Mechanisms of fatty acid-induced inhibition of glucose uptake

Obesity, insulin resistance and free fatty acids

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

An oral lipid challenge and acute intake of caffeinated coffee additively decrease glucose tolerance in healthy men

Lipid-induced insulin resistance has been investigated primarily with i.v. infusions, and caffeine-induced insulin resistance, with alkaloid caffeine. The effects of orally consumed lipids and coffee have not been established and to our knowledge have never been simultaneously investigated. The goals of this study were to determine whether an oral lipid challenge and caffeinated coffee would disrupt glucose homeostasis and to characterize their respective incretin responses. It was hypothesized that oral ingestion of saturated lipids would impair glucose tolerance and that caffeinated coffee would further hinder glucose management. Ten young, healthy males participated in 5 trials in a randomized, cross-over design. At time 0 h, they underwent an oral fat tolerance test (OFTT: 1 g lipid/kg body weight) or consumed water, followed 5 h later by caffeinated (5 mg/kg) coffee, decaffeinated coffee, or water. At 6 h, volunteers underwent an oral glucose tolerance test (OGTT). Consumption of the OFTT increased glucose concentrations (P < 0.05) after a subsequent OGTT. At 7 h, caffeinated coffee produced the highest glucose concentrations (P < 0.05). Glucagon-like peptide-1 active (GLP-1a) and glucose-dependent insulinotropic polypeptide (GIP) were both increased for up to 6 h in all OFTT trials (P < 0.05). Compared to all other treatments, caffeinated and decaffeinated coffee produced higher GLP-1a response at 6.25 h (P < 0.05), whereas only caffeinated coffee increased GIP secretion (P < 0.05). These results show that oral consumption of lipids and caffeinated coffee can independently and additively decrease glucose tolerance. Incretin hormones could explain at least in part this impaired glucose homeostasis.

Gastric bypass surgery is associated with near-normal insulin suppression of lipolysis in nondiabetic individuals

We hypothesized that individuals who have undergone gastric bypass have greater insulin sensitivity that obese subjects but less compared with lean. We measured free fatty acid (FFA) and glucose kinetics during a two-step, hyperinsulinemic euglycemic clamp in nondiabetic subjects who were 38 ± 5 mo post-gastric bypass surgery (GB; n = 15), in lean subjects (L; n = 15), and in obese subjects (O; n = 16). Fasting FFAa were not significantly different between the three study groups but during both doses of insulin were significantly higher in O than in either GB or L. The effective insulin concentration resulting in half-maximal suppression of FFA was similar in L and GB and significantly less in both groups compared with O. Glucose infusion rates during low-dose insulin were not significantly different in GB compared with either L or O. During high-dose insulin, glucose infusion rates were significantly greater in GB than in O but less than in L. Endogenous glucose production in GB was significantly lower than O only during low dose of insulin. We conclude that gastric bypass is associated with improvements in adipose tissue insulin sensitivity to levels similar to lean, healthy persons and also with improvements in the response of glucose metabolism to insulin. These changes may be due to preferential reduction in visceral fat and decreased FFA availability. However, some differences in insulin sensitivity in GB remain compared with L. Residual insulin resistance may be related to excess total body fat or abnormal lipolysis and requires further study.

Acute high-fat feeding does not prevent the improvement in glucose tolerance after resistance exercise in lean individuals

Our first aim was to investigate whether the ingestion of a single high-fat meal impairs glucose tolerance. Our second aim was to investigate whether improvements in glucose tolerance that are seen after resistance exercise remain when exercise is performed after ingestion of a high-fat meal. Eight young males consumed either a high fat (HF) or an isocaloric control (CON) meal in the morning and underwent an oral glucose tolerance test (OGTT) 6 h later. On two other occasions, a single 1 h bout of resistance exercise was completed 2 h after consumption of each meal (HFE and CONE). There were no significant differences in plasma glucose and plasma insulin areas under the curve (AUC) or estimates of insulin sensitivity between the HF and CON trials (P > 0.05). The HFE and CONE trials elicited a ~20% lower plasma glucose AUC (P < 0.05) compared to their respective control trials. The HFE also elicited a ~25% lower plasma insulin AUC (P < 0.05) in comparison to the HF trial. The HFE trial also significantly improved estimates of insulin sensitivity in comparison to the HF condition (P < 0.05). In conclusion, this study demonstrates that consumption of a single HF meal does not impair glucose tolerance in the resting state in lean individuals and that an acute bout of resistance exercise remains effective in enhancing glucose tolerance following the ingestion of a single high-fat meal.

Dyslipidemia in the elderly: should it be treated?

Elderly or older adults constitute a rapidly growing segment of the United States population, thus resulting in an increase in morbidity and mortality related to cardiovascular disease-an increase that is reaching epidemic proportions. Dyslipidemia is a well established risk factor for cardiovascular disease and is estimated to account for more than half of the global cases of coronary artery disease. Despite the increased prevalence of dyslipidemia in the older adult population, controversy persists regarding the benefits of treatment in this group. Epidemiologic studies have shown that dyslipidemia is often underdiagnosed and under treated in this population probably as a result of a paucity of evidence regarding the impact of treatment in delaying the progression of atherosclerotic disease, concerns involving increased likelihood of adverse events or drug interactions, or doubts regarding the cost effectiveness of lipid-lowering therapy in older adults. In conclusion, despite the proven efficacy of lipid-lowering therapy in decreasing cardiovascular morbidity and mortality, these therapies have been underutilized in older patients.

Discovery of new inhibitor for PDE3 by virtual screening

In this work, we tried to find a new scaffold for a PDE3 using virtual screening for the obesity treatment. We first analyzed structural features for the known PDE3 inhibitors based on the PDE3B-ligand complex structure, and then carried out a docking study based on PDE3B 3D structure. We obtained a compound as potent PDE3 inhibitor stimulating lipolysis in murine adipocytes and human adipocytes.

Brain insulin controls adipose tissue lipolysis and lipogenesis

White adipose tissue (WAT) dysfunction plays a key role in the pathogenesis of type 2 diabetes (DM2). Unrestrained WAT lipolysis results in increased fatty acid release, leading to insulin resistance and lipotoxicity, while impaired de novo lipogenesis in WAT decreases the synthesis of insulin-sensitizing fatty acid species like palmitoleate. Here, we show that insulin infused into the mediobasal hypothalamus (MBH) of Sprague-Dawley rats increases WAT lipogenic protein expression, inactivates hormone-sensitive lipase (Hsl), and suppresses lipolysis. Conversely, mice that lack the neuronal insulin receptor exhibit unrestrained lipolysis and decreased de novo lipogenesis in WAT. Thus, brain and, in particular, hypothalamic insulin action play a pivotal role in WAT functionality.

Nonoxidative free fatty acid disposal is greater in young women than men

CONTEXT

Large increases in systemic free fatty acid (FFA) availability in the absence of a corresponding increase in fatty acid oxidation can create a host of metabolic abnormalities. These adverse responses are thought to be the result of fatty acids being shunted into hepatic very low-density lipoprotein-triglyceride production and/or intracellular lipid storage and signaling pathways because tissues are forced to increase nonoxidative FFA disposal.

OBJECTIVE

The objective of the study was to examine whether variations in postabsorptive nonoxidative FFA disposal within the usual range predict insulin resistance and hypertriglyceridemia.

DESIGN

We measured: systemic FFA turnover using a continuous iv infusion of [9-10, (3)H]palmitate; substrate oxidation with indirect calorimetry combined with urinary nitrogen excretion; whole-body and peripheral insulin sensitivity with the labeled iv glucose tolerance test minimal model.

SETTING

the study was conducted at the Mayo Clinic General Clinical Research Center.

PARTICIPANTS

Participants included healthy, postabsorptive, nonobese adults (21 women and 21 men).

INTERVENTIONS

There were no interventions.

MAIN OUTCOME MEASURES

Nonoxidative FFA disposal (micromoles per minute), defined as the FFA disappearance rate minus fatty acid oxidation.

RESULTS

Women had 64% greater nonoxidative FFA disposal rate than men but a better lipid profile and similar insulin sensitivity. There was no significant correlation between nonoxidative FFA disposal and whole-body sensitivity, peripheral insulin sensitivity, or fasting serum triglyceride concentrations in men or women.

CONCLUSIONS

Healthy nonobese women have greater rates of nonoxidative FFA disposal than men, but this does not appear to relate to adverse health consequences. Understanding the sex-specific interaction between adipose tissue lipolysis and peripheral FFA removal will help to discover new approaches to treat FFA-induced abnormalities.

