JNK and tumor necrosis factor-alpha mediate free fatty acid-induced insulin resistance in 3T3-L1 adipocytes

Inflammation and lipid signaling in the etiology of insulin resistance

Inflammation and lipid signaling are intertwined modulators of homeostasis and immunity. In addition to the extensively studied eicosanoids and inositol phospholipids, emerging studies indicate that many other lipid species act to positively and negatively regulate inflammatory responses. Conversely, inflammatory signaling can significantly alter lipid metabolism in the liver, adipose tissue, skeletal muscle, and macrophage in the context of infection, diabetes, and atherosclerosis. Here, we review recent findings related to this interconnected network from the perspective of immunity and metabolic disease.

Regulation of chemokine and chemokine receptor expression by PPARγ in adipocytes and macrophages

Background

PPARγ plays a key role in adipocyte biology, and Rosiglitazone (Rosi), a thiazolidinedione (TZD)/PPARγ agonist, is a potent insulin-sensitizing agent. Recent evidences demonstrate that adipose tissue inflammation links obesity with insulin resistance and that the insulin-sensitizing effects of TZDs result, in part, from their anti-inflammatory properties. However the underlying mechanisms are unclear.

Methodology and Principal Findings

In this study, we establish a link between free fatty acids (FFAs) and PPARγ in the context of obesity-associated inflammation. We show that treatment of adipocytes with FFAs, in particular Arachidonic Acid (ARA), downregulates PPARγ protein and mRNA levels. Furthermore, we demonstrate that the downregulation of PPARγ by ARA requires the activation the of Endoplamsic Reticulum (ER) stress by the TLR4 pathway. Knockdown of adipocyte PPARγ resulted in upregulation of MCP1 gene expression and secretion, leading to enhanced macrophage chemotaxis. Rosi inhibited these effects. In a high fat feeding mouse model, we show that Rosi treatment decreases recruitment of proinflammatory macrophages to epididymal fat. This correlates with decreased chemokine and decreased chemokine receptor expression in adipocytes and macrophages, respectively.

Conclusions and Significance

In summary, we describe a novel link between FAs, the TLR4/ER stress pathway and PPARγ, and adipocyte-driven recruitment of macrophages. We thus both describe an additional potential mechanism for the anti-inflammatory and insulin-sensitizing actions of TZDs and an additional detrimental property associated with the activation of the TLR4 pathway by FA.

Garlic-derived S-allylmercaptocysteine is a hepato-protective agent in non-alcoholic fatty liver disease in vivo animal model

PURPOSE

To investigate the hepato-protective properties and underlying mechanisms of SAMC in a non-alcoholic fatty liver disease (NAFLD) rat model.

METHODS

Female rats were fed with a diet comprising highly unsaturated fat diet (30% fish oil) for 8 weeks to develop NAFLD with or without an intraperitoneal injection of 200 mg/kg SAMC three times per week. After euthanasia, blood and liver samples of rats were collected for histological and biochemical analyses.

RESULTS

Co-treatment of SAMC attenuated NAFLD-induced liver injury, fat accumulation, collagen formation and free fatty acids (FFAs). At the molecular level, SAMC decreased the lipogenesis marker and restored the lipolysis marker. SAMC also reduced the expression levels of pro-fibrogenic factors and diminished liver oxidative stress partly through the inhibition in the activity of cytochrome P450 2E1-dependent pathway. NAFLD-induced inflammation was also partially mitigated by SAMC treatment via reduction in the pro-inflammatory mediators, chemokines and suppressor of cytokine signaling. The protective effect of SAMC is also shown partly through the restoration of altered phosphorylation status of FFAs-dependent MAP kinase pathways and diminished in the nuclear transcription factors (NF-κB and AP-1) activity during NAFLD development.

CONCLUSIONS

SAMC is a novel hepato-protective agent against NAFLD caused by abnormal liver functions. Garlic or garlic derivatives could be considered as a potent food supplement in the prevention of fatty liver disease.

Adipocyte NCoR knockout decreases PPARγ phosphorylation and enhances PPARγ activity and insulin sensitivity

Insulin resistance, tissue inflammation, and adipose tissue dysfunction are features of obesity and Type 2 diabetes. We generated adipocyte-specific Nuclear Receptor Corepressor (NCoR) knockout (AKO) mice to investigate the function of NCoR in adipocyte biology, glucose and insulin homeostasis. Despite increased obesity, glucose tolerance was improved in AKO mice, and clamp studies demonstrated enhanced insulin sensitivity in liver, muscle, and fat. Adipose tissue macrophage infiltration and inflammation were also decreased. PPARγ response genes were upregulated in adipose tissue from AKO mice and CDK5-mediated PPARγ ser-273 phosphorylation was reduced, creating a constitutively active PPARγ state. This identifies NCoR as an adaptor protein that enhances the ability of CDK5 to associate with and phosphorylate PPARγ. The dominant function of adipocyte NCoR is to transrepress PPARγ and promote PPARγ ser-273 phosphorylation, such that NCoR deletion leads to adipogenesis, reduced inflammation, and enhanced systemic insulin sensitivity, phenocopying the TZD-treated state.

The multi-level action of fatty acids on adiponectin production by fat cells

Current epidemics of diabetes mellitus is largely caused by wide spread obesity. The best-established connection between obesity and insulin resistance is the elevated and/or dysregulated levels of circulating free fatty acids that cause and aggravate insulin resistance, type 2 diabetes, cardiovascular disease and other hazardous metabolic conditions. Here, we investigated the effect of a major dietary saturated fatty acid, palmitate, on the insulin-sensitizing adipokine adiponectin produced by cultured adipocytes. We have found that palmitate rapidly inhibits transcription of the adiponectin gene and the release of adiponectin from adipocytes. Adiponectin gene expression is controlled primarily by PPARγ and C/EBPα. Using mouse embryonic fibroblasts from C/EBPα-null mice, we have determined that the latter transcription factor may not solely mediate the inhibitory effect of palmitate on adiponectin transcription leaving PPARγ as a likely target of palmitate. In agreement with this model, palmitate increases phosphorylation of PPARγ on Ser273, and substitution of PPARγ for the unphosphorylated mutant Ser273Ala blocks the effect of palmitate on adiponectin transcription. The inhibitory effect of palmitate on adiponectin gene expression requires its intracellular metabolism via the acyl-CoA synthetase 1-mediated pathway. In addition, we found that palmitate stimulates degradation of intracellular adiponectin by lysosomes, and the lysosomal inhibitor, chloroquine, suppressed the effect of palmitate on adiponectin release from adipocytes. We present evidence suggesting that the intracellular sorting receptor, sortilin, plays an important role in targeting of adiponectin to lysosomes. Thus, palmitate not only decreases adiponectin expression at the level of transcription but may also stimulate lysosomal degradation of newly synthesized adiponectin.

Global IRS-1 phosphorylation analysis in insulin resistance

AIMS/HYPOTHESIS

IRS-1 serine phosphorylation is often elevated in insulin resistance models, but confirmation in vivo in humans is lacking. We therefore analysed IRS-1 phosphorylation in human muscle in vivo.

METHODS

We used HPLC-electrospray ionisation (ESI)-MS/MS to quantify IRS-1 phosphorylation basally and after insulin infusion in vastus lateralis muscle from lean healthy, obese non-diabetic and type 2 diabetic volunteers.

