid artery. SM SUVmax was highest in AMI, intermediate in CSA, and lowest in control group. SM SUVmax was significantly correlated with carotid artery TBR and systemic inflammatory surrogate markers. Furthermore, SM SUVmax was independently associated with carotid artery TBR and showed better predictability than SM area for the prediction of AMI. Metabolic activity of psoas muscle assessed by ¹⁸F-FDG PET/CT was associated with coronary plaque vulnerability and synchronized with the carotid artery inflammation in the participants with CAD. Furthermore, it may also be useful to predict AMI.

Effect of Exercise on Inflamed Psoas Muscle in Women with Obesity: A Pilot Prospective 18F-FDG PET/CT Study

Obesity increases inflammation in skeletal muscle thereby promoting systemic inflammation which leads to increased risk of cardiometabolic disease. This prospective study aimed to evaluate whether the metabolic activity of psoas muscle (PM) was associated with systemic inflammation, and whether physical exercise could reduce the PM metabolic activity evaluated by ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) in women with obesity. A total of 23 women with obesity who participated in a 3-month physical exercise program were enrolled. ¹⁸F-FDG PET/CT was performed before the start of the program (baseline) and after completion of the program. The maximum standardized uptake value of psoas muscle (PM SUVmax) was used for the PM metabolic activity. The SUVmax of spleen and bone marrow, and the high-sensitivity C-reactive protein were used to evaluate the systemic inflammation. At baseline, PM SUVmax was strongly correlated with the systemic inflammation. The exercise program significantly reduced the PM SUVmax, in addition to adiposity and systemic inflammation. Furthermore, we found that the association between PM SUVmax and the systemic inflammation disappeared after completion of the exercise program. In women with obesity, PM SUVmax, assessed by ¹⁸F-FDG PET/CT, was associated with obesity-induced systemic inflammation and exercise reduced the PM SUVmax and eliminated its association with systemic inflammation.

The chemokine CXCL16 can rescue the defects in insulin signaling and sensitivity caused by palmitate in C2C12 myotubes

In obesity, macrophages infiltrate peripheral tissues and secrete pro-inflammatory cytokines that impact local insulin sensitivity. Lipopolysaccharide (LPS) and the saturated fatty acid (FA) palmitate polarise macrophages towards a pro-inflammatory phenotype in vitro and indirectly cause insulin resistance (IR) in myotubes. In contrast, unsaturated FAs confer an anti-inflammatory phenotype and counteract the actions of palmitate. To explore paracrine mechanisms of interest, J774 macrophages were exposed to palmitate ± palmitoleate or control medium and the conditioned media generated were screened using a cytokine array. Of the 62 cytokines examined, 8 were significantly differentially expressed following FA treatments. Notably, CXCL16 secretion was downregulated by palmitate. In follow-up experiments using ELISAs, this downregulation was confirmed and reversed by simultaneous addition of palmitoleate or oleate, while LPS also diminished CXCL16 secretion. To dissect potential effects of CXCL16, C2C12 myotubes were treated with palmitate to induce IR, recombinant soluble CXCL16 (sCXCL16), combined treatment, or control medium. Palmitate caused the expected reduction of insulin-stimulated Akt activation and glycogen synthesis, whereas simultaneous treatment with sCXCL16 attenuated these effects. These data indicate a putative role for CXCL16 in preservation of Akt activation and insulin signaling in the context of chronic low-grade inflammation in skeletal muscle.

Hydrogen Sulfide Alleviates Lipopolysaccharide-Induced Diaphragm Dysfunction in Rats by Reducing Apoptosis and Inflammation through ROS/MAPK and TLR4/NF-κB Signaling Pathways

Diaphragm dysfunction is an important clinical problem worldwide. Hydrogen sulfide (H₂S) is involved in many physiological and pathological processes in mammals. However, the effect and mechanism of H₂S in diaphragm dysfunction have not been fully elucidated. In this study, we detected that the level of H₂S was decreased in lipopolysaccharide- (LPS-) treated L6 cells. Treatment with H₂S increased the proliferation and viability of LPS-treated L6 cells. We found that H₂S decreased reactive oxygen species- (ROS-) induced apoptosis through the mitogen-activated protein kinase (MAPK) signaling pathway in LPS-treated L6 cells. Administration of H₂S alleviated LPS-induced inflammation by mediating the toll-like receptor-4 (TLR-4)/nuclear factor-kappa B (NF-κB) signaling pathway in L6 cells. Furthermore, H₂S improved diaphragmatic function and structure through the reduction of inflammation and apoptosis in the diaphragm of septic rats. In conclusion, these findings indicate that H₂S ameliorates LPS-induced diaphragm dysfunction in rats by reducing apoptosis and inflammation through ROS/MAPK and TLR4/NF-κB signaling pathways. Novel slow-releasing H₂S donors can be designed and applied for the treatment of diaphragm dysfunction.

Transgenic mice with ectopic expression of constitutively active TLR4 in adipose tissues do not show impaired insulin sensitivity

INTRODUCTION

Chronic low-grade inflammation is associated with obesity and diabetes. However, what causes and mediates chronic inflammation in metabolic disorders is not well understood. Toll-like receptor 4 (TLR4) mediates both infection-induced and sterile inflammation by recognizing pathogen-associated molecular patterns and endogenous molecules, respectively. Saturated fatty acids can activate TLR4, and TLR4-deficient mice were protected from high fat diet (HFD)-induced obesity and insulin resistance, suggesting that TLR4-mediated inflammation may cause metabolic dysfunction, such as obesity and insulin resistance.

METHODS

We generated two transgenic (TG) mouse lines expressing a constitutively active TLR4 in adipose tissue and determined whether these TG mice would show increased insulin resistance.

RESULTS

TG mice fed a high fat or a normal chow diet did not exhibit increased insulin resistance compared to their wild-type controls despite increased localized inflammation in white adipose tissue. Furthermore, females of one TG line fed a normal chow diet had improved insulin sensitivity with reduction in both adiposity and body weight when compared with wild-type littermates. There were significant differences between female and male mice in metabolic biomarkers and mRNA expression in proinflammatory genes and negative regulators of TLR4 signaling, regardless of genotype and diet.

CONCLUSIONS

Together, these results suggest that constitutively active TLR4-induced inflammation in white adipose tissue is not sufficient to induce systemic insulin resistance, and that high fat diet-induced insulin resistance may require other signals in addition to TLR4-mediated inflammation.

Involvement of receptor-interacting protein 140 in palmitate-stimulated macrophage infiltration of pancreatic beta cells

