Partial inhibition of adipose tissue lipolysis improves glucose metabolism and insulin sensitivity without alteration of fat mass. PLoS Biol. 2013;11:e1001485. [PMID: 23431266 PMCID: PMC3576369 DOI: 10.1371/journal.pbio.1001485]

Catabolic and anabolic faces of insulin resistance and their disorders: a new insight into circadian control of metabolic disorders leading to diabetes

Maintenance of glucose homeostasis during circadian behavioral cycles is critical. The processes controlling the switch between predominant lipolysis/fatty oxidation during fasting and predominant lipid storage/glucose oxidation following feeding are determined principally by insulin. Chronic elevated threshold of insulin resistance (IR) is a key pathological feature of obesity, Type 2 diabetes, sepsis and cancer cachexia; however, a temporal reduced threshold of IR is widely met in fasting/hibernation, pregnancy, antibacterial immunity, exercise and stress. Paradoxically, some of these cases are associated with catabolic metabolism, whereas others are related to anabolic pathways. This article considers the possible causes of circadian disorders in glucose and lipid metabolism that act as a driving force for obesity-promoted development of Type 2 diabetes. This is intended to provide improved insight into the pathogenesis of chronic circadian disorders that increase the risk of diabetes, and consider new targets for its metabolic and drug correction.

Insulin resistance (IR) is a common adaptive mechanism, acting under opposite anabolic and catabolic conditions. However, chronic IR is a key pathological feature of obesity, Type 2 diabetes and cancer cachexia, whereas a temporal IR is widely seen in fasting, pregnancy, exercise and stress. Therefore, it is important to understand when this transient IR-mechanism shifts to chronic IR-associated diseases. What factors result in the switch between the anabolic and catabolic conditions and what defect(s) in this switch is associated with chronic IR induction? The present opinion article aimed to address these questions to the metabolic changes typical for circadian regulation in lean, obese and diabetic patients.

Graphical abstract: Early circadian IR disorders caused by overweight and obesity are associated with increased risk for diabetes via formation of a vicious cycle between lipid anabolic and catabolic programs thus distorting insulin and lipid levels in day/night period.

GPR120 suppresses adipose tissue lipolysis and synergizes with GPR40 in antidiabetic efficacy

GPR40 and GPR120 are fatty acid sensors that play important roles in glucose and energy homeostasis. GPR40 potentiates glucose-dependent insulin secretion and demonstrated in clinical studies robust glucose lowering in type 2 diabetes. GPR120 improves insulin sensitivity in rodents, albeit its mechanism of action is not fully understood. Here, we postulated that the antidiabetic efficacy of GPR40 could be enhanced by coactivating GPR120. A combination of GPR40 and GPR120 agonists in db/db mice, as well as a single molecule with dual agonist activities, achieved superior glycemic control compared with either monotherapy. Compared with a GPR40 selective agonist, the dual agonist improved insulin sensitivity in ob/ob mice measured by hyperinsulinemic-euglycemic clamp, preserved islet morphology, and increased expression of several key lipolytic genes in adipose tissue of Zucker diabetic fatty rats. Novel insights into the mechanism of action for GPR120 were obtained. Selective GPR120 activation suppressed lipolysis in primary white adipocytes, although this effect was attenuated in adipocytes from obese rats and obese rhesus, and sensitized the antilipolytic effect of insulin in rat and rhesus primary adipocytes. In conclusion, GPR120 agonism enhances insulin action in adipose tissue and yields a synergistic efficacy when combined with GPR40 agonism.

Targeting the NO/superoxide ratio in adipose tissue: relevance to obesity and diabetes management

Insulin sensitivity and metabolic homeostasis depend on the capacity of adipose tissue to take up and utilize excess glucose and fatty acids. The key aspects that determine the fuel-buffering capacity of adipose tissue depend on the physiological levels of the small redox molecule, nitric oxide (NO). In addition to impairment of NO synthesis, excessive formation of the superoxide anion (О2•- ) in adipose tissue may be an important interfering factor diverting the signalling of NO and other reactive oxygen and nitrogen species in obesity, resulting in metabolic dysfunction of adipose tissue over time. Besides its role in relief from superoxide burst, enhanced NO signalling may be responsible for the therapeutic benefits of different superoxide dismutase mimetics, in obesity and experimental diabetes models. This review summarizes the role of NO in adipose tissue and highlights the effects of NO/О2•- ratio 'teetering' as a promising pharmacological target in the metabolic syndrome.

LINKED ARTICLES

This article is part of a themed section on Redox Biology and Oxidative Stress in Health and Disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.12/issuetoc.

Colonic infusions of short-chain fatty acid mixtures promote energy metabolism in overweight/obese men: a randomized crossover trial

Short-chain fatty acids (SCFA), formed by microbial fermentation, are believed to be involved in the aetiology of obesity and diabetes. This study investigated the effects of colonic administration of physiologically relevant SCFA mixtures on human substrate and energy metabolism. In this randomized, double-blind, crossover study, twelve normoglycaemic men (BMI 25–35 kg/m²) underwent four investigational days, during which SCFA mixtures (200 mmol/L) high in either acetate (HA), propionate (HP), butyrate (HB) or placebo (PLA) were rectally administered during fasting and postprandial conditions (oral glucose load). Before and for two hours after colonic infusions, indirect calorimetry was performed and blood samples were collected. All three SCFA mixtures increased fasting fat oxidation (P < 0.01), whilst resting energy expenditure increased after HA and HP compared with PLA (P < 0.05). In addition, all three SCFA mixtures increased fasting and postprandial plasma peptide YY (PYY) concentrations, and attenuated fasting free glycerol concentrations versus PLA (P < 0.05). Colonic infusions of SCFA mixtures, in concentrations and ratios reached after fibre intake, increased fat oxidation, energy expenditure and PYY, and decreased lipolysis in overweight/obese men. Human intervention studies are warranted to investigate whether these effects translate into long-term benefits for body weight control and insulin sensitivity in the obese insulin resistant state.

MondoA/ChREBP: The usual suspects of transcriptional glucose sensing; Implication in pathophysiology

Identification of the Mondo glucose-responsive transcription factors family, including the MondoA and MondoB/ChREBP paralogs, has shed light on the mechanism whereby glucose affects gene transcription. They have clearly emerged, in recent years, as key mediators of glucose sensing by multiple cell types. MondoA and ChREBP have overlapping yet distinct expression profiles, which underlie their downstream targets and separate roles in regulating genes involved in glucose metabolism. MondoA can restrict glucose uptake and influences energy utilization in skeletal muscle, while ChREBP signals energy storage through de novo lipogenesis in liver and white adipose tissue. Because Mondo proteins mediate metabolic adaptations to changing glucose levels, a better understanding of cellular glucose sensing through Mondo proteins will likely uncover new therapeutic opportunities in the context of the imbalanced glucose homeostasis that accompanies metabolic diseases such as type 2 diabetes and cancer. Here, we provide an overview of structural homologies, transcriptional partners as well as the nutrient and hormonal mechanisms underlying Mondo proteins regulation. We next summarize their relative contribution to energy metabolism changes in physiological states and the evolutionary conservation of these pathways. Finally, we discuss their possible targeting in human pathologies.

Oral β-hydroxybutyrate increases ketonemia, decreases visceral adipocyte volume and improves serum lipid profile in Wistar rats

Background

Ketosis can be induced in humans and in animals by fasting or dietary interventions, such as ketogenic diets. However, the increasing interest on the ketogenic state has motivated the development of alternative approaches to rapidly increase ketonemia using less drastic interventions. Here, it was tested whether oral intake of a β-hydroxybutyrate (βHB) mineral salt mixture could increase ketonemia in Wistar rats without any other dietary changes, thereby being a useful model to study ketones effects alone on metabolism.

Methods

βHB salts were orally administered to provoke elevation in the ketonemia. Effects of this intervention were tested acutely (by gavage) and chronically (4 weeks in drinking water). Acutely, a concomitant glucose overload was used to suppress endogenous ketogenesis and verify whether βHB salts were really absorbed or not. Long-term administration allowed to weekly evaluate the impact on ketonemia, blood glucose and, after 4 weeks, on body weight, visceral fat mass, lipid blood profile, serum lipolysis products and adiponectinemia.

Results

βHB salts increased ketonemia in acute and long-term administrations, improved blood lipid profile by raising HDL-cholesterol concentration and decreasing LDL/HDL ratio, while reduced visceral adipocyte volume. Mean ketonemia correlated positively with HDLc and negatively with adipocyte volume and serum lipolysis products.

