The role of lipids in the pathogenesis of muscle insulin resistance and beta cell failure in type II diabetes and obesity

From Obesity-Induced Low-Grade Inflammation to Lipotoxicity and Mitochondrial Dysfunction: Altered Multi-Crosstalk between Adipose Tissue and Metabolically Active Organs

Obesity is a major risk factor for several metabolic diseases, including type 2 diabetes, hyperlipidemia, cardiovascular diseases, and brain disorders. Growing evidence suggests the importance of inter-organ metabolic communication for the progression of obesity and the subsequent onset of related disorders. This review provides a broad overview of the pathophysiological processes that from adipose tissue dysfunction leading to altered multi-tissue crosstalk relevant to regulating energy homeostasis and the etiology of obesity. First, a comprehensive description of the role of adipose tissue was reported. Then, attention was turned toward the unhealthy expansion of adipose tissue, low-grade inflammatory state, metabolic inflexibility, and mitochondrial dysfunction as root causes of systemic metabolic alterations. In addition, a short spot was devoted to iron deficiency in obese conditions and the role of the hepcidin-ferroportin relationship in the management of this issue. Finally, different classes of bioactive food components were described with a perspective to enhance their potential preventive and therapeutic use against obesity-related diseases.

Short-term changes in dietary fat levels and starch sources affect weight management, glucose and lipid metabolism, and gut microbiota in adult cats

A 2 × 2 factorial randomized design was utilized to investigate the effects of fat level (8% or 16% fat on a fed basis) and starch source (pea starch or corn starch) on body weight, glycolipid metabolism, hematology, and fecal microbiota in cats. The study lasted for 28 d and included a low fat and pea starch diet (LFPS), a high fat and pea starch diet, a low fat and corn starch diet, and a high fat and corn starch diet. In this study, hematological analysis showed that all cats were healthy. The apparent total tract digestibility of gross energy, crude protein, and crude fat was above 85% in the four diets. After 28 d, cats fed the high fat diets (HF) gained an average of 50 g more than those fed the low fat diets (LF). The hematological results showed that the HF diets increased the body inflammation in cats, while the LFPS group improved the glucolipid metabolism. The levels of glucose and insulin were lower in cats fed the LF diets than those in cats fed the HF diets (P < 0.05). Meanwhile, compared with the LF, the concentrations of total cholesterol, triglyceride, and high-density lipoprotein cholesterol in serum were greater in the cats fed the HF diets (P < 0.05). Additionally, both fat level and starch source influenced the fecal microbiota, with the relative abundance of beneficial bacteria, such as Blautia being significantly greater in the LFPS group than in the other three groups (P < 0.05). Reducing energy density and using pea starch in foods are both valuable design additions to aid in the management of weight control and improve gut health in cats. This study highlights the importance of fat level and starch in weight management in cats.

Evidence-Based Anti-Diabetic Properties of Plant from the Occitan Valleys of the Piedmont Alps

Data on urban and rural diabetes prevalence ratios show a significantly lower presence of diabetes in rural areas. Several bioactive compounds of plant origin are known to exert anti-diabetic properties. Interestingly, most of them naturally occur in different plants present in mountainous areas and are linked to traditions of herbal use. This review will aim to evaluate the last 10 years of evidence-based data on the potential anti-diabetic properties of 9 plants used in the Piedmont Alps (North-Western Italy) and identified through an ethnobotanical approach, based on the Occitan language minority of the Cuneo province (Sambucus nigra L., Achillea millefolium L., Cornus mas L., Vaccinium myrtillus L., Fragaria vesca L., Rosa canina L., Rubus idaeus L., Rubus fruticosus/ulmifolius L., Urtica dioica L.), where there is a long history of herbal remedies. The mechanism underlying the anti-hyperglycemic effects and the clinical evidence available are discussed. Overall, this review points to the possible use of these plants as preventive or add-on therapy in treating diabetes. However, studies of a single variety grown in the geographical area, with strict standardization and titration of all the active ingredients, are warranted before applying the WHO strategy 2014-2023.

Longitudinal Plasma Lipidome and Risk of Type 2 Diabetes in a Large Sample of American Indians With Normal Fasting Glucose: The Strong Heart Family Study

OBJECTIVE

Comprehensive assessment of alterations in lipid species preceding type 2 diabetes (T2D) is largely unknown. We aimed to identify plasma molecular lipids associated with risk of T2D in American Indians.

RESEARCH DESIGN AND METHODS

Using untargeted liquid chromatography-mass spectrometry, we repeatedly measured 3,907 fasting plasma samples from 1,958 participants who attended two examinations (∼5.5 years apart) and were followed up to 16 years in the Strong Heart Family Study. Mixed-effects logistic regression was used to identify lipids associated with risk of T2D, adjusting for traditional risk factors. Repeated measurement analysis was performed to examine the association between change in lipidome and change in continuous measures of T2D, adjusting for baseline lipids. Multiple testing was controlled by false discovery rate at 0.05.

RESULTS

Higher baseline level of 33 lipid species, including triacylglycerols, diacylglycerols, phosphoethanolamines, and phosphocholines, was significantly associated with increased risk of T2D (odds ratio [OR] per SD increase in log2-transformed baseline lipids 1.50-2.85) at 5-year follow-up. Of these, 21 lipids were also associated with risk of T2D at 16-year follow-up. Aberrant lipid profiles were also observed in prediabetes (OR per SD increase in log2-transformed baseline lipids 1.30-2.19 for risk lipids and 0.70-0.78 for protective lipids). Longitudinal changes in 568 lipids were significantly associated with changes in continuous measures of T2D. Multivariate analysis identified distinct lipidomic signatures differentiating high- from low-risk groups.

CONCLUSIONS

Lipid dysregulation occurs many years preceding T2D, and novel molecular lipids (both baseline level and longitudinal change over time) are significantly associated with risk of T2D beyond traditional risk factors. Our findings shed light on the mechanisms linking dyslipidemia to T2D and may yield novel therapeutic targets for early intervention tailored to American Indians.

Aerobic training associated with an active lifestyle exerts a protective effect against oxidative damage in hypothalamus and liver: The involvement of energy metabolism

BACKGROUND

Oxidation resistance protein 1 (OXR1) is of scientific interest due its role in protecting tissues against oxidative stress, DNA mutations and tumorigenesis, but little is known regarding strategies to increase OXR1 in different tissues. As an improved antioxidant defense may result from a high total amount of physical activity, the present study was designed to determine whether an active lifestyle including aerobic training exercise and spontaneous physical activity (SPA) can increase OXR1. We have built a large cage (LC) that allows animals to move freely, promoting an increase in SPA in comparison to a small cage (SC).

METHODS

We examined the effects of aerobic training applied for 8 weeks on SPA and OXR1 of C57BL/6 J mice living in two types of housing (SC and LC). OXR1 protein was studied in hypothalamus, muscle and liver, which were chosen due to their important role in energy and metabolic homeostasis.

RESULTS

LC-mice were more active than SC-mice as determined by SPA values. Despite both trained groups exhibiting similar gains in aerobic capacity, only trained mice kept in a large cage (but not for trained mice housed in SC) exhibited high OXR1 in the hypothalamus and liver. Trained mice housed in LC that exhibited an up-regulation of OXR1 also were those who exhibited an energy-expensive metabolism (based on metabolic parameters).

CONCLUSIONS

These results suggest that aerobic training associated with a more active lifestyle exerts a protective effect against oxidative damage and may be induced by changes in energy metabolism.