Receptor-interacting protein 140 (RIP140) in macrophages stimulates the nuclear factor-κB subunit RelA to activate tumor necrosis factor (TNF)-α and interleukin (IL)-6 transcription. However, under lipotoxic conditions, the involvement of RIP140 in the infiltration of beta cells by macrophages remains unknown. In the present study, murine RAW264.7 macrophages were transfected with a RIP140 overexpression plasmid or siRNA prior to macrophage activation with 500 µM palmitate. Palmitate-free conditioned media was then collected and added to murine insulinoma MIN6 cells. Significant decreases were observed in cell viability (P<0.01), glucose-stimulated insulin secretion (P<0.01) and levels of peroxisome proliferator-activated receptor-γ coactivator-1α (P<0.05), phosphoenolpyruvate carboxykinase and proliferating cell nuclear antigen mRNA (P<0.01) in MIN6 cells. In addition, conditioned media from palmitate-treated and RIP140-upregulated macrophages significantly increased the levels of uncoupling protein-2 (P<0.01), inducible nitric oxide synthase 1 (P<0.01) and pancreatic and duodenal homeobox 1 (P<0.05) mRNA and levels of activated Jun N-terminal kinase (JNK) (P<0.01) and extracellular signal-regulated kinase (ERK) 1/2 (P<0.01). In turn, the conditioned media was found to be significantly enriched in TNF-α and IL-6 (both P<0.05). These results were the opposite of those obtained from MIN6 cells treated with conditioned media from palmitate-treated and RIP140-knockdown macrophages. MIN6 cells were transfected with RIP140 overexpression plasmid or siRNA prior to treatment with 500 µM palmitate and supernatant was collected for use in macrophage chemotaxis assays. In the palmitate-activated and RIP140-overexpressing MIN6 cells, TNF-α and IL-6 secretion increased significantly (both P<0.05) and macrophage chemotaxis towards MIN6 cells was enhanced. By contrast, downregulating RIP140 in MIN6 cells had the opposite effect. These data suggest that RIP140 in macrophages mediates the transcription of inflammatory cytokines when concentration of palmitate is high. Macrophage RIP140 may also impair beta cell function by activating the JNK and ERK1/2 signaling pathways and promoting specific gene transcription. Furthermore, expression of RIP140 in pancreatic beta cells may stimulate macrophage chemotaxis, thus triggering local low-grade inflammation.

Long-lasting effect of obesity on skeletal muscle transcriptome

BACKGROUND

Reduced physical activity and increased intake of calorically-dense diets are the main risk factors for obesity, glucose intolerance, and type 2 diabetes. Chronic overnutrition and hyperglycemia can alter gene expression, contributing to long-term obesity complications. While caloric restriction can reduce obesity and glucose intolerance, it is currently unknown whether it can effectively reprogram transcriptome to a pre-obesity level. The present study addressed this question by the preliminary examination of the transcriptional dynamics in skeletal muscle after exposure to overnutrition and following caloric restriction.

RESULTS

Six male rhesus macaques of 12-13 years of age consumed a high-fat western-style diet for 6 months and then were calorically restricted for 4 months without exercise. Skeletal muscle biopsies were subjected to longitudinal gene expression analysis using next-generation whole-genome RNA sequencing. In spite of significant weight loss and normalized insulin sensitivity, the majority of WSD-induced (n = 457) and WSD-suppressed (n = 47) genes remained significantly dysregulated after caloric restriction (FDR ≤0.05). The Metacore^TM pathway analysis reveals that western-style diet induced the sustained activation of the transforming growth factor-β gene network, associated with extracellular matrix remodeling, and the downregulation of genes involved in muscle structure development and nutritional processes.

CONCLUSIONS

Western-style diet, in the absence of exercise, induced skeletal muscle transcriptional programing, which persisted even after insulin resistance and glucose intolerance were completely reversed with caloric restriction.

Deconstructing metabolic inflammation using cellular systems

Over the past years, we have embarked in a systematic analysis of the effect of obesity or fatty acids on circulating monocytes, microvascular endothelial cells, macrophages, and skeletal muscle cells. With the use of cell culture strategies, we have deconstructed complex physiological systems and then reconstructed "partial equations" to better understand cell-to-cell communication. Through these approaches, we identified that in high saturated fat environments, cell-autonomous proinflammatory pathways are activated in monocytes and endothelial cells, promoting monocyte adhesion and transmigration. We think of this as a paradigm of the conditions promoting immune cell infiltration into tissues during obesity. In concert, it is possible that muscle and adipose tissue secrete immune cell chemoattractants, and indeed, our tissue culture reconstructions reveal that myotubes treated with the saturated fatty acid palmitate, but not the unsaturated fatty acid palmitoleate, release nucleotides that attract monocytes and other compounds that promote proinflammatory classically activated "(M1)-like" polarization in macrophages. In addition, palmitate directly triggers an M1-like macrophage phenotype, and secretions from these activated macrophages confer insulin resistance to target muscle cells. Together, these studies suggest that in pathophysiological conditions of excess fat, the muscle, endothelial and immune cells engage in a synergistic crosstalk that exacerbates tissue inflammation, leukocyte infiltration, polarization, and consequent insulin resistance.

Sea Buckthorn Fruit Oil Extract Alleviates Insulin Resistance through the PI3K/Akt Signaling Pathway in Type 2 Diabetes Mellitus Cells and Rats

Sea buckthorn fruit oil is rich in palmitoleic acid (POA), which has been reported to play roles in many metabolic processes. In this study, a sea buckthorn fruit oil (SBFO) extract was evaluated through in vitro experiments (the doses were 50, 100, 200, and 400 μM) and in vivo experiments (the doses were 100, 200, and 300 mg/kg·day) to explore its mechanism of action in the treatment of type 2 diabetes mellitus (T2DM). The results revealed that the SBFO extract effectively increased the glucose uptake from 12.23 ± 1.09 to 14.90 ± 1.48 mmol/L in insulin resistance (IR) HepG2 cells, lowered blood glucose (the reductions rates of blood glucose in groups treated with SBFO extract at 200 and 300 mg/kg·day were 10.47% and 13.79%, respectively) and improved insulin indices from -6.11 ± 0.10 to -5.45 ± 0.31 after 4 weeks treatment with SBFO extract at 300 mg/kg·day in T2DM SD rats. RT-PCR and Western blotting analyses suggested that the SBFO extract could promote the expression of phosphatidylinositol-3-kinase (PI3K) and glycogen synthesis (GS) while inhibiting the expression of glycogen synthesis kinase-3β (GSK-3β). Thus, the SBFO extract played a positive role in alleviating T2DM through the PI3K/Akt signaling pathway in HepG2 cells, and diabetic rats and could be used for the future development of functional food and dietary supplements.

Tissue resident macrophages: Key players in the pathogenesis of type 2 diabetes and its complications

There is increasing evidence showing that chronic inflammation is an important pathogenic mediator of the development of type 2 diabetes (T2D). It is now generally accepted that tissue-resident macrophages play a major role in regulation of tissue inflammation. T2D-associated inflammation is characterized by an increased abundance of macrophages in different tissues along with production of inflammatory cytokines. The complexity of macrophage phenotypes has been reported from different human tissues. Macrophages exhibit a phenotypic range that is intermediate between two extremes, M1 (pro-inflammatory) and M2 (anti-inflammatory). Cytokines and chemokines produced by macrophages generate local and systemic inflammation and this condition leads to pancreatic β-cell dysfunction and insulin resistance in liver, adipose and skeletal muscle tissues. Data from human and animal studies also suggest that macrophages contribute to T2D complications such as nephropathy, neuropathy, retinopathy and cardiovascular diseases through cell-cell interactions and the release of pro-inflammatory cytokines, chemokines, and proteases to induce inflammatory cell recruitment, cell apoptosis, angiogenesis, and matrix protein remodeling. In this review we focus on the functions of macrophages and the importance of these cells in the pathogenesis of T2D. In addition, the contribution of macrophages to diabetes complications such as nephropathy, neuropathy, retinopathy and cardiovascular diseases is discussed.

Adipose Tissue Insulin Action and IL-6 Signaling after Exercise in Obese Mice

INTRODUCTION

Adipose tissue insulin action is impaired in obesity and is associated with inflammation, macrophage infiltration, and polarization toward a proinflammatory phenotype. Acute exercise can reduce markers of adipose inflammation, including interleukin (IL) 6, in parallel with improvements in insulin action; however, others have provided evidence that IL-6 has anti-inflammatory properties.

PURPOSE

This study aimed to examine the relation between IL-6 signaling, macrophage infiltration, and polarization and insulin action in inguinal fat after acute exercise in obese, insulin-resistant mice.