Conclusions

Oral βHB can rapidly increase ketonemia and, therefore, be used as an acute and long-term animal model of ketosis. Long-term treatment points to important beneficial effects of ketone bodies in serum lipid concentrations and visceral fat mass. These results may help to explain the metabolic adaptations following ketogenic diets, such as a better body fat control and a serum lipid profile improvement.

Insulin-mimetic compound hexaquis (benzylammonium) decavanadate is antilipolytic in human fat cells

AIM

To assess in rodent and human adipocytes the antilipolytic capacity of hexaquis(benzylammonium) decavanadate (B6V10), previously shown to exert antidiabetic effects in rodent models, such as lowering free fatty acids (FFA) and glucose circulating levels.

METHODS

Adipose tissue (AT) samples were obtained after informed consent from overweight women undergoing plastic surgery. Comparison of the effects of B6V10 and reference antilipolytic agents (insulin, benzylamine, vanadate) on the lipolytic activity was performed on adipocytes freshly isolated from rat, mouse and human AT. Glycerol release was measured using colorimetric assay as an index of lipolytic activity. The influence of B6V10 and reference agents on glucose transport into human fat cells was determined using the radiolabelled 2-deoxyglucose uptake assay.

RESULTS

In all the species studied, B6V10 exhibited a dose-dependent inhibition of adipocyte lipolysis when triglyceride breakdown was moderately enhanced by β-adrenergic receptor stimulation. B6V10 exerted on human adipocyte a maximal lipolysis inhibition of glycerol release that was stronger than that elicited by insulin. However, B6V10 did not inhibit basal and maximally stimulated lipolysis. When incubated at dose ≥ 10 μmol/L, B6V10 stimulated by twofold the glucose uptake in human fat cells, but - similarly to benzylamine - without reaching the maximal effect of insulin, while it reproduced one-half of the insulin-stimulation of lipogenesis in mouse fat cells.

CONCLUSION

B6V10 exerts insulin-like actions in adipocytes, including lipolysis inhibition and glucose transport activation. B6V10 may be useful in limiting lipotoxicity related to obesity and insulin resistance.

Pharmacological inhibition of adipose triglyceride lipase corrects high-fat diet-induced insulin resistance and hepatosteatosis in mice

Elevated circulating fatty acids (FAs) contribute to the development of obesity-associated metabolic complications such as insulin resistance (IR) and non-alcoholic fatty liver disease (NAFLD). Hence, reducing adipose tissue lipolysis to diminish the mobilization of FAs and lower their respective plasma concentrations represents a potential treatment strategy to counteract obesity-associated disorders. Here we show that specific inhibition of adipose triglyceride lipase (Atgl) with the chemical inhibitor, Atglistatin, effectively reduces adipose tissue lipolysis, weight gain, IR and NAFLD in mice fed a high-fat diet. Importantly, even long-term treatment does not lead to lipid accumulation in ectopic tissues such as the skeletal muscle or heart. Thus, the severe cardiac steatosis and cardiomyopathy that is observed in genetic models of Atgl deficiency does not occur in Atglistatin-treated mice. Our data validate the pharmacological inhibition of Atgl as a potentially powerful therapeutic strategy to treat obesity and associated metabolic disorders.

The enzyme Atgl participates in the breakdown of lipids in adipose tissue. Here the authors show that pharmacological inhibition of Atgl reduces weight gain and improves metabolic health in mice fed a high-fat diet, without causing adverse effects in cardiac muscle associated with genetic depletion of Atgl.

Hydroxy-Carboxylic Acid Receptor Actions in Metabolism

Lactic acid, the ketone body 3-hydroxy-butyric acid, also known as β-hydroxybutyrate, and the β-oxidation intermediate 3-hydroxy-octanoic acid are hydroxy-carboxylic acids (HCAs) that serve as intermediates of energy metabolism. However, they also regulate cellular functions, in part by directly activating the G protein-coupled receptors HCA₁/GPR81, HCA₂/GPR109A, and HCA₃/GPR109B. During the past decade, it has become clear that HCA receptors help to maintain homeostasis under changing metabolic and dietary conditions, by controlling metabolic, immune, and other body functions. Work based on genetic mouse models and synthetic ligands of HCA receptors has, in addition, shown that members of this receptor family can serve as targets for the prevention and therapy of diseases such as metabolic and inflammatory disorders.

Short-Chain Fatty Acids Differentially Affect Intracellular Lipolysis in a Human White Adipocyte Model

BACKGROUND AND AIMS

Gut-derived short-chain fatty acids (SCFA), formed by microbial fermentation of dietary fibers, are believed to be involved in the etiology of obesity and diabetes. Previous data from our group showed that colonic infusions of physiologically relevant SCFA mixtures attenuated whole-body lipolysis in overweight men. To further study potential mechanisms involved in the antilipolytic properties of SCFA, we aimed to investigate the in vitro effects of SCFA incubations on intracellular lipolysis and signaling using a human white adipocyte model, the human multipotent adipose tissue-derived stem (hMADS) cells.

METHODS

hMADS adipocytes were incubated with mixtures of acetate, propionate, and butyrate or single SCFA (acetate, propionate and butyrate) in concentrations ranging between 1 µmol/L and 1 mmol/L. Glycerol release and lipase activation was investigated during basal conditions and following β-adrenergic stimulation.

RESULTS

SCFA mixtures high in acetate and propionate decreased basal glycerol release, when compared to control (P < 0.05), while mixtures high in butyrate had no effect. Also, β-adrenergic receptor mediated glycerol release was not significantly altered following incubation with SCFA mixtures. Incubation with only acetate decreased basal (1 µmol/L) and β-adrenergically (1 µmol/L and 1 mmol/L) mediated glycerol release when compared with control (P < 0.05). In contrast, butyrate (1 µmol/L) slightly increased basal and β-adrenergically mediated glycerol release compared with control (P < 0.05), while propionate had no effect on lipolysis. The antilipolytic effect of acetate was accompanied by a reduced phosphorylation of hormone-sensitive lipase (HSL) at serine residue 650. In addition, inhibition of Gi G proteins following pertussis toxin treatment prevented the antilipolytic effect of acetate.

CONCLUSION

The present data demonstrated that acetate was mainly responsible for the antilipolytic effects of SCFA and acts via attenuation of HSL phosphorylation in a Gi-coupled manner in hMADS adipocytes. Therefore, the modulation of colonic and circulating acetate may be an important target to modulate human adipose tissue lipid metabolism.

Long-term Protective Changes in Adipose Tissue After Gastric Bypass

OBJECTIVE

Although long-term weight regain may occur after bariatric surgery, many patients are protected against relapse or development of type 2 diabetes. The study objective was to investigate whether this involves beneficial changes in adipose function.

RESEARCH DESIGN AND METHODS

Forty-nine obese women were investigated before and 2 and 5 years after Roux-en-Y gastric bypass (RYGB). At the 5-year follow-up, 30 subjects were pairwise matched for BMI and age to 30 control women. Clinical parameters and fine-needle biopsies from subcutaneous abdominal adipose tissue were obtained; fat cell size and number, lipolysis, adiponectin, and proinflammatory protein secretion were determined.

RESULTS

After 2 years, BMI decreased from 43 to 29 kg/m², which was accompanied by improvements in insulin sensitivity (HOMA of insulin resistance [HOMA-IR]), increased circulating and adipose secreted adiponectin, and decreased adipose lipolysis and fat cell size but no change in adipocyte number. Between 2 and 5 years after surgery, BMI had increased to 31 kg/m². This was associated with slightly increased HOMA-IR and unaltered circulating or adipose secreted adiponectin but higher secretion of tumor necrosis factor-α and increased lipolysis and number of fat cells but no change in adipocyte size. All these parameters, except lipolysis, were significantly more favorable compared with those in matched control subjects. Furthermore, the relationship between HOMA-IR and circulating adiponectin was less steep than in control subjects.

CONCLUSIONS

RYGB improves long-term insulin sensitivity and adipose phenotypes beyond the control state despite weight regain. Postoperative beneficial alterations in adipose function may be involved in the diabetes-protective effect of bariatric surgery.

Visceral adiposity and metabolic syndrome after very high-fat and low-fat isocaloric diets: a randomized controlled trial

BACKGROUND

Different aspects of dietary pattern, including macronutrient and food profiles, may affect visceral fat mass and metabolic syndrome.

OBJECTIVE

We hypothesized that consuming energy primarily from carbohydrate or fat in diets with similar food profiles would differentially affect the ability to reverse visceral adiposity and metabolic syndrome.

DESIGN

Forty-six men (aged 30-50 y) with body mass index (in kg/m²) >29 and waist circumference >98 cm were randomly assigned to a very high-fat, low-carbohydrate (VHFLC; 73% of energy fat and 10% of energy carbohydrate) or low-fat, high-carbohydrate (LFHC; 30% of energy fat and 53% of energy carbohydrate) diet for 12 wk. The diets were equal in energy (8750 kJ/d), protein (17% of energy), and food profile, emphasizing low-processed, lower-glycemic foods. Fat mass was quantified with computed tomography imaging.