Ectopic fat storage in the pancreas, liver, and abdominal fat depots: impact on β-cell function in individuals with impaired glucose metabolism

CONTEXT

Pancreatic fat content (PFC) may have deleterious effects on β-cell function.

OBJECTIVE

We hypothesized that ectopic fat deposition, in particular pancreatic fat accumulation, is related to β-cell dysfunction in individuals with impaired fasting glucose (IFG) and/or impaired glucose tolerance (IGT).

DESIGN, SETTING AND PARTICIPANTS

This was a cross-sectional study in 64 age- and body mass index-matched individuals, with normal glucose tolerance (NGT; n = 16, 60% males), IFG (n = 29, 52% males), or IFG/IGT (n = 19, 63% males) was conducted.

INTERVENTION AND MAIN OUTCOME MEASURES

Participants underwent the following: 1) a combined hyperinsulinemic-euglycemic and hyperglycemic clamp, with subsequent arginine stimulation to quantify insulin sensitivity and β-cell function; 2) proton-magnetic resonance spectroscopy to assess PFC and liver fat content (LFC); and 3) magnetic resonance imaging to quantify visceral (VAT) and sc (SAT) adipose tissue. The disposition index (DI; insulin sensitivity adjusted β-cell function) was assessed.

RESULTS

IFG and IFG/IGT were more insulin resistant (P < 0.001) compared with NGT. Individuals with IFG/IGT had the lowest values of glucose- and arginine-stimulated C-peptide secretion (both P < 0.03) and DI (P < 0.001), relative to IFG and NGT. PFC and LFC gradually increased between NGT, IFG, and IFG/IGT (P = 0.02 and P = 0.01, respectively), whereas VAT and SAT were similar between groups. No direct associations were found between PFC, LFC, VAT, and SAT and C-peptide secretion. The DI was inversely correlated with PFC, LFC, and VAT (all P < 0.05).

CONCLUSIONS

PFC was increased in individuals with IFG and/or IGT, without a direct relation with β-cell function.

Lipid-induced pancreatic β-cell dysfunction: focus on in vivo studies

The phenomenon of lipid-induced pancreatic β-cell dysfunction ("lipotoxicity") has been very well documented in numerous in vitro experimental systems and has become widely accepted. In vivo demonstration of β-cell lipotoxicity, on the other hand, has not been consistently demonstrated, and there remains a lack of consensus regarding the in vivo effects of chronically elevated free fatty acids (FFA) on β-cell function. Much of the disagreement relates to how insulin secretion is quantified in vivo and in particular whether insulin secretion is assessed in relation to whole body insulin sensitivity, which is clearly reduced by elevated FFA. By correcting for changes in in vivo insulin sensitivity, we and others have shown that prolonged elevation of FFA impairs β-cell secretory function. Prediabetic animal models and humans with a positive family history of type 2 diabetes are more susceptible to this impairment, whereas those with severe impairment of β-cell function (such as individuals with type 2 diabetes) demonstrate no additional impairment of β-cell function when FFA are experimentally raised. Glucolipotoxicity (i.e., the combined β-cell toxicity of elevated glucose and FFA) has been amply demonstrated in vitro and in some animal studies but not in humans, perhaps because there are limitations in experimentally raising plasma glucose to sufficiently high levels for prolonged periods of time. We and others have shown that therapies directed toward diminishing oxidative stress and ER stress have the potential to reduce lipid-induced β-cell dysfunction in animals and humans. In conclusion, lipid-induced pancreatic β-cell dysfunction is likely to be one contributor to the complex array of genetic and metabolic insults that result in the relentless decline in pancreatic β-cell function in those destined to develop type 2 diabetes, and mechanisms involved in this lipotoxicity are promising therapeutic targets.

A newly identified CG301269 improves lipid and glucose metabolism without body weight gain through activation of peroxisome proliferator-activated receptor alpha and gamma

OBJECTIVE

Peroxisome proliferator-activated receptor (PPAR)-α/γ dual agonists have been developed to alleviate metabolic disorders. However, several PPARα/γ dual agonists are accompanied with unwanted side effects, including body weight gain, edema, and tissue failure. This study investigated the effects of a novel PPARα/γ dual agonist, CG301269, on metabolic disorders both in vitro and in vivo.

RESEARCH DESIGN AND METHODS

Function of CG301269 as a PPARα/γ dual agonist was assessed in vitro by luciferase reporter assay, mammalian one-hybrid assay, and analyses of PPAR target genes. In vitro profiles on fatty acid oxidation and inflammatory responses were acquired by fatty acid oxidation assay and quantitative (q)RT-PCR of proinflammatory genes. In vivo effect of CG301269 was examined in db/db mice. Total body weight and various tissue weights were measured, and hepatic lipid profiles were analyzed. Systemic glucose and insulin tolerance were measured, and the in vivo effect of CG301269 on metabolic genes and proinflammatory genes was examined by qRT-PCR.

RESULTS

CG301269 selectively stimulated the transcriptional activities of PPARα and PPARγ. CG301269 enhanced fatty acid oxidation in vitro and ameliorated insulin resistance and hyperlipidemia in vivo. In db/db mice, CG301269 reduced inflammatory responses and fatty liver, without body weight gain.

CONCLUSIONS

We demonstrate that CG301269 exhibits beneficial effects on glucose and lipid metabolism by simultaneous activation of both PPARα and PPARγ. Our data suggest that CG301269 would be a potential lead compound against obesity and related metabolic disorders.

Reversible high affinity inhibition of phosphofructokinase-1 by acyl-CoA: a mechanism integrating glycolytic flux with lipid metabolism

The enzyme phosphofructokinase-1 (PFK-1) catalyzes the first committed step of glycolysis and is regulated by a complex array of allosteric effectors that integrate glycolytic flux with cellular bioenergetics. Here, we demonstrate the direct, potent, and reversible inhibition of purified rabbit muscle PFK-1 by low micromolar concentrations of long chain fatty acyl-CoAs (apparent Ki∼1 μM). In sharp contrast, short chain acyl-CoAs, palmitoylcarnitine, and palmitic acid in the presence of CoASH were without effect. Remarkably, MgAMP and MgADP but not MgATP protected PFK-1 against inhibition by palmitoyl-CoA indicating that acyl-CoAs regulate PFK-1 activity in concert with cellular high energy phosphate status. Furthermore, incubation of PFK-1 with [1-(14)C]palmitoyl-CoA resulted in robust acylation of the enzyme that was reversible by incubation with acyl-protein thioesterase-1 (APT1). Importantly, APT1 reversed palmitoyl-CoA-mediated inhibition of PFK-1 activity. Mass spectrometric analyses of palmitoylated PFK-1 revealed four sites of acylation, including Cys-114, Cys-170, Cys-351, and Cys-577. PFK-1 in both skeletal muscle extracts and in purified form was inhibited by S-hexadecyl-CoA, a nonhydrolyzable palmitoyl-CoA analog, demonstrating that covalent acylation of PFK-1 was not required for inhibition. Tryptic footprinting suggested that S-hexadecyl-CoA induced a conformational change in PFK-1. Both palmitoyl-CoA and S-hexadecyl-CoA increased the association of PFK-1 with Ca2+/calmodulin, which attenuated the binding of palmitoylated PFK-1 to membrane vesicles. Collectively, these results demonstrate that fatty acyl-CoA modulates phosphofructokinase activity through both covalent and noncovalent interactions to regulate glycolytic flux and enzyme membrane localization via the branch point metabolic node that mediates lipid flux through anabolic and catabolic pathways.

Measurement of the increase in endoplasmic reticulum stress-related proteins and genes in adipose tissue of obese, insulin-resistant individuals

Here, we provide a detailed description of proteomic, Western blot and RT-PCR analyses performed to examine fat biopsy samples from lean insulin-sensitive and obese insulin-resistant nondiabetic individuals for evidence of endoplasmic reticulum (ER) stress. Subcutaneous fat biopsies were obtained from the upper thighs of six lean and six obese nondiabetic subjects. Fat homogenates were used for proteomic (two-dimensional gel (2DE) and MALDI-TOF/TOF), Western blot, and RT-PCR analysis. Proteomic analysis revealed 19 differentially upregulated proteins in fat of obese subjects. Three of these proteins were the ER stress-related unfolded protein response (UPR) proteins calreticulin, protein disulfide-isomerase A3, and glutathione-S-transferase P; Western blotting revealed upregulation of several other UPR stress-related proteins, including calnexin, a membrane-bound chaperone, and phospho c-jun NH(2)-terminal kinase (JNK)-1, a downstream effector protein of ER stress; RT-PCR analysis revealed upregulation of the spliced form of X-box-binding protein-1s, a potent transcription factor and part of the proximal ER stress sensor inositol-requiring enzyme-1 pathway. These findings demonstrate of UPR activation in subcutaneous adipose tissue of obese human subjects. As JNK can inhibit insulin action and activate proinflammatory pathways, ER stress activation of JNK may be a link between obesity, insulin resistance, and inflammation.