RESULTS

Basal Ser323 phosphorylation was increased in type 2 diabetic patients (2.1 ± 0.43, p ≤ 0.05, fold change vs lean controls). Thr495 phosphorylation was decreased in type 2 diabetic patients (p ≤ 0.05). Insulin increased IRS-1 phosphorylation at Ser527 (1.4 ± 0.17, p ≤ 0.01, fold change, 60 min after insulin infusion vs basal) and Ser531 (1.3 ± 0.16, p ≤ 0.01, fold change, 60 min after insulin infusion vs basal) in the lean controls and suppressed phosphorylation at Ser348 (0.56 ± 0.11, p ≤ 0.01, fold change, 240 min after insulin infusion vs basal), Thr446 (0.64 ± 0.16, p ≤ 0.05, fold change, 60 min after insulin infusion vs basal), Ser1100 (0.77 ± 0.22, p ≤ 0.05, fold change, 240 min after insulin infusion vs basal) and Ser1142 (1.3 ± 0.2, p ≤ 0.05, fold change, 60 min after insulin infusion vs basal).

CONCLUSIONS/INTERPRETATION

We conclude that, unlike some aspects of insulin signalling, the ability of insulin to increase or suppress certain IRS-1 phosphorylation sites is intact in insulin resistance. However, some IRS-1 phosphorylation sites do not respond to insulin, whereas other Ser/Thr phosphorylation sites are either increased or decreased in insulin resistance.

JNKs, insulin resistance and inflammation: A possible link between NAFLD and coronary artery disease

The incidence of obesity has dramatically increased in recent years. Consequently, obesity and associated disorders such as nonalcoholic fatty liver disease constitute a serious problem. Therefore, the contribution of adipose tissue to metabolic homeostasis has become a focus of interest. In this review, we discuss the latest discoveries that support the role of lipids in nonalcoholic fatty liver disease. We describe the common mechanisms (c-Jun amino-terminal kinases, endoplasmic reticulum stress, unfolded protein response, ceramide, low-grade chronic inflammation) by which lipids and their derivatives impair insulin responsiveness and contribute to inflammatory liver and promote plaque instability in the arterial wall. Presenting the molecular mechanism of lipid activation of pro-inflammatory pathways, we attempt to find a link between nonalcoholic fatty liver disease, metabolic syndrome and cardiovascular diseases. Describing the common mechanisms by which lipid derivatives, through modulation of macrophage function, promote plaque instability in the arterial wall, impair insulin responsiveness and contribute to inflammatory liver and discussing the molecular mechanism of lipid activation of pro-inflammatory pathways, the key roles played by the proliferator-activated receptor and liver X receptor α, nuclear receptors-lipid sensors that link lipid metabolism and inflammation, should be emphasized. Further studies are warranted of anti-inflammatory drugs such as aspirin, anti-interleukin-6 receptors, immune-modulators (calcineurin inhibitors), substances enhancing the expression of heat shock proteins (which protect cells from endoplasmic reticulum stress-induced apoptosis), and anti- c-Jun amino-terminal kinases in well-designed trials to try to minimize the high impact of these illnesses, and the different expressions of the diseases, on the whole population.

Genetic variants in candidate genes influencing NAFLD progression

Nonalcoholic fatty liver disease (NAFLD) is a metabolic disorder including simple steatosis and nonalcoholic steatohepatitis (NASH). Advanced stages of NASH result ultimately in fibrosis, cirrhosis, and hepatocarcinoma. A diagnosis of NASH entails an increased risk of both liver-related and cardiovascular mortality as worsening of the metabolic syndrome. Because of its escalation, many investigations have been performed to elucidate the pathophysiologic origins of the disease progression. Human epidemiologic studies describing polymorphisms in a number of genes involved in metabolic dysfunctions have contributed to clarify the causes leading to the disease evolution. In this review, we attempt to outline critically the most recently identified genetic variants in NAFLD patients to identify possible risk factors promoting the progression of the disease. The evaluation of altered genotypes together with other clinical variables may facilitate the clinical management of these patients.

Macrophage deletion of SOCS1 increases sensitivity to LPS and palmitic acid and results in systemic inflammation and hepatic insulin resistance

OBJECTIVE

Macrophage secretion of proinflammatory cytokines contributes to the pathogenesis of obesity-related insulin resistance. An important regulator of inflammation is the suppressor of cytokine signaling-1 (SOCS1), which inhibits the JAK-STAT and toll-like receptor-4 (TLR4) pathways. Despite the reported role of SOCS1 in inhibiting insulin signaling, it is surprising that a SOCS1 polymorphism that increases SOCS1 promoter activity is associated with enhanced insulin sensitivity despite obesity. In the current study, we investigated the physiological role of myeloid and lymphoid cell SOCS1 in regulating inflammation and insulin sensitivity.

RESEARCH DESIGN AND METHODS

We used mice generated by crossing SOCS1 floxed mice with mice expressing Cre recombinase under the control of the LysM-Cre promoter (SOCS1 LysM-Cre). These mice have deletion of SOCS1 in macrophages and lymphocytes. We assessed macrophage inflammation using flow cytometry and serum cytokine levels using Bioplex assays. We then measured insulin sensitivity using glucose tolerance tests and the euglycemic-hyperinsulinemic clamp. Using bone marrow-derived macrophages, we tested the effects of SOCS1 deletion in regulating responses to the TLR4 ligands: lipopolysaccharide (LPS) and palmitic acid.

RESULTS

SOCS1 LysM-Cre mice had increased macrophage expression of CD11c, enhanced sensitivity to LPS, and palmitic acid and increased serum concentrations of tumor necrosis factor-α, interleukin-6, and monocyte chemoattractant protein. Increased inflammation was associated with impaired glucose tolerance and hyperinsulinemia as a result of reduced hepatic but not skeletal muscle insulin sensitivity.

CONCLUSIONS

The expression of SOCS1 in hematopoietic cells protects mice against systemic inflammation and hepatic insulin resistance potentially by inhibiting LPS and palmitate-induced TLR4 signaling in macrophages.

PPARs, Obesity, and Inflammation

The worldwide prevalence of obesity and related metabolic disorders is rising rapidly, increasing the burden on our healthcare system. Obesity is often accompanied by excess fat storage in tissues other than adipose tissue, including liver and skeletal muscle, which may lead to local insulin resistance and may stimulate inflammation, as in steatohepatitis. In addition, obesity changes the morphology and composition of adipose tissue, leading to changes in protein production and secretion. Some of these secreted proteins, including several proinflammatory mediators, may be produced by macrophages resident in the adipose tissue. The changes in inflammatory status of adipose tissue and liver with obesity feed a growing recognition that obesity represents a state of chronic low-level inflammation. Various molecular mechanisms have been implicated in obesity-induced inflammation, some of which are modulated by the peroxisome proliferator-activated receptors (PPARs). PPARs are ligand-activated transcription factors involved in the regulation of numerous biological processes, including lipid and glucose metabolism, and overall energy homeostasis. Importantly, PPARs also modulate the inflammatory response, which makes them an interesting therapeutic target to mitigate obesity-induced inflammation and its consequences. This review will address the role of PPARs in obesity-induced inflammation specifically in adipose tissue, liver, and the vascular wall.

Adipose tissue remodeling and obesity

To fulfill its role as the major energy-storing tissue, adipose has several unique properties that cannot be seen in any other organ, including an almost unlimited capacity to expand in a non-transformed state. As such, the tissue requires potent mechanisms to remodel, acutely and chronically. Adipocytes can rapidly reach the diffusional limit of oxygen during growth; hypoxia is therefore an early determinant that limits healthy expansion. Proper expansion requires a highly coordinated response among many different cell types, including endothelial precursor cells, immune cells, and preadipocytes. There are therefore remarkable similarities between adipose expansion and growth of solid tumors, a phenomenon that presents both an opportunity and a challenge, since pharmacological interventions supporting healthy adipose tissue adaptation can also facilitate tumor growth.