Potential roles of bone morphogenetic protein-9 in glucose and lipid homeostasis

Bone morphogenetic protein-9 (BMP-9) is a novel cytokine which is cloned from the fetal mouse liver cDNA library and belongs to the member of the transforming growth factor-β (TGF-β) superfamily. BMP-9 is mainly secreted by the liver and exerts a variety of physiological functions. In this review, we present the latest knowledge on the biochemistry of BMP-9 and its role in glucose metabolism and lipid homeostasis. We introduced the expression site, structure, synthesis, and secretion of BMP-9, as well as BMP-9 signaling pathway. We also discuss the effects of BMP-9 on glucose metabolism and lipid metabolism in different organs. BMP-9 can regulate glucose and lipid homeostasis in the body by inhibiting liver gluconeogenesis, transforming white adipose tissue to brown adipose tissue, promoting muscle glycogen synthesis, increasing the uptake and utilization of glucose by muscle tissue, increasing liver and adipose tissue insulin sensitivity, promoting insulin synthesis and secretion, inhibiting liver lipid deposition, and playing a leptin-like role. Finally, through the results of animal intervention studies and human clinical studies in the review, we deeply understand the association of BMP-9 with obesity, insulin resistance (IR), type 2 diabetes, and non-alcoholic fatty liver disease (NAFLD), which provides new ideas for the prevention and treatment of diseases.

New Insights into Inflammatory Bowel Diseases from Proteomic and Lipidomic Studies

Ulcerative colitis (UC) and Crohn's disease (CD) represent the two main forms of chronic inflammatory bowel diseases (IBD). The exact IBD etiology is not yet revealed but CD and UC are likely induced by an excessive immune response against normal constituents of the intestinal microbial flora. IBD diagnosis is based on clinical symptoms often combined with invasive and costly procedures. Thus, the need for more non-invasive markers is urgent. Several routine laboratory investigations have been explored as indicators of intestinal inflammation in IBD, including blood testing for C-reactive protein, erythrocyte sedimentation rate, and specific antibodies, in addition to stool testing for calprotectin and lactoferrin. However, none has been universally adopted, some have been well-characterized, and others hold great promise. In recent years, the technological developments within the field of mass spectrometry (MS) and bioinformatics have greatly enhanced the ability to retrieve, characterize, and analyze large amounts of data. High-throughput research allowed enhancing the understanding of the biology of IBD permitting a more accurate biomarker discovery than ever before. In this review, we summarize currently used IBD serological and stool biomarkers and how proteomics and lipidomics are contributing to the identification of IBD biomarkers.

Obesity and Mechanisms of its Negative Impact on the Cardiovascular System

Currently, the number of obese people in the world is constantly increasing. Obesity has a direct negative impact on the heart and blood vessels, which can be considered not only as an appropriate response to an increase in the volume of circulating blood due to an increase in body weight, but also as a side tissue reaction of the myocardium to hormonal and metabolic changes inherent in obesity. Our review is devoted to the description of the mechanisms of influence of obesity on the structural and functional parameters of the heart, which create prerequisites for the development of cardiovascular diseases, as well as the existing contradictions. Currently, the accumulated data suggest that an excessive amount of adipose tissue, in addition to metabolic disorders, including insulin resistance, imbalance of adipokines and inflammation markers, leading to the development of lipotoxicity, can directly penetrate the myocardium and cause violations of its contractile properties, as well as affect the conduction of excitation pulses and provoke the development of rhythm and conduction disorders. The development of endothelial dysfunction in obesity ultimately leads to the development of atherosclerosis and coronary heart disease. In addition, obesity contributes to the emergence of risk factors for hypertension, diabetes, atrial fibrillation, chronic heart failure, obstructive sleep apnea syndrome. Given the differences in the literature on the effect of obesity on long-term outcomes in patients with cardiovascular diseases, it is important to conduct prospective studies on the role of individual factors and their combinations that affect the mortality of patients with cardiovascular diseases.

Sphingolipids in adipose tissue: What's tipping the scale?

Adipose tissue lies at the heart of obesity, mediating its many effects upon the rest of the body, with its unique capacity to expand and regenerate, throughout the lifespan of the organism. Adipose is appreciated as an endocrine organ, with its myriad adipokines that elicit both physiological and pathological outcomes. Sphingolipids, bioactive signaling molecules, affect many aspects of obesity and the metabolic syndrome. While sphingolipids are appreciated in the context of these diseases in other tissues, there are many discoveries yet to be uncovered in the adipose tissue. This review focuses on the effects of sphingolipids on various aspects of adipose function and dysfunction. The processes of adipogenesis, metabolism and thermogenesis, in addition to inflammation and insulin resistance are intimately linked to sphingolipids as discussed below.

Lipidomics Profiling of Human Adipose Tissue Identifies a Pattern of Lipids Associated with Fish Oil Supplementation

To understand the interaction between diet and health, biomarkers that accurately reflect consumption of foods of perceived health relevance are needed. The aim of this investigation was to use direct infusion-mass spectrometry (DI-MS) lipidomics to determine the effects of fish oil supplementation on lipid profiles of human adipose tissue. Adipose tissue samples from an n-3 polyunsaturated fatty acid (PUFA) supplementation study (n = 66) were analyzed to compare the pattern following supplementation equivalent to zero or four portions of oily fish per week. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) were incorporated into highly unsaturated (≥5 double bonds) triglycerides (TGs), phosphocholines, and phosphoethanolamines as well as being detected directly as the nonesterified fatty acid forms. Multivariate statistics demonstrated that phospholipids were the most accurate and sensitive lipids for the assessing EPA and DHA incorporation into adipose tissue. Potential confounding factors (adiposity, age, and sex of the subject) were also considered in the analysis, and adiposity was also associated with an increase in highly unsaturated TGs as a result of incorporation of the n-6 PUFA arachidonic acid. DI-MS provides a high-throughput analysis of fatty acid status that can monitor oily fish consumption, suitable for use in cohort studies.

Metabolic fate of glucose and candidate signaling and excess-fuel detoxification pathways in pancreatic β-cells

Glucose metabolism promotes insulin secretion in β-cells via metabolic coupling factors that are incompletely defined. Moreover, chronically elevated glucose causes β-cell dysfunction, but little is known about how cells handle excess fuels to avoid toxicity. Here we sought to determine which among the candidate pathways and coupling factors best correlates with glucose-stimulated insulin secretion (GSIS), define the fate of glucose in the β-cell, and identify pathways possibly involved in excess-fuel detoxification. We exposed isolated rat islets for 1 h to increasing glucose concentrations and measured various pathways and metabolites. Glucose oxidation, oxygen consumption, and ATP production correlated well with GSIS and saturated at 16 mm glucose. However, glucose utilization, glycerol release, triglyceride and glycogen contents, free fatty acid (FFA) content and release, and cholesterol and cholesterol esters increased linearly up to 25 mm glucose. Besides being oxidized, glucose was mainly metabolized via glycerol production and release and lipid synthesis (particularly FFA, triglycerides, and cholesterol), whereas glycogen production was comparatively low. Using targeted metabolomics in INS-1(832/13) cells, we found that several metabolites correlated well with GSIS, in particular some Krebs cycle intermediates, malonyl-CoA, and lower ADP levels. Glucose dose-dependently increased the dihydroxyacetone phosphate/glycerol 3-phosphate ratio in INS-1(832/13) cells, indicating a more oxidized state of NAD in the cytosol upon glucose stimulation. Overall, the data support a role for accelerated oxidative mitochondrial metabolism, anaplerosis, and malonyl-CoA/lipid signaling in β-cell metabolic signaling and suggest that a decrease in ADP levels is important in GSIS. The results also suggest that excess-fuel detoxification pathways in β-cells possibly comprise glycerol and FFA formation and release extracellularly and the diversion of glucose carbons to triglycerides and cholesterol esters.