RESULTS

Recorded intake of carbohydrate and total and saturated fat in the LFHC and VHFLC groups were 51% and 11% of energy, 29% and 71% of energy, and 12% and 34% of energy, respectively, with no difference in protein and polyunsaturated fatty acids. Mean energy intake decreased by 22% and 14% in the LFHC and VHFLC groups. The diets similarly reduced waist circumference (11-13 cm), abdominal subcutaneous fat mass (1650-1850 cm³), visceral fat mass (1350-1650 cm³), and total body weight (11-12 kg). Both groups improved dyslipidemia, with reduced circulating triglycerides, but showed differential responses in total and low-density lipoprotein cholesterol (decreased in LFHC group only), and high-density lipoprotein cholesterol (increased in VHFLC group only). The groups showed similar reductions in insulin, insulin C-peptide, glycated hemoglobin, and homeostasis model assessment of insulin resistance. Notably, improvements in circulating metabolic markers in the VHFLC group mainly were observed first after 8 wk, in contrast to more acute and gradual effects in the LFHC group.

CONCLUSIONS

Consuming energy primarily as carbohydrate or fat for 3 mo did not differentially influence visceral fat and metabolic syndrome in a low-processed, lower-glycemic dietary context. Our data do not support the idea that dietary fat per se promotes ectopic adiposity and cardiometabolic syndrome in humans. This study was registered at clinicaltrials.gov as NCT01750021.

Dyslipidemia: Obese or Not Obese-That Is Not the Question

PURPOSE OF REVIEW

Purpose of review: It is becoming increasingly clear that some obese individuals do not develop dyslipidemia and instead remain healthy, while some normal weight individuals become dyslipidemic and unhealthy.

RECENT FINDINGS

The present review examines the similarities and differences between healthy and unhealthy individuals with and without obesity and discusses putative underlying mechanisms of dyslipidemia. The presence of dyslipidemia and compromised metabolic health in both lean and obese individuals suggests that the obese phenotype per se does not represent a main independent risk factor for the development of dyslipidemia and that dyslipidemia, rather than obesity, may be the driver of metabolic diseases. Notably, adipose tissue dysfunction and ectopic lipid deposition, in particular in the liver, seems a common trait of unhealthy individuals.

Impaired adipose tissue lipid storage, but not altered lipolysis, contributes to elevated levels of NEFA in type 2 diabetes. Degree of hyperglycemia and adiposity are important factors

BACKGROUND

Elevated levels of circulating non-esterified fatty acids (NEFA) mediate many adverse metabolic effects. In this work we aim to determine the impact of type 2 diabetes (T2D), glycemic control and obesity on lipolysis regulation.

DESIGN AND PARTICIPANTS

20 control and 20 metformin-treated T2D subjects were matched for sex (10M/10 F), age (58±11 vs 58±9 y) and BMI (30.8±4.6 vs 30.7±4.9kg/m²). In vivo lipolysis was assessed during a 3h-OGTT with plasma glycerol and NEFA levels. Subcutaneous adipose tissue (SAT) biopsies were obtained to measure mRNA and metabolite levels of factors related to lipolysis and lipid storage and to assess in vitro lipolysis in isolated subcutaneous adipocytes.

RESULTS

Plasma NEFA AUC during the OGTT where higher 30% (P=0.005) in T2D than in control subjects, but plasma glycerol AUC and subcutaneous adipocyte lipolysis in vitro were similar, suggesting that adipose tissue lipolysis is not altered. Expression in SAT of genes involved in lipid storage (FABP4, DGAT1, FASN) were reduced in T2D subjects compared with controls, but no differences were seen for genes involved in lipolysis. T2D subjects had elevated markers of beta-oxidation, α-hydroxybutyrate (1.4-fold, P<0.01) and β-hydroxybutyrate (1.7-fold, P<0.05) in plasma. In multivariate analysis, HbA1c, visceral adipose tissue volume and sex (male) were significantly associated with NEFA AUC in T2D subjects.

CONCLUSIONS

In T2D subjects, NEFA turnover is impaired, but not due to defects in lipolysis or lipid beta-oxidation. Impaired adipose NEFA re-esterification or de novo lipogenesis is likely to contribute to higher NEFA plasma levels in T2D. The data suggest that hyperglycemia and adiposity are important contributing factors for the regulation of plasma NEFA concentrations.

Nicotinic acid timed to feeding reverses tissue lipid accumulation and improves glucose control in obese Zucker rats[S]

Nicotinic acid (NiAc) is a potent inhibitor of lipolysis, acutely reducing plasma free fatty acid (FFA) concentrations. However, a major FFA rebound is seen during rapid NiAc washout, and sustained exposure is associated with tolerance development, with FFAs returning to pretreatment levels. Our aim was to find a rational NiAc dosing regimen that preserves FFA lowering, sufficient to reverse nonadipose tissue lipid accumulation and improve metabolic control, in obese Zucker rats. We compared feeding-period versus fasting-period NiAc dosing for 5 days: 12 h subcutaneous infusion (programmable, implantable mini-pumps) terminated by gradual withdrawal. It was found that NiAc timed to feeding decreased triglycerides in liver (-47%; P < 0.01) and heart (-38%; P < 0.05) and reduced plasma fructosamine versus vehicle. During oral glucose tolerance test, plasma FFA levels were reduced with amelioration of hyperglycemia and hypertriglyceridemia. Furthermore, timing NiAc to feeding resulted in a general downregulation of de novo lipogenesis (DNL) genes in liver. By contrast, NiAc timed to fasting did not reduce tissue lipids, ameliorate glucose intolerance or dyslipidemia, or alter hepatic DNL genes. In conclusion, NiAc dosing regimen has a major impact on metabolic control in obese Zucker rats. Specifically, a well-defined NiAc exposure, timed to feeding periods, profoundly improves the metabolic phenotype of this animal model.

Role for hepatic CEACAM1 in regulating fatty acid metabolism along the adipocyte-hepatocyte axis

Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) regulates insulin sensitivity by promoting hepatic insulin clearance and mediating suppression of fatty acid synthase activity. Feeding C57BL/6J male mice with a high-fat (HF) diet for 3-4 weeks triggered a >60% decrease in hepatic CEACAM1 levels to subsequently impair insulin clearance and cause systemic insulin resistance and hepatic steatosis. This study aimed at investigating whether lipolysis drives reduction in hepatic CEACAM1 and whether this constitutes a key mechanism leading to diet-induced metabolic abnormalities. Blocking lipolysis with a daily intraperitoneal injection of nicotinic acid in the last two days of a 30-day HF feeding regimen demonstrated that white adipose tissue (WAT)-derived fatty acids repressed hepatic CEACAM1-dependent regulation of insulin and lipid metabolism in 3-month-old male C57BL/6J mice. Adenoviral-mediated CEACAM1 redelivery countered the adverse metabolic effect of the HF diet on insulin resistance, hepatic steatosis, visceral obesity, and energy expenditure. It also reversed the effect of HF diet on inflammation and fibrosis in WAT and liver. This assigns a causative role for lipolysis-driven decrease in hepatic CEACAM1 level and its regulation of insulin and lipid metabolism in sustaining systemic insulin resistance, hepatic steatosis, and other abnormalities associated with excessive energy supply.

DNA Damage and the Activation of the p53 Pathway Mediate Alterations in Metabolic and Secretory Functions of Adipocytes

Activation of the p53 pathway in adipose tissue contributes to insulin resistance associated with obesity. However, the mechanisms of p53 activation and the effect on adipocyte functions are still elusive. Here we found a higher level of DNA oxidation and a reduction in telomere length in adipose tissue of mice fed a high-fat diet and an increase in DNA damage and activation of the p53 pathway in adipocytes. Interestingly, hallmarks of chronic DNA damage are visible at the onset of obesity. Furthermore, injection of lean mice with doxorubicin, a DNA damage-inducing drug, increased the expression of chemokines in adipose tissue and promoted its infiltration by proinflammatory macrophages and neutrophils together with adipocyte insulin resistance. In vitro, DNA damage in adipocytes increased the expression of chemokines and triggered the production of chemotactic factors for macrophages and neutrophils. Insulin signaling and effect on glucose uptake and Glut4 translocation were decreased, and lipolysis was increased. These events were prevented by p53 inhibition, whereas its activation by nutlin-3 reproduced the DNA damage-induced adverse effects. This study reveals that DNA damage in obese adipocytes could trigger p53-dependent signals involved in alteration of adipocyte metabolism and secretory function leading to adipose tissue inflammation, adipocyte dysfunction, and insulin resistance.