METHODS

Male C57BL/6 mice were fed a low-fat diet (10% kcal lard) or a high-fat diet (HFD, 60% kcal lard) for 7 wk and then underwent an acute bout of exercise (2-h treadmill running: 15 m·min, 5% incline).

RESULTS

The HFD resulted in increased body mass, glucose intolerance, and attenuated insulin-induced AKT Thr308 phosphorylation in inguinal fat. This was accompanied by increases in indices of macrophage infiltration (F4/80, CD68, and monocyte chemoattractant protein-1 expression) and polarization toward an M1 phenotype (increased expression of CD11c, CD11c/galactose-type C-type lectin 1, and inducible nitric oxide synthase). Immunofluorescence imaging demonstrated increased F4/80- and CD11c-positive cells with the HFD. Two hours after exercise, the insulin-induced activation of AKT Th308 phosphorylation was recovered in HFD mice. This was accompanied by an upregulation of IL-6 and IL-10 signaling, as demonstrated by increased expression of IL-6, IL-10, and SOCS3 as well as STAT3 phosphorylation. Furthermore, acute exercise resulted in a shift toward reduction in M1 polarization indicated by a decrease in the ratio of CD11c to galactose-type C-type lectin 1 mRNA as well as a decline in F4/80- and CD11c-positive cells.

CONCLUSIONS

The results suggest a link between exercise-induced increases in IL-6, reductions in indices of M1 macrophages, and increased IL-10, a reputed anti-inflammatory cytokine with insulin-sensitizing properties.

Nutrient and immune sensing are obligate pathways in metabolism, immunity, and disease

The growth and survival of multicellular organisms depend upon their abilities to acquire and metabolize nutrients, efficiently store and harness energy, and sense and fight infection. Systems for sensing and using nutrients have consequently coevolved alongside systems for sensing and responding to danger signals, including pathogens, and share many of the same cell signaling proteins and networks. Diets rich in carbohydrates and fats can overload these systems, leading to obesity, metabolic dysfunction, impaired immunity, and cardiovascular disease. Excessive nutrient intake promotes adiposity, typically altering adipocyte function and immune cell distribution, both of which trigger metabolic dysfunction. Here, we discuss novel mechanistic links between metabolism and immunity that underlie metabolic dysfunction in obesity. We aim to stimulate debate about how the endocrine and immune systems are connected through autocrine, paracrine, and neuroendocrine signaling in sophisticated networks that are only now beginning to be resolved. Understanding the expression and action of signaling proteins, together with modulating their receptors or pattern recognition using agonists or antagonists, will enable rational intervention in immunometabolism that may lead to novel treatments for obesity and metabolic dysfunction.

Opposite cross-talk by oleate and palmitate on insulin signaling in hepatocytes through macrophage activation

Chronic low grade inflammation in adipose tissue during obesity is associated with an impairment of the insulin signaling cascade. In this study, we have evaluated the impact of palmitate or oleate overload of macrophage/Kupffer cells in triggering stress-mediated signaling pathways, in lipoapoptosis, and in the cross-talk with insulin signaling in hepatocytes. RAW 264.7 macrophages or Kupffer cells were stimulated with oleate or palmitate, and levels of M1/M2 polarization markers and the lipidomic profile of eicosanoids were analyzed. Whereas proinflammatory cytokines and total eicosanoids were elevated in macrophages/Kupffer cells stimulated with palmitate, enhanced arginase 1 and lower leukotriene B4 (LTB4) levels were detected in macrophages stimulated with oleate. When hepatocytes were pretreated with conditioned medium (CM) from RAW 264.7 or Kupffer cells loaded with palmitate (CM-P), phosphorylation of stress kinases and endoplasmic reticulum stress signaling was increased, insulin signaling was impaired, and lipoapoptosis was detected. Conversely, enhanced insulin receptor-mediated signaling and reduced levels of the phosphatases protein tyrosine phosphatase 1B (PTP1B) and phosphatase and tensin homolog (PTEN) were found in hepatocytes treated with CM from macrophages stimulated with oleate (CM-O). Supplementation of CM-O with LTB4 suppressed insulin sensitization and increased PTP1B and PTEN. Furthermore, LTB4 decreased insulin receptor tyrosine phosphorylation in hepatocytes, activated the NFκB pathway, and up-regulated PTP1B and PTEN, these effects being mediated by LTB4 receptor BTL1. In conclusion, oleate and palmitate elicit an opposite cross-talk between macrophages/Kupffer cells and hepatocytes. Whereas CM-P interferes at the early steps of insulin signaling, CM-O increases insulin sensitization, possibly by reducing LTB4.

Nucleotides released from palmitate-challenged muscle cells through pannexin-3 attract monocytes

Obesity-associated low-grade inflammation in metabolically relevant tissues contributes to insulin resistance. We recently reported monocyte/macrophage infiltration in mouse and human skeletal muscles. However, the molecular triggers of this infiltration are unknown, and the role of muscle cells in this context is poorly understood. Animal studies are not amenable to the specific investigation of this vectorial cellular communication. Using cell cultures, we investigated the crosstalk between myotubes and monocytes exposed to physiological levels of saturated and unsaturated fatty acids. Media from L6 myotubes treated with palmitate-but not palmitoleate-induced THP1 monocyte migration across transwells. Palmitate activated the Toll-like receptor 4 (TLR4)/nuclear factor-κB (NF-κB) pathway in myotubes and elevated cytokine expression, but the monocyte chemoattracting agent was not a polypeptide. Instead, nucleotide degradation eliminated the chemoattracting properties of the myotube-conditioned media. Moreover, palmitate-induced expression and activity of pannexin-3 channels in myotubes were mediated by TLR4-NF-κB, and TLR4-NF-κB inhibition or pannexin-3 knockdown prevented monocyte chemoattraction. In mice, the expression of pannexin channels increased in adipose tissue and skeletal muscle in response to high-fat feeding. These findings identify pannexins as new targets of saturated fatty acid-induced inflammation in myotubes, and point to nucleotides as possible mediators of immune cell chemoattraction toward muscle in the context of obesity.

Palmitoleic acid prevents palmitic acid-induced macrophage activation and consequent p38 MAPK-mediated skeletal muscle insulin resistance

Obesity and saturated fatty acid (SFA) treatment are both associated with skeletal muscle insulin resistance (IR) and increased macrophage infiltration. However, the relative effects of SFA and unsaturated fatty acid (UFA)-activated macrophages on muscle are unknown. Here, macrophages were treated with palmitic acid, palmitoleic acid or both and the effects of the conditioned medium (CM) on C2C12 myotubes investigated. CM from palmitic acid-treated J774s (palm-mac-CM) impaired insulin signalling and insulin-stimulated glycogen synthesis, reduced Inhibitor κBα and increased phosphorylation of p38 mitogen-activated protein kinase (MAPK) and c-Jun N-terminal kinase in myotubes. p38 MAPK inhibition or siRNA partially ameliorated these defects, as did addition of tumour necrosis factor-α blocking antibody to the CM. Macrophages incubated with both FAs generated CM that did not induce IR, while palmitoleic acid-mac-CM alone was insulin sensitising. Thus UFAs may improve muscle insulin sensitivity and counteract SFA-mediated IR through an effect on macrophage activation.

Artemisia dracunculus L. extract ameliorates insulin sensitivity by attenuating inflammatory signalling in human skeletal muscle culture

AIMS

Bioactives of Artemisia dracunculus L. (termed PMI 5011) have been shown to improve insulin action by increasing insulin signalling in skeletal muscle. However, it was not known if PMI 5011's effects are retained during an inflammatory condition. We examined the attenuation of insulin action and whether PMI 5011 enhances insulin signalling in the inflammatory environment with elevated cytokines.