Lipid-induced insulin resistance affects women less than men and is not accompanied by inflammation or impaired proximal insulin signaling

OBJECTIVE

We have previously shown that overnight fasted women have higher insulin-stimulated whole body and leg glucose uptake despite a higher intramyocellular triacylglycerol concentration than men. Women also express higher muscle mRNA levels of proteins related to lipid metabolism than men. We therefore hypothesized that women would be less prone to lipid-induced insulin resistance.

RESEARCH DESIGN AND METHODS

Insulin sensitivity of whole-body and leg glucose disposal was studied in 16 young well-matched healthy men and women infused with intralipid or saline for 7 h. Muscle biopsies were obtained before and during a euglycemic-hyperinsulinemic clamp (1.42 mU · kg⁻¹ · min⁻¹).

RESULTS

Intralipid infusion reduced whole-body glucose infusion rate by 26% in women and 38% in men (P < 0.05), and insulin-stimulated leg glucose uptake was reduced significantly less in women (45%) than men (60%) after intralipid infusion. Hepatic glucose production was decreased during the clamp similarly in women and men irrespective of intralipid infusion. Intralipid did not impair insulin or AMPK signaling in muscle and subcutaneous fat, did not cause accumulation of muscle lipid intermediates, and did not impair insulin-stimulated glycogen synthase activity in muscle or increase plasma concentrations of inflammatory cytokines. In vitro glucose transport in giant sarcolemmal vesicles was not decreased by acute exposure to fatty acids. Leg lactate release was increased and respiratory exchange ratio was decreased by intralipid.

CONCLUSIONS

Intralipid infusion causes less insulin resistance of muscle glucose uptake in women than in men. This insulin resistance is not due to decreased canonical insulin signaling, accumulation of lipid intermediates, inflammation, or direct inhibition of GLUT activity. Rather, a higher leg lactate release and lower glucose oxidation with intralipid infusion may suggest a metabolic feedback regulation of glucose metabolism.

Adipose triglyceride lipase-null mice are resistant to high-fat diet-induced insulin resistance despite reduced energy expenditure and ectopic lipid accumulation

Adipose triglyceride lipase (ATGL) null (-/-) mice store vast amounts of triacylglycerol in key glucoregulatory tissues yet exhibit enhanced insulin sensitivity and glucose tolerance. The mechanisms underpinning these divergent observations are unknown but may relate to the reduced availability of circulating fatty acids. The aim of this study was to determine whether the enhancements in insulin stimulated glucose metabolism in ATGL-/- mice persist when challenged with a high-fat diet. ATGL-/- mice fed a low-fat diet exhibit improved whole-body insulin sensitivity and glucose tolerance compared with wild-type mice. Wild-type mice became hyperlipidemic and insulin-resistant when challenged with a high-fat diet (HFD, 60% fat) for 4 wk. ATGL-/- mice fed a HFD had elevated circulating fatty acids but had reduced fasting glycemia compared to pre-high-fat diet levels and were refractory to glucose intolerance and insulin resistance. This protection from high-fat diet-induced metabolic perturbations was associated with a preference for fatty acid utilization but reduced energy expenditure and no change in markers of mitochondrial capacity or density. The protection from high-fat diet-induced insulin resistance in ATGL-/- mice was due to increased cardiac and liver insulin-stimulated glucose clearance despite increased lipid content in these tissues. Additionally, there was no difference in skeletal muscle insulin-stimulated glucose disposal, but there was a reduction observed in brown adipose tissue. Overall, these results show that ATGL-/- mice are protected from HFD-induced insulin resistance and reveal a tissue specific disparity between lipid accumulation and insulin sensitivity.

Effects of Mixed Isoenergetic Meals on Fat and Carbohydrate Metabolism during Exercise in Older Men

The present study was designed to investigate the effects of four different meals on fat and CHO metabolism during subsequent exercise in elderly males. Eight healthy males (age: 63.3 ± 5.2 years) reported to the physiology laboratory on four separate occasions, each of which was allocated for the performance of a 30-minute exercise on a cycle ergometer at 60% after having normal (N), high fat (HF), high carbohydrate high glycaemic index (HGI) and high carbohydrate low glycaemic index (LGI) meals. Fat oxidation during exercise after the meals ( g/min;  g/min;  g/min;  g/min) was not significant (), and neither were the rates of carbohydrate oxidation (, , , and  g/m). NEFA concentration increased after HF () but decreased after HGI and LGI (). Glucose concentration decreased as a result of exercise after HF, and LGI () whereas insulin concentration decreased significantly during exercise after N, HF, and HGI (). It can be concluded that, in elderly males, feeding isoenergetic meals containing different proportions of carbohydrate and fat do not significantly alter oxidation of fat and CHO during exercise in spite of changes in some circulating metabolites.

Effects of oral and intravenous fat load on blood pressure, endothelial function, sympathetic activity, and oxidative stress in obese healthy subjects

We compared the effects of high and low oral and intravenous (iv) fat load on blood pressure (BP), endothelial function, autonomic nervous system, and oxidative stress in obese healthy subjects. Thirteen obese subjects randomly received five 8-h infusions of iv saline, 20 (32 g, low iv fat) or 40 ml/h intralipid (64 g, high iv fat), and oral fat load at 32 (low oral) or 64 g (high oral). Systolic BP increased by 14 ± 10 (P = 0.007) and 12 ± 9 mmHg (P = 0.007) after low and high iv lipid infusions and by 13 ± 17 (P = 0.045) and 11 ± 11 mmHg (P = 0.040) after low and high oral fat loads, respectively. The baseline flow-mediated dilation was 9.4%, and it decreased by 3.8 ± 2.1 (P = 0.002) and 4.1 ± 3.1% (P < 0.001) after low and high iv lipid infusion and by 3.8 ± 1.8 (P = 0.002) and 5.0 ± 2.5% (P < 0.001) after low and high oral fat load, respectively. Oral and iv fat load stimulated oxidative stress, increased heart rate, and decreased R-R interval variability. Acute iv fat load decreased blood glucose by 6-10 mg/dl (P < 0.05) without changes in insulin concentration, whereas oral fat increased plasma insulin by 3.7-4.0 μU/ml (P < 0.01) without glycemic variations. Intravenous saline and both oral and iv fat load reduced leptin concentration from baseline (P < 0.01). In conclusion, acute fat load administered orally or intravenously significantly increased blood pressure, altered endothelial function, and activated sympathetic nervous system by mechanisms not likely depending on changes in leptin, glucose, and insulin levels in obese healthy subjects. Thus, fat load, independent of its source, has deleterious hemodynamic effects in obese subjects.

Insulin resistance, serum leptin, and adiponectin levels and outcomes of viral hepatitis C cirrhosis

BACKGROUND & AIMS

Mechanisms linking obesity and unfavourable outcomes in patients with viral hepatitis C (HCV) cirrhosis are not well understood. Obesity is associated with insulin resistance, increased leptin, and decreased adiponectin serum levels.

METHODS

We assessed the predictive value of those factors for the occurrence of hepatocellular carcinoma (HCC) and liver-related death or transplantation in a cohort of 248 patients (mean age 58 (12 years, BMI 25.4 ± 4.4 kg/m(2)) with compensated HCV cirrhosis and persistent infection prospectively followed and screened for HCC.

RESULTS

The mean baseline serum levels of adiponectin and leptin were 16.8 ± 15 mg/L and 16.8 ± 19 ng/ml, respectively. The mean homeostasis model assessment of insulin resistance (HOMA) index was 3.8 ± 3; median 2.9. After a median follow-up of 72 months, 61 patients developed HCC, 58 died of liver causes, and 17 were transplanted. The incidences (Kaplan Meier) of HCC were 7%, 18%, and 27% at 5 years (p=0.017) and of liver-related death or transplantation 15%, 15% and 29% (p=0.002) according to the lowest, middle and highest tertile of HOMA, respectively. In multivariate analysis, the HOMA index was associated with HCC occurrence (HR=1.10, [1.01-1.21] p=0.026) and was a strong predictor of liver-related death or transplantation (HR=1.13, [1.07-1.21] p<0.0001). Serum levels of adiponectin and leptin were not associated with the outcome.