Cyanidin 3-glucoside attenuates obesity-associated insulin resistance and hepatic steatosis in high-fat diet-fed and db/db mice via the transcription factor FoxO1

Obesity is a major risk factor for the development of type 2 diabetes, and both conditions are now recognized to possess significant inflammatory components underlying their pathophysiologies. Here, we hypothesized that cyanidin 3-glucoside (C3G), a typical anthocyanin reported to possess potent anti-inflammatory properties, would ameliorate obesity-associated inflammation and metabolic disorders, such as insulin resistance and hepatic steatosis in mouse models of diabesity. Male C57BL/6J obese mice fed a high-fat diet for 12 weeks and genetically diabetic db/db mice at an age of 6 weeks received dietary C3G supplementation (0.2%) for 5 weeks. We found that dietary C3G lowered fasting glucose levels and markedly improved the insulin sensitivity in both high-fat diet fed and db/db mice as compared with unsupplemented controls. White adipose tissue messenger RNA levels and serum concentrations of inflammatory cytokines (tumor necrosis factor-α, interleukin-6, and monocyte chemoattractant protein-1) were reduced by C3G, as did macrophage infiltration in adipose tissue. Concomitantly, hepatic triglyceride content and steatosis were alleviated by C3G. Moreover, C3G treatment decreased c-Jun N-terminal kinase activation and promoted phosphorylation and nuclear exclusion of forkhead box O1 after refeeding. These findings clearly indicate that C3G has significant potency in antidiabetic effects by modulating the c-Jun N-terminal kinase/forkhead box O1 signaling pathway and the related inflammatory adipocytokines.

Carnitine palmitoyltransferase 1A prevents fatty acid-induced adipocyte dysfunction through suppression of c-Jun N-terminal kinase

The adipocyte is the principal cell type for fat storage. CPT1 (carnitine palmitoyltransferase-1) is the rate-limiting enzyme for fatty acid β-oxidation, but the physiological role of CPT1 in adipocytes remains unclear. In the present study, we focused on the specific role of CPT1A in the normal functioning of adipocytes. Three 3T3-L1 adipocyte cell lines stably expressing hCPT1A (human CPT1A) cDNA, mouse CPT1A shRNA (short-hairpin RNA) or GFP (green fluorescent protein) were generated and the biological functions of these cell lines were characterized. Alteration in CPT1 activity, either by ectopic overexpression or pharmacological inhibition using etomoxir, did not affect adipocyte differentiation. However, overexpression of hCPT1A significantly reduced the content of intracellular NEFAs (non-esterified fatty acids) compared with the control cells when adipocytes were challenged with fatty acids. The changes were accompanied by an increase in fatty acid uptake and a decrease in fatty acid release. Interestingly, CPT1A protected against fatty acid-induced insulin resistance and expression of pro-inflammatory adipokines such as TNF-α (tumour necrosis factor-α) and IL-6 (interleukin-6) in adipocytes. Further studies demonstrated that JNK (c-Jun N terminal kinase) activity was substantially suppressed upon CPT1A overexpression, whereas knockdown or pharmacological inhibition of CPT1 caused a significant enhancement of JNK activity. The specific inhibitor of JNK SP600125 largely abolished the changes caused by the shRNA- and etomoxir-mediated decrease in CPT1 activity. Moreover, C2C12 myocytes co-cultured with adipocytes pre-treated with fatty acids displayed altered insulin sensitivity. Taken together, our findings have identified a favourable role for CPT1A in adipocytes to attenuate fatty acid-evoked insulin resistance and inflammation via suppression of JNK.

Apelin stimulates glucose uptake through the PI3K/Akt pathway and improves insulin resistance in 3T3-L1 adipocytes

Apelin, a cytokine mainly secreted by adipocytes, is closely related with insulin resistance. The underlying molecular mechanisms of how apelin affects insulin resistance, however, are poorly understood. This study aimed to investigate the effect of apelin on glucose metabolism and insulin resistance in 3T3-L1 adipocytes. After 10 ng/ml TNF-α treatment for 24 h, insulin-stimulated glucose uptake was reduced by 47% in 3T3-L1 adipocytes. Apelin treatment improved glucose uptake in a time- and dose-dependent manner. Treatment of 1,000 nM apelin for 60 min maximally augmented glucose uptake in insulin-resistant 3T3-L1 adipocytes. Furthermore, apelin pre-incubation also increased adipocytes' insulin-stimulated glucose uptake, and PI3K/Akt pathway were involved in these effects. In addition, immunocytochemistry staining and western blotting analysis indicated that apelin could increase glucose transporter 4 translocation from the cytoplasm to the plasma membrane. Apelin also increased the anti-inflammatory adipokine adiponectin mRNA expression while reducing that of pro-inflammatory adipokine interleukin-6 in insulin-resistant 3T3-L1 adipocytes. These results suggest that apelin stimulates glucose uptake through the PI3K/Akt pathway, promotes GLUT4 translocation from the cytoplasm to the plasma membrane, and modulates inflammatory responses in insulin-resistant 3T3-L1 adipocytes.

Obesity, insulin resistance and free fatty acids

PURPOSE OF REVIEW

To describe the role of free fatty acid (FFA) as a cause for insulin resistance in obese people.

RECENT FINDINGS

Elevated plasma FFA levels can account for a large part of insulin resistance in obese patients with type 2 diabetes. Insulin resistance is clinically important because it is closely associated with several diseases including type 2 diabetes, hypertension, dyslipidemia and abnormalities in blood coagulation and fibrinolysis. These disorders are all independent risk factors for cardiovascular disease (heart attacks, strokes and peripheral arterial disease). The mechanisms by which FFA can cause insulin resistance, although not completely known, include generation of lipid metabolites (diacylglycerol), proinflammatory cytokines (TNF-α, IL-1β, IL-6, MCP1) and cellular stress including oxidative and endoplasmic reticulum stress.

SUMMARY

Increased plasma FFA levels are an important cause of obesity-associated insulin resistance and cardiovascular disease. Therapeutic application of this knowledge is hampered by the lack of readily accessible methods to measure FFA and by the lack of medications to lower plasma FFA levels.

Lipid-heparin infusion suppresses the IL-10 response to trauma in subcutaneous adipose tissue in humans

An imbalance between pro- and anti-inflammatory cytokine productions in adipose tissue is thought to contribute to chronic, systemic, low-grade inflammation and consequently to an increased risk of cardiovascular complications in obese and type 2 diabetic patients. Nonesterified fatty acids (NEFA), whose serum levels are elevated in such patients, have been shown to interfere with cytokine production in vitro. In order to evaluate the effects of elevated NEFA levels on cytokine production in adipose tissue in vivo we used an 18-gauge open-flow microperfusion (OFM) catheter to induce local inflammation in the subcutaneous adipose tissue (SAT) of healthy volunteers and to sample interstitial fluid (IF) specifically from the inflamed tissue. In two crossover studies, nine subjects received either an intravenous lipid-heparin infusion to elevate circulating NEFA levels or saline over a period of 28 h. The former increased the circulating levels of triglycerides (TGs), NEFA, glucose, and insulin over the study period. NEFA effects on locally induced inflammation were estimated by measuring the levels of a panel adipokines in the OFM probe effluent. Interleukin-6 (IL-6), IL-8, tumor necrosis factor-α (TNF-α) and monocyte chemoattractant protein-1 (MCP-1) levels increased during the study period but were not affected by lipid-heparin infusion. In contrast, the level of IL-10, an anti-inflammatory cytokine, was significantly reduced during the final hour of lipid-heparin infusion (saline: 449.2 ± 105.9 vs. lipid-heparin: 65.4 ± 15.4 pg/ml; P = 0.02). These data provide the first in vivo evidence that elevated NEFA can modulate cytokine production by adipose tissue.