METHODS

Muscle cell cultures derived from lean, overweight and diabetic-obese subjects were used. Expression of pro-inflammatory genes and inflammatory response of human myotubes were evaluated by real-time polymerase chain reaction (RT-PCR). Insulin signalling and activation of inflammatory pathways in human myotubes were evaluated by multiplex protein assays.

RESULTS

We found increased gene expression of monocyte chemoattractant protein 1 (MCP1) and TNFα (tumour necrosis factor alpha), and basal activity of the NFkB (nuclear factor kappa-light-chain-enhancer of activated B cells) pathway in myotubes derived from diabetic-obese subjects as compared with myotubes derived from normal-lean subjects. In line with this, basal Akt phosphorylation (Ser473) was significantly higher, while insulin-stimulated phosphorylation of Akt (Ser473) was lower in myotubes from normal-overweight and diabetic-obese subjects compared with normal-lean subjects. PMI 5011 treatment reduced basal phosphorylation of Akt and enhanced insulin-stimulated phosphorylation of Akt in the presence of cytokines in human myotubes. PMI 5011 treatment led to an inhibition of cytokine-induced activation of inflammatory signalling pathways such as Erk1/2 and IkBα (nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor, alpha)-NFkB and moreover, NFkB target gene expression, possibly by preventing further propagation of the inflammatory response within muscle tissue.

CONCLUSIONS

PMI 5011 improved insulin sensitivity in diabetic-obese myotubes to the level of normal-lean myotubes despite the presence of pro-inflammatory cytokines.

Fas (CD95) expression in myeloid cells promotes obesity‐induced muscle insulin resistance

Low-grade inflammation in adipose tissue and liver has been implicated in obesity-associated insulin resistance and type 2 diabetes. Yet, the contribution of inflammatory cells to the pathogenesis of skeletal muscle insulin resistance remains elusive. In a large cohort of obese human individuals, blood monocyte Fas (CD95) expression correlated with systemic and skeletal muscle insulin resistance. To test a causal role for myeloid cell Fas expression in the development of skeletal muscle insulin resistance, we generated myeloid/haematopoietic cell-specific Fas-depleted mice. Myeloid/haematopoietic Fas deficiency prevented the development of glucose intolerance in high fat-fed mice, in ob/ob mice, and in mice acutely challenged by LPS. In vivo, ex vivo and in vitro studies demonstrated preservation of muscle insulin responsiveness with no effect on adipose tissue or liver. Studies using neutralizing antibodies demonstrated a role for TNFα as mediator between myeloid Fas and skeletal muscle insulin resistance, supported by significant correlations between monocyte Fas expression and circulating TNFα in humans. In conclusion, our results demonstrate an unanticipated crosstalk between myeloid cells and skeletal muscle in the development of obesity-associated insulin resistance.

Cross-talk between skeletal muscle and immune cells: muscle-derived mediators and metabolic implications

Skeletal muscles contain resident immune cell populations and their abundance and type is altered in inflammatory myopathies, endotoxemia or different types of muscle injury/insult. Within tissues, monocytes differentiate into macrophages and polarize to acquire pro- or anti-inflammatory phenotypes. Skeletal muscle macrophages play a fundamental role in repair and pathogen clearance. These events require a precisely regulated cross-talk between myofibers and immune cells, involving paracrine/autocrine and contact interactions. Skeletal muscle also undergoes continuous repair as a result of contractile activity that involves participation of myokines and anti-inflammatory input. Finally, skeletal muscle is the major site of dietary glucose disposal; therefore, muscle insulin resistance is essential to the development of whole body insulin resistance. Notably, muscle inflammation is emerging as a potential contributor to insulin resistance. Recent reports show that inflammatory macrophage numbers within muscle are elevated during obesity and that muscle cells in vitro can mount autonomous inflammatory responses under metabolic challenge. Here, we review the nature of skeletal muscle inflammation associated with muscle exercise, damage, and regeneration, endotoxin presence, and myopathies, as well as the new evidence of local inflammation arising with obesity that potentially contributes to insulin resistance.

GLP-1 secretion by microglial cells and decreased CNS expression in obesity

Background

Type 2 diabetes (T2D) is a strong risk factor for developing neurodegenerative pathologies. T2D patients have a deficiency in the intestinal incretin hormone GLP-1, which has been shown to exert neuroprotective and anti-inflammatory properties in the brain.

Methods

Here we investigate potential sources of GLP-1 in the CNS and the effect of diabetic conditions on the proglucagon mRNA expression in the CNS. The obese mouse model ob/ob, characterized by its high levels of free fatty acids, and the microglia cell line BV-2 were used as models. mRNA expression and protein secretion were analyzed by qPCR, immunofluorescence and ELISA.

Results

We show evidence for microglia as a central source of GLP-1 secretion. Furthermore, we observed that expression and secretion are stimulated by cAMP and dependent on microglial activation state. We also show that insulin-resistant conditions reduce the central mRNA expression of proglucagon.

Conclusion

The findings that microglial mRNA expression of proglucagon and GLP-1 protein expression are affected by high levels of free fatty acids and that both mRNA expression levels of proglucagon and secretion levels of GLP-1 are affected by inflammatory stimuli could be of pathogenic importance for the premature neurodegeneration and cognitive decline commonly seen in T2D patients, and they may also be harnessed to advantage in therapeutic efforts to prevent or treat such disorders.

Fatty acids do not pay the toll: effect of SFA and PUFA on human adipose tissue and mature adipocytes inflammation

Background

On the basis that high fat diet induces inflammation in adipose tissue, we wanted to test the effect of dietary saturated and polysunsaturated fatty acids on human adipose tissue and adipocytes inflammation. Moreover we wanted to determine if TLR2 and TLR4 are involved in this pathway.

Methods

Human adipose tissue and adipocytes primary cultures were treated with endotoxin-free BSA conjugated with SFA (lauric acid and palmitic acid - LA and PA) and PUFA (eicosapentaeneic acid, docosahexaenoic acid and oleic acid - EPA, DHA and OA) with or without LPS. Cytokines were then assayed by ELISA (TNF-alpha, IL-6 and MCP-1). In order to determine if TLR2 and TLR4 are activated by fatty acid (FA), we used HEK-Blue cells transfected by genes from TLR2 or TLR4 pathways associated with secreted alkaline phosphatase reporter gene.

Results

None of the FA tested in HEK-Blue cells were able to activate TLR2 or TLR4, which is concordant with the fact that after FA treatment, adipose tissue and adipocytes cytokines levels remain the same as controls. However, all the PUFA tested: DHA, EPA and to a lesser extent OA down-regulated TNF-alpha, IL-6 and MCP-1 secretion in human adipose tissue and adipocytes cultures.

Conclusions

This study first confirms that FA do not activate TLR2 and TLR4. Moreover by using endotoxin-free BSA, both SFA and PUFA tested were not proinflammatory in human adipose tissue and adipocytes model. More interestingly we showed that some PUFA exert an anti-inflammatory action in human adipose tissue and adipocytes model. These results are important since they clarify the relationship between dietary fatty acids and inflammation linked to obesity.