Sphingolipids in obesity and related complications

Sphingolipids are biologically active lipids ubiquitously produced in all vertebrate cells. Asides from structural components of cell membrane, sphingolipids also function as intracellular and extracellular mediators that regulate many important physiological cellular processes including cell survival, proliferation, apoptosis, differentiation, migration and immune processes. Recent studies have also indicated that disruption of sphingolipid metabolism is strongly associated with different diseases that exhibit diverse neurological and metabolic consequences. Here, we briefly summarize current evidence for understanding of sphingolipid pathways in obesity and associated complications. The regulation of sphingolipids and their enzymes may have a great impact in the development of novel therapeutic modalities for a variety of metabolic diseases.

An extract of Urtica dioica L. mitigates obesity induced insulin resistance in mice skeletal muscle via protein phosphatase 2A (PP2A)

The leaf extract of Urtica dioica L. (UT) has been reported to improve glucose homeostasis in vivo, but definitive studies on efficacy and mechanism of action are lacking. We investigated the effects of UT on obesity- induced insulin resistance in skeletal muscle. Male C57BL/6J mice were divided into three groups: low-fat diet (LFD), high-fat diet (HFD) and HFD supplemented with UT. Body weight, body composition, plasma glucose and plasma insulin were monitored. Skeletal muscle (gastrocnemius) was analyzed for insulin sensitivity, ceramide accumulation and the post translational modification and activity of protein phosphatase 2A (PP2A). PP2A is activated by ceramides and dephosphorylates Akt. C2C12 myotubes exposed to excess free fatty acids with or without UT were also evaluated for insulin signaling and modulation of PP2A. The HFD induced insulin resistance, increased fasting plasma glucose, enhanced ceramide accumulation and PP2A activity in skeletal muscle. Supplementation with UT improved plasma glucose homeostasis and enhanced skeletal muscle insulin sensitivity without affecting body weight and body composition. In myotubes, UT attenuated the ability of FFAs to induce insulin resistance and PP2A hyperactivity without affecting ceramide accumulation and PP2A expression. UT decreased PP2A activity through posttranslational modification that was accompanied by a reduction in Akt dephosphorylation.

Lipolytic and thermogenic depletion of adipose tissue in cancer cachexia

Although muscle wasting is the obvious manifestation of cancer cachexia that impacts on patient quality of life, the loss of lipid reserves and metabolic imbalance in adipose tissue also contribute to the devastating impact of cachexia. Depletion of fat depots in cancer patients is more pronounced than loss of muscle and often precedes, or even occurs in the absence of, reduced lean body mass. Rapid mobilisation of triglycerides stored within adipocytes to supply the body with fatty acids in periods of high-energy demand is normally mediated through a well-defined process of lipolysis involving the lipases ATGL, HSL and MGL. Studies into how these lipases contribute to fat loss in cancer cachexia have revealed the prominent role for ATGL in initiating lipolysis during adipose tissue atrophy, together with links between tumour-derived factors and the signalling pathways that control lipid flux within fat cells. The recent findings of increased thermogenesis in brown fat during cancer cachexia indicate that metabolically active adipose tissue contributes to the imbalance in energy homeostasis involved in catabolic wasting. Such energetically futile use of fatty acids liberated from adipose tissue to generate heat represents a maladaptive response in conjunction with anorexia experienced by cancer patients. As IL-6 release by tumours provokes lipolysis and activates the thermogenic programme in brown fat, this review explores the overlap in dysregulated metabolic processes due to inflammatory mediators in cancer cachexia and other disease states characterised by elevated cytokines such as obesity and diabetes.

Palmitate Inhibits SIRT1-Dependent BMAL1/CLOCK Interaction and Disrupts Circadian Gene Oscillations in Hepatocytes

Elevated levels of serum saturated fatty acid palmitate have been shown to promote insulin resistance, increase cellular ROS production, and trigger cell apoptosis in hepatocytes during the development of obesity. However, it remains unclear whether palmitate directly impacts the circadian clock in hepatocytes, which coordinates nutritional inputs and hormonal signaling with downstream metabolic outputs. Here we presented evidence that the molecular clock is a novel target of palmitate in hepatocytes. Palmitate exposure at low dose inhibits the molecular clock activity and suppresses the cyclic expression of circadian targets including Dbp, Nr1d1 and Per2 in hepatocytes. Palmitate treatment does not seem to alter localization or reduce protein expression of BMAL1 and CLOCK, the two core components of the molecular clock in hepatocytes. Instead, palmitate destabilizes the protein-protein interaction between BMAL1-CLOCK in a dose and time-dependent manner. Furthermore, we showed that SIRT1 activators could reverse the inhibitory action of palmitate on BMAL1-CLOCK interaction and the clock gene expression, whereas inhibitors of NAD synthesis mimic the palmitate effects on the clock function. In summary, our findings demonstrated that palmitate inhibits the clock function by suppressing SIRT1 function in hepatocytes.

Proteomics, lipidomics, metabolomics: a mass spectrometry tutorial from a computer scientist's point of view

Background

For decades, mass spectrometry data has been analyzed to investigate a wide array of research interests, including disease diagnostics, biological and chemical theory, genomics, and drug development. Progress towards solving any of these disparate problems depends upon overcoming the common challenge of interpreting the large data sets generated. Despite interim successes, many data interpretation problems in mass spectrometry are still challenging. Further, though these challenges are inherently interdisciplinary in nature, the significant domain-specific knowledge gap between disciplines makes interdisciplinary contributions difficult.

Results

This paper provides an introduction to the burgeoning field of computational mass spectrometry. We illustrate key concepts, vocabulary, and open problems in MS-omics, as well as provide invaluable resources such as open data sets and key search terms and references.

Conclusions

This paper will facilitate contributions from mathematicians, computer scientists, and statisticians to MS-omics that will fundamentally improve results over existing approaches and inform novel algorithmic solutions to open problems.

Regulation of glucose homoeostasis by brown adipose tissue

Brown adipose tissue (BAT) has emerged as a therapeutic target for the treatment of obesity. Activation of BAT in human beings could also have beneficial metabolic effects that might resolve common complications of obesity, such as type 2 diabetes, by ameliorating the glucolipotoxic pathological changes that underlie the development of peripheral insulin resistance and impaired insulin secretion due to pancreatic β-cell failure. Evidence from rodent models suggests that BAT activation improves glucose homoeostasis through several mechanisms, which could point to new strategies to optimise stimulation of BAT in human beings and reverse insulin resistance in peripheral tissues.

Role of p53 in the cellular response following oleic acid accumulation in Chang liver cells

Abnormal accumulation of fatty acids triggers the harmful cellular response called lipotoxicity. In this study, we investigated the cellular response following accumulation of oleic acid (OA), a monounsaturated fatty acid, in human Chang liver cells. OA droplets were distributed freely in the cytoplasm and/or degraded within lysosomes. OA exposure increased ATP production and concomitantly dilated mitochondria. At 24h after OA exposure, cell viability decreased slightly and was coupled with a reduction in mitochondrial Ca(2+) concentration, the alteration in cell viability was also associated with the generation of reactive oxygen species and changes in the cell cycle. Moreover, OA treatment increased the expression of autophagy- and apoptotic cell death-related proteins in a dose-dependent manner. Furthermore, we investigated the role of p53, a tumor suppressor protein, in the cellular response elicited by OA accumulation. OA-induced changes in cell viability and ATP production were rescued to control levels when cells were pretreated with pifithrin-alpha (PTA), a p53 inhibitor. By contrast, the expressions of LC3-II and perilipin, proteins required for lipophagy, were down-regulated by PTA pretreatment. Taken together, our results suggest that p53 plays a key role in the cellular response elicited by OA accumulation in Chang liver cells.