CONCLUSIONS

In patients with compensated HCV cirrhosis, insulin resistance but not serum levels of adiponectin and leptin predicted the occurrence of HCC and of liver-related death or transplantation.

Metformin regulates palmitate-induced apoptosis and ER stress response in HepG2 liver cells

The excessive supply of fatty acids to the liver contributes to hepatic insulin resistance and endoplasmic reticulum (ER) stress associated with obesity or type 2 diabetes mellitus. Furthermore, excess and/or prolonged ER stress contributes to hepatic cell death deteriorating nonalcoholic fatty liver disease to steatohepatitis. The aim of this study was to investigate the effects of metformin on palmitate-induced ER stress and hepatic insulin resistance in HepG2 cells. Metformin significantly inhibited palmitate-induced cell death and apoptosis via caspase-3 activation. Metformin also blocked the induction of ER stress proteins (GRP78, Chop, Cleaved ATF-6, p-eIF2 alpha and XBP-1) and regulated serine phosphorylation of IRS-1. Metformin may therefore protect hepatocytes from death induced by saturated fatty acids. These data may also provide a further rationale for exploring the use of metformin in the treatment of non-alcoholic fatty liver disease, revealing its blocking effect for hepatic insulin resistance evoked by saturated fatty acids.

Relationship of mid-thigh adiposity to the metabolic syndrome in postmenopausal women

BACKGROUND

In postmenopausal women, a population at risk for the metabolic syndrome, the relative contribution of central fat versus peripheral muscle fat to the metabolic risk profile is unknown. This study explored the relationship between muscle fat infiltration derived from computed tomography (CT) scans and metabolic syndrome.

METHODS

Mid-thigh CT scans measured the surface of muscle with low attenuation (LAMS) [0-34 Hounsfield units (HU)], which represented the specific component of fat-rich muscle. Insulin sensitivity was evaluated by an euglycemic-hyperinsulinemic clamp. National Cholesterol Education Program Adult Treatment Panel III (NCEP ATP III) criteria were used to determine the presence of the metabolic syndrome.

RESULTS

A total of 103 postmenopausal women were studied. Metabolic syndrome was found in 43 women with significantly higher levels of abdominal adiposity, higher LAMS (27 +/- 8 vs. 23 +/- 7 cm(2)), and lower insulin sensitivity compared to those without the metabolic syndrome. Women with higher levels of LAMS presented higher metabolic risk features such as higher blood pressure, abdominal adiposity, inflammatory markers, and blood lipid levels. LAMS and visceral adipose tissue correlated significantly with the presence of metabolic syndrome, but these relationships were lost when LAMS was adjusted for visceral adipose tissue but not when visceral adipose tissue was adjusted for LAMS.

CONCLUSIONS

These results suggest that postmenopausal women who present with metabolic syndrome had increased fat-rich mid-thigh muscle. Moreover, women with more fat-rich muscle had many features of the metabolic syndrome. These relations were weakened when visceral adipose tissue was taken into account suggesting that LAMS may play a relatively smaller role, compared to VAT, in the contribution to the metabolic syndrome.

Intensive insulin therapy improves insulin sensitivity and mitochondrial function in severely burned children

OBJECTIVE

To institute intensive insulin therapy protocol in an acute pediatric burn unit and study the mechanisms underlying its benefits.

DESIGN

Prospective, randomized study.

SETTING

An acute pediatric burn unit in a tertiary teaching hospital.

PATIENTS

Children, 4-18 yrs old, with total body surface area burned > or =40% and who arrived within 1 wk after injury were enrolled in the study.

INTERVENTIONS

Patients were randomized to one of two groups. Intensive insulin therapy maintained blood glucose levels between 80 and 110 mg/dL. Conventional insulin therapy maintained blood glucose < or =215 mg/dL.

MEASUREMENTS AND MAIN RESULTS

Twenty patients were included in the data analysis consisting of resting energy expenditure, whole body and liver insulin sensitivity, and skeletal muscle mitochondrial function. Studies were performed at 7 days postburn (pretreatment) and at 21 days postburn (posttreatment). Resting energy expenditure significantly increased posttreatment (1476 +/- 124 to 1925 +/- 291 kcal/m(2) x day; p = .02) in conventional insulin therapy as compared with a decline in intensive insulin therapy. Glucose infusion rate was identical between groups before treatment (6.0 +/- 0.8 conventional insulin therapy vs. 6.8 +/- 0.9 mg/kg x min intensive insulin therapy; p = .5). Intensive insulin therapy displayed a significantly higher glucose clamp infusion rate posttreatment (9.1 +/- 1.3 intensive insulin therapy versus 4.8 +/- 0.6 mg/kg x min conventional insulin therapy, p = .005). Suppression of hepatic glucose release was significantly greater in the intensive insulin therapy after treatment compared with conventional insulin therapy (5.0 +/- 0.9 vs. 2.5 +/- 0.6 mg/kg x min; intensive insulin therapy vs. conventional insulin therapy; p = .03). States 3 and 4 mitochondrial oxidation of palmitate significantly improved in intensive insulin therapy (0.9 +/- 0.1 to 1.7 +/- 0.1 microm O(2)/CS/mg protein/min for state 3, p = .004; and 0.7 +/- 0.1 to 1.3 +/- 0.1 microm O(2)/CS/mg protein/min for state 4, p < .002), whereas conventional insulin therapy remained at the same level of activity (0.9 +/- 0.1 to 0.8 +/- 0.1 microm O(2)/CS/mg protein/min for state 3, p = .4; 0.6 +/- 0.03 to 0.7 +/- 0.1 microm O(2)/CS/mg protein/min, p = .6).

CONCLUSION

Controlling blood glucose levels < or =120 mg/dL using an intensive insulin therapy protocol improves insulin sensitivity and mitochondrial oxidative capacity while decreasing resting energy expenditure in severely burned children.

Metabolic actions of insulin in men and women

Insulin is an important regulator of glucose, lipid, and protein metabolism. It suppresses hepatic glucose and triglyceride production, inhibits adipose tissue lipolysis and whole-body and muscle proteolysis, and stimulates glucose uptake in muscle. In this review we discuss what is currently known about the control of substrate metabolism by insulin in men and women. The data available so far indicate that women are more sensitive to insulin with regards to glucose metabolism (both in the liver and in muscle), whereas there are no differences between men and women in insulin action on lipolysis. Potential differences exist in the regulation of plasma triglyceride concentration and protein metabolism by insulin and in changes in insulin action in response to stimuli (e.g., weight loss and exercise) that are known to alter insulin sensitivity. However, these areas have not been studied comprehensively enough to draw firm conclusions.

Lipid-induced insulin resistance: unravelling the mechanism

Insulin resistance has long been associated with obesity. More than 40 years ago, Randle and colleagues postulated that lipids impaired insulin-stimulated glucose use by muscles through inhibition of glycolysis at key points. However, work over the past two decades has shown that lipid-induced insulin resistance in skeletal muscle stems from defects in insulin-stimulated glucose transport activity. The steatotic liver is also resistant to insulin in terms of inhibition of hepatic glucose production and stimulation of glycogen synthesis. In muscle and liver, the intracellular accumulation of lipids-namely, diacylglycerol-triggers activation of novel protein kinases C with subsequent impairments in insulin signalling. This unifying hypothesis accounts for the mechanism of insulin resistance in obesity, type 2 diabetes, lipodystrophy, and ageing; and the insulin-sensitising effects of thiazolidinediones.