Inhibition of malonyl-CoA decarboxylase reduces the inflammatory response associated with insulin resistance

We previously showed that genetic inactivation of malonyl-CoA decarboxylase (MCD), which regulates fatty acid oxidation, protects mice against high-fat diet-induced insulin resistance. Development of insulin resistance has been associated with activation of the inflammatory response. Therefore, we hypothesized that the protective effect of MCD inhibition might be caused by a favorable effect on the inflammatory response. We examined if pharmacological inhibition of MCD protects neonatal cardiomyocytes and peritoneal macrophages against inflammatory-induced metabolic perturbations. Cardiomyocytes and macrophages were treated with LPS to induce an inflammatory response, in the presence or absence of an MCD inhibitor (CBM-301106, 10 μM). Inhibition of MCD attenuated the LPS-induced inflammatory response in cardiomyocytes and macrophages. MCD inhibition also prevented LPS impairment of insulin-stimulated glucose uptake in cardiomyocytes and increased phosphorylation of Akt. Additionally, inhibition of MCD strongly diminished LPS-induced activation of palmitate oxidation. We also found that treatment with an MCD inhibitor prevented LPS-induced collapse of total cellular antioxidant capacity. Interestingly, treatment with LPS or an MCD inhibitor did not alter intracellular triacylglycerol content. Furthermore, inhibition of MCD prevented LPS-induced increases in the level of ceramide in cardiomyocytes and macrophages while also ameliorating LPS-initiated decreases in PPAR binding. This suggests that the anti-inflammatory effect of MCD inhibition is mediated via accumulation of long-chain acyl-CoA, which in turn stimulates PPAR binding. Our results also demonstrate that pharmacological inhibition of MCD is a novel and promising approach to treat insulin resistance and its associated metabolic complications.

Muscle cells challenged with saturated fatty acids mount an autonomous inflammatory response that activates macrophages

Obesity is associated with chronic low-grade inflammation. Within adipose tissue of mice fed a high fat diet, resident and infiltrating macrophages assume a pro-inflammatory phenotype characterized by the production of cytokines which in turn impact on the surrounding tissue. However, inflammation is not restricted to adipose tissue and high fat-feeding is responsible for a significant increase in pro-inflammatory cytokine expression in muscle. Although skeletal muscle is the major disposer of dietary glucose and a major determinant of glycemia, the origin and consequence of muscle inflammation in the development of insulin resistance are poorly understood.

We used a cell culture approach to investigate the vectorial crosstalk between muscle cells and macrophages upon exposure to physiological, low levels of saturated and unsaturated fatty acids. Inflammatory pathway activation and cytokine expression were analyzed in L6 muscle cells expressing myc-tagged GLUT4 (L6GLUT4myc) exposed to 0.2 mM palmitate or palmitoleate. Conditioned media thereof, free of fatty acids, were then tested for their ability to activate RAW264.7 macrophages.

Palmitate -but not palmitoleate- induced IL-6, TNFα and CCL2 expression in muscle cells, through activation of the NF-κB pathway. Palmitate (0.2 mM) alone did not induce insulin resistance in muscle cells, yet conditioned media from palmitate-challenged muscle cells selectively activated macrophages towards a pro-inflammatory phenotype.

These results demonstrate that low concentrations of palmitate activate autonomous inflammation in muscle cells to release factors that turn macrophages pro-inflammatory. We hypothesize that saturated fat-induced, low-grade muscle cell inflammation may trigger resident skeletal muscle macrophage polarization, possibly contributing to insulin resistance in vivo.

The emerging role of HDL in glucose metabolism

A low plasma level of HDL cholesterol is an atherosclerotic risk factor; however, emerging evidence suggests that low HDL levels might also contribute to the pathophysiology of type 2 diabetes mellitus (T2DM) through direct effects on plasma glucose. In the past decade, animal and clinical studies have uncovered a previously undescribed spectrum of HDL actions, indicating that HDL may control glucose homeostasis through mechanisms including insulin secretion, direct glucose uptake by muscle via the AMP-activated protein kinase, and possibly enhanced insulin sensitivity. These effects are mediated by multiple cell types via mechanisms including preservation of cell function through cellular lipid removal and also via direct signaling events. We suggest a paradigm shift from HDL being a bystander to being an active player in diabetic pathophysiology, which raises the possibility that HDL elevation could be a novel therapeutic avenue for T2DM. The entry of HDL-raising agents of the cholesteryl ester transfer protein (CETP) inhibitor class into late-phase clinical trials creates potential for rapid clinical translation. This Review will discuss the emerging evidence for a role of HDL-mediated glucose regulation in the pathophysiology of T2DM, and will also outline the therapeutic potential for HDL elevation for the prevention and management of T2DM.

The macrophage at the intersection of immunity and metabolism in obesity

Obesity is a worldwide pandemic representing one of the major challenges that societies face around the globe. Identifying the mechanisms involved in its development and propagation will help the development of preventative and therapeutic strategies that may help control its rising rates.Obesity is associated with chronic low-grade inflammation, and this is believed to be one of the major contributors to the development of insulin resistance, which is an early event in obesity and leads to type 2 diabetes when the pancreas fails to keep up with increased demand for insulin. In this review, we discuss the role of macrophages in mediation of inflammation in obesity in metabolic organs including adipose tissue, skeletal muscle and liver. The presence of immune cells at the interface with metabolic organs modulates both metabolic function and inflammatory responses in these organs, and may provide a potential therapeutic target to modulate metabolic function in obesity.

Palmitate-activated macrophages confer insulin resistance to muscle cells by a mechanism involving protein kinase C θ and ε

Background

Macrophage-derived factors contribute to whole-body insulin resistance, partly by impinging on metabolically active tissues. As proof of principle for this interaction, conditioned medium from macrophages treated with palmitate (CM-PA) reduces insulin action and glucose uptake in muscle cells. However, the mechanism whereby CM-PA confers this negative response onto muscle cells remains unknown.

Methodology/Principal Findings

L6-GLUT4myc myoblasts were exposed for 24 h to palmitate-free conditioned medium from RAW 264.7 macrophages pre-treated with 0.5 mM palmitate for 6 h. This palmitate-free CM-PA, containing selective cytokines and chemokines, inhibited myoblast insulin-stimulated insulin receptor substrate 1 (IRS1) tyrosine phosphorylation, AS160 phosphorylation, GLUT4 translocation and glucose uptake. These effects were accompanied by a rise in c-Jun N-terminal kinase (JNK) activation, degradation of Inhibitor of κBα (IκBα), and elevated expression of proinflammatory cytokines in myoblasts. Notably, CM-PA caused IRS1 phosphorylation on Ser1101, and phosphorylation of novel PKCθ and ε. Co-incubation of myoblasts with CM-PA and the novel and conventional PKC inhibitor Gö6983 (but not with the conventional PKC inhibitor Gö6976) prevented PKCθ and ε activation, JNK phosphorylation, restored IκBα mass and reduced proinflammatory cytokine production. Gö6983 also restored insulin signalling and glucose uptake in myoblasts. Moreover, co-silencing both novel PKC θ and ε isoforms in myoblasts by RNA interference, but not their individual silencing, prevented the inflammatory response and restored insulin sensitivity to CM-PA-treated myoblasts.

Conclusions/Clinical Significance

The results suggest that the block in muscle insulin action caused by CM-PA is mediated by novel PKCθ and PKCε. This study re-establishes the participation of macrophages as a relay in the action of fatty acids on muscle cells, and further identifies PKCθ and PKCε as key elements in the inflammatory and insulin resistance responses of muscle cells to macrophage products. Furthermore, it portrays these PKC isoforms as potential targets for the treatment of fatty acid-induced, inflammation-linked insulin resistance.