The Dichotomous Effect of Chronic Stress on Obesity

Obesity and metabolic diseases are linked to chronic stress and low socioeconomic status. The mechanistic link between stress and obesity has not been clarified, partly due to the inherent complexity exemplified by the bidirectional effect of stress on eating and body weight. Recent studies focusing on adaptive thermogenesis and brown adipose tissue (BAT) function support a dichotomous relation to explain the impact of stress on obesity: stress promotes obesity in the presence of hyperphagia and unchanged BAT function; stress results in weight loss and/or obesity resistance in the presence of hypophagia, or when hyperphagia is associated with BAT recruitment and enhanced thermogenesis. Mechanistically dissecting the bidirectional effects of stress on metabolic outcomes might open new avenues for innovative pharmacotherapies for the treatment of obesity-associated diseases.

Effects of Genetic Loci Associated with Central Obesity on Adipocyte Lipolysis

OBJECTIVES

Numerous genetic loci have been associated with measures of central fat accumulation, such as waist-to-hip ratio adjusted for body mass index (WHRadjBMI). However the mechanisms by which genetic variations influence obesity remain largely elusive. Lipolysis is a key process for regulation of lipid storage in adipocytes, thus is implicated in obesity and its metabolic complications. Here, genetic variants at 36 WHRadjBMI-associated loci were examined for their influence on abdominal subcutaneous adipocyte lipolysis.

SUBJECTS AND METHODS

Fasting subcutaneous adipose tissue biopsies were collected from 789 volunteers (587 women and 202 men, body mass index (BMI) range 17.7-62.3 kg/m2). We quantified subcutaneous adipocyte lipolysis, both spontaneous and stimulated by the catecholamine isoprenaline or a cyclic AMP analogue. DNA was extracted from peripheral blood mononuclear cells and genotyping of SNPs associated with WHRadjBMI conducted. The effects on adipocyte lipolysis measures were assessed for SNPs individually and combined in a SNP score.

RESULTS

The WHRadjBMI-associated loci CMIP, PLXND1, VEGFA and ZNRF3-KREMEN1 demonstrated nominal associations with spontaneous and/or stimulated lipolysis. Candidate genes in these loci have been reported to influence NFκB-signaling, fat cell size and Wnt signalling, all of which may influence lipolysis.

SIGNIFICANCE

This report provides evidence for specific WHRadjBMI-associated loci as candidates to modulate adipocyte lipolysis. Additionally, our data suggests that genetically increased central fat accumulation is unlikely to be a major cause of altered lipolysis in abdominal adipocytes.

White-to-brite conversion in human adipocytes promotes metabolic reprogramming towards fatty acid anabolic and catabolic pathways

Objective

Fat depots with thermogenic activity have been identified in humans. In mice, the appearance of thermogenic adipocytes within white adipose depots (so-called brown-in-white i.e., brite or beige adipocytes) protects from obesity and insulin resistance. Brite adipocytes may originate from direct conversion of white adipocytes. The purpose of this work was to characterize the metabolism of human brite adipocytes.

Methods

Human multipotent adipose-derived stem cells were differentiated into white adipocytes and then treated with peroxisome proliferator-activated receptor (PPAR)γ or PPARα agonists between day 14 and day 18. Gene expression profiling was determined using DNA microarrays and RT-qPCR. Variations of mRNA levels were confirmed in differentiated human preadipocytes from primary cultures. Fatty acid and glucose metabolism was investigated using radiolabelled tracers, Western blot analyses and assessment of oxygen consumption. Pyruvate dehydrogenase kinase 4 (PDK4) knockdown was achieved using siRNA. In vivo, wild type and PPARα-null mice were treated with a β₃-adrenergic receptor agonist (CL316,243) to induce appearance of brite adipocytes in white fat depot. Determination of mRNA and protein levels was performed on inguinal white adipose tissue.

Results

PPAR agonists promote a conversion of white adipocytes into cells displaying a brite molecular pattern. This conversion is associated with transcriptional changes leading to major metabolic adaptations. Fatty acid anabolism i.e., fatty acid esterification into triglycerides, and catabolism i.e., lipolysis and fatty acid oxidation, are increased. Glucose utilization is redirected from oxidation towards glycerol-3-phophate production for triglyceride synthesis. This metabolic shift is dependent on the activation of PDK4 through inactivation of the pyruvate dehydrogenase complex. In vivo, PDK4 expression is markedly induced in wild-type mice in response to CL316,243, while this increase is blunted in PPARα-null mice displaying an impaired britening response.

Conclusions

Conversion of human white fat cells into brite adipocytes results in a major metabolic reprogramming inducing fatty acid anabolic and catabolic pathways. PDK4 redirects glucose from oxidation towards triglyceride synthesis and favors the use of fatty acids as energy source for uncoupling mitochondria.

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PPARγ and α agonists induce conversion of human white into brite adipocytes.

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Fatty acid anabolism and catabolism are activated in human brite adipocytes.

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Glucose use in brite adipocytes is redirected from oxidation to glyceroneogenesis.

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PDK4 induction is responsible for the shift from glucose to fatty acid oxidation.

Lipid metabolism and signaling in cardiac lipotoxicity

The heart balances uptake, metabolism and oxidation of fatty acids (FAs) to maintain ATP production, membrane biosynthesis and lipid signaling. Under conditions where FA uptake outpaces FA oxidation and FA sequestration as triacylglycerols in lipid droplets, toxic FA metabolites such as ceramides, diacylglycerols, long-chain acyl-CoAs, and acylcarnitines can accumulate in cardiomyocytes and cause cardiomyopathy. Moreover, studies using mutant mice have shown that dysregulation of enzymes involved in triacylglycerol, phospholipid, and sphingolipid metabolism in the heart can lead to the excess deposition of toxic lipid species that adversely affect cardiomyocyte function. This review summarizes our current understanding of lipid uptake, metabolism and signaling pathways that have been implicated in the development of lipotoxic cardiomyopathy under conditions including obesity, diabetes, aging, and myocardial ischemia-reperfusion. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.

Gsα deficiency in adipose tissue improves glucose metabolism and insulin sensitivity without an effect on body weight

Gsα, the G protein that transduces receptor-stimulated cAMP generation, mediates sympathetic nervous system stimulation of brown adipose tissue (BAT) thermogenesis and browning of white adipose tissue (WAT), which are both potential targets for treating obesity, as well as lipolysis. We generated a mouse line with Gsα deficiency in mature BAT and WAT adipocytes (Ad-GsKO). Ad-GsKO mice had impaired BAT function, absent browning of WAT, and reduced lipolysis, and were therefore cold-intolerant. Despite the presence of these abnormalities, Ad-GsKO mice maintained normal energy balance on both standard and high-fat diets, associated with decreases in both lipolysis and lipid synthesis. In addition, Ad-GsKO mice maintained at thermoneutrality on a standard diet also had normal energy balance. Ad-GsKO mice had improved insulin sensitivity and glucose metabolism, possibly secondary to the effects of reduced lipolysis and lower circulating fatty acid binding protein 4 levels. Gsα signaling in adipose tissues may therefore affect whole-body glucose metabolism in the absence of an effect on body weight.

FAT FLUX: enzymes, regulators, and pathophysiology of intracellular lipolysis

The great 19^th century French physiologist Claude Bernard reasoned “Man can learn nothing except by going from the known to the unknown”. This premise is particularly applicable to the progression of discoveries made in the field of fat metabolism since Bernard's time. Beginning with his groundbreaking discovery of fat digestion (later termed “lipolysis”) in 1848, research addressing the basic processes of cellular storage and mobilization of fat has steadily advanced. Even after 150 years of research dedicated to lipolysis, exciting new principles have continued to emerge in the last 10 years. This Perspective summarizes these recent landmark discoveries in the field and emphasizes their relevance for the pathogenesis of extremely prevalent diseases such as obesity, heart disease, and cancer.