Analyzing serum-stimulated prostate cancer cell lines after low-fat, high-fiber diet and exercise intervention

Serum from men undergoing a low-fat, high-fiber diet and exercise intervention has previously been shown to decrease growth and increase apoptosis in serum-stimulated, androgen-dependent LNCaP cells associated with a reduction in serum IGF-I. Here we sought to determine the underlying mechanisms for these anticancer effects. Again, the intervention slowed growth and increased apoptosis in LNCaP cells; responses that were eliminated when IGF-I was added back to the post-intervention samples. The p53 protein content was increased and NFκB activation reduced in the post serum-stimulated LNCaP cells. Similar results were observed when the IGF-I receptor was blocked in the pre-intervention serum. In androgen-independent PC-3 cells, growth was reduced while none of the other factors were changed by the intervention. We conclude that diet and exercise intervention might help prevent clinical PCa as well as aid in the treatment of PCa during the early stages of development.

Physical exercise reduces circulating lipopolysaccharide and TLR4 activation and improves insulin signaling in tissues of DIO rats

OBJECTIVE

Insulin resistance in diet-induced obesity (DIO) is associated with a chronic systemic low-grade inflammation, and Toll-like receptor 4 (TLR4) plays an important role in the link among insulin resistance, inflammation, and obesity. The current study aimed to analyze the effect of exercise on TLR4 expression and activation in obese rats and its consequences on insulin sensitivity and signaling.

RESEARCH DESIGN AND METHODS

The effect of chronic and acute exercise was investigated on insulin sensitivity, insulin signaling, TLR4 activation, c-Jun NH(2)-terminal kinase (JNK) and IκB kinase (IKKβ) activity, and lipopolysaccharide (LPS) serum levels in tissues of DIO rats.

RESULTS

The results showed that chronic exercise reduced TLR4 mRNA and protein expression in liver, muscle, and adipose tissue. However, both acute and chronic exercise blunted TLR4 signaling in these tissues, including a reduction in JNK and IKKβ phosphorylation and IRS-1 serine 307 phosphorylation, and, in parallel, improved insulin-induced IR, IRS-1 tyrosine phosphorylation, and Akt serine phosphorylation, and reduced LPS serum levels.

CONCLUSIONS

Our results show that physical exercise in DIO rats, both acute and chronic, induces an important suppression in the TLR4 signaling pathway in the liver, muscle, and adipose tissue, reduces LPS serum levels, and improves insulin signaling and sensitivity. These data provide considerable progress in our understanding of the molecular events that link physical exercise to an improvement in inflammation and insulin resistance.

FFA-induced adipocyte inflammation and insulin resistance: involvement of ER stress and IKKβ pathways

Free-fatty acids (FFAs) are well-characterized factor for causing production of inflammatory factors and insulin resistance in adipocytes. Using cultured adipocytes, we demonstrate that FFAs can activate endoplasmic reticulum (ER) stress pathway by examination of ER stress sensor activation and marker gene expression. Chemical chaperone tauroursodeoxycholic acid (TUDCA) can reduce FFA-induced adipocyte inflammation and improve insulin signaling whereas overexpression of spliced X-box protein 1 (XBP-1s) only attenuates FFA-induced inflammation. PKR-like eukaryotic initiation factor 2α kinase (PERK) is one of the three major ER stress sensor proteins and deficiency of PERK alleviates FFA-induced inflammation and insulin resistance. The key downstream target of FFA-induced ER stress is IκB kinase β (IKKβ), a master kinase for regulating expression of inflammatory genes. Deficiency of PERK attenuates FFA-induced activation of IKKβ and deficiency of IKKβ alleviates FFA-induced inflammation and insulin resistance. Consistently, overexpression of IKKβ in 3T3-L1 CAR adipocytes causes inflammation and insulin resistance. In addition, IKKβ overexpression has profound effect on adipocyte lipid metabolism, including inhibition of lipogenesis and promotion of lipolysis. Furthermore, increased endogenous IKKβ expression and activation is also observed in isolated primary adipocytes from mice injected with lipids or fed on high-fat diet (HFD) acutely. These results indicate that ER stress pathway is a key mediator for FFA-induced inflammation and insulin resistance in adipocytes with PERK and IKKβ as the critical signaling components.

The role of JNK proteins in metabolism

The stress-activated c-Jun amino-terminal kinase (JNK) plays a pivotal role in metabolic conditions such as obesity, insulin resistance, and type 2 diabetes. Intricate tissue-specific tweaking of JNK activity in preclinical models of metabolic diseases reveals a complex interplay among local and systemic effects on carbohydrate and lipid metabolism. Synthesis of these entangled effects illustrates that for JNK inhibitors to have therapeutic impact, they must function in multiple cell types to modulate JNK activity.

Loss of function mutation in toll-like receptor-4 does not offer protection against obesity and insulin resistance induced by a diet high in trans fat in mice

Background

Toll-like receptor-4 (TLR4) triggers inflammatory signaling in response to microbial lipoploysaccharide. It has been reported that loss of TLR4 protected against saturated fat-induced inflammation and insulin resistance. It is not known whether loss of TLR4 function offers protection against trans fat (TF) induced obesity, inflammation, and insulin resistance. We investigated whether mice with loss of function mutation in TLR4 were resistant to TF-induced pathologies such as obesity, inflammation, hyperglycemia, and hyperinsulinemia.

Methods

C57BL/6j and C57BL/10 mice were cross bred to generate TLR4 mutant and wild type (WT). TLR4 mutant (n = 12) and WT (n = 12) mice were fed either low fat (LF) (13.5% fat energy) or high TF diets (60% fat energy) for 12 weeks. In vitro experiments were conducted on mouse macrophage cells (RAW 264.7 and J774A.1) to investigate whether elaidic (trans 18:1) or oleic acid (cis 18:1) would upregulate inflammatory markers.

Results

TLR4 mutant mice were ~26.4% heavier than WT mice. In both genotypes, mice that received TF diet were significantly heavier than those mice that received LF diet (P < 0.01). TLR4 mutant mice compared to WT mice had significantly higher fasting blood glucose, serum insulin, insulin resistance, serum leptin, and serum cholesterol when they received TF diet (P < 0.05). No upregulation of iNOS or COX2 in response to either elaidic or oleic acid in macrophage cells was observed.

Conclusions

Loss of function mutation in TLR4 not only did not protect mice from TF-induced obesity, hyperglycemia, hyperinsulinemia, and hypercholesterolemia but also exacerbated the above pathologies suggesting that functional TLR4 is necessary in attenuating TF-induced deleterious effects. It is likely that TF induces pathologies through pathways independent of TLR4.

Acute heat treatment improves insulin-stimulated glucose uptake in aged skeletal muscle

Aging is associated with insulin resistance and decreased insulin-stimulated glucose uptake into skeletal muscle. Although the mechanisms underlying age-related insulin resistance are not clearly defined, impaired defense against inflammation and tissue oxidative stress are likely causes. Heat shock proteins (HSPs) have been shown to protect tissue from oxidative stress and inhibit the activation of stress kinases such as JNK, known to interfere with the insulin signaling pathway. While the induction of HSPs via chronic heat treatment has been shown to protect skeletal muscle from obesity-related insulin resistance, the ability of heat treatment to improve insulin action in aged skeletal muscle is not known. In the present study, one bout of in vivo heat treatment applied to 24-mo-old Fischer 344 rats improved insulin-stimulated glucose uptake after 24 h in slow-twitch soleus muscles. In vitro heat treatment applied to young (3-mo-old) and aged (24-mo-old) soleus muscles increased expression of HSP72 and inhibited anisomycin-induced activation of JNK. In contrast, heat treatment had no effect on p38 MAPK, a MAPK strongly activated with anisomycin. Prior inhibition of HSP72 transcription with the pharmacological inhibitor KNK437 eliminated the ability of heat treatment to blunt JNK activation. This suggests that the ability of heat treatment to inhibit JNK activation in skeletal muscle is dependent on increased HSP72 expression. In conclusion, an acute bout of heat treatment can increase insulin-stimulated glucose uptake in aged skeletal muscle, with the underlying mechanism likely to be HSP72-mediated JNK inhibition.