Conditioned medium from hypoxia-treated adipocytes renders muscle cells insulin resistant

Adipose tissue hypoxia is an early phenotype in obesity, associated with macrophage infiltration and local inflammation. Here we test the hypothesis that adipocytes in culture respond to a hypoxic environment with the release of pro-inflammatory factors that stimulate macrophage migration and cause muscle insulin resistance. 3T3-L1 adipocytes cultured in a 1% O2 atmosphere responded with a classic hypoxia response by elevating protein expression of HIF-1α. This was associated with elevated mRNA expression and peptide release of cytokines TNFα, IL-6 and the chemokine monocyte chemoattractant protein-1 (MCP-1). The mRNA and protein expression of the anti-inflammatory adipokine adiponectin was reduced. Conditioned medium from hypoxia-treated adipocytes (CM-H), inhibited insulin-stimulated and raised basal cell surface levels of GLUT4myc stably expressed in C2C12 myotubes. Insulin stimulation of Akt and AS160 phosphorylation, key regulators of GLUT4myc exocytosis, was markedly impaired. CM-H also caused activation of JNK and S6K, and elevated serine phosphorylation of IRS1 in the C2C12 myotubes. These effects were implicated in reducing propagation of insulin signaling to Akt and AS160. Heat inactivation of CM-H reversed its dual effects on GLUT4myc traffic in muscle cells. Interestingly, antibody-mediated neutralization of IL-6 in CM-H lowered its effect on both the basal and insulin-stimulated cell surface GLUT4myc compared to unmodified CM-H. IL-6 may have regulated GLUT4myc traffic through its action on AMPK. Additionally, antibody-mediated neutralization of MCP-1 partly reversed the inhibition of insulin-stimulated GLUT4myc exocytosis caused by unmodified CM-H. In Transwell co-culture, hypoxia-challenged adipocytes attracted RAW 264.7 macrophages, consistent with elevated release of MCP-1 from adipocytes during hypoxia. Neutralization of MCP-1 in adipocyte CM-H prevented macrophage migration towards it and partly reversed the effect of CM-H on insulin response in muscle cells. We conclude that adipose tissue hypoxia may be an important trigger of its inflammatory response observed in obesity, and the elevated chemokine MCP-1 may contribute to increased macrophage migration towards adipose tissue and subsequent decreased insulin responsiveness of glucose uptake in muscle.

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.

DHA reduces the atrophy-associated Fn14 protein in differentiated myotubes during coculture with macrophages

Macrophages are an important component of muscle where they are involved in complex processes such as repair, regeneration and hypertrophy. We recently reported that macrophage numbers increase in the muscle of obese patients, suggesting that muscle-resident macrophages could be involved in the development of muscle insulin resistance that is associated with obesity. Coculture of activated macrophages with human muscle cells impairs insulin signaling and induces atrophy signaling pathways in the human muscle cells; this is exacerbated by the addition of palmitic acid. In this study, we tested the hypothesis that docosahexaenoic acid (DHA), a polyunsaturated fatty acid that has anti-inflammatory properties, would have the opposite effect of palmitic acid on muscle-macrophage cocultures. Surprisingly, DHA did not stimulate insulin signaling in human muscle myotubes that were cocultured with fibroblasts or macrophages. However, DHA inhibited Fn14, the TNF-like weak inducer of apoptosis receptor that increases the expression of the muscle-specific ubiquitin ligase MuRF-1 (muscle ring-finger protein-1). DHA treatment also increased the apparent molecular mass of MuRF-1 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels, suggesting that DHA causes MuRF-1 to be posttranslationally modified. In conclusion, these results suggest that DHA may have a beneficial effect on muscle mass in humans by inhibiting the induction of Fn14 by infiltrating macrophages.

Capsaicin attenuates palmitate-induced expression of macrophage inflammatory protein 1 and interleukin 8 by increasing palmitate oxidation and reducing c-Jun activation in THP-1 (human acute monocytic leukemia cell) cells

Capsaicin, a spicy component of hot peppers, has been shown to improve inflammatory disease and obesity. In this study, we tested the hypothesis that the anti-inflammatory activity of capsaicin can be used to improve free fatty acid (FFA)-induced inflammation by reducing gene expression of macrophage inflammatory protein 1 (MIP-1) and interleukin 8 (IL-8) in THP-1 (human acute monocytic leukemia cell) macrophages. To investigate whether capsaicin ameliorates palmitate-induced MIP-1 and IL-8 gene expressions, we treated THP-1 cells with palmitate in the presence or absence of capsaicin and measured MIP-1 and IL-8 by real-time polymerase chain reaction. To elucidate the mechanism by which capsaicin effects on palmitate-induced MIP-1 and IL-8 gene expressions, we performed immunoblotting with stress kinase-related antibodies and measured palmitate oxidation and palmitate oxidation-related gene expression. Palmitate and stearate but not the unsaturated FFA oleate significantly increased MIP-1 and IL-8 expressions in THP-1 macrophages. Treatment with capsaicin or FFA oxidation stimulators inhibited palmitate-induced MIP-1 and IL-8 expressions in THP-1 macrophages. Capsaicin increased the gene expression of carnitine palmitoyltransferase 1 and the β-oxidation of palmitate. Furthermore, capsaicin significantly reduced palmitate-stimulated activation of c-Jun N-terminal kinase, c-Jun, and p38. Our data suggest that the attenuation of palmitate-induced MIP-1 and IL-8 gene expressions by capsaicin is associated with reduced activation of c-Jun N-terminal kinase, c-Jun, and p38 and preserved β-oxidation activity.

Deficiency of haematopoietic-cell-derived IL-10 does not exacerbate high-fat-diet-induced inflammation or insulin resistance in mice

AIMS/HYPOTHESIS

Recent work has identified the important roles of M1 pro-inflammatory and M2 anti-inflammatory macrophages in the regulation of insulin sensitivity. Specifically, increased numbers of M2 macrophages and a decrease in M1 macrophages within the adipose tissue are associated with a state of enhanced insulin sensitivity. IL-10 is an anti-inflammatory cytokine and is a critical effector molecule of M2 macrophages.

METHODS

In the present study, we examined the contribution of haematopoietic-cell-derived IL-10 to the development of obesity-induced inflammation and insulin resistance. We hypothesised that haematopoietic-cell-restricted deletion of IL-10 would exacerbate obesity-induced inflammation and insulin resistance. Lethally irradiated wild-type recipient mice receiving bone marrow from either wild-type or Il10-knockout mice were placed on either a chow or a high-fat diet for a period of 12 weeks and assessed for alterations in body composition, tissue inflammation and glucose and insulin tolerance.

RESULTS

Contrary to our hypothesis, neither inflammation, as measured by the activation of pro-inflammatory stress kinases and gene expression of several pro-inflammatory cytokines in the adipose tissue and liver, nor diet-induced obesity and insulin resistance were exacerbated by the deletion of haematopoietic-cell-derived IL-10. Interestingly, however, Il10 mRNA expression and IL-10 protein production in liver and/or adipose tissue were markedly elevated in Il10-knockout bone-marrow-transplanted mice relative to wild-type bone marrow-transplanted mice.

CONCLUSIONS/INTERPRETATION

These data show that deletion of IL-10 from the haematopoietic system does not potentiate high-fat diet-induced inflammation or insulin resistance.