Adipocyte lipolysis and insulin resistance

Obesity-induced insulin resistance is a major risk factor for the development of type 2 diabetes. Basal fat cell lipolysis (i.e., fat cell triacylglycerol breakdown into fatty acids and glycerol in the absence of stimulatory factors) is elevated during obesity and is closely associated with insulin resistance. Inhibition of adipocyte lipolysis may therefore be a promising therapeutic strategy for treating insulin resistance and preventing obesity-associated type 2 diabetes. In this review, we explore the relationship between adipose lipolysis and insulin sensitivity. After providing an overview of the components of fat cell lipolytic machinery, we describe the hypotheses that may support the causality between lipolysis and insulin resistance. Excessive circulating fatty acids may ectopically accumulate in insulin-sensitive tissues and impair insulin action. Increased basal lipolysis may also modify the secretory profile of adipose tissue, influencing whole body insulin sensitivity. Finally, excessive fatty acid release may also worsen adipose tissue inflammation, a well-known parameter contributing to insulin resistance. Partial genetic or pharmacologic inhibition of fat cell lipases in mice as well as short term clinical trials using antilipolytic drugs in humans support the benefit of fat cell lipolysis inhibition on systemic insulin sensitivity and glucose metabolism, which occurs without an increase of fat mass. Modulation of fatty acid fluxes and, putatively, of fat cell secretory pattern may explain the amelioration of insulin sensitivity whereas changes in adipose tissue immune response do not seem involved.

Fatty acids from fat cell lipolysis do not activate an inflammatory response but are stored as triacylglycerols in adipose tissue macrophages

AIMS/HYPOTHESIS

Activation of macrophages by fatty acids (FAs) is a potential mechanism linking obesity to adipose tissue (AT) inflammation and insulin resistance. Here, we investigated the effects of FAs released during adipocyte lipolysis on AT macrophages (ATMs).

METHODS

Human THP-1 macrophages were treated with media from human multipotent adipose-derived stem (hMADS) adipocytes stimulated with lipolytic drugs. Macrophages were also treated with mixtures of FAs and an inhibitor of Toll-like receptor 4, since this receptor is activated by saturated FAs. Levels of mRNA and the secretion of inflammation-related molecules were measured in macrophages. FA composition was determined in adipocytes, conditioned media and macrophages. The effect of chronic inhibition or acute activation of fat cell lipolysis on ATM response was investigated in vivo in mice.

RESULTS

Whereas palmitic acid alone activates THP-1, conditioned media from hMADS adipocyte lipolysis had no effect on IL, chemokine and cytokine gene expression, and secretion by macrophages. Mixtures of FAs representing de novo lipogenesis or habitual dietary conditions also had no effect. FAs derived from adipocyte lipolysis were taken up by macrophages and stored as triacylglycerol droplets. In vivo, chronic treatment with an antilipolytic drug did not modify gene expression and number of ATMs in mice with intact or defective Tlr4. Stimulation of adipocyte lipolysis increased storage of neutral lipids by macrophages without change in number and phenotype.

CONCLUSIONS/INTERPRETATION

Our data suggest that adipocyte lipolysis does not activate inflammatory pathways in ATMs, which instead may act as scavengers of FAs.

An apolipoprotein B100 mimotope prevents obesity in mice

Immunization with an ApoB100 mimotope prevents high-fat-diet-induced obesity in mice. Antibody titres parallel the weight decrements. In vitro data implicate increased lipolysis and reduced lipoprotein uptake by adipocytes, as well as increased uptake and metabolism of native lipoprotein by macrophages.

Although apolipoprotein B100 (ApoB100) plays a key role in peripheral fat deposition, it is not considered a suitable therapeutic target in obesity. In the present study we describe a novel ApoB100 mimotope, peptide pB1, and the use of pB1-based vaccine-like formulations (BVFs) against high-fat diet (HFD)-induced obesity. In HFD- compared with chow-fed adolescent mice, BVFs reduced the 3-month body-weight gains attributable to increased dietary fat by 44–65%, and prevented mesenteric fat accumulation and liver steatosis. The body-weight reductions paralleled the titres of pB1-reactive immunoglobulin G (IgG) antibodies, and pB1-reactive antibodies specifically recognized native ApoB100 and a synthetic peptide from the C-terminal half of ApoB100. In cultured 3T3L1 adipocytes, anti-pB1 antibodies increased lipolysis and inhibited low-density lipoprotein (LDL) uptake. In cultured RAW 264.7 macrophages, the same antibodies enhanced LDL uptake (without causing foam cell formation). These findings make ApoB100 a promising target for an immunization strategy against HFD-induced obesity.

Hypophagia and metabolic adaptations in mice with defective ATGL-mediated lipolysis cause resistance to HFD-induced obesity

Adipose triglyceride lipase (ATGL) initiates intracellular triglyceride (TG) catabolism. In humans, ATGL deficiency causes neutral lipid storage disease with myopathy (NLSDM) characterized by a systemic TG accumulation. Mice with a genetic deletion of ATGL (AKO) also accumulate TG in many tissues. However, neither NLSDM patients nor AKO mice are exceedingly obese. This phenotype is unexpected considering the importance of the enzyme for TG catabolism in white adipose tissue (WAT). In this study, we identified the counteracting mechanisms that prevent excessive obesity in the absence of ATGL. We used "healthy" AKO mice expressing ATGL exclusively in cardiomyocytes (AKO/cTg) to circumvent the cardiomyopathy and premature lethality observed in AKO mice. AKO/cTg mice were protected from high-fat diet (HFD)-induced obesity despite complete ATGL deficiency in WAT and normal adipocyte differentiation. AKO/cTg mice were highly insulin sensitive under hyperinsulinemic-euglycemic clamp conditions, eliminating insulin insensitivity as a possible protective mechanism. Instead, reduced food intake and altered signaling by peroxisome proliferator-activated receptor-gamma (PPAR-γ) and sterol regulatory element binding protein-1c in WAT accounted for the phenotype. These adaptations led to reduced lipid synthesis and storage in WAT of HFD-fed AKO/cTg mice. Treatment with the PPAR-γ agonist rosiglitazone reversed the phenotype. These results argue for the existence of an adaptive interdependence between lipolysis and lipid synthesis. Pharmacological inhibition of ATGL may prove useful to prevent HFD-induced obesity and insulin resistance.

Contribution of lipase deficiency to mitochondrial dysfunction and insulin resistance in hMADS adipocytes

BACKGROUND/OBJECTIVES

Adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) are key enzymes involved in intracellular lipid catabolism. We have previously shown decreased expression and activity of these lipases in adipose tissue of obese insulin resistant individuals. Here we hypothesized that lipase deficiency might impact on insulin sensitivity and metabolic homeostasis in adipocytes not just by enhancing lipid accumulation, but also by altering lipid and carbohydrate catabolism in a peroxisome proliferator-activated nuclear receptor (PPAR)-dependent manner.

METHODS

To address our hypothesis, we performed a series of in vitro experiments in a human white adipocyte model, the human multipotent adipose-derived stem (hMADS) cells, using genetic (siRNA) and pharmacological knockdown of ATGL and/or HSL.

RESULTS

We show that ATGL and HSL knockdown in hMADS adipocytes disrupted mitochondrial respiration, which was accompanied by a decreased oxidative phosphorylation (OxPhos) protein content. This lead to a reduced exogenous and endogenous palmitate oxidation following ATGL knockdown, but not in HSL deficient adipocytes. ATGL deficiency was followed by excessive triacylglycerol accumulation, and HSL deficiency further increased diacylglycerol accumulation. Both single and double lipase knockdown reduced insulin-stimulated glucose uptake, which was attributable to impaired insulin signaling. These effects were accompanied by impaired activation of the nuclear receptor PPARα, and restored on PPARα agonist treatment.

CONCLUSIONS

The present study indicates that lipase deficiency in human white adipocytes contributes to mitochondrial dysfunction and insulin resistance, in a PPARα-dependent manner. Therefore, modulation of adipose tissue lipases may provide a promising strategy to reverse insulin resistance in obese and type 2 diabetic patients.

Lipopolysaccharide binding protein is an adipokine involved in the resilience of the mouse adipocyte to inflammation

AIMS/HYPOTHESIS

Lipopolysaccharide (LPS) binding protein (LBP) is a novel 65 kDa adipokine, linked to adipose tissue (AT) inflammation, obesity and insulin resistance, that inhibits adipocyte differentiation. Here, we investigated the molecular mechanisms behind these detrimental effects on adipogenesis through whole-genome transcriptomics and in vitro experiments.

METHODS

Permanent and transient knockdown (KD) and co-culture experiments were performed in 3T3-L1 and 3T3-F442A cell lines during adipocyte differentiation. Microarray gene expression was performed using Genechip Affymetrix technology and validated by real-time PCR.

RESULTS

LBP KD of 3T3-L1 cells led to a potentiated adipocyte differentiation with a dose-response relationship; genes involved in mitochondrial biogenesis, fatty acid metabolism and peroxisome proliferator-activated receptor γ (PPAR-γ) action were dramatically upregulated in parallel to increased insulin signalling. Cells with LBP KD became refractory to proinflammatory cytokines and other inflammatory stimuli (LPS and palmitate). This phenotype, mediated through disrupted nuclear factor κB (NFκB) signalling, was reversed by a soluble factor present in a co-culture with native cells and by exogenous LBP. Double-silencing of LBP and toll-like receptor 4 (TLR4) again rendered these cells insensitive to co-culture, LBP and inflammatory factors.