AMPK regulation of fatty acid metabolism and mitochondrial biogenesis: implications for obesity

Skeletal muscle plays an important role in regulating whole-body energy expenditure given it is a major site for glucose and lipid oxidation. Obesity and type 2 diabetes are causally linked through their association with skeletal muscle insulin resistance, while conversely exercise is known to improve whole body glucose homeostasis simultaneously with muscle insulin sensitivity. Exercise activates skeletal muscle AMP-activated protein kinase (AMPK). AMPK plays a role in regulating exercise capacity, skeletal muscle mitochondrial content and contraction-stimulated glucose uptake. Skeletal muscle AMPK is also thought to be important for regulating fatty acid metabolism; however, direct genetic evidence in this area is currently lacking. This review will discuss the current paradigms regarding the influence of AMPK in regulating skeletal muscle fatty acid metabolism and mitochondrial biogenesis at rest and during exercise, and highlight the potential implications in the development of insulin resistance.

Sphingolipid metabolism and obesity-induced inflammation

Obesity is a metabolic disorder developed by overnutrition and a major cause for insulin resistance and cardiovascular events. Since adipose tissue is one of the major sites for the synthesis and secretion of cytokines, enlarged adipose tissue in obese condition alters inflammatory state leading to pathophysiological conditions such as type 2 diabetes and increased cardiovascular risk. A plausible theory for development of metabolic dysregulation is that obesity increases secretion of inflammatory cytokines from adipose tissue and causes a chronic inflammation in the whole body. Additionally accumulation of lipids in non-adipose tissues elevates the cellular levels of bioactive lipids that inhibit the signaling pathways implicated in metabolic regulation together with activated inflammatory response. Recent findings suggest that obesity-induced inflammatory response leads to modulation of sphingolipid metabolism and these bioactive lipids may function as mediators for increased risk of metabolic dysfunction. Importantly, elucidation of mechanism regarding sphingolipid metabolism and inflammatory disease will provide crucial information to development of new therapeutic strategies for the treatment of obesity-induced pathological inflammation.

Insulin Action, Regional Fat, and Myocyte Lipid: Altered Relationships with Increased Adiposity

OBJECTIVE

Abdominal fat and myocyte triglyceride levels relate negatively to insulin sensitivity, but their interrelationships are inadequately characterized in the overweight. Using recent methods for measuring intramyocyte triglyceride, these relationships were studied in men with a broad range of adiposity.

RESEARCH METHODS AND PROCEDURES

Myocyte triglyceride content ((1)H-magnetic resonance spectroscopy of soleus and tibialis anterior muscles and biochemical assessment of vastus lateralis biopsies), regional fat distribution (DXA and abdominal magnetic resonance imaging), serum lipids, insulin action (euglycemic hyperinsulinemic clamp), and substrate oxidation rates (indirect calorimetry) were measured in 39 nondiabetic men (35.1 +/- 7.8 years) with a broad range of adiposity (BMI 28.6 +/- 4.1 kg/m(2), range 20.1 to 37.6 kg/m(2)).

RESULTS

Relationships between insulin-stimulated glucose disposal and regional body fat depots appeared more appropriately described by nonlinear than linear models. When the group was subdivided using median total body fat as the cut-point, insulin-stimulated glucose disposal correlated negatively to all regional body fat measures (all p < or = 0.004), serum triglycerides and free fatty acids (p < 0.02), and both soleus intramyocellular lipid (p = 0.003) and vastus lateralis triglyceride (p = 0.04) in the normal/less overweight group. In contrast, only visceral abdominal fat showed significant negative correlation with insulin-stimulated glucose disposal in more overweight men (r = -0.576, p = 0.01), some of whom surprisingly had lower than expected myocyte lipid levels. These findings persisted when the group was subdivided using different cut-points or measures of adiposity.

DISCUSSION

Interrelationships among body fat depots, myocyte triglyceride, serum lipids, and insulin action are generally absent with increased adiposity. However, visceral abdominal fat, which corresponds less closely to total adiposity, remains an important predictor of insulin resistance in men with both normal and increased adiposity.

Insulin resistance, ceramide accumulation, and endoplasmic reticulum stress in human chronic alcohol-related liver disease

Background. Chronic alcohol-related liver disease (ALD) is mediated by insulin resistance, mitochondrial dysfunction, inflammation, oxidative stress, and DNA damage. Recent studies suggest that dysregulated lipid metabolism with accumulation of ceramides, together with ER stress potentiate hepatic insulin resistance and may cause steatohepatitis to progress. Objective. We examined the degree to which hepatic insulin resistance in advanced human ALD is correlated with ER stress, dysregulated lipid metabolism, and ceramide accumulation. Methods. We assessed the integrity of insulin signaling through the Akt pathway and measured proceramide and ER stress gene expression, ER stress signaling proteins, and ceramide profiles in liver tissue. Results. Chronic ALD was associated with increased expression of insulin, IGF-1, and IGF-2 receptors, impaired signaling through IGF-1R and IRS1, increased expression of multiple proceramide and ER stress genes and proteins, and higher levels of the C14, C16, C18, and C20 ceramide species relative to control. Conclusions. In human chronic ALD, persistent hepatic insulin resistance is associated with dysregulated lipid metabolism, ceramide accumulation, and striking upregulation of multiple ER stress signaling molecules. Given the role of ceramides as mediators of ER stress and insulin resistance, treatment with ceramide enzyme inhibitors may help reverse or halt progression of chronic ALD.

High fat diet induced hepatic steatosis and insulin resistance: Role of dysregulated ceramide metabolism

AIM

  Non-alcoholic fatty liver disease (NAFLD) is an insulin resistance disease that can progress to cirrhosis and liver failure. We hypothesized that in NAFLD, insulin resistance dysregulates lipid metabolism, increasing production of cytotoxic lipids including ceramides, which exacerbate hepatic insulin resistance and injury.

METHODS

  Long Evans rats were pair-fed low (LFD) or high (HFD) fat diets for 8 weeks. Livers were used to measure lipids, gene expression, insulin receptor binding, integrity of insulin signaling, and pro-inflammatory cytokines. In vitro experiments characterized effects of ceramides on Huh7 cell viability, mitochondrial function, and insulin signaling.

RESULTS

  High fat diet feeding caused NAFLD with peripheral and hepatic insulin resistance, increased hepatic expression of pro-ceramide genes, sphingomyelinase activity, and lipid peroxidation, and increased serum ceramide. Ceramide treatment impaired Huh7 cell viability, mitochondrial function, and insulin signaling.

CONCLUSIONS

  Increased hepatic ceramide generation and release may mediate both hepatic and peripheral insulin resistance in NAFLD.