Computational model of cellular metabolic dynamics: effect of insulin on glucose disposal in human skeletal muscle

Identifying the mechanisms by which insulin regulates glucose metabolism in skeletal muscle is critical to understanding the etiology of insulin resistance and type 2 diabetes. Our knowledge of these mechanisms is limited by the difficulty of obtaining in vivo intracellular data. To quantitatively distinguish significant transport and metabolic mechanisms from limited experimental data, we developed a physiologically based, multiscale mathematical model of cellular metabolic dynamics in skeletal muscle. The model describes mass transport and metabolic processes including distinctive processes of the cytosol and mitochondria. The model simulated skeletal muscle metabolic responses to insulin corresponding to human hyperinsulinemic-euglycemic clamp studies. Insulin-mediated rate of glucose disposal was the primary model input. For model validation, simulations were compared with experimental data: intracellular metabolite concentrations and patterns of glucose disposal. Model variations were simulated to investigate three alternative mechanisms to explain insulin enhancements: Model 1 (M.1), simple mass action; M.2, insulin-mediated activation of key metabolic enzymes (i.e., hexokinase, glycogen synthase, pyruvate dehydrogenase); or M.3, parallel activation by a phenomenological insulin-mediated intracellular signal that modifies reaction rate coefficients. These simulations indicated that models M.1 and M.2 were not sufficient to explain the experimentally measured metabolic responses. However, by application of mechanism M.3, the model predicts metabolite concentration changes and glucose partitioning patterns consistent with experimental data. The reaction rate fluxes quantified by this detailed model of insulin/glucose metabolism provide information that can be used to evaluate the development of type 2 diabetes.

Aging per se increases the susceptibility to free fatty acid-induced insulin resistance

Elevations in systemic free fatty acids (FFA) contribute to insulin resistance. To determine the effects of an acute elevation in FFA on insulin action with aging, we infused saline or intralipid (IL) during a hyperinsulinemic-euglycemic clamp in three groups of rats: young ad libitum-fed (YAL), old ad libitum-fed (OAL), and old on lifelong calorie restriction (OCR). The OCR group was included to distinguish between aging per se and age-related changes in body fat distribution. IL induced marked insulin resistance in both YAL and OCR, but the onset of insulin resistance was approximately two to three times more rapid in OCR as compared with YAL. In response to IL infusion, plasminogen-activating inhibitor-1 (PAI-1) expression was increased in subcutaneous fat from OAL animals. In visceral fat, a marked increase in PAI-1 and interleukin-6 expression was observed in OAL and OCR rats, but not YAL, in response to IL treatment. Thus, aging per se increases the inflammatory response to excess nutrients and vulnerability to FFA-induced insulin resistance with aging.

Leukocyte peroxidase and leptin: an associated link of glycemic tolerance and bronchial asthma?

Recent observations suggest the presence of an interaction between leptin and the inflammatory system during bronchial asthma. Although there is evidence of a positive association between asthma and obesity in adults and children, little is yet known about the role of serum leptin, as a potential mediator for bronchial epithelial homeostasis, and intraleukocyte myeloperoxidase (MPO), a hemoprotein with a molecular weight of 140 kDa, expression of the inflammatory system, in asthmatic children. Glycemic tolerance is an important pathogenetic element in developing type 2 mellitus diabetes and a confirmed predictor of incident asthma-like symptoms in adults. This work is aimed at assessing a possible correlation between basal leukocyte myeloperoxidase levels, basal leptin and insulin-glycemic tolerance in obese children. Thirty obese children aged between 7 and 15 years were examined. The analyzed data showed a normal response to the insulinemic stimulus in children of both sexes whose basal leptin and MPO values, expressed as MPO intracellular index, were within the normal range.

Twenty-eight-day bed rest with hypercortisolemia induces peripheral insulin resistance and increases intramuscular triglycerides

Spaceflight represents a unique physiologic challenge to humans, altering hormonal profiles and tissue insulin sensitivity. Among these hormonal alterations, hypercortisolemia and insulin insensitivity are thought to negatively affect muscle mass and function with spaceflight. As insulin sensitivity influences the accumulation of muscle triglycerides, we examined this relationship during hypercortisolemia and inactivity. Six young healthy volunteers were confined to bed rest for 28 days. To mimic the stress response observed during spaceflight, hypercortisolemia (20-24 mg/dL) was induced and maintained by oral ingestion of hydrocortisone. On days 1 and 28 of bed rest, insulin sensitivity across the leg was assessed with a local (femoral arterial insulin infusion) 2-stage hyperinsulinemic-euglycemic clamp (stage 1, 35 microU/min per milliliter of leg; stage 2, 70 microU/min per milliliter of leg). Intramuscular lipid was measured with magnetic resonance spectroscopy. After bed rest, there was a decrease in insulin sensitivity, as assessed by glucose uptake during hyperinsulinemia (from 9.1 +/- 1.3 [mean +/- SEM] to 5.2 +/- 0.7 mg/kg of leg per minute [P = .015]). Intramuscular triglyceride increased from 0.077 +/- 0.011 to 0.136 +/- 0.018 (signal area of fat/signal area of standard, P = .009). Intramuscular lipid content correlated with the glucose uptake at day 28 (R = -0.85, P = .035). These data demonstrate that muscular inactivity and hypercortisolemia are associated with an increase in intramuscular triglyceride and skeletal muscle insulin resistance in previously healthy subjects.

The fermentable fibre inulin increases postprandial serum short-chain fatty acids and reduces free-fatty acids and ghrelin in healthy subjects

It is thought that diets high in dietary fibre are associated with reduced risk for type 2 diabetes, at least in part because the short-chain fatty acids (SCFAs) produced during the colonic fermentation of fibre beneficially influence circulating concentrations of free-fatty acids (FFAs) and gut hormones involved in the regulation of blood glucose and body mass. However, there is a paucity of data showing this sequence of events in humans. Thus, our objective was to determine the effect of the fermentable fibre inulin on postprandial glucose, insulin, SCFA, FFA, and gut hormone responses in healthy subjects. Overnight fasted healthy subjects (n = 12) were studied for 6 h after consuming 400 mL drinks, containing 80 g high-fructose corn syrup (80HFCS), 56 g HFCS (56HFCS), or 56 g HFCS plus 24 g inulin (Inulin), using a randomized, single-blind, crossover design. A standard lunch was served 4 h after the test drink. Glucose and insulin responses after Inulin did not differ significantly from those after 80HFCS or 56HFCS. Serum acetate, propionate, and butyrate were significantly higher after Inulin than after HFCS drinks from 4-6 h. FFAs fell at a similar rate after all 3 test drinks, but were lower after Inulin than after 56HFCS at 4 h (0.40 +/- 0.06 vs. 0.51 +/- 0.06 mmol*L-1; p < 0.05). Compared with 56HFCS, Inulin significantly increased plasma glucagon-like peptide-1 concentrations at 30 min, and reduced ghrelin at 4.5 h and 6 h. The results are consistent with the hypothesis that dietary fibre increases the production of colonic SCFAs, which may reduce type 2 diabetes risk by reducing postprandial FFAs and favorably affecting gut hormones, which regulate food intake.

Live performance, carcass composition, and blood metabolite responses to dietary nutrient density in two distinct broiler breeds of male chickens

A study was conducted to evaluate the effects of varying nutrient density with constant ME:CP ratio on growing performance, carcass characteristics, and blood responses in 2 distinct broiler breeds of male chickens (Arbor Acres, a commercial line, and Beijing-You, a Chinese nonimproved line). Experimental diets were formulated with high, medium, or low nutrient densities for 3 growing phases. Starter diets (1 to 21 d) contained 23, 21, and 19% CP with 3,059, 2,793, and 2,527 kcal/kg of ME; grower diets (22 to 35 d) contained 21, 19, and 17% CP with 3,150, 2,850, and 2,550 kcal/kg of ME; and finisher diets (36 to 42 d for Arbor Acres and 36 to 91 d for Beijing-You) had 19, 17, and 15% CP with 3,230, 2,890, and 2,550 kcal/kg of ME. Male hatchlings (216 of each breed) were randomly assigned to 6 replicates of 12 birds in each treatment. Arbor Acres broilers had better (P < 0.001) BW gain, feed conversion ratio (FCR), and carcass yield, but had greater (P < 0.001) abdominal and carcass fat deposition. In both breeds, the higher nutrient density increased (P < 0.05) BW gain, protein efficiency ratio, and energy efficiency ratio while decreasing (P < 0.05) feed intake and FCR. The breed differences were increased for FCR, protein efficiency ratio, and energy efficiency ratio in the starter period and decreased for carcass chemical composition, respectively, by higher nutrient density. These findings indicate that 1) genetic improvement has a significant effect on broiler responses to dietary nutrient density, 2) performance differences between breeds are lessened with diets of low nutrient density, 3) carcass quality differences are less when birds were fed diets of high nutrient density, 4) carcass composition is hardly modified by nutrient density and both breeds exhibit similar metabolite responses to dietary concentrations, and 5) optimal diets are deduced for these breeds for the 3 growing phases.