CONCLUSIONS/INTERPRETATION

In summary, LBP is a proinflammatory soluble adipokine that acts as a brake for adipogenesis, strengthening the negative effects of palmitate and LPS on adipocyte differentiation.

Targeting fatty acid metabolism to improve glucose metabolism

Disturbances in fatty acid metabolism in adipose tissue, liver, skeletal muscle, gut and pancreas play an important role in the development of insulin resistance, impaired glucose metabolism and type 2 diabetes mellitus. Alterations in diet composition may contribute to prevent and/or reverse these disturbances through modulation of fatty acid metabolism. Besides an increased fat mass, adipose tissue dysfunction, characterized by an altered capacity to store lipids and an altered secretion of adipokines, may result in lipid overflow, systemic inflammation and excessive lipid accumulation in non-adipose tissues like liver, skeletal muscle and the pancreas. These impairments together promote the development of impaired glucose metabolism, insulin resistance and type 2 diabetes mellitus. Furthermore, intrinsic functional impairments in either of these organs may contribute to lipotoxicity and insulin resistance. The present review provides an overview of fatty acid metabolism-related pathways in adipose tissue, liver, skeletal muscle, pancreas and gut, which can be targeted by diet or food components, thereby improving glucose metabolism.

Reversal of diet-induced obesity and insulin resistance by inducible genetic ablation of GRK2

Insulin resistance is a common feature of obesity and predisposes individuals to various prevalent pathological conditions. G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptor kinase 2 (GRK2) integrates several signal transduction pathways and is emerging as a physiologically relevant inhibitor of insulin signaling. GRK2 abundance is increased in humans with metabolic syndrome and in different murine models of insulin resistance. To support GRK2 as a potential drug target in type 2 diabetes and obesity, we investigated whether lowering GRK2 abundance reversed an ongoing systemic insulin-resistant phenotype, using a mouse model of tamoxifen-induced GRK2 ablation after high-fat diet-dependent obesity and insulin resistance. Tamoxifen-triggered GRK2 deletion impeded further body weight gain, normalized fasting glycemia, improved glucose tolerance, and was associated with preserved insulin sensitivity in skeletal muscle and liver, thereby maintaining whole-body glucose homeostasis. Moreover, when continued to be fed a high-fat diet, these animals displayed reduced fat mass and smaller adipocytes, were resistant to the development of liver steatosis, and showed reduced expression of proinflammatory markers in the liver. Our results indicate that GRK2 acts as a hub to control metabolic functions in different tissues, which is key to controlling insulin resistance development in vivo. These data suggest that inhibiting GRK2 could reverse an established insulin-resistant and obese phenotype, thereby putting forward this enzyme as a potential therapeutic target linking glucose homeostasis and regulation of adiposity.

Dosing profile profoundly influences nicotinic acid's ability to improve metabolic control in rats

Acute nicotinic acid (NiAc) administration results in rapid reduction of plasma FFA concentrations. However, sustained NiAc exposure is associated with tolerance development resulting in return of FFA to pretreatment levels. The aim of this study was to determine whether a 12 h rectangular exposure profile (intermittent dose group) could avoid tolerance development and thereby reverse insulin resistance induced by lipid overload. FFA lowering was assessed in male Sprague Dawley (lean) and obese Zucker rats (obese) in response to a 5 h NiAc infusion, in either NiAc-naïve animals or after 5 days of continuous (24 h/day) or intermittent (12 h/day) NiAc dosing (via implantable, programmable minipump). We found that intermittent dosing over 5 days preserved NiAc-induced FFA lowering, comparable to dosing in NiAc-naïve animals. By contrast, following 5 days continuous administration, NiAc-induced FFA lowering was lost. The effect of intermittent NiAc infusion on insulin sensitivity was assessed in obese Zucker rats using hyperinsulinemic-isoglycemic clamps. The acute effect of NiAc to elevate glucose infusion rate (vs. saline control) was indeed preserved with intermittent dosing, while being lost upon continuous infusion. In conclusion, an intermittent but not continuous NiAc dosing strategy succeeded in retaining NiAc's ability to lower FFA and improve insulin sensitivity in obese Zucker rats.-Kroon, T., A. Kjellstedt, P. Thalén, J. Gabrielsson, and N. D. Oakes.

Antiobesity Effect of a Small Molecule Repressor of RORγ

The orphan nuclear receptor RORγ is a key regulator for T helper 17 (TH17) cell differentiation, which regulates metabolic and circadian rhythm genes in peripheral tissues. Previously, it was shown that the small molecule inverse agonist of RORγ SR1555 [1-(4-((4'-(1,1,1,3,3,3-hexafluoro-2-hydroxypropan-2-yl)-[1,1'-biphenyl]-4-yl)methyl)piperazin-1-yl) ethanone] suppressed TH17 differentiation and stimulated induced T regulatory (iTreg) cells. Here, we show that treatment of cultured pre-adipocyctes with SR1555 represses the expression of RORγ while leading to increased expression of FGF21 and adipoQ. Chronic administration of SR1555 to obese diabetic mice resulted in a modest reduction in food intake accompanied with significant reduction in fat mass, resulting in reduced body weight and improved insulin sensitivity. Analysis ex vivo of treated mice demonstrates that SR1555 induced expression of the thermogenic gene program in fat depots. Further studies in cultured cells showed that SR1555 inhibited activation of hormone-sensitive lipase and increased fatty acid oxidation. Combined, these results suggest that pharmacological repression of RORγ may represent a strategy for treatment of obesity by increasing thermogenesis and fatty acid oxidation, while inhibition of hormone-sensitive lipase activity results in a reduction of serum free fatty acids, leading to improved peripheral insulin sensitivity.

Nox2 mediates skeletal muscle insulin resistance induced by a high fat diet

Inflammation and oxidative stress through the production of reactive oxygen species (ROS) are consistently associated with metabolic syndrome/type 2 diabetes. Although the role of Nox2, a major ROS-generating enzyme, is well described in host defense and inflammation, little is known about its potential role in insulin resistance in skeletal muscle. Insulin resistance induced by a high fat diet was mitigated in Nox2-null mice compared with wild-type mice after 3 or 9 months on the diet. High fat feeding increased Nox2 expression, superoxide production, and impaired insulin signaling in skeletal muscle tissue of wild-type mice but not in Nox2-null mice. Exposure of C2C12 cultured myotubes to either high glucose concentration, palmitate, or H2O2 decreases insulin-induced Akt phosphorylation and glucose uptake. Pretreatment with catalase abrogated these effects, indicating a key role for H2O2 in mediating insulin resistance. Down-regulation of Nox2 in C2C12 cells by shRNA prevented insulin resistance induced by high glucose or palmitate but not H2O2. These data indicate that increased production of ROS in insulin resistance induced by high glucose in skeletal muscle cells is a consequence of Nox2 activation. This is the first report to show that Nox2 is a key mediator of insulin resistance in skeletal muscle.

Low early B-cell factor 1 (EBF1) activity in human subcutaneous adipose tissue is linked to a pernicious metabolic profile

AIM

Recently, in both human and murine white adipose tissue (WAT), transcription factor early B-cell factor 1 (EBF1) has been shown to regulate adipocyte differentiation, adipose morphology and triglyceride hydrolysis (lipolysis). This study investigated whether EBF1 expression and biological activity in WAT is related to different metabolic parameters.

METHODS

In this cross-sectional study of abdominal subcutaneous WAT, EBF1 protein levels were examined in 18 non-obese subjects, while biological activity was determined in 56 obese and non-obese subjects. Results were assessed by anthropometric measures and blood pressure as well as by plasma lipid levels and insulin sensitivity.

RESULTS

EBF1 protein levels were negatively associated with waist circumference (r=-0.56; P=0.015), but not with body mass index (BMI) or body fat (P=0.10-0.29). Biological activity of EBF1 correlated negatively with plasma triglycerides (r=-0.46; P=0.0005) and plasma insulin (r=-0.39; P=0.0027), but positively with plasma HDL cholesterol (r=0.48; P=0.0002) and insulin sensitivity, as assessed by intravenous insulin tolerance test (r=0.64; P<0.0001). These relationships, except for plasma insulin, remained statistically significant after adjusting for BMI and adipose morphology. EBF1 activity was not associated with age, systolic/diastolic blood pressure or total plasma cholesterol (P=0.17-0.48). In contrast to EBF1 activity, after adjusting for BMI, EBF1 mRNA levels displayed only an association with plasma triglycerides.