Insulin effects in muscle and adipose tissue

The major effects of insulin on muscle and adipose tissue are: (1) Carbohydrate metabolism: (a) it increases the rate of glucose transport across the cell membrane, (b) it increases the rate of glycolysis by increasing hexokinase and 6-phosphofructokinase activity, (c) it stimulates the rate of glycogen synthesis and decreases the rate of glycogen breakdown. (2) Lipid metabolism: (a) it decreases the rate of lipolysis in adipose tissue and hence lowers the plasma fatty acid level, (b) it stimulates fatty acid and triacylglycerol synthesis in tissues, (c) it increases the uptake of triglycerides from the blood into adipose tissue and muscle, (d) it decreases the rate of fatty acid oxidation in muscle and liver. (3) Protein metabolism: (a) it increases the rate of transport of some amino acids into tissues, (b) it increases the rate of protein synthesis in muscle, adipose tissue, liver, and other tissues, (c) it decreases the rate of protein degradation in muscle (and perhaps other tissues). These insulin effects serve to encourage the synthesis of carbohydrate, fat and protein, therefore, insulin can be considered to be an anabolic hormone.

The role of G-protein-coupled receptors in mediating the effect of fatty acids on inflammation and insulin sensitivity

PURPOSE OF REVIEW

Chronic activation of inflammatory pathways mediates the pathogenesis of insulin resistance, and the macrophage/adipocyte nexus provides a key mechanism underlying decreased insulin sensitivity. Free fatty acids are important in the pathogenesis of insulin resistance, although their precise mechanisms of action have yet to be fully elucidated. Recently, a family of G-protein-coupled receptors has been identified that exhibits high affinity for fatty acids. This review summarizes recent findings on six of these receptors, their ligands, and their potential physiological functions in vivo.

RECENT FINDINGS

Upon activation, the free fatty acid receptors affect inflammation, glucose metabolism, and insulin sensitivity. Genetic deletion of GPR40 and GPR41, receptors for long-chain and short-chain fatty acids, respectively, results in resistance to diet-induced obesity. Deletion of GPR43 and GPR84 exacerbates inflammation, and deletion of the long-chain fatty acid receptors GPR119 and GPR120 reduces or is predicted to reduce glucose tolerance.

SUMMARY

These studies provide a new understanding of the general biology of gastric motility and also shed valuable insight into some potentially beneficial therapeutic targets. Furthermore, highly selective agonists or antagonists for the free fatty acid receptors have been developed and look promising for treating various metabolic diseases.

Ceramide formation as a target in beta-cell survival and function

INTRODUCTION

Ceramide may be synthesized de novo or generated by sphingomyelinase-dependent hydrolysis of sphingomyelin.

AREAS COVERED

The role of ceramide, ceramide-sensitive signaling and ion channels in β-cell apoptosis, lipotoxicity and amyloid-induced β-cell death.

EXPERT OPINION

Ceramide participates in β-cell dysfunction and apoptosis after exposure to TNFα, IL-1β and IFN-γ, excessive amyloid and islet amyloid polypeptide or non-esterified fatty acids (lipotoxicity). Knockout of sphingomyelin synthase 1, which converts ceramide to sphingomyelin, leads to impairment of insulin secretion. Increased ceramidase activity or pharmacological inhibition of ceramide synthetase, inhibits β-cell apoptosis. Ceramide contributes to endoplasmatic reticulum (ER) stress, decreased mitochondrial membrane potential in insulin-secreting cells and mitochondrial release of cytochrome c into the cytosol, which are all triggers of apoptotic cell death. Ceramide-dependent signaling involves activation of extracellularly regulated kinases 1 and 2 (ERK1/2), downregulation of Period (Per)-aryl hydrocarbon receptor nuclear translocator (Arnt)-single-minded (Sim) kinase (PASK), activation of okadaic-acid-sensitive protein phosphatase 2A (PP2A) and stimulation of NADPH-oxidase with generation of superoxides and lipid peroxides. Ceramide reduces the activity of voltage gated potassium (Kv)-channels in insulin-secreting cells. The role of ceramide in β-cell survival and function may be therapeutically relevant, because ceramide formation can be suppressed by pharmacological inhibition of ceramide synthetase and/or sphingomyelinase.

HLA class II alleles susceptibility markers of type 1 diabetes fail to specify phenotypes of ketosis-prone diabetes in adult Tunisian patients

We aimed to characterize the different subgroups of ketosis-prone diabetes (KPD) in a sample of Tunisian patients using the Aβ scheme based on the presence or absence of β-cell autoantibodies (A+ or A-) and β-cell functional reserve (β+ or β-) and we investigated whether HLA class II alleles could contribute to distinct KPD phenotypes. We enrolled 43 adult patients with a first episode of ketosis. For all patients we evaluated clinical parameters, β-cell autoimmunity, β-cell function and HLA class II alleles. Frequency distribution of the 4 subgroups was 23.3% A+β-, 23.3% A-β-, 11.6% A+β+ and 41.9% A-β+. Patients from the group A+β- were significantly younger than those from the group A-β- (P = .002). HLA susceptibility markers were significantly more frequent in patients with autoantibodies (P = .003). These patients also had resistance alleles but they were more frequent in A+β+ than A+β- patients (P = .04). Insulin requirement was not associated to the presence or the absence of HLA susceptibility markers. HLA class II alleles associated with susceptibility to autoimmune diabetes have not allowed us to further define Tunisian KPD groups. However, high prevalence of HLA resistance alleles in our patients may reflect a particular genetic background of Tunisian KPD population.

Adipose tissue and ceramide biosynthesis in the pathogenesis of obesity

Although obesity is a complex metabolic disorder often associated with insulin resistance, hyperinsulinemia and Type 2 diabetes, as well as with accelerated atherosclerosis, the molecular changes in obesity that promote these disorders are not completely understood. Several mechanisms have been proposed to explain how increased adipose tissue mass affects whole body insulin resistance and cardiovascular risk. One theory is that increased adipose derived inflammatory cytokines induces a chronic inflammatory state that not only increases cardiovascular risk, but also antagonizes insulin signaling and mitochondrial function and thereby impair glucose hemostasis. Another suggests that lipid accumulation in nonadipose tissues not suited for fat storage leads to the buildup of bioactive lipids that inhibit insulin signaling and metabolism. Recent evidence demonstrates that sphingolipid metabolism is dysregulated in obesity and specific sphingolipids may provide a common pathway that link excess nutrients and inflammation to increased metabolic and cardiovascular risk. This chapter will focus primarily on the expression and regulation of adipose and plasma ceramide biosynthesis in obesity and, its potential contribution to the pathogenesis of obesity and the metabolic syndrome.

Hepatic steatosis, low-grade chronic inflammation and hormone/growth factor/adipokine imbalance

Non-alcoholic fatty liver disease (NAFLD), a further expression of metabolic syndrome, strictly linked to obesity and diabetes mellitus, is characterized by insulin resistance (IR), elevated serum levels of free fatty acids and fatty infiltration of the liver, which is known as hepatic steatosis. Hepatocyte apoptosis is a key feature of this disease and correlates with its severity. Free-fatty-acid-induced toxicity represents one of mechanisms for the pathogenesis of NAFLD and hormones, growth factors and adipokines influence also play a key role. This review highlights the various pathways that contribute to the development of hepatic steatosis. Circulating concentrations of inflammatory cytokines are reckoned to be the most important factor in causing and maintaining IR. Low-grade chronic inflammation is fundamental in the progression of NAFLD toward higher risk cirrhotic states.