Direct involvement of tumor necrosis factor-&alpha; in the regulation of glucose uptake in rainbow trout muscle cells

The proinflammatory cytokine TNF-α is known to have a direct action on skeletal muscle in mammals. However, little is known regarding the potential effects of cytokines on nonimmune tissues, particularly in skeletal muscle, in fish. The aim of this study was to investigate the effects of recombinant trout TNF-α (rtTNF-α) on skeletal muscle carbohydrate metabolism in rainbow trout (Oncorhynchus mykiss). We used a primary cell culture of muscle cells from rainbow trout to show that rtTNF-α stimulates glucose uptake in myoblasts and myotubes at concentrations that do not affect the viability of the cells, requiring de novo protein synthesis as shown by the impairment of rtTNF-α-stimulated glucose uptake by cycloheximide. With the use of specific inhibitors, we show that rtTNF-α-stimulated glucose uptake is mediated by the p38MAPK, NF-κB, and JNK pathways. Additionally, we provide evidence that the stimulatory effects of rtTNF-α on glucose uptake in trout skeletal muscle cells may be caused, at least in part, by an increase in the amount of GLUT4 at the plasma membrane. Incubation of trout muscle cells with conditioned medium from LPS-stimulated trout macrophages, enriched in TNF-α, increased glucose uptake. Our results indicate that recombinant, as well as native trout TNF-α, directly stimulates glucose uptake in trout muscle cells and provide evidence, for the first time in nonmammalian vertebrates, for a potential regulatory role of TNF-α in skeletal muscle metabolism.

Gut microbiota, lipopolysaccharides, and innate immunity in the pathogenesis of obesity and cardiovascular risk

Compelling evidence supports the concepts that gut microbiota actively promotes weight gain and fat accumulation and sustains, indirectly, a condition of low-grade inflammation, thus enhancing the cardiovascular risk. Fewer Bacteroidetes and more Firmicutes seem to characterize the gut microbiota of obese people as compared with that of lean individuals. This difference translates into an increased efficiency of microbiota of obese individuals in harvesting energy from otherwise indigestible carbohydrates. Furthermore, the microbiota also seems able to favor fat accumulation. Indeed, studies performed in germ-free animals have demonstrated that conventionalization of sterile intestine with gut microbiota is associated with an enhanced expression of various lipogenic genes in different tissues, i.e., hepatic, adipose, and muscle tissues. Finally, the microbiota favors systemic exposure to the lipopolysaccharides (LPSs), large glycolipids derived from the outer membrane of Gram-negative bacteria. LPSs can cause a condition of "metabolic endotoxemia" characterized by low-grade inflammation, insulin resistance, and augmented cardiovascular risk. LPSs are a powerful trigger for the innate immune system response. Upon binding to the Toll-like receptor 4 and its coreceptors, LPSs trigger a cascade of responses ultimately resulting in the release of proinflammatory molecules that interfere with modulation of glucose and insulin metabolism, promote development and rupture of the atherosclerotic plaque, and favor progression of fatty liver disease to steatohepatitis. This review gives a comprehensive breakdown of the interaction among gut microbiota, LPSs, and the innate immune system in the development of obesity and promotion of an individual's cardiovascular risk.

NOD2 activation induces muscle cell-autonomous innate immune responses and insulin resistance

Insulin resistance is associated with chronic low-grade inflammation in vivo, largely mediated by activated innate immune cells. Cytokines and pathogen-derived ligands of surface toll-like receptors can directly cause insulin resistance in muscle cells. However, it is not known if intracellular pathogen sensors can, on their own, provoke insulin resistance. Here, we show that the cytosolic pattern recognition receptors nucleotide-binding oligomerization domain-containing protein (NOD)1 and NOD2 are expressed in immune and metabolic tissues and hypothesize that their activation in muscle cells would result in cell-autonomous responses leading to insulin resistance. Bacterial peptidoglycan motifs that selectively activate NOD2 were directly administered to L6- GLUT4myc myotubes in culture. Within 3 h, insulin resistance arose, characterized by reductions in each insulin-stimulated glucose uptake, GLUT4 translocation, Akt Ser(473) phosphorylation, and insulin receptor substrate 1 tyrosine phosphorylation. Muscle cell-autonomous responses to NOD2 ligand included activation of the stress/inflammation markers c-Jun N-terminal kinase, ERK1/2, p38 MAPK, degradation of inhibitor of κBα, and production of proinflammatory cytokines. These results show that NOD2 alone is capable of acutely inducing insulin resistance within muscle cells, possibly by activating endogenous inflammatory signals and/or through cytokine production, curbing upstream insulin signals. NOD2 is hence a new inflammation target connected to insulin resistance, and this link occurs without the need of additional contributing cell types. This study provides supporting evidence for the integration of innate immune and metabolic responses through the involvement of NOD proteins and suggests the possible participation of cell autonomous immune responses in the development of insulin resistance in skeletal muscle, the major depot for postprandial glucose utilization.

Inflammatory signaling in skeletal muscle insulin resistance: green signal for nutritional intervention?

PURPOSE OF REVIEW

To review the evidence implying a role of inflammatory signaling pathways, specifically nuclear factor-κB and c-Jun NH2-terminal kinase, in fatty acid-induced skeletal muscle insulin resistance and to discuss the potential of dietary interventions to interfere with these processes.

RECENT FINDINGS

Fatty acids can induce skeletal muscle insulin resistance via inflammatory signaling after binding Toll-like receptors at the cell membrane of muscle cells or after accumulating as intramyocellular lipid metabolites. In both processes, activation of intracellular inflammatory signaling is involved. The majority of literature addressing the causality of muscle nuclear factor-κB activation in skeletal muscle insulin resistance suggests that insulin resistance does not require muscle nuclear factor-κB activation. Recently, strong evidence was given that c-Jun NH2-terminal kinase signaling is an important inflammatory pathway involved in skeletal muscle insulin resistance. Furthermore, it is well established that proinflammatory cytokines originating from the enlarged adipose tissue or from activated adipose tissue macrophages can cause muscle insulin resistance. Recently, also macrophages resided in the muscle have been proposed to play an important role in muscle insulin resistance. Because of their anti-inflammatory characteristics, several dietary components like polyphenols may be interesting candidates for manipulating skeletal muscle insulin resistance.

SUMMARY

Several dietary components, like polyphenols, have been reported to interfere with inflammatory signaling. To test whether these compounds can be used to prevent or reverse insulin resistance, well controlled human intervention studies have to be designed.

Muscle insulin resistance: assault by lipids, cytokines and local macrophages

PURPOSE OF REVIEW

The present review outlines possible mechanisms by which high fatty acids, associated with high-fat diet and obesity, impose insulin resistance on glucose uptake into skeletal muscle.

RECENT FINDINGS

It is well established that muscle insulin resistance arises in conditions of high-fatty acid availability, and correlates with accumulation of triglycerides within skeletal muscle fibres. However, it is debated whether triglycerides or other lipid metabolites such as diacylglycerols and ceramides are directly responsible. These lipid metabolites can activate serine kinases that impair insulin signalling. Accumulation of acylcarnitines and reactive oxygen species could be additional causative agents of insulin resistance. Further, the precise defects in insulin signalling in muscle caused by high intramuscular lipid (i.e. lipotoxicity) remain unclear. In parallel, proinflammatory activation within the adipose tissue of obese and high-fat fed animals or humans causes muscle insulin resistance, and is ascribed to circulating inflammatory cytokines. Recent evidence also shows proinflammatory macrophages infiltrating muscle tissue and/or intermuscular adipose tissue, and there is growing evidence that fatty acids trigger macrophages to secrete factors that directly impair insulin actions. These factors are postulated to activate stress-signalling pathways in muscle that act on the same insulin-signalling components affected by lipotoxicity.

SUMMARY

Altered intramuscular lipid metabolism, circulating cytokines, and inflammatory macrophage infiltration of muscle tissue have been recently linked to muscle insulin resistance provoked by fatty acids. Each is analysed separately in this review, but they may act simultaneously and synergistically to render skeletal muscle insulin-resistant.