CONCLUSION

Low EBF1 protein expression and activity in abdominal subcutaneous WAT is a BMI-independent marker for several traits associated with the metabolic syndrome. However, whether EBF1 constitutes a novel treatment target remains to be demonstrated.

Progesterone-induced down-regulation of hormone sensitive lipase (Lipe) and up-regulation of G0/G1 switch 2 (G0s2) genes expression in inguinal adipose tissue of female rats is reflected by diminished rate of lipolysis

Decreased lipolytic activity in adipose tissue may be one of the reasons behind excess accumulation of body fat during pregnancy. The aim of this study was to analyze the effect of progesterone on the expression of: (a) Lipe (encoding hormone-sensitive lipase, HSL), (b) Pnpla2 (encoding adipose triglyceride lipase, ATGL), (c) abhydrolase domain containing 5 (Abhd5), and (d) G0/G1 switch 2 (G0s2) genes in white adipose tissue (WAT), as potential targets for progesterone action during the course of pregnancy. Administration of progesterone to female rats, which was reflected by approximately 2.5-fold increase in circulating progesterone concentration, is associated with a decrease in Lipe gene expression in the inguinal WAT. The expression of Pnpla2 gene in all main fat depots of females and males remained unchanged after progesterone administration. Administration of progesterone resulted in an increase in the expression of Abhd5 gene (whose product increases ATGL activity) and G0s2 gene (whose product decreases ATGL activity) in the inguinal WAT of female rats. Mifepristone, a selective antagonist of progesterone receptor, abolished the effect of progesterone on Lipe, Abhd5 and G0s2 genes expression in the inguinal WAT. The decrease in Lipe and the increase in Abhd5 and G0s2 genes expression was associated with lower rate of stimulated lipolysis. Administration of progesterone exerted no effect on Lipe, Abhd5 and G0s2 genes expression and stimulated lipolysis in the retroperitoneal WAT of females, as well as in the inguinal, epididymal and retroperitoneal WAT of males. In conclusion, our findings suggest that progesterone decreases the rate of lipolysis in the inguinal WAT of female rats, inhibiting the activity of both ATGL (by stimulating synthesis of G0S2 - specific inhibitor of the enzyme) and HSL (due to inhibition of Lipe gene expression).

System model network for adipose tissue signatures related to weight changes in response to calorie restriction and subsequent weight maintenance

Nutrigenomics investigates relationships between nutrients and all genome-encoded molecular entities. This holistic approach requires systems biology to scrutinize the effects of diet on tissue biology. To decipher the adipose tissue (AT) response to diet induced weight changes we focused on key molecular (lipids and transcripts) AT species during a longitudinal dietary intervention. To obtain a systems model, a network approach was used to combine all sets of variables (bio-clinical, fatty acids and mRNA levels) and get an overview of their interactions. AT fatty acids and mRNA levels were quantified in 135 obese women at baseline, after an 8-week low calorie diet (LCD) and after 6 months of ad libitum weight maintenance diet (WMD). After LCD, individuals were stratified a posteriori according to weight change during WMD. A 3 steps approach was used to infer a global model involving the 3 sets of variables. It consisted in inferring intra-omic networks with sparse partial correlations and inter-omic networks with regularized canonical correlation analysis and finally combining the obtained omic-specific network in a single global model. The resulting networks were analyzed using node clustering, systematic important node extraction and cluster comparisons. Overall, AT showed both constant and phase-specific biological signatures in response to dietary intervention. AT from women regaining weight displayed growth factors, angiogenesis and proliferation signaling signatures, suggesting unfavorable tissue hyperplasia. By contrast, after LCD a strong positive relationship between AT myristoleic acid (a fatty acid with low AT level) content and de novo lipogenesis mRNAs was found. This relationship was also observed, after WMD, in the group of women that continued to lose weight. This original system biology approach provides novel insight in the AT response to weight control by highlighting the central role of myristoleic acid that may account for the beneficial effects of weight loss.

Obesity is an excess fat mass leading to metabolic diseases. Dietary management is a conventional strategy to promote weight loss. As energy buffering, in the form of esterified fatty acids, and secretory organ, the adipose tissue has a pivotal role in obesity and its related complications. A comprehensive insight of adipose tissue response during and after calorie restriction might improve obesity management. Modern nutrition research study the impact of diet on health by combining multiple datasets to provide an holistic view of tissue physiopathology. To identify significant clusters of fatty acids, transcripts or bio-clinical parameters related to weight change along calorie restriction and subsequent weight follow-up in obese individuals, the issue of different datasets integration must be resolved. Here, we implemented an innovative multistep approach to infer multi-data networks and compare clusters of network components. This original strategy highlighted an unexpected central role of a minor adipose tissue fatty acid, myristoleic acid, which is not provided by food. Its link to transcripts encoding enzymes from a pathway converting glucose into fat that mediates favorable metabolic effects makes myristoleic acid a key factor of the positive impact of fat mass reduction.

Fatty Acids, Obesity and Insulin Resistance

OBJECTIVE

Although elevated free fatty acid (FFA) levels in obesity have been considered to be of importance for insulin resistance, a recent meta-analysis suggested normal FFA levels in obese subjects. We investigated fasting circulating FFA and glycerol levels in a large cohort of non-obese and obese subjects.

METHODS

Subjects recruited for a study on obesity genetics were investigated in the morning after an overnight fast (n = 3,888). Serum FFA (n = 3,306), plasma glycerol (n = 3,776), and insulin sensitivity index (HOMA-IR,n = 3,469) were determined. Obesity was defined as BMI ≥ 30 kg/m 2 and insulin resistance as HOMA-IR ≥ 2.21.

RESULTS

In obese subjects, circulating FFA and glycerol levels were higher than in non-obese individuals (by 26% and 47%, respectively; both p < 0.0001). Similar results were obtained if only men, women or medication-free subjects were investigated. Insulin resistance and type 2 diabetes were associated with a further minor increase in FFA/glycerol among obese subjects. When comparing insulin-sensitive non-obese with insulin-sensitive or -resistant obese individuals, FFA and glycerol were 21–29% and 43–49% higher in obese individuals, respectively.

CONCLUSION

Circulating FFA and glycerol levels are markedly elevated in obesity but only marginally influenced by insulin resistance and type 2 diabetes. Whether these differences persist during diurnal variations in circulating FFA/glycerol, remains to be established

Secretion of fatty acid binding protein aP2 from adipocytes through a nonclassical pathway in response to adipocyte lipase activity

Adipocyte fatty acid binding protein 4, aP2, contributes to the pathogenesis of several common diseases including type 2 diabetes, atherosclerosis, fatty liver disease, asthma, and cancer. Although the biological functions of aP2 have classically been attributed to its intracellular action, recent studies demonstrated that aP2 acts as an adipokine to regulate systemic metabolism. However, the mechanism and regulation of aP2 secretion remain unknown. Here, we demonstrate a specific role for lipase activity in aP2 secretion from adipocytes in vitro and ex vivo. Our results show that chemical inhibition of lipase activity, genetic deficiency of adipose triglyceride lipase and, to a lesser extent, hormone-sensitive lipase blocked aP2 secretion from adipocytes. Increased lipolysis and lipid availability also contributed to aP2 release as determined in perilipin1-deficient adipose tissue explants ex vivo and upon treatment with lipids in vivo and in vitro. In addition, we identify a nonclassical route for aP2 secretion in exosome-like vesicles and show that aP2 is recruited to this pathway upon stimulation of lipolysis. Given the effect of circulating aP2 on glucose metabolism, these data support that targeting aP2 or the lipolysis-dependent secretory pathway may present novel mechanistic and translational opportunities in metabolic disease.

SIRT1 enhances glucose tolerance by potentiating brown adipose tissue function

OBJECTIVE

SIRT1 has been proposed to be a key signaling node linking changes in energy metabolism to transcriptional adaptations. Although SIRT1 overexpression is protective against diverse metabolic complications, especially in response to high-fat diets, studies aiming to understand the etiology of such benefits are scarce. Here, we aimed to identify the key tissues and mechanisms implicated in the beneficial effects of SIRT1 on glucose homeostasis.

METHODS

We have used a mouse model of moderate SIRT1 overexpression, under the control of its natural promoter, to evaluate glucose homeostasis and thoroughly characterize how different tissues could influence insulin sensitivity.

RESULTS

Mice with moderate overexpression of SIRT1 exhibit better glucose tolerance and insulin sensitivity even on a low fat diet. Euglycemic-hyperinsulinemic clamps and in-depth tissue analyses revealed that enhanced insulin sensitivity was achieved through a higher brown adipose tissue activity and was fully reversed by housing the mice at thermoneutrality. SIRT1 did not influence brown adipocyte differentiation, but dramatically enhanced the metabolic transcriptional responses to β3-adrenergic stimuli in differentiated adipocytes.