Modulation of pattern recognition receptor-mediated inflammation and risk of chronic diseases by dietary fatty acids

Chronic inflammation is known to promote the development of many chronic diseases. Pattern recognition receptors (PRRs), Toll-like receptors (TLRs), and nucleotide-binding oligomerization domain proteins (NODs) mediate both infection-induced inflammation and sterile inflammation by recognizing pathogen- associated molecular patterns and endogenous molecules, respectively. PRR-mediated inflammation is an important determinant in altering the risk of many chronic diseases. Saturated fatty acids (SFAs) can activate PRRs, leading to enhanced expression of pro-inflammatory target gene products. However, n-3 polyunsaturated fatty acids (PUFAs) inhibit agonist-induced activation of PRRs. These results suggest that SFAs and n-3 PUFAs can reciprocally modulate PRR-mediated inflammation, and that PRRs and their downstream signaling components are molecular targets for dietary strategies to reduce chronic inflammation and subsequent risk of chronic diseases. This advancement in knowledge provides a new paradigm for understanding the mechanism by which different dietary fatty acids modify risk of chronic diseases including insulin resistance, atherosclerosis, and cancer.

[Adipose tissue as a therapeutic target in obesity]

Obesity is characterized by an increase of adipose tissue as a result of a positive imbalance between food intake and energy expenditure. Recent studies have indicated that adipocyte function is more complex than expected, since these cells have multiple functions and are integrated in a homeostatic network to optimize energy resources. As metabolic sensors in the body, adipocytes and the surrounding stromal vascular cells produce and secrete autocrine, paracrine and endocrine factors, able to regulate aspects involved in the development of adipocytes, as well as effects in peripheral organs important for metabolism. Regulation of these endocrine factors could lead to new therapeutic approaches targeted at aspects related to adipogenesis, preadipocyte proliferation and differentiation, inflammatory cytokine release and secretion, adipose tissue vascularization, and regulation of lipid metabolism or, alternatively, regulation of energy dissipation in mitochondria. In the study of the mechanisms of adipogenesis and remodulation of adipose tissue with respect to adipocyte size and function, an alternative and unorthodox strategy to improve obesity-associated metabolic complications could consist of increasing the storage capacity of adipose tissue to prevent a toxic response known as lipotoxicity.

Central role of ceramide biosynthesis in body weight regulation, energy metabolism, and the metabolic syndrome

Although obesity is associated with multiple features of the metabolic syndrome (insulin resistance, leptin resistance, hepatic steatosis, chronic inflammation, etc.), the molecular changes that promote these conditions are not completely understood. Here, we tested the hypothesis that elevated ceramide biosynthesis contributes to the pathogenesis of obesity and the metabolic syndrome. Chronic treatment for 8 wk of genetically obese (ob/ob), and, high-fat diet-induced obese (DIO) mice with myriocin, an inhibitor of de novo ceramide synthesis, decreased circulating ceramides. Decreased ceramide was associated with reduced weight, enhanced metabolism and energy expenditure, decreased hepatic steatosis, and improved glucose hemostasis via enhancement of insulin signaling in the liver and muscle. Inhibition of de novo ceramide biosynthesis decreased adipose expression of suppressor of cytokine signaling-3 (SOCS-3) and induced adipose uncoupling protein-3 (UCP3). Moreover, ceramide directly induced SOCS-3 and inhibited UCP3 mRNA in cultured adipocytes suggesting a direct role for ceramide in regulation of metabolism and energy expenditure. Inhibition of de novo ceramide synthesis had no effect on adipose tumor necrosis factor-alpha (TNF-alpha) expression but dramatically reduced adipose plasminogen activator inhibitor-1 (PAI-1) and monocyte chemoattactant protein-1 (MCP-1). This study highlights a novel role for ceramide biosynthesis in body weight regulation, energy expenditure, and the metabolic syndrome.

The islet beta-cell: fuel responsive and vulnerable

The pancreatic beta-cell senses blood nutrient levels and is modulated by neurohormonal signals so that it secretes insulin according to the need of the organism. Nutrient sensing involves marked metabolic activation, resulting in the production of coupling signals that promote insulin biosynthesis and secretion. The beta-cell's high capacity for nutrient sensing, however, necessitates reduced protection to nutrient toxicity. This potentially explains why in susceptible individuals, chronic fuel surfeit results in beta-cell failure and type 2 diabetes. Here we discuss recent insights into first, the biochemical basis of beta-cell signaling in response to glucose, amino acids and fatty acids, and second, beta-cell nutrient detoxification. We emphasize the emerging role of glycerolipid/fatty acid cycling in these processes.

Serum leptin and its relationship with metabolic variables in Arabs with type 2 diabetes mellitus

BACKGROUND AND OBJECTIVES

Most studies on serum leptin in type 2 diabetes mellitus have focused on white populations. We studied serum leptin concentrations and parameters related to glycemic control and the association between leptin levels and anthropometric and metabolic factors in Arab patients with type 2 diabetes and in Arab control subjects.

SUBJECTS AND METHODS

Ninety-two patients (65 females and 27 males) with type 2 diabetes and 69 matched normal control subjects (48 females and 21 males) were included. Anthropometric measures (including body mass index [BMI] and waist:hip ratio) were assessed in all subjects. After an overnight fast, blood was collected for serum leptin assay. Other metabolic parameters including glucose, insulin, C-peptide, intact proinsulin, insulin resistance index (HOMA-IR), insulin-like growth factor 1 (IGF-1), lipids and hemoglobin A1c (HbA1c) were determined.

RESULTS

Fasting serum leptin levels, IGF-1 and high-density lipoprotein (HDL) cholesterol were similar in patients with type 2 diabetes and control subjects. When obese subjects (BMI > or =30 kg/m2) were analyzed separately, serum levels of leptin were significantly lower in patients compared to controls. In contrast, patients had higher fasting glucose, insulin, C-peptide, intact proinsulin, insulin resistance, total cholesterol, triglycerides, HbA1c, and a larger waist circumference and waist-to-hip ratio than controls. Serum leptin correlated positively with BMI, negatively with waist-to-hip ratio, and demonstrated no relationship to other parameters.

CONCLUSION

Patients with type 2 diabetes in an Arab ethnic population showed evidence of an unfavorable metabolic profile despite having leptin levels similar to controls. Obesity influences serum leptin levels more significantly in type 2 diabetes, in which leptin levels tends to be low.

Physiological, pharmacological, and nutritional regulation of circulating adiponectin concentrations in humans

Adiponectin is an adipocyte hormone that links visceral adiposity with insulin resistance and atherosclerosis. It is unique among adipocyte-derived hormones in that its circulating concentrations are inversely proportional to adiposity, and low adiponectin concentrations predict the development of type 2 diabetes and cardiovascular disease. Consequently, in the decade since its discovery, adiponectin has generated immense interest as a potential therapeutic target for the metabolic syndrome and diabetes. This review summarizes current research regarding the regulation of circulating adiponectin concentrations by physiological, pharmacological, and nutritional factors, with an emphasis on human studies. In humans, plasma adiponectin concentrations are influenced by age and gender, and are inversely proportional to visceral adiposity. In vitro studies suggest that adiponectin production may be determined primarily by adipocyte size and insulin sensitivity, with larger, insulin-resistant adipocytes producing less adiponectin. While adiponectin concentrations are unchanged after meal ingestion, they are increased by significant weight loss, such as after bariatric surgery. In addition, adiponectin production is inhibited by a number of hormones, including testosterone, prolactin, glucocorticoids and growth hormone, and by inflammation and oxidative stress in adipose tissue. Smoking decreases, while moderate alcohol consumption increases, circulating adiponectin concentrations. Dietary fatty acid composition in rodents influences adiponectin production via ligand-activated nuclear receptors (PPARs); however, current evidence in humans is equivocal. In addition to PPAR agonists (such as thiazolidinediones and fibrates), a number of pharmacological agents (angiotensin receptor type 1 blockers, ACE inhibitors, and cannabinoid receptor antagonists) used in treatment of the metabolic syndrome also increase adiponectin concentrations in humans.