Targeting type 2 diabetes

The evolving concept of how nutrient excess and inflammation modulate metabolism provides new opportunities for strategies to correct the detrimental health consequences of obesity. In this review, we focus on the complex interplay among lipid overload, immune response, proinflammatory pathways and organelle dysfunction through which excess adiposity might lead to type 2 diabetes. We then consider evidence linking dysregulated CNS circuits to insulin resistance and results on nutrient-sensing pathways emerging from studies with calorie restriction. Subsequently, recent recommendations for the management of type 2 diabetes are discussed with emphasis on prevailing current therapeutic classes of biguanides, thiazolidinediones and incretin-based approaches.

Toll-like receptor 2 deficiency improves insulin sensitivity and hepatic insulin signalling in the mouse

AIMS/HYPOTHESIS

Substantial evidence suggests a link between elevated inflammation and development of insulin resistance. Toll-like receptor 2 (TLR2) recognises a large number of lipid-containing molecules and transduces inflammatory signalling in a variety of cell types, including insulin-responsive cells. Considering the contribution of the fatty acid composition in TLR2-depedent signalling, we hypothesised that the inflammatory signals transduced by TLR2 contribute to insulin resistance.

METHODS

Mice deficient in TLR2 were used to investigate the in vivo roles of TLR2 in initiating and maintaining inflammation-associated insulin resistance and energy homeostasis.

RESULTS

We first recapitulated the observation with elevated expression of TLR2 and inflammatory cytokines in white adipose tissue and liver of ob/ob mice. Aged or high-fat-fed TLR2-deficient mice were protected from obesity and adipocyte hypertrophy compared with wild-type mice. Moreover, mice lacking TLR2 exhibited improved glucose tolerance and insulin sensitivity regardless of feeding them regular chow or a high-fat diet. This is accompanied by reductions in expression of inflammatory cytokines and activation of extracellular signal-regulated kinase (ERK) in a liver-specific manner. The attenuated hepatic inflammatory cytokine expression and related signalling are correlated with increased insulin action specifically in the liver in TLR2-deficient mice, reflected by increased insulin-stimulated protein kinase B (Akt) phosphorylation and IRS1 tyrosine phosphorylation and increased insulin-suppressed hepatocyte glucose production.

CONCLUSIONS/INTERPRETATION

The absence of TLR2 attenuates local inflammatory cytokine expression and related signalling and increases insulin action specifically in the liver. Thus, our work has identified TLR2 as a key mediator of hepatic inflammation-related signalling and insulin resistance.

Interruption of triacylglycerol synthesis in the endoplasmic reticulum is the initiating event for saturated fatty acid-induced lipotoxicity in liver cells

The aim of the present study was to investigate in detail the molecular mechanisms by which free fatty acids induce liver toxicity in liver cells. HepG2 and Huh7 human liver cell lines were exposed to varying concentrations of stearate (18:0), oleate (18:1), or mixtures of the two fatty acids, and the effects on cell proliferation, lipid droplet accumulation and induction of endoplasmic reticulum stress and apoptosis were evaluated. It was observed that: (a) stearate, but not oleate, inhibited cell proliferation and induced cell death; (b) stearate-induced cell death had the characteristics of endoplasmic reticulum stress-mediated and mitochondrial-mediated apoptosis; (c) the activation of stearate in the form of stearoyl-CoA was a necessary step for the lipotoxic effect; (d) the capacity of cells to produce and accumulate triacylglycerols in the form of lipid droplets was interrupted following exposure to stearate, whereas it proceeded normally in oleate-treated cells; and (e) the presence of relatively low amounts of oleate protected cells from stearate-induced toxicity and restored the ability of the cells to accumulate triacylglycerols. Our data suggest that interruption of triacylglycerol synthesis in the endoplasmic reticulum, apparently because of the formation of a pool of oversaturated intermediates, represents the key initiating event in the mechanism of saturated fatty acid-induced lipotoxicity.

GPR120 is an omega-3 fatty acid receptor mediating potent anti-inflammatory and insulin-sensitizing effects

Omega-3 fatty acids (omega-3 FAs), DHA and EPA, exert anti-inflammatory effects, but the mechanisms are poorly understood. Here, we show that the G protein-coupled receptor 120 (GPR120) functions as an omega-3 FA receptor/sensor. Stimulation of GPR120 with omega-3 FAs or a chemical agonist causes broad anti-inflammatory effects in monocytic RAW 264.7 cells and in primary intraperitoneal macrophages. All of these effects are abrogated by GPR120 knockdown. Since chronic macrophage-mediated tissue inflammation is a key mechanism for insulin resistance in obesity, we fed obese WT and GPR120 knockout mice a high-fat diet with or without omega-3 FA supplementation. The omega-3 FA treatment inhibited inflammation and enhanced systemic insulin sensitivity in WT mice, but was without effect in GPR120 knockout mice. In conclusion, GPR120 is a functional omega-3 FA receptor/sensor and mediates potent insulin sensitizing and antidiabetic effects in vivo by repressing macrophage-induced tissue inflammation.

The protective effect of thalidomide on left ventricular function in a rat model of diabetic cardiomyopathy

AIMS

To evaluate the protective effect of thalidomide, a potent anti-inflammatory drug, on the development of diabetic cardiomyopathy (DMCMP).

METHODS AND RESULTS

We induced type 1 diabetes using streptozocin in 8-week-old Sprague-Dawley rats, divided them into two groups-a thalidomide treatment group (DM-T, n = 15) and a non-treatment group (DM-N, n = 15)-and compared them with a normal control (n = 10). Ten weeks after diabetes induction, heart and lung mass indices were higher in the DM-N group compared with the control group. In the DM-T group, increases in heart and lung mass indices were attenuated compared with the DM-N group. On echocardiographic examination, systolic and diastolic mitral annulus velocities were impaired in the DM-N group, but they remained normal in the DM-T group. On haemodynamic analyses, left ventricular (LV) systolic function, represented by end-systolic elastance (0.35 ± 0.14 vs. 0.18 ± 0.07 mmHg/μl, P < 0.001) and preload-recruitable stroke work (90.5 ± 24.3 vs. 51.8 ± 22.0 mmHg, P < 0.001), was preserved in the DM-T group compared with the DM-N group. Likewise, deterioration of LV diastolic function was attenuated in the DM-T group. Increases in serum levels of TNF-α were attenuated in the DM-T group compared with the DM-N group. On histological analysis, thalidomide treatment lowered total myocardial collagen content and the expression of TNF-α, IL-1β, ICAM-1, and VCAM-1.

CONCLUSION

In an animal model of DMCMP, deterioration of LV systolic and diastolic function was partially prevented by thalidomide treatment.