CONCLUSIONS

Our work demonstrates that SIRT1 improves glucose homeostasis by enhancing BAT function. This is not consequent to an alteration in the brown adipocyte differentiation process, but as a result of potentiating the response to β3-adrenergic stimuli.

Increased subcutaneous adipose tissue expression of genes involved in glycerolipid-fatty acid cycling in obese insulin-resistant versus -sensitive individuals

CONTEXT

A subpopulation of obese individuals remains insulin sensitive (ISO). They represent a unique human model to investigate factors underlying insulin resistance (IR) without the confounding effect of major differences in weight/adiposity. Altered fatty-acid (FA) metabolism in sc adipose tissue (SAT) contributes to obesity-associated IR.

OBJECTIVE

To test the hypothesis that ISO and body mass index-matched insulin-resistant obese (IRO) patients demonstrate differential SAT expression profiles of genes involved in glycerolipid-FA metabolism and that weight loss-induced improvement of IR ameliorates these changes.

DESIGN AND SETTING

A cross-sectional and longitudinal study.

PATIENTS AND INTERVENTION

Thirty-eight nondiabetic obese women were stratified into ISO (n = 25) or IRO (n = 13) groups based on hyperinsulinemic-euglycemic clamp results. Subjects were studied before and after a 6-month hypocaloric diet intervention.

MAIN OUTCOME MEASURES

mRNA (quantitative RT-PCR) and protein (mass spectrometry and immunoblots) levels were measured in SAT biopsies.

RESULTS

Despite having age, body mass index, and fat mass similar to ISO individuals, IRO patients had lower insulin sensitivity and glucose tolerance (P < .05). Baseline SAT mRNA and protein levels of genes involved in both the synthesis and lipolysis of glycerolipid-FAs were higher in IRO individuals (P < .05), even when groups were matched for visceral adipose tissue content. The dietary intervention resulted in approximately 6% weight loss in both the IRO and ISO groups (P < .05) but only ameliorated insulin sensitivity in IRO individuals (P < .05). Likewise, the intervention reduced the expression of most glycerolipid-FA metabolism genes (P < .05), with expression levels in IRO individuals being restored to ISO levels.

CONCLUSIONS

Increased SAT expression of genes involved in both the synthesis and hydrolysis of glycerolipid-FAs is closely associated with IR in obese women. The results suggest that enhanced glycerolipid-FA cycling in SAT contributes to obesity-associated IR.

Is bisphenol S a safe substitute for bisphenol A in terms of metabolic function? An in vitro study

As bisphenol A (BPA) has been shown to induce adverse effects on human health, especially through the activation of endocrine pathways, it is about to be withdrawn from the European market and replaced by analogues such as bisphenol S (BPS). However, toxicological data on BPS is scarce, and so it is necessary to evaluate the possible effects of this compound on human health. We compared the effect of BPA and BPS on obesity and hepatic steatosis processes using low doses in the same range as those found in the environment. Two in vitro models were used, the adipose cell line 3T3-L1 and HepG2 cells, representative of hepatic functions. We analyzed different parameters such as lipid and glucose uptakes, lipolysis, leptin production and the modulation of genes involved in lipid metabolism and energy balance. BPA and BPS induced an increase in the lipid content in the 3T3-L1 cell line and more moderately in the hepatic cells. We also observed a decrease in lipolysis after bisphenol treatment of adipocytes, but only BPS was involved in the increase in glucose uptake and leptin production. These latter effects could be linked to the modulation of SREBP-1c, PPARγ, aP2 and ERRα and γ genes after exposure to BPA, whereas BPS seems to target the PGC1α and the ERRγ genes. The findings suggest that both BPA and BPS could be involved in obesity and steatosis processes, but through two different metabolic pathways.

Hormone-sensitive lipase deficiency in humans

The breakdown of cellular fat stores fuels energy production and multiple anabolic processes. Albert et al. (2014) demonstrate that the lack of hormone-sensitive lipase, a member of the enzyme trio that catabolizes fat, has pronounced effects on lipid metabolism, glucose homeostasis, and cell signaling in humans.

Grizzly bears exhibit augmented insulin sensitivity while obese prior to a reversible insulin resistance during hibernation

The confluence of obesity and diabetes as a worldwide epidemic necessitates the discovery of new therapies. Success in this endeavor requires translatable preclinical studies, which traditionally employ rodent models. As an alternative approach, we explored hibernation where obesity is a natural adaptation to survive months of fasting. Here we report that grizzly bears exhibit seasonal tripartite insulin responsiveness such that obese animals augment insulin sensitivity but only weeks later enter hibernation-specific insulin resistance (IR) and subsequently reinitiate responsiveness upon awakening. Preparation for hibernation is characterized by adiposity coupled to increased insulin sensitivity via modified PTEN/AKT signaling specifically in adipose tissue, suggesting a state of "healthy" obesity analogous to humans with PTEN haploinsufficiency. Collectively, we show that bears reversibly cope with homeostatic perturbations considered detrimental to humans and describe a mechanism whereby IR functions not as a late-stage metabolic adaptation to obesity, but rather a gatekeeper of the fed-fasting transition.

Adipocyte size threshold matters: link with risk of type 2 diabetes and improved insulin resistance after gastric bypass

CONTEXT

Adipocyte volume has been associated with insulin resistance and type 2 diabetes.

OBJECTIVE

Our purpose was to identify an adipocyte volume threshold linked with increased insulin resistance risk, and to examine its association with insulin resistance improvement after bariatric surgery.

SETTING AND DESIGN

We investigated two cohorts of Caucasian women, candidates for bariatric surgery, from two institutional centers in France (age 42.0 ± 11.5 years; body mass index, 47.6 ± 6.9 kg/m(2)) and Germany (age 41.3 ± 11.2 years; body mass index, 49.5 ± 8.1 kg/m(2)). 38% of the subjects had gastric bypass surgery and were followed for 6 months after the intervention. We defined a group of subjects with type 2 diabetes or at risk of type 2 diabetes (DRD) and investigated the relations between adipocyte volume and this status before and after surgery.

RESULTS

In both cohorts, subjects with DRD presented enlarged adipocytes (France, P = 3×10(-4); Germany, P =3×10(-10)) and we were able to determine thresholds in each cohort above which diabetes risk was potentially increased (France: 1003±42 pL, Germany: 798±32 pL). Subjects above those adipocyte thresholds were less prone to disappearance of the DRD status after bypass surgery (France, risk ratio = 2.1, P = .024; Germany, risk ratio = 1.3, P = .05).

CONCLUSIONS

We show in two cohorts of morbidly obese subjects that a specific adipocyte volume threshold may predict an increased risk for obesity-associated type 2 diabetes. However, this threshold might be established for each specific investigation site. Having a high adipocyte size is associated with a lower improvement of insulin resistance after bypass surgery in both cohorts.

Adipose tissue CIDEA is associated, independently of weight variation, to change in insulin resistance during a longitudinal weight control dietary program in obese individuals

AIM

Weight loss reduces risk factors associated with obesity. However, long-term metabolic improvement remains a challenge. We investigated quantitative gene expression of subcutaneous adipose tissue in obese individuals and its relationship with low calorie diet and long term weight maintenance induced changes in insulin resistance.

RESEARCH DESIGN

Three hundred eleven overweight and obese individuals followed a dietary protocol consisting of an 8-week low calorie diet followed by a 6-month ad libitum weight-maintenance diet. Individuals were clustered according to insulin resistance trajectories assessed using homeostasis model assessment of insulin resistance (HOMA-IR) index. Adipose tissue mRNA levels of 267 genes selected for regulation according to obesity, metabolic status and response to dieting was assessed using high throughput RT-qPCR. A combination of discriminant analyses was used to identify genes with regulation according to insulin resistance trajectories. Partial correlation was used to control for change in body mass index.

RESULTS

Three different HOMA-IR profile groups were determined. HOMA-IR improved during low calorie diet in the 3 groups. At the end of the 6-month follow-up, groups A and B had reduced HOMA-IR by 50%. In group C, HOMA-IR had returned to baseline values. Genes were differentially expressed in the adipose tissue of individuals according to groups but a single gene, CIDEA, was common to all phases of the dietary intervention. Changes in adipose tissue CIDEA mRNA levels paralleled variations in insulin sensitivity independently of change in body mass index. Overall, CIDEA was up-regulated in adipose tissue of individuals with successful long term insulin resistance relapse and not in adipose tissue of unsuccessful individuals.

CONCLUSION

The concomitant change in adipose tissue CIDEA mRNA levels and insulin sensitivity suggests a beneficial role of adipose tissue CIDEA in long term glucose homeostasis, independently of weight variation.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00390637.