SIRT1 inhibits inflammatory pathways in macrophages and modulates insulin sensitivity

Chronic inflammation is an important etiology underlying obesity-related disorders such as insulin resistance and type 2 diabetes, and recent findings indicate that the macrophage can be the initiating cell type responsible for this chronic inflammatory state. The mammalian silent information regulator 2 homolog SIRT1 modulates several physiological processes important for life span, and a potential role of SIRT1 in the regulation of insulin sensitivity has been shown. However, with respect to inflammation, the role of SIRT1 in regulating the proinflammatory pathway within macrophages is poorly understood. Here, we show that knockdown of SIRT1 in the mouse macrophage RAW264.7 cell line and in intraperitoneal macrophages broadly activates the JNK and IKK inflammatory pathways and increases LPS-stimulated TNFalpha secretion. Moreover, gene expression profiles reveal that SIRT1 knockdown leads to an increase in inflammatory gene expression. We also demonstrate that SIRT1 activators inhibit LPS-stimulated inflammatory pathways, as well as secretion of TNFalpha, in a SIRT1-dependent manner in RAW264.7 cells and in primary intraperitoneal macrophages. Treatment of Zucker fatty rats with a SIRT1 activator leads to greatly improved glucose tolerance, reduced hyperinsulinemia, and enhanced systemic insulin sensitivity during glucose clamp studies. These in vivo insulin-sensitizing effects were accompanied by a reduction in tissue inflammation markers and a decrease in the adipose tissue macrophage proinflammatory state, fully consistent with the in vitro effects of SIRT1 in macrophages. In conclusion, these results define a novel role for SIRT1 as an important regulator of macrophage inflammatory responses in the context of insulin resistance and raise the possibility that targeting of SIRT1 might be a useful strategy for treating the inflammatory component of metabolic diseases.

Desialylation of insulin receptors and IGF-1 receptors by neuraminidase-1 controls the net proliferative response of L6 myoblasts to insulin

We recently established that the subunit of cell surface-residing elastin receptor, neuraminidase-1 (Neu1), can desialylate adjacent insulin-like growth factor 1 receptors (IGF-1R) of arterial smooth muscle cells, thereby quenching their proliferative response to insulin-like growth factor II. In this study, we explored whether Neu1 would also desialylate the insulin receptors (IR), as well as the IGF-1R on rat skeletal L6 myoblasts, and whether desialylation of IR and IGF-1R would affect a net proliferative effect of insulin. First, we found that physiological (0.5-1 nM) and high therapeutic (10 nM) insulin concentrations induced a modest increase in proliferation rate of cultured L6 myoblasts. While IR kinase inhibitor could abolish the mitogenic effect of these insulin concentrations, the observed more pronounced proliferative response to supraphysiological concentration (100 nM) of insulin could be eliminated only by specific inhibition of IGF-1R. Then, we found that treatment of L6 cells with mouse-derived Neu1 or with Clostridium perfringens neuraminidase caused desialylation of IR, which coincided with a significant increase of their proliferative response to lower (0.5-10 nM) concentrations of insulin. In contrast, experimental desialylation of IGF-1R coincided with elimination of the heightened proliferative response of L6 myoblasts to 100 nM insulin. Importantly, we also found that inhibition of endogenous Neu1 abolished the increase in proliferation of L6 cells induced by 1 and 10 nM of insulin, but amplified the proliferative effect of 100 nM insulin. We therefore conclude that desialylation of both IR and IGF-1R by Neu1 controls the net proliferative response of skeletal myoblasts to insulin.

SIRT1 inhibits inflammatory pathways in macrophages and modulates insulin sensitivity

Chronic inflammation is an important etiology underlying obesity-related disorders such as insulin resistance and type 2 diabetes, and recent findings indicate that the macrophage can be the initiating cell type responsible for this chronic inflammatory state. The mammalian silent information regulator 2 homolog SIRT1 modulates several physiological processes important for life span, and a potential role of SIRT1 in the regulation of insulin sensitivity has been shown. However, with respect to inflammation, the role of SIRT1 in regulating the proinflammatory pathway within macrophages is poorly understood. Here, we show that knockdown of SIRT1 in the mouse macrophage RAW264.7 cell line and in intraperitoneal macrophages broadly activates the JNK and IKK inflammatory pathways and increases LPS-stimulated TNFalpha secretion. Moreover, gene expression profiles reveal that SIRT1 knockdown leads to an increase in inflammatory gene expression. We also demonstrate that SIRT1 activators inhibit LPS-stimulated inflammatory pathways, as well as secretion of TNFalpha, in a SIRT1-dependent manner in RAW264.7 cells and in primary intraperitoneal macrophages. Treatment of Zucker fatty rats with a SIRT1 activator leads to greatly improved glucose tolerance, reduced hyperinsulinemia, and enhanced systemic insulin sensitivity during glucose clamp studies. These in vivo insulin-sensitizing effects were accompanied by a reduction in tissue inflammation markers and a decrease in the adipose tissue macrophage proinflammatory state, fully consistent with the in vitro effects of SIRT1 in macrophages. In conclusion, these results define a novel role for SIRT1 as an important regulator of macrophage inflammatory responses in the context of insulin resistance and raise the possibility that targeting of SIRT1 might be a useful strategy for treating the inflammatory component of metabolic diseases.

Direct and macrophage-mediated actions of fatty acids causing insulin resistance in muscle cells

Obesity is associated with insulin resistance and increased risk for developing type 2 diabetes. Enlarged adipocytes develop resistance to the anti-lipolytic action of insulin. Elevated levels of fatty acids in the plasma and interstitial fluids lead to whole-body insulin resistance by disrupting normal insulin-regulated glucose uptake and glycogen storage in skeletal muscle. A new understanding has been cultivated in the past 5 to 10 years that adipocytes and macrophages (resident or bone marrow-derived) in adipose tissue of obese animals and humans are activated in a pro-inflammatory capacity and secrete insulin resistance-inducing factors. However, only recently have fatty acids themselves been identified as agents that engage toll-like receptors of the innate immunity systems of macrophages, adipocytes and muscle cells to trigger pro-inflammatory responses. This review summarizes our observations that fatty acids evoke the release of pro-inflammatory factors from macrophages that consequently induce insulin resistance in muscle cells.

Muscle inflammatory response and insulin resistance: synergistic interaction between macrophages and fatty acids leads to impaired insulin action

Obesity is characterized by adipose tissue expansion as well as macrophage infiltration of adipose tissue. This results in an increase in circulating inflammatory cytokines and nonesterified fatty acids, factors that cause skeletal muscle insulin resistance. Whether obesity also results in skeletal muscle inflammation is not known. In this study, we quantified macrophages immunohistochemically in vastus lateralis biopsies from eight obese and eight lean subjects. Our study demonstrates that macrophages infiltrate skeletal muscle in obesity, and we developed an in vitro system to study this mechanistically. Myoblasts were isolated from vastus lateralis biopsies and differentiated in culture. Coculture of differentiated human myotubes with macrophages in the presence of palmitic acid, to mimic an obese environment, revealed that macrophages in the presence of palmitic acid synergistically augment cytokine and chemokine expression in myotubes, decrease IkappaB-alpha protein expression, increase phosphorylated JNK, decrease phosphorylated Akt, and increase markers of muscle atrophy. These results suggest that macrophages alter the inflammatory state of muscle cells in an obese milieu, inhibiting insulin signaling. Thus in obesity both adipose tissue and skeletal muscle inflammation may contribute to insulin resistance.