Calcium deficiency in the early stages after weaning is associated with the enhancement of a low level of adrenaline-stimulated lipolysis and reduction of adiponectin release in isolated rat mesenteric adipocytes

Dysregulation of visceral adipocytes increases the incidence of metabolic syndrome. Higher production of nonesterified fatty acid and changes in adipocytokine release may trigger insulin resistance. Many studies have suggested that calcium (Ca) deficiency is associated with insulin resistance; however, the mechanisms are poorly understood. We examined the effects of Ca deficiency on adrenaline-induced lipolysis and adipocytokine release in the early stages after weaning using freshly isolated adipocytes from mesenteric fat tissue of 3-week-old male Sprague-Dawley rats fed a normal-Ca (5 g/kg diet) or low-Ca (1 g/kg diet) diet for 4 weeks. The release rate of nonesterified fatty acid in the mesenteric adipocytes after stimulation with a low level of adrenaline (0.2 microg/mL) was much higher in the Ca-deficient group than in the control group. In contrast, adiponectin release in the mesenteric fat cells was lower in Ca-deficient rats. Leptin and tumor necrosis factor-alpha secretion showed a similar tendency without significant intergroup differences, and monocyte chemoattractant protein-1 release was not affected by Ca deficiency. We found that Ca deficiency reduced the average size of fat cells through a large increase in the number of cells slightly smaller than the average size, which may be associated with the changes in the properties of the mesenteric adipose tissue. Our present results suggest that a low intake of Ca in the early stages after weaning is associated with changes in the properties of mesenteric adipocytes, which may be linked to insulin resistance in the future.

The role of adipose tissue and adipokines in obesity-related inflammatory diseases

Obesity is an energy-rich condition associated with overnutrition, which impairs systemic metabolic homeostasis and elicits stress. It also activates an inflammatory process in metabolically active sites, such as white adipose tissue, liver, and immune cells. As consequence, increased circulating levels of proinflammatory cytokines, hormone-like molecules, and other inflammatory markers are induced. This determines a chronic active inflammatory condition, associated with the development of the obesity-related inflammatory diseases. This paper describes the role of adipose tissue and the biological effects of many adipokines in these diseases.

Curcumin-induced suppression of adipogenic differentiation is accompanied by activation of Wnt/β-catenin signaling

Curcumin, a polyphenol found in the rhizomes of Curcuma longa , improves obesity-associated inflammation and diabetes in obese mice. Curcumin also suppresses adipocyte differentiation, although the underlying mechanism remains unclear. Here, we used 3T3-L1 cells to investigate the details of the mechanism underlying the anti-adipogenic effects of curcumin. Curcumin inhibited mitogen-activated protein kinase (MAPK) (ERK, JNK, and p38) phosphorylation that was associated with differentiation of 3T3-L1 cells into adipocytes. During differentiation, curcumin also restored nuclear translocation of the integral Wnt signaling component β-catenin in a dose-dependent manner. In parallel, curcumin reduced differentiation-stimulated expression of CK1α, GSK-3β, and Axin, components of the destruction complex targeting β-catenin. Accordingly, quantitative PCR analysis revealed that curcumin inhibited the mRNA expression of AP2 (mature adipocyte marker) and increased the mRNA expression of Wnt10b, Fz2 (Wnt direct receptor), and LRP5 (Wnt coreceptor). Curcumin also increased mRNA levels of c-Myc and cyclin D1, well-known Wnt targets. These results suggest that the Wnt signaling pathway participates in curcumin-induced suppression of adipogenesis in 3T3-L1 cells.

IGF-I stimulates reactive oxygen species (ROS) production and inhibits insulin-dependent glucose uptake via ROS in 3T3-L1 adipocytes

OBJECTIVE

IGF-I is known to enhance insulin sensitivity in whole body mainly via the IGF-I receptors in muscles. However, the effect of IGF-I on the regulation of insulin sensitivity in the adipose tissue is yet unclear. Insulin sensitivity was found to be higher in the IGF-I receptor-deficient adipocytes than that in wild-type adipocytes, suggesting that IGF-I signaling induces insulin resistance in adipocytes. However, the underlying mechanism has not yet been elucidated. In addition, the effect of superphysiological levels of IGF-I, as is observed in patients with acromegaly, on insulin sensitivity remains unclear.

DESIGN

To clarify the role of IGF-I on insulin sensitivity in adipocytes, we determined insulin-induced glucose uptake and IRS-1 status in 3T3-L1 adipocytes treated with IGF-I. Since reactive oxygen species (ROS) are causally related to insulin resistance, we investigated the effect of IGF-I on ROS production to elucidate the molecular mechanism underlying insulin resistance.

RESULTS

Preincubation of the adipocytes with IGF-I attenuated insulin-dependent glucose uptake. Interestingly, we found that IGF-I significantly stimulated ROS production. Furthermore, preincubation of adipocytes with an antioxidant, N-acetyl-cysteine (NAC) restored the IGF-I-induced attenuation of insulin-dependent glucose uptake; this indicates that IGF-I induces insulin resistance via ROS. Serine phosphorylation of IRS-1 was strongly induced and the insulin-dependent tyrosine phosphorylation of IRS-1 was suppressed by preincubating the adipocytes with IGF-I. Further, NAC restored these changes induced by IGF-I on both serine and tyrosine phosphorylation of IRS-1.

CONCLUSIONS

These data indicate that IGF-I inhibited insulin activity in the 3T3-L1 adipocytes via ROS production, which affects IRS-1 phosphorylation status.

cJun NH2-terminal kinase 1 (JNK1): roles in metabolic regulation of insulin resistance

The cJun NH(2)-terminal kinase isoform JNK1 is implicated in the mechanism of obesity-induced insulin resistance. Feeding a high-fat diet causes activation of the JNK1 signaling pathway, insulin resistance, and obesity in mice. Germ-line ablation of Jnk1 prevents both diet-induced obesity and insulin resistance. Genetic analysis indicates that the effects of JNK1 on insulin resistance can be separated from effects of JNK1 on obesity. Emerging research indicates that JNK1 plays multiple roles in the regulation of insulin resistance, including altered gene expression, hormone/cytokine production, and lipid metabolism. Together, these studies establish JNK1 as a potential pharmacological target for the development of drugs that might be useful for the treatment of insulin resistance, metabolic syndrome, and type 2 diabetes.

Adipocytokine orosomucoid integrates inflammatory and metabolic signals to preserve energy homeostasis by resolving immoderate inflammation

Orosomucoid (ORM), also called alpha-1 acid glycoprotein, is an abundant plasma protein that is an immunomodulator induced by stressful conditions such as infections. In this study, we reveal that Orm is induced selectively in the adipose tissue of obese mice to suppress excess inflammation that otherwise disturbs energy homeostasis. Adipose Orm levels were elevated by metabolic signals, including insulin, high glucose, and free fatty acid, as well as by the proinflammatory cytokine tumor necrosis factor-alpha, which is found in increased levels in the adipose tissue of morbid obese subjects. In both adipocytes and macrophages, ORM suppressed proinflammatory gene expression and pathways such as NF-kappaB and mitogen-activated protein kinase signalings and reactive oxygen species generation. Concomitantly, ORM relieved hyperglycemia-induced insulin resistance as well as tumor necrosis factor-alpha-mediated lipolysis in adipocytes. Accordingly, ORM improved glucose and insulin tolerance in obese and diabetic db/db mice. Taken together, our results suggest that ORM integrates inflammatory and metabolic signals to modulate immune responses to protect adipose tissue from excessive inflammation and thereby from metabolic dysfunction.