Low muscle glycogen and elevated plasma free fatty acid modify but do not prevent exercise-induced PDH activation in human skeletal muscle

OBJECTIVE

To test the hypothesis that free fatty acid (FFA) and muscle glycogen modify exercise-induced regulation of PDH (pyruvate dehydrogenase) in human skeletal muscle through regulation of PDK4 expression.

RESEARCH DESIGN AND METHODS

On two occasions, healthy male subjects lowered (by exercise) muscle glycogen in one leg (LOW) relative to the contra-lateral leg (CON) the day before the experimental day. On the experimental days, plasma FFA was ensured normal or remained elevated by consuming breakfast rich (low FFA) or poor (high FFA) in carbohydrate, 2 h before performing 20 min of two-legged knee extensor exercise. Vastus lateralis biopsies were obtained before and after exercise.

RESULTS

PDK4 protein content was approximately 2.2- and approximately 1.5-fold higher in LOW than CON leg in high FFA and low FFA, respectively, and the PDK4 protein content in the CON leg was approximately twofold higher in high FFA than in low FFA. In all conditions, exercise increased PDHa (PDH in the active form) activity, resulting in similar levels in LOW leg in both trials and CON leg in high FFA, but higher level in CON leg in low FFA. PDHa activity was closely associated with the PDH-E1alpha phosphorylation level.

CONCLUSIONS

Muscle glycogen and plasma FFA attenuate exercise-induced PDH regulation in human skeletal muscle in a nonadditive manner. This might be through regulation of PDK4 expression. The activation of PDH by exercise independent of changes in muscle glycogen or plasma FFA suggests that exercise overrules FFA-mediated inhibition of PDH (i.e., carbohydrate oxidation), and this may thus be one mechanism behind the health-promoting effects of exercise.

Insulin resistance is associated with higher intramyocellular triglycerides in type I but not type II myocytes concomitant with higher ceramide content

OBJECTIVE

We tested the primary hypotheses that sphingolipid and diacylglycerol (DAG) content is higher within insulin-resistant muscle and that the association between intramyocellular triglycerides (IMTG) and insulin resistance is muscle fiber type specific.

RESEARCH DESIGN AND METHODS

A nested case-control analysis was conducted in 22 obese (BMI >30 kg/m(2)) women who were classified as insulin-resistant (IR; n = 12) or insulin-sensitive (IS; n = 10), determined by hyperinsulinemic-euglycemic clamp (>30% greater in IS compared with IR, P < 0.01). Sphingolipid and DAG content was determined by high-performance liquid chromatography-tandem mass spectrometry. Fiber type-specific IMTG content was histologically determined. Gene expression was determined by quantitative PCR.

RESULTS

Total (555 +/- 53 vs. 293 +/- 54 pmol/mg protein, P = 0.004), saturated (361 +/- 29 vs. 179 +/- 34 pmol/mg protein, P = 0.001), and unsaturated (198 +/- 29 vs. 114 +/- 21 pmol/mg protein, P = 0.034) ceramides were higher in IR compared with IS. DAG concentrations, however, were similar. IMTG content within type I myocytes, but not type II myocytes, was higher in IR compared with IS subjects (P = 0.005). Insulin sensitivity was negatively correlated with IMTG within type I myocytes (R = -0.51, P = 0.026), but not with IMTG within type II myocytes. The proportion of type I myocytes was lower (41 vs. 59%, P < 0.01) in IR subjects. Several genes involved in lipid droplet and fatty acid metabolism were differentially expressed in IR compared with IS subjects.

CONCLUSIONS

Human skeletal muscle insulin resistance is related to greater IMTG content in type I but not type II myocytes, to greater ceramide content, and to alterations in gene expression associated with lipid metabolism.

Association of serum leptin levels with homeostasis model assessment-estimated insulin resistance and metabolic syndrome: the key role of central obesity

BACKGROUND

Leptin is correlated with several features of metabolic syndrome; however, possible confounders (eg, obesity) of this association are not known. This study evaluated the relationship between leptin, metabolic syndrome, and insulin resistance in an Iranian population and further investigated whether this relationship is confounded by obesity or central obesity.

METHODS

A total of 387 participants (18-65 years old) who referred to a large university general hospital for routine health examinations were categorized into 2 groups with (n = 130) and without (n = 257) metabolic syndrome. Fasting plasma glucose, insulin, lipids, and leptin levels were measured and the homeostasis model assessment of insulin resistance (HOMA-IR) was calculated. Metabolic syndrome was defined according to the National Cholesterol Education Program Adult Treatment Panel III (NCEP ATP III) criteria.

RESULTS

Age- and sex-adjusted leptin levels were significantly higher in patients with than those without metabolic syndrome (29.62 +/- 1.67 vs. 18.50 +/- 1.21 ng/mL, P < 0.001). After adjustment for age, sex, and body mass index (BMI), leptin values were significantly correlated with HOMA-IR (P < 0.001), metabolic syndrome, and its components (P < 0.05). After adjustment for waist circumference, however, these associations were no longer statistically significant.

CONCLUSIONS

We demonstrated that high leptin levels are associated with insulin resistance and metabolic syndrome independent of BMI but these associations are significantly mediated through the effects of central obesity.

Berberine improves free-fatty-acid-induced insulin resistance in L6 myotubes through inhibiting peroxisome proliferator-activated receptor gamma and fatty acid transferase expressions

The plant alkaloid berberine (BBR) has been reported to have antidiabetic effect in humans and animals. However, the mechanism of action is not well understood. The present study was conducted to determine the effect and mechanism of action of BBR on the free-fatty-acid (FFA)-induced insulin resistance in muscle cells. The FFA-induced insulin-resistant cell model was established in L6 myotubes by treating them with 250 mumol/L of palmitic acid. The inclusion of FFA in the medium increased peroxisome proliferator-activated receptor gamma (PPARgamma) and fatty acid transferase (FAT/CD36) expressions by 26% and 50% and decreased glucose consumption by 43% and insulin-mediated glucose uptake by 63%, respectively. Berberine treatment increased the glucose consumption and insulin-stimulated glucose uptake in normal cells and improved glucose uptake in the FFA-induced insulin-resistant cells. The improved glucose uptake by BBR was accompanied with a dose-dependent decrease in PPARgamma and FAT/CD36 protein expressions. In insulin-resistant myotubes, BBR (5 micromol/L) decreased PPARgamma and FAT/CD36 proteins by 31% and 24%, whereas PPARgamma antagonist GW9662 reduced both proteins by 56% and 46%, respectively. In contrast, PPARgamma agonist rosiglitazone increased the expression of PPARgamma and FAT/CD36 by 34% and 21%, respectively. Our results suggest that BBR improves the FFA-induced insulin resistance in myotubes through inhibiting fatty acid uptake at least in part by reducing PPARgamma and FAT/CD36 expressions.

The effect of high-dose sodium salicylate on chronically elevated plasma nonesterified fatty acid-induced insulin resistance and β-cell dysfunction in overweight and obese nondiabetic men

Prolonged elevation of plasma nonesterified fatty acids (NEFA) induces insulin resistance and impairs pancreatic β-cell adaptation to insulin resistance. Studies in rodents suggest that inflammation may play a role in this "lipotoxicity." We studied the effects of sodium salicylate, an anti-inflammatory agent, on lipid-induced alterations in β-cell function and insulin sensitivity in six overweight and obese nondiabetic men. Each subject underwent four separate studies, 4-6 wk apart, in random order: 1) SAL, 1-wk placebo followed by intravenous (iv) infusion of saline for 48 h; 2) IH, 1-wk placebo followed by iv infusion of intralipid plus heparin for 48 h to raise plasma NEFA approximately twofold; 3) IH + SS, 1-wk sodium salicylate (4.5 g/day) followed by 48-h IH infusion; and 4) SS, 1-wk oral sodium salicylate followed by 48-h saline infusion. After 48-h saline or lipid infusion, insulin secretion and sensitivity were assessed by hyperglycemic clamp and euglycemic hyperinsulinemic clamp, respectively, in sequential order. Insulin sensitivity was reduced by lipid infusion (IH = 67% of SAL) and was not improved by salicylate (IH + SS = 56% of SAL). Lipid infusion also reduced the disposition index (P < 0.05), which was not prevented by sodium salicylate. Salicylate reduced insulin clearance. These data suggest that oral sodium salicylate at this dose impairs insulin clearance but does not ameliorate lipid-induced insulin resistance and β-cell dysfunction in overweight and obese nondiabetic men.