Adipocyte dysfunctions linking obesity to insulin resistance and type 2 diabetes

Acquired resistance to the action of insulin to stimulate glucose transport in skeletal muscle is associated with obesity and promotes the development of type 2 diabetes. In skeletal muscle, insulin resistance can result from high levels of circulating fatty acids that disrupt insulin signalling pathways. However, the severity of insulin resistance varies greatly among obese people. Here we postulate that this variability might reflect differences in levels of lipid-droplet proteins that promote the sequestration of fatty acids within adipocytes in the form of triglycerides, thereby lowering exposure of skeletal muscle to the inhibitory effects of fatty acids.

Elevated hepatic fatty acid elongase-5 activity affects multiple pathways controlling hepatic lipid and carbohydrate composition

Hepatic fatty acid elongase-5 (Elovl-5) plays an important role in long chain monounsaturated and polyunsaturated fatty acid synthesis. Elovl-5 activity is regulated during development, by diet, hormones, and drugs, and in chronic disease. This report examines the impact of elevated Elovl-5 activity on hepatic function. Adenovirus-mediated induction of Elovl5 activity in livers of C57BL/6 mice increased hepatic and plasma levels of dihomo-gamma-linolenic acid (20:3,n-6) while suppressing hepatic arachidonic acid (20:4,n-6) and docosahexaenoic acid (22:6,n-3) content. The fasting-refeeding response of peroxisome proliferator-activated receptor alpha-regulated genes was attenuated in mice with elevated Elovl5 activity. In contrast, the fasting-refeeding response of hepatic sterol-regulatory element binding protein-1 (SREBP-1)-regulated and carbohydrate-regulatory element binding protein/Max-like factor X-regulated genes, Akt and glycogen synthase kinase (Gsk)-3beta phosphorylation, and the accumulation of hepatic glycogen content and nuclear SREBP-1 were not impaired by elevated Elovl5 activity. Hepatic triglyceride content and the phosphorylation of AMP-activated kinase alpha and Jun kinase 1/2 were reduced by elevated Elovl5 activity. Hepatic phosphoenolpyruvate carboxykinase expression was suppressed, while hepatic glycogen content and phosphorylated Gsk-3beta were significantly increased, in livers of fasted mice with increased Elovl5 activity. As such, hepatic Elovl5 activity may affect hepatic glucose production during fasting. In summary, Elovl5-induced changes in hepatic fatty acid content affect multiple pathways regulating hepatic lipid and carbohydrate composition.

Roles of plasma interleukin-6 and tumor necrosis factor-alpha and FFA and TG in the development of insulin resistance induced by high-fat diet

The role of adipokines in development of insulin resistance still remains controversial. The purpose of the present study was to examine the dynamic changes of fasting plasma levels of interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-alpha), free fatty acids (FFA) and insulin in a Sprague-Dawley rat insulin resistant model induced by high-fat diet. Heterotopic deposition of triglycerides (TG) in liver, skeletal muscles and pancreatic islet was also investigated. The fasting plasma level of insulin in rats in the high-fat diet group was significantly higher than that in the normal diet group on day 21 (P<0.01), suggesting that an increased insulin resistance developed in the high-fat diet group. However, no significant difference in the plasma IL-6 level was observed between the two groups (P>0.05), although in both groups, the plasma IL-6 level was significantly higher on day 21 than that of the day 0 (P<0.05). The plasma FFA level in the high-fat diet group began to increase significantly on day 21 (P<0.05), and elevated markedly on day 28, was positively correlated to the fasting plasma insulin level. Histological study revealed a more abundant TG deposition in liver and skeletal muscles (from quadriceps femoris) in the high-fat diet group than in the normal diet group on day 21, and the liver deposition was even higher on day 28. However, no deposition was observed in pancreatic islets. The plasma TNF-alpha level remained unchanged throughout the duration of the experiment. These results indicate that the progression of insulin resistance in high-fat diet rats is closely related to the plasma FFA elevation and the heterotopic deposition of TG in liver and skeletal muscles, but is unrelated to the plasma TNF-alpha and IL-6 levels.

Adipogenesis and lipotoxicity: role of peroxisome proliferator-activated receptor gamma (PPARgamma) and PPARgammacoactivator-1 (PGC1)

Obesity is characterised by an increase in the adipose deposits, resulting from an imbalance between food intake and energy expenditure. When expansion of the adipose tissue reaches its maximum limit, as in obesity, fat accumulates in non-adipose tissues such as liver, heart, muscle and pancreas, developing a toxic response known as lipotoxicity, a condition that promotes the development of insulin resistance and other metabolic complications. Thus, the lipotoxic state may contribute to the increased risk of insulin resistance, diabetes, fatty liver and cardiovascular complications associated with obesity. We are interested in studying adipose tissue, specifically how mechanisms of adipogenesis and remodelling of adipose tissue, in terms of size and function of the adipocytes, could be considered a strategy to increase the capacity for lipid storage and prevent lipotoxicity. The peroxisome proliferator-activated receptors (PPARs) are a family of transcription factors that regulate energy balance by promoting either energy deposition or energy dissipation. Under normal physiological conditions, PPARgamma is mainly expressed in adipose tissue and regulates diverse functions such as the development of fat cells and their capacity to store lipids. The generation of PPARgamma knockout mice, either tissue specific or isoform specific, has provided new models to study PPARgamma's role in adipose tissue differentiation and function and have highlighted the essential role of PPARgamma in adipogenesis and lipogenesis.A second strategy to prevent lipotoxicity is to increase the capacity of tissues to oxidise fatty acids. PPARgammacoactivator-1alpha is a coactivator of PPARgamma that induces the expression of genes that promote the differentiation of preadipocytes to brown adipocytes. Recently, it has been implicated in increasing the oxidation of fatty acids via increasing mitochondrial capacity and function, making this co-factor a key candidate for the treatment of lipotoxicity.

Oxidation of Intracellular and Extracellular Fatty Acids in Skeletal Muscle: Application of kinetic modeling, stable isotopes and liquid chromatography/electrospray ionization ion-trap tandem mass spectrometry technology

Fatty acids are a major fuel for many tissues and abnormal utilization is implicated in diseases. However, tissue fatty acid oxidation has not been determined reliably in vivo. Furthermore, fatty acid oxidation has not been partitioned into intracellular and extracellular components. In this report, a one-pool model is described that enables direct quantitation of fluxes of intracellular and plasma fatty acids to mitochondria in skeletal muscle using dual stable isotopes and liquid chromatography/electrospray ionization ion-trap tandem mass spectrometry (LC/ESI-itMS²) technology. It is validated by the determination of palmitate oxidation by skeletal muscle in lean and obese rats and the regulation by insulin. Resting postabsorptive intramyocellular and plasma palmitate oxidation by gastrocnemius muscle was determined to be 3.47±0.8 and 2.06±0.5 nmol/g min in lean and 6.96±1.8 and 1.34±0.2 nmol/g min in obese rats, respectively. In obese rats, hyperinsulinemia (1 nmol/l) suppressed intramyocellular (by 59±5% to 2.88±0.3 nmol/g min P<0.05) but not plasma (1.41±0.14 nmol/g min, P>0.05) palmitate oxidation. The fractional turnover rate of palmitoylcarnitine (0.34±0.1/min vs. 0.83±0.2/min, P<0.05) was also suppressed by insulin. In obese and lean rats, there are 83% and 51%, respectively (P=0.08), of plasma fatty acids traverse triglyceride pool before being oxidized. The results demonstrated that the methodology is feasible and sensitive to metabolic alterations and thus can be used to study fatty acid utilization at tissue level in a compartmentalized manner for the firs time.