Macrophage alpha1 AMP-activated protein kinase (alpha1AMPK) antagonizes fatty acid-induced inflammation through SIRT1

In this study, we aim to determine cellular mechanisms linking nutrient metabolism to the regulation of inflammation and insulin resistance. The nutrient sensors AMP-activated protein kinase (AMPK) and SIRT1 show striking similarities in nutrient sensing and regulation of metabolic pathways. We find that the expression, activity, and signaling of the major isoform alpha1AMPK in adipose tissue and macrophages are substantially down-regulated by inflammatory stimuli and in nutrient-rich conditions, such as exposure to lipopolysaccharide (LPS), free fatty acids (FFAs), and diet-induced obesity. Activating AMPK signaling in macrophages by 5-aminoimidazole-4-carboxamide-1-beta4-ribofuranoside or constitutively active alpha1AMPK (CA-alpha1) significantly inhibits; although inhibiting alpha1AMPK by short hairpin RNA knock-down or dominant-negative alpha1AMPK (DN-alpha1) increases LPS- and FFA-induced tumor necrosis factor alpha expression. Chromatin immunoprecipitation and luciferase reporter assays show that activation of AMPK by CA-alpha1 in macrophages significantly inhibits LPS- or FFA-induced NF-kappaB signaling. More importantly, in a macrophage-adipocyte co-culture system, we find that inactivation of macrophage AMPK signaling inhibits adipocyte insulin signaling and glucose uptake. Activation of AMPK by CA-alpha1 increases the SIRT1 activator NAD(+) content and SIRT1 expression in macrophages. Furthermore, alpha1AMPK activation mimics the effect of SIRT1 on deacetylating NF-kappaB, and the full capacity of AMPK to deacetylate NF-kappaB and inhibit its signaling requires SIRT1. In conclusion, AMPK negatively regulates lipid-induced inflammation, which acts through SIRT1, thereby contributing to the protection against obesity, inflammation, and insulin resistance. Our study defines a novel role for AMPK in bridging the signaling between nutrient metabolism and inflammation.

JNK1 and IKKbeta: molecular links between obesity and metabolic dysfunction

Inflammation is thought to underlie the pathogenesis of many chronic diseases. It is now established that obesity results in a state of chronic low-grade inflammation thought to contribute to several metabolic disorders, including insulin resistance and pancreatic islet dysfunction. The protein kinases JNK1 and IKKbeta have been found to serve as critical molecular links between obesity, metabolic inflammation, and disorders of glucose homeostasis. The precise mechanisms of these linkages are still being investigated. However, as we discuss here, JNK1 and IKKbeta are activated by almost all forms of metabolic stress that have been implicated in insulin resistance or islet dysfunction. Furthermore, both JNK1 and IKKbeta are critically involved in the promotion of diet-induced obesity, metabolic inflammation, insulin resistance, and beta-cell dysfunction. Understanding the molecular mechanisms by which JNK1 and IKKbeta mediate obesity-induced metabolic stress is likely to be of importance for the development of new treatments for a variety of obesity-associated diseases.

The association of tumor necrosis factor alpha receptor 2 and tumor necrosis factor alpha with insulin resistance and the influence of adipose tissue biomarkers in humans

Tumor necrosis factor alpha (TNFalpha) is a proinflammatory adipokine hypothesized to link obesity with insulin resistance. Functional studies suggest that TNFalpha acts through pathways involving adipokines and fatty acids to induce insulin resistance. We tested the hypothesis that the association of measures of TNFalpha activity with insulin resistance is independent of obesity and adipose tissue biomarkers. We analyzed data from 2131 participants (without diabetes) of the Framingham Offspring Study examination 7. The outcome of interest was insulin resistance, measured using the homeostasis model assessment (HOMA-IR). Tumor necrosis factor alpha activity was measured by plasma tumor necrosis factor alpha receptor 2 (TNFr2) or TNFalpha; possible confounders included adipose tissue biomarkers (plasma adiponectin, resistin, and triglycerides). We used multivariable age- and sex-adjusted linear regression analyses to adjust for waist circumference and for biomarkers individually and simultaneously, and in biomarker-stratified (above and below median) models. We found that TNFr2 was positively associated with HOMA-IR (r = 0.21, P < .0001). In age- and sex-adjusted model, for each increase of 1 standard deviation of TNFr2 (SD = 746 pg/mL), the log (HOMA-IR) value was increased by 0.11 units (P < .0001). Adjustment for waist circumference reduced the TNFr2 beta-coefficient (by about 45%), but the association between TNFr2 and HOMA-IR remained significant (P < .0001). Tumor necrosis factor alpha receptor 2 was still associated to HOMA-IR after adding adiponectin, resistin, and triglycerides (individually and simultaneously). We found similar associations with plasma levels of TNFalpha. We conclude that, in a representative community sample, measures of TNFalpha activity are associated with insulin resistance, even after accounting for central adiposity and other adipose tissue biomarkers.

Hypothalamic and pituitary c-Jun N-terminal kinase 1 signaling coordinately regulates glucose metabolism

c-Jun N-terminal kinase (JNK) 1-dependent signaling plays a crucial role in the development of obesity-associated insulin resistance. Here we demonstrate that JNK activation not only occurs in peripheral tissues, but also in the hypothalamus and pituitary of obese mice. To resolve the importance of JNK1 signaling in the hypothalamic/pituitary circuitry, we have generated mice with a conditional inactivation of JNK1 in nestin-expressing cells (JNK1(DeltaNES) mice). JNK1(DeltaNES) mice exhibit improved insulin sensitivity both in the CNS and in peripheral tissues, improved glucose metabolism, as well as protection from hepatic steatosis and adipose tissue dysfunction upon high-fat feeding. Moreover, JNK1(DeltaNES) mice also show reduced somatic growth in the presence of reduced circulating growth hormone (GH) and insulin-like growth factor 1 (IGF1) concentrations, as well as increased thyroid axis activity. Collectively, these experiments reveal an unexpected, critical role for hypothalamic/pituitary JNK1 signaling in the coordination of metabolic/endocrine homeostasis.

Chronic inhibition of the mTORC1/S6K1 pathway increases insulin-induced PI3K activity but inhibits Akt2 and glucose transport stimulation in 3T3-L1 adipocytes

The mammalian target of rapamycin complex 1 (mTORC)1 pathway has emerged as a critical signaling component in the modulation of insulin's metabolic action. This effect is triggered by a nutrient- and insulin-mediated negative feedback loop in which mTOR and S6 kinase (S6K)1 phosphorylate insulin receptor substrate (IRS)-1 on serine residues, which blunts phosphatidylinositol 3-kinase (PI3K) activation. Acute inhibition of mTORC1/S6K1 by rapamycin increases insulin signaling and glucose uptake in myocytes and adipocytes, but whether these effects can be maintained under chronic inhibition of mTORC1 or S6K1 remains unclear. Here, we analyzed the effect of chronic rapamycin inhibition or small interfering RNA-based down-regulation of specific elements of the mTORC1/S6K1 pathway on insulin signaling and glucose transport in adipocytes. Both chronic inhibition of mTORC1 by rapamycin or knockdown of either mTOR, raptor, or S6K1 reduced inhibitory serine phosphorylation of IRS-1, while increasing its insulin-stimulated tyrosine phosphorylation and associated PI3K activity. However, knockdown of either mTOR or raptor selectively blunted IRS-1 phosphorylation on Ser636/639, whereas only S6K1 knockdown was found to reduce phosphorylation of IRS-1 on Ser1101. Unexpectedly, insulin-induced activation of Akt2 and glucose transporter 4 expression were reduced after chronic disruption of the mTORC1/S6K1 pathway, impairing insulin-mediated glucose uptake despite increased PI3K activation. In conclusion, these data indicate that both mTORC1 and S6K1 are key elements of the negative feedback loop but inhibit insulin-induced PI3K activity through phosphorylation of specific serine residues in IRS-1. However, this study also shows that chronic inhibition of the mTORC1/S6K1 pathway uncouples IRS-1/PI3K signaling from insulin-induced glucose transport due to impaired activation of Akt2 and blunted glucose transporter 4 expression.