Insulin resistance due to lipid-induced signaling defects could be prevented by mahanine

It is well known that free fatty acids (FFAs) play a key role in implementing insulin resistance and type 2 diabetes. Resources of chemical compounds that intervene the derogatory effect of FFAs are indeed very limited. We have isolated mahanine, a carbazole alkaloid, from the leaves of Murraya koenegii that prevented palmitate-induced inhibition of insulin-stimulated phosphorylation of IRbeta, PI3K, PDK1, and Akt in L6 myotubes. This was also reflected in the palmitate-induced inhibition of insulin-stimulated [(3)H] 2-DOG uptake by L6 myotubes, where palmitate adverse effect was significantly blocked by mahanine. Previous reports indicated that one of the major targets of lipid-induced damage in insulin signaling pathway resulting impairment of insulin sensitivity is insulin receptor (IR). Here, we have observed that palmitate significantly increased pPKCepsilon in both cytosol and nuclear region of L6 myotubes in comparison to control. Translocation of pPKCepsilon to the nucleus was associated with the impairment of HMGA1, the architectural transcription factor of IR gene and all these were reversed by mahanine. Palmitate-induced activation of IKK/IkappaBeta/NF-kappaBeta pathway was also attenuated by mahanine. Taken together, mahanine showed encouraging possibility to deal with lipid induced insulin resistance. In order to examine it further, mahanine was administered on nutritionally induced type 2 diabetic golden hamsters; it significantly improved hyperglycemia in all the treated animals. Our results, therefore, suggest that mahanine acts on two important sites of lipid induced insulin resistance (i) impairment of IR gene expression and (ii) activation of NF-kappaBeta pathway, thus, showing promise for its therapeutic choice for type 2 diabetes.

Article Commentary: A Role for IR-β in the Free Fatty Acid Mediated Development of Hepatic Insulin Resistance?

Several studies have been conducted to elucidate the role of free fatty acids (FFAs) in the pathogenesis of type 2 diabetes, but the exact molecular mechanism by which FFAs alter glucose metabolism in the liver is still not completely understood. 1 – 4 In a recent publication, Ragheb and coworkers have examined the effect of free fatty acid (FFA) treatment on insulin signaling and insulin resistance by using immunoprecipitation and immunoblotting to study the effect of high concentrations of insulin and FFAs on insulin receptor-beta (IR-β) and downstream elements in the PI3K pathway using the fructose-fed hamster model. 5 Their results clearly show that free fatty acids have an insignificant effect on IR-β and supports previous findings that FFAs lead to insulin resistance in the liver via the PKC-NFκB pathway. 2 , 3

Relationship between body fat mass and free fatty acid kinetics in men and women

An increased release of free fatty acids (FFAs) into plasma likely contributes to the metabolic complications associated with obesity. However, the relationship between body fat and FFA metabolism is unclear because of conflicting results from different studies. The goal of our study was to determine the inter-relationships between body fat, sex, and plasma FFA kinetics. We determined FFA rate of appearance (Ra) in plasma, by using stable isotopically labeled tracer techniques, during basal conditions in 106 lean, overweight, and obese, nondiabetic subjects (43 men and 63 women who had 7.0-56.0% body fat). Correlation analyses demonstrated: (i) no differences between men and women in the relationship between fat mass (FM) and total FFA Ra (micromol/min); (ii) total FFA Ra increased linearly with increasing FM (r=0.652, P<0.001); (iii) FFA Ra per kg FM decreased in a curvilinear fashion with increasing FM (r=-0.806; P<0.001); (iv) FFA Ra in relationship to fat-free mass (FFM) was greater in obese than lean subjects and greater in women than in men; and (v) abdominal fat itself was not an important determinant of total FFA Ra. We conclude that total body fat, not regional fat distribution or sex, is an important modulator of the rate of FFA release into plasma. Although increased adiposity is associated with a decrease in fatty acid release in relationship to FM, this downregulation is unable to completely compensate for the increase in FM, so total FFA Ra and FFA Ra with respect to FFM are greater in women than in men and in obese than in lean subjects.

Lipid induced overexpression of NF-kappaB in skeletal muscle cells is linked to insulin resistance

Lipid induced NF-kappaB activation is known to be associated with insulin resistance and type2 diabetes. Here we show that incubation of L6 skeletal muscle cells with palmitate significantly increased NF-kappaB p65 and NF-kappaB p50 expression along with their phosphorylation. NF-kappaB p65 siRNA inhibited palmitate induced overexpression of NF-kappaB p65 indicating palmitate effect on transcriptional activation. RT-PCR and real time PCR experiments also showed a significant increase in NF-kappaB p65 gene expression due to palmitate. Overexpression of NF-kappaB p65 by palmitate was linked to impairment of insulin activity. Palmitate effect on NF-kappaB gene and protein expression was found to be mediated by phospho-PKCepsilon as calphostin C (an inhibitor of PKC) and epsilonV1 (PKCepsilon translocation inhibitor) significantly reduced NF-kappaB expression. To understand the underlying mechanism, we purified NF-kappaB and pPKCepsilon from palmitate incubated skeletal muscle cells and their interaction in cell free system demonstrated the transfer of phosphate from PKCepsilon to NF-kappaB. This prompted us to transduct pPKCepsilon to the skeletal muscle cells. These cells showed increased amount of pNF-kappaB and NF-kappaB. Excess of NF-kappaB p65 pool thus created in the cells made them insulin resistant. Addition of NF-kappaB p65 siRNA and SN50 inhibited palmitate induced NF-kappaB p65 expression indicating NF-kappaB regulation of its gene expression. Increase of NF-kappaB did not affect the activation of IKK/IkappaB indicating NF-kappaB p65 expression to be a distinct effect of palmitate. Since NF-kappaB p65 is linked to several diseases, including type2 diabetes, this report may be important in understanding the pathogenicity of these diseases.

Effects of growth hormone and free fatty acids on insulin sensitivity in patients with type 1 diabetes

CONTEXT

Because GH stimulates lipolysis, an increase in circulating free fatty acid levels, as opposed to a direct effect of high GH levels, could underlie the development of insulin resistance in type 1 diabetes (T1D). Our aim was to explore the relative contributions of GH and free fatty acids to the development of insulin resistance in patients with T1D.

PATIENTS

Seven (four females, three males) nonobese patients with T1D aged 21-30 yr were studied on four occasions in random order. On each visit, overnight endogenous GH production was suppressed by octreotide. Three 1-h pulses of recombinant human GH (rhGH) or placebo were administered on two visits each. Acipimox, an antilipolytic drug, or a placebo were ingested every 4 h on two visits each. Stable glucose and glycerol isotopes were used to assess glucose and glycerol turnover. The overnight protocol was concluded by a two-step hyperinsulinemic euglycemic clamp on each visit.

MAIN OUTCOME

rhGH administration led to increases in the insulin infusion rate required to maintain euglycemia overnight (P = 0.008), elevated basal endogenous glucose production (P = 0.007), decreased basal peripheral glucose uptake (P = 0.03), and reduced glucose uptake during step 1 of the clamp (P < 0.0001). Coadministration of rhGH and acipimox reversed these effects and suppression of lipolysis in the absence of GH replacement led to further increases in insulin sensitivity.

RESULTS

GH pulses were associated with an increase in endogenous glucose production and decreased rates of peripheral glucose uptake, which was entirely reversed by acipimox. Therefore, GH-driven decreases in insulin sensitivity are mainly determined by the effect of GH on lipolysis.