Diet-induced modulation of mitochondrial activity in rat muscle

Growing evidence supports the theory that mitochondrial dysfunction is an underlying cause of intramyocellular lipid (IMCL) accumulation and insulin resistance. Here, we hypothesized that high dietary fat (HF) intake could trigger changes in mitochondrial activity such that fatty acid oxidation is impaired in muscle and contributes to an elevation in intramyocellular lipid (IMCL) levels. Muscle mitochondrial activity was determined in vivo through measurement of the F(1)F(0) ATP synthase flux, the terminal step in the oxidative phosphorylation process. An initial study comparing rats on normal chow diet with rats on an HF diet revealed strong correlations between muscle ATP synthesis rates, IMCL levels and whole body glucose tolerance. Results obtained from two latter studies showed multiphasic responses to dietary intervention. Initially, the ATP synthesis rates decreased as much as 50% within 24 h of raising the fat content in the diet to 60% of the caloric intake. These rates eventually returned to normal values after 2-3 wk on the HF regimen, seemingly to prevent further IMCL accumulation. Only beyond 1 mo on the HF diet did results consistently show ATP synthesis rates to diminish by 30-50% accompanied by steadily augmenting IMCL levels. Interestingly, switching back to a chow diet after 3 wk of HF feeding reversed the initial diet-induced changes. Although the muscle mitochondrial system may initially offer enough compliance to counteract lipid surplus, these in vivo data suggest a vicious long-term cycle among mitochondrial dysfunction, IMCL accumulation, and glucose intolerance in the rat.

A subpopulation of macrophages infiltrates hypertrophic adipose tissue and is activated by free fatty acids via Toll-like receptors 2 and 4 and JNK-dependent pathways

Obesity and type 2 diabetes are characterized by decreased insulin sensitivity, elevated concentrations of free fatty acids (FFAs), and increased macrophage infiltration in adipose tissue (AT). Here, we show that FFAs can cause activation of RAW264.7 cells primarily via the JNK signaling cascade and that TLR2 and TLR4 are upstream of JNK and help transduce FFA proinflammatory signals. We also demonstrate that F4/80(+)CD11b(+)CD11c(+) bone marrow-derived dendritic cells (BMDCs) have heightened proinflammatory activity compared with F4/80(+)CD11b(+)CD11c(-) bone marrow-derived macrophages and that the proinflammatory activity and JNK phosphorylation of BMDCs, but not bone marrow-derived macrophages, was further increased by FFA treatment. F4/80(+)CD11b(+)CD11c(+) cells were found in AT, and the proportion and number of these cells in AT is increased in ob/ob mice and by feeding wild type mice a high fat diet for 1 and 12 weeks. AT F4/80(+)CD11b(+)CD11c(+) cells express increased inflammatory markers compared with F4/80(+)CD11b(+)CD11c(-) cells, and FFA treatment increased inflammatory responses in these cells. In addition, we found that CD11c expression is increased in skeletal muscle of high fat diet-fed mice and that conditioned medium from FFA-treated wild type BMDCs, but not TLR2/4 DKO BMDCs, can induce insulin resistance in L6 myotubes. Together our results show that FFAs can activate CD11c(+) myeloid proinflammatory cells via TLR2/4 and JNK signaling pathways, thereby promoting inflammation and subsequent cellular insulin resistance.

PPAR gamma 2 prevents lipotoxicity by controlling adipose tissue expandability and peripheral lipid metabolism

Peroxisome proliferator activated receptor gamma 2 (PPARg2) is the nutritionally regulated isoform of PPARg. Ablation of PPARg2 in the ob/ob background, PPARg2^−/− Lep^ob/Lep^ob (POKO mouse), resulted in decreased fat mass, severe insulin resistance, β-cell failure, and dyslipidaemia. Our results indicate that the PPARg2 isoform plays an important role, mediating adipose tissue expansion in response to positive energy balance. Lipidomic analyses suggest that PPARg2 plays an important antilipotoxic role when induced ectopically in liver and muscle by facilitating deposition of fat as relatively harmless triacylglycerol species and thus preventing accumulation of reactive lipid species. Our data also indicate that PPARg2 may be required for the β-cell hypertrophic adaptive response to insulin resistance. In summary, the PPARg2 isoform prevents lipotoxicity by (a) promoting adipose tissue expansion, (b) increasing the lipid-buffering capacity of peripheral organs, and (c) facilitating the adaptive proliferative response of β-cells to insulin resistance.

It is known that obesity is linked to type 2 diabetes, however how obesity causes insulin resistance and diabetes is not well understood. Some extremely obese people are not diabetic, while other less obese people develop severe insulin resistance and diabetes. We believe diabetes occurs when adipose tissue becomes “full,” and fat overflows into other organs such as liver, pancreas, and muscle, causing insulin resistance and diabetes. Peroxisome proliferator activated receptor gamma (PPARg) is essential for the development of adipose tissue and control of insulin sensitivity. PPARg2 is the isoform of PPARg regulated by nutrition. Here we investigate the role of PPARg2 under conditions of excess nutrients by removing the PPARg2 isoform in genetically obese mice, the POKO mouse. We report that removing PPARg2 decreases adipose tissue's capacity to expand and prevents the mouse from making as much fat as a normal obese mouse, despite eating similarly. Our studies suggest that PPARg plays an important antitoxic role when it is induced in liver, muscle, and beta cells by facilitating deposition of fat as relatively harmless lipids and thus prevents accumulation of toxic lipid species. We also show that PPARg2 may be involved in the adaptive response of beta cells to insulin resistance.

Hepatic overexpression of glycerol-sn-3-phosphate acyltransferase 1 in rats causes insulin resistance

Fatty liver is commonly associated with insulin resistance and type 2 diabetes, but it is unclear whether triacylglycerol accumulation or an excess flux of lipid intermediates in the pathway of triacyglycerol synthesis are sufficient to cause insulin resistance in the absence of genetic or diet-induced obesity. To determine whether increased glycerolipid flux can, by itself, cause hepatic insulin resistance, we used an adenoviral construct to overexpress glycerol-sn-3-phosphate acyltransferase-1 (Ad-GPAT1), the committed step in de novo triacylglycerol synthesis. After 5-7 days, food intake, body weight, and fat pad weight did not differ between Ad-GPAT1 and Ad-enhanced green fluorescent protein control rats, but the chow-fed Ad-GPAT1 rats developed fatty liver, hyperlipidemia, and insulin resistance. Liver was the predominant site of insulin resistance; Ad-GPAT1 rats had 2.5-fold higher hepatic glucose output than controls during a hyperinsulinemic-euglycemic clamp. Hepatic diacylglycerol and lysophosphatidate were elevated in Ad-GPAT1 rats, suggesting a role for these lipid metabolites in the development of hepatic insulin resistance, and hepatic protein kinase Cepsilon was activated, providing a potential mechanism for insulin resistance. Ad-GPAT1-treated rats had 50% lower hepatic NF-kappaB activity and no difference in expression of tumor necrosis factor-alpha and interleukin-beta, consistent with hepatic insulin resistance in the absence of increased hepatic inflammation. Glycogen synthesis and uptake of 2-deoxyglucose were reduced in skeletal muscle, suggesting mild peripheral insulin resistance associated with a higher content of skeletal muscle triacylglycerol. These results indicate that increased flux through the pathway of hepatic de novo triacylglycerol synthesis can cause hepatic and systemic insulin resistance in the absence of obesity or a lipogenic diet.