The role of inflammation and macrophage accumulation in the development of obesity-induced type 2 diabetes mellitus and the possible therapeutic effects of long-chain n-3 PUFA

The WHO estimate that >1 x 10(6) deaths in Europe annually can be attributed to diseases related to excess body weight, and with the rising global obesity levels this death rate is set to drastically increase. Obesity plays a central role in the metabolic syndrome, a state of insulin resistance that predisposes patients to the development of CVD and type 2 diabetes mellitus. Obesity is associated with low-grade chronic inflammation characterised by inflamed adipose tissue with increased macrophage infiltration. This inflammation is now widely believed to be the key link between obesity and development of insulin resistance. In recent years it has been established that activation of pro-inflammatory pathways can cross talk with insulin signalling pathways via a number of mechanisms including (a) down-regulation of insulin signalling pathway proteins (e.g. GLUT4 and insulin receptor substrate (IRS)-1), (b) serine phosphorylation of IRS-1 blocking its tyrosine phosphorylation in response to insulin and (c) induction of cytokine signalling molecules that sterically hinder insulin signalling by blocking coupling of the insulin receptor to IRS-1. Long-chain (LC) n-3 PUFA regulate gene expression (a) through transcription factors such as PPAR and NF-kappaB and (b) via eicosanoid production, reducing pro-inflammatory cytokine production from many different cells including the macrophage. LC n-3 PUFA may therefore offer a useful anti-inflammatory strategy to decrease obesity-induced insulin resistance, which will be examined in the present review.

Carbohydrate-responsive gene expression in the adipose tissue of rats

Although obesity is often associated with high-fat diets, it can develop from a variety of meal patterns. Excessive intake of simple carbohydrates is one consistent eating behavior leading to obesity. However, the impact of overconsumption of diets with high carbohydrate to fat ratios (C/F) on body composition and global adipose tissue gene expression remains unclear. We used total enteral nutrition to evaluate the effects of caloric intake and C/F on body weight gain and development of obesity. Female Sprague Dawley rats were fed diets with either low C/F or high C/F (HC) (reflecting a 19.5-fold increase in C/F) at two levels of caloric intake: 187 or 220 kcal/kg(3/4) x d (15% excess) for 4 wk. At the end of the study period, rats fed HC diets had about 20% higher body weight at either caloric intake compared with rats fed low C/F diets (P < 0.05). Body composition (assessed by nuclear magnetic resonance, computerized tomography, and adipose tissue weights) revealed higher percent fat mass (P < 0.05) in HC rats. Obesity was associated with increased serum resistin, leptin, fasting hyperinsulinemia, and insulin resistance after an oral glucose challenge (P < 0.05). Microarray analyses of adipose tissues revealed HC diets led to changes in 270 and 464 transcripts at 187 and 220 kcal/kg(3/4) x d intakes. Genes regulating glucose transport, glycolysis, fatty acid and triglyceride biosynthesis, desaturation and elongation, adipogenesis, and adipokines were affected by HC diets. These results suggest that C/F and interactions with excessive caloric intake per se may regulate body composition and play important roles in the development of obesity and metabolic syndrome.

TNF-alpha, a potent lipid metabolism regulator

As a multifunctional cytokine, tumor necrosis factor alpha (TNF-alpha) exerts a series of biological actions in different cells, tissues, organs, and species and has been demonstrated to regulate and interfere with energy metabolism, especially lipid homeostasis. A large body of researches suggested that the effects of TNF-alpha on lipid metabolism mainly include five aspects: (1) suppresses free fatty acid (FFA) uptake and promotes lipogenesis; (2) induces lipolysis; (3) inhibits lipid-metabolism-related enzymes activity; (4) regulates cholesterol metabolism; (5) regulates other adipocyte-derived adipokines. The molecular mechanisms underlying these actions are complex and several signal transduction pathways might be involved. Regulation of metabolism-related gene expression at transcriptional and protein levels and impact on enzymes activity might be of importance. Identification and verification of these pathways might provide novel potential strategies and drug targets for dyslipidemia therapy. However, the inconsistent and even conflict conclusions on lipid profile drawn from human subjects after infliximab therapy poses the possibility that the effect of TNF-alpha on lipid metabolism might be more complicated than it appeared to be.

The influence of sex, body composition, and nonesterified fatty acids on serum adipokine concentrations

Serum adiponectin concentrations are higher in women than men. The sexual dimorphism for adiponectin has been attributed to the direct effects of testosterone on adipose tissue adiponectin secretion. However, serum testosterone and adiponectin concentrations are generally lower in obese men than lean men, suggesting that sex steroids may not be the only factor that contributes to sex differences in serum adiponectin. The primary objective of this study was to examine the influence of sex, body composition, and nonesterified fatty acids (NEFAs) on serum adiponectin concentrations. Women and men between the ages of 18 and 35 years were consecutively accrued into the study. Sixty-one participants were partitioned into normal-weight (15 female and 16 male) or obese (14 female and 16 male) groups. Blood samples were obtained after a 12-hour fast. Differences between groups were determined by analysis of variance with Tukey-Kramer post hoc testing. Serum adiponectin was 26% higher in women compared with men. Body mass index was associated with total serum adiponectin in men (r = -0.63, P < .05) but not women. Adiponectin was correlated with the homeostasis model assessment index in women (r = -0.56, P < .05) and men (r = -0.58, P < .05) and with NEFAs (r = -0.68, P < .05) in men only. After partitioning men and women into normal-weight and obese groups, serum adiponectin was lower and NEFAs were higher in obese men only. Homeostasis model assessment was similar between obese women and men despite higher NEFAs in the obese men. Leptin and plasminogen activator inhibitor-1 were higher in obese participants but were not associated with serum NEFAs. These results suggest that serum NEFAs may reduce adiponectin concentrations independent of their effects on insulin sensitivity in obese young men.

Improved insulin sensitivity after weight loss and exercise training is mediated by a reduction in plasma fatty acid mobilization, not enhanced oxidative capacity

Obesity is characterized by excessive rates of plasma fatty acid mobilization and uptake, which play a key role in mediating insulin resistance. While weight loss via diet-only or a diet + exercise program clearly improves insulin sensitivity, the precise mechanisms modulating this improvement are not completely understood. The purpose of the present study was to determine the role of the reduced fatty acid mobilization and uptake after weight loss in obese women who were randomly assigned to lifestyle interventions of either weight loss without exercise (WL) (n = 7) or a weight loss + exercise program (WL + EX) (n = 10). Before and after losing 12% of their body weight, we measured insulin sensitivity (S(I)), systemic fatty acid rate of appearance (Ra) and disappearance (Rd), oxidative capacity, and markers for pro-inflammatory pathways in skeletal muscle. Fatty acid Ra and Rd were reduced by 30% after both interventions (P < 0.05). While oxidative capacity increased 25% in WL + EX (compared with no increase after WL), the improvement in S(I) was identical in both groups (60%; P < 0.05), and skeletal muscle pro-inflammatory pathways were reduced (P < 0.05) similarly in both groups. When we artificially increased fatty acid mobilization after weight loss to pre-weight-loss levels via an overnight lipid infusion, the improvement in S(I) was almost completely reversed. Importantly, WL + EX did not protect against this lipid-induced reversal in S(I) despite a significant increase in resting whole-body fat oxidation and a marked increase in skeletal muscle oxidative capacity. In conclusion, reduced fatty acid mobilization and uptake appears to be a primary mediator of improved insulin sensitivity after weight loss. Moreover, enhancing fatty acid oxidative capacity via exercise training is not sufficient to prevent the insulin resistance caused by high fatty acid mobilization, such as that found in obesity.