Activation of SIRT1 by resveratrol represses transcription of the gene for the cytosolic form of phosphoenolpyruvate carboxykinase (GTP) by deacetylating hepatic nuclear factor 4alpha

The SIRT1 activators isonicotinamide (IsoNAM), resveratrol, fisetin, and butein repressed transcription of the gene for the cytosolic form of phosphoenolpyruvate carboxykinase (GTP) (PEPCK-C). An evolutionarily conserved binding site for hepatic nuclear factor (HNF) 4alpha (-272/-252) was identified, which was required for transcriptional repression of the PEPCK-C gene promoter caused by these compounds. This site contains an overlapping AP-1 binding site and is adjacent to the C/EBP binding element (-248/-234); the latter is necessary for hepatic transcription of PEPCK-C. AP-1 competed with HNF4alpha for binding to this site and also decreased HNF4alpha stimulation of transcription from the PEPCK-C gene promoter. Chromatin immunoprecipitation experiments demonstrated that HNF4alpha and AP-1, but not C/EBPbeta, reciprocally bound to this site prior to and after treating HepG2 cells with IsoNAM. IsoNAM treatment resulted in deacetylation of HNF4alpha, which decreased its binding affinity to the PEPCK-C gene promoter. In HNF4alpha-null Chinese hamster ovary cells, IsoNAM and resveratrol failed to repress transcription from the PEPCK-C gene promoter; overexpression of HNF4alpha in Chinese hamster ovary cells re-established transcriptional inhibition. Exogenous SIRT1 expression repressed transcription, whereas knockdown of SIRT1 by RNA interference reversed this effect. IsoNAM decreased the level of mRNA for PEPCK-C but had no effect on mRNA for glucose-6-phosphatase in AML12 mouse hepatocytes. We conclude that SIRT1 activation inhibited transcription of the gene for PEPCK-C in part by deacetylation of HNF4alpha. However, SIRT1 deacetylation of other key regulatory proteins that control PEPCK-C gene transcription also likely contributed to the inhibitory effect.

Intermuscular adipose tissue (IMAT): association with other adipose tissue compartments and insulin sensitivity

PURPOSE

To quantify intermuscular adipose tissue (IMAT) of the lower leg as well as to investigate associations with other adipose tissue (AT) compartments. The relationship between IMAT and insulin sensitivity was also examined.

MATERIALS AND METHODS

Standardized quantification of IMAT was performed in a large cohort (N = 249) at increased risk for type 2 diabetes in the right calf by T1-weighted fast spin-echo imaging at 1.5T (Magnetom Sonata; Siemens Healthcare). Additionally, whole-body AT distribution was assessed. Insulin sensitivity was determined by glucose clamp.

RESULTS

Males showed significantly more IMAT than females (2.1 +/- 1.1 cm(2) vs. 1.5 +/- 0.9 cm(2); P < 0.001). IMAT correlated well with other AT depots, especially with visceral AT (VAT; r(females) = 0.52, P < 0.0001 vs. r(males) = 0.42, P < 0.0001). Moreover, IMAT showed a negative correlation with the glucose infusion rate (GIR; r(females) = -0.43, P = 0.0002 vs. r(males) = -0.40, P = 0.0007).

CONCLUSION

Quantification of IMAT is possible by standard MR techniques. AT distribution of the lower leg is comparable to the visceral compartment with males having higher IMAT/VAT but lower subcutaneous AT (SCAT). IMAT seems to be involved in the pathogenesis of insulin resistance, as shown by the significant negative correlation with GIR.

Fenofibrate inhibits adipocyte hypertrophy and insulin resistance by activating adipose PPARalpha in high fat diet-induced obese mice

Peroxisome proliferator-activated receptor alpha (PPARalpha) activation in rodents is thought to improve insulin sensitivity by decreasing ectopic lipids in non-adipose tissues. Fenofibrate, a lipid-modifying agent that acts as a PPARalpha agonist, may prevent adipocyte hypertrophy and insulin resistance by increasing intracellular lipolysis from adipose tissue. Consistent with this hypothesis, fenofibrate decreased visceral fat mass and adipocyte size in high fat diet-fed obese mice, and concomitantly increased the expression of PPARalpha target genes involved in fatty acid beta-oxidation in both epididymal adipose tissue and differentiated 3T3-L1 adipocytes. However, mRNA levels of adipose marker genes, such as leptin and TNFalpha, were decreased in epididymal adipose tissue by fenofibrate treatment. Fenofibrate not only reduced circulating levels of free fatty acids and triglycerides, but also normalized hyperinsulinemia and hyperglycemia in obese mice. Blood glucose levels of fenofibrate-treated mice were significantly reduced during intraperitoneal glucose tolerance test compared with obese controls. These results suggest that fenofibrate-induced fatty acid beta-oxidation in visceral adipose tissue may be one of the major factors leading to decreased adipocyte size and improved insulin sensitivity.

Oleate protects against palmitate-induced insulin resistance in L6 myotubes

Oleate has been shown to protect against palmitate-induced insulin resistance. The present study investigates mechanisms involved in the interaction between oleate and palmitate on insulin-stimulated glucose uptake by L6 skeletal muscle cells. L6 myotubes were cultured for 6 h with palmitate or oleate alone, and combinations of palmitate with oleate, with and without phosphatidylinositol 3-kinase (PI3-kinase) inhibition. Insulin-stimulated glucose uptake, measured by uptake of 2-deoxy-d-[3H]glucose, was almost completely prevented by 300 microm-palmitate. Cells incubated with oleate up to 750 micromol/l maintained a significant increase in insulin-stimulated glucose uptake. Co-incubation of 50-300 microm-oleate with 300 microm-palmitate partially prevented the decrease in insulin-stimulated glucose uptake associated with palmitate. Adding the PI3-kinase inhibitors wortmannin (10- 7 mol/l) or LY294002 (25 micromol/l) to 50 microm-oleate plus 300 microm-palmitate significantly reduced the beneficial effect of oleate against palmitate-induced insulin resistance, indicating that activation of PI3-kinase is involved in the protective effect of oleate. Thus, the prevention of palmitate-induced insulin resistance by oleate in L6 muscle cells is associated with the ability of oleate to maintain insulin signalling through PI3-kinase.

Antioxidant Tempol Lowers Age-Related Increases in Insulin Resistance in Fischer 344 Rats

It is well documented that both oxidative stress and insulin resistance increase with advancing age. In the present study, a hypothesis was tested that an increase in oxidative stress leads to an age-associated increase in insulin resistance. Adult (6-month) and old (24-month) Fischer 344 rats were supplemented with vehicle and antioxidant (tempol, 1 mmol/L in drinking water, four weeks). Markers of oxidative stress and insulin resistance were measured. The level of malondialdehyde (MDA) showed an increase in the plasma and renal proximal tubules (RPT) of vehicle-supplemented old rats but not adult rats. Also, the carboxymethyllysine (CML) level increased in the RPT of vehicle-supplemented old rats. Tempol-supplementation to old rats decreased the levels of MDA and CML compared to vehicle-supplemented old rats. Further, plasma glucose, insulin, and triglycerides were higher in the vehicle-supplemented old rats than the adult rats. Tempol-supplementation to old rats decreased plasma glucose, insulin, and triglycerides, unlike vehicle-supplemented old rats. Homeostasis model assessment, an index of insulin resistance, was increased in vehicle-supplemented old rats but decreased following tempol-supplementation. This study suggests that there are age-related increases in oxidative stress and insulin resistance in Fischer 344 rats. It is speculated that increased oxidative stress may be responsible for the development of insulin resistance in old Fischer 344 rats.

Apolipoprotein E deficiency abrogates insulin resistance in a mouse model of type 2 diabetes mellitus

AIMS/HYPOTHESIS

Although it is known that lipid metabolism plays a role in insulin resistance in type 2 diabetes and in obesity, the mechanism is still largely unknown. Apolipoprotein E (ApoE) regulates plasma lipid levels and also plays a role in the uptake of lipids into various tissues. To investigate whether the suppression of whole-particle lipoprotein uptake into tissues affects insulin responsiveness and the diabetic condition, we examined the effect of an ApoE (also known as Apoe) gene deletion in MKR mice, a mouse model of type 2 diabetes.

METHODS

ApoE ( -/- ), MKR, ApoE ( -/- )/MKR and control mice were placed on a high-fat, high-cholesterol diet for 16 weeks. Glucose tolerance, serum insulin, blood glucose, insulin tolerance, tissue triacylglycerol content and atherosclerotic lesions were assessed.

RESULTS

ApoE ( -/- )/MKR and ApoE ( -/- ) mice showed significantly improved blood glucose, glucose tolerance and insulin sensitivity. Reduced triacylglycerol content in liver and reduced fat accumulation in liver and adipose tissue were found in ApoE ( -/- )/MKR and ApoE ( -/- ) mice compared with control and MKR mice. ApoE ( -/- ) and ApoE ( -/- )/MKR mice demonstrated similarly large atherosclerotic lesions, whereas MKR and control mice had small atherosclerotic lesions.

CONCLUSIONS/INTERPRETATION

We demonstrated that ApoE deficiency abrogates insulin resistance in a mouse model of type 2 diabetes, suggesting that lipid accumulation in tissue is a major cause of insulin resistance in this mouse model.