Obesity, free fatty acids, and insulin resistance:

Impaired glucose tolerance and metformin: clinical and mechanistic aspects

The Diabetes Prevention Program (DPP) showed that metformin reduced the incidence of diabetes in subjects with impaired glucose tolerance (IGT) who were at high risk of progression to type 2 diabetes. Metformin was not as efficient as intensive life style intervention, but had a clinically significant effect in obese individuals and in those with impaired fasting glucose (IFG). This review discusses the clinical implications and the mechanistic aspects of the effect of metformin in IGT and IFG. Acute actions of metformin on postprandial metabolism to improve hepatic glucose handling and improve the lipid profile could contribute to the lower incidence of diabetes. Longer term improvements in haemodynamic parameters and reduced oxidative stress are also implicated. Metformin offers a potential alternative or complement to lifestyle intervention for IGT, and deserves further evaluation in this respect.

Discovery of a Potent 9-Deazaxanthine-based Agent for the Treatment of Obesity-Related Non-alcoholic Fatty Liver Disease

A series of deazaxanthine-based derivatives were rationally prepared and evaluated. 8g exhibited the most potent glucose-lowering effect on HepG2 cell line and modulated adiponectin and leptin expression in 3T3-L1 adipocytes. Oral administration of 8g at 25 mg/kg/day for 4 weeks manifested therapeutic effects on high-fat diet-induced non-alcoholic fatty liver disease (NAFLD) by decreasing the weights of the body, liver, and fat. 8g also modulated the serum levels of fasting glucose and adiponectin, triglycerides, low-density lipoprotein-cholesterol, and alanine aminotransferase, as well as the hepatic concentrations of triglycerides, total cholesterol. Moreover, 8g significantly decreased steatosis and blocked the increase of adipocytes and the size of adipose tissues from NAFLD. In the DIO mice model, 8g ameliorated the obesity-related symptoms and normalized serum biomarkers.

Effects of physical training with different intensities of effort on lipid metabolism in rats submitted to the neonatal application of alloxan

BACKGROUND

Type 2 diabetes mellitus (T2DM) is a chronic disease that is characterized by insulin resistance. Its development is directly connected with the inability of insulin to exert its action, not just on carbohydrate metabolism but also on primarily on lipid metabolism. The present study aimed to compare the effects of continuous, intermittent, and strength training on serum and tissue variables on the lipid metabolism of alloxan rats.

METHODS

Wistar rats were divided into eight groups: sedentary alloxan (SA), sedentary control (SC), continuous training alloxan (CA), intermittent training alloxan (IA), strength training alloxan (StA), continuous training control (CC), intermittent training control (IC) and strength training control (StC). Alloxan (250 mg/kg bw) was injected into neonatal rats at 6 days of age. The continuous training protocol consisted of 12 weeks of swimming training for 1 uninterrupted hour/day, five days/week, supporting a load that was 5% bw. The intermittent training protocol consisted of 12 weeks of swimming training with 30 s of activity interrupted by 30 s of rest, for a total of 20 min/day, five days/week, supporting a load that was 15% bw. The strength-training protocol consisted of 12 weeks of training, five days/week with 4 sets of 10 jumps in water with 1 min rest between sets, supporting a load that was a 50% bw.

RESULTS

At 28 days, the alloxan animals exhibited higher insulin resistance as measured by the disappearance of glucose serum (% Kitt/min) during the ITT. At 120 days, the sedentary alloxan animals showed higher FFA values than continuous and intermittent training alloxan. In addition, the alloxan animals that underwent intermittent and strength training showed lower FFA values compared to the corresponding controls. The continuous training protocol was less effective than the strength training protocol for reducing the levels of total cholesterol in the alloxan animals. Serum total lipid values revealed that intermittent training increased serum levels in alloxan animals

CONCLUSION

Thus, it was concluded that physical training at different intensities of effort is of great importance in attenuation and control of changes in the lipid metabolism in alloxan animals.

Subclinical metabolic abnormalities associated with obesity in prepubertal Mexican schoolchildren

Childhood obesity has increased to epidemic levels and is considered a public health problem due to its association with a number of metabolic abnormalities, which are being detected at earlier stages of life. The objective was to evaluate the association between the presence of subclinical metabolic abnormalities (SMA) and obesity in a sample of pre-pubertal Mexican schoolchildren. Children of both sexes and 6 to 13 years old were questioned for signs of puberty, underwent anthropometric measurement and had their Body Mass Index (BMI) calculated. Two groups were formed: those with obesity (case group) and those with normal weight paired by age and chosen randomly (control group). Fasting insulin, glucose and cholesterol were measured. 92 children were included, 46 in each group, mean age 9.9 and 9.5 years old, respectively (p = 0.97). A higher frequency of hyperinsulinism was found in the case group: Fasting insulin > 15 mU/ml, 75% vs. 21% (case group vs. control group, respectively); fasting glucose to insulin ratio < 6, 72% vs. 24%; HOMA IR > 2.7, 83% vs. 14%; and decrease in QUICKI (< 0.3), 80% vs. 19% (p = 0.000). Hypercholesterolemia was 25% vs. 15% (p = 0.22), impaired fasting glucose 28% vs. 8% (p = 0.01), and family history of diabetes mellitus (DM) 35% vs. 9% (OR = 5.6; 95% CI = 1.5-22.2; p = 0.002). In this sample of Mexican schoolchildren, obesity was associated to a higher frequency of SMA, such as hyperinsulinism and impaired fasting glucose, and to a family history of DM.

Inflammation and insulin resistance: an old story with new ideas

Years before insulin was discovered, anti-inflammatory sodium salicylate was used to treat diabetes in 1901. Intriguingly for many years that followed, diabetes was viewed as a disorder of glucose metabolism, and then it was described as a disease of dysregulated lipid metabolism. The diabetes research focused on the causal relationship between obesity and insulin resistance, a major characteristic of type 2 diabetes. It is only within the past 20 years when the notion of inflammation as a cause of insulin resistance began to surface. In obesity, inflammation develops when macrophages infiltrate adipose tissue and stimulate adipocyte secretion of inflammatory cytokines, that in turn affect energy balance, glucose and lipid metabolism, leading to insulin resistance. This report reviews recent discoveries of stress kinase signaling involving molecular scaffolds and endoplasmic reticulum chaperones that regulate energy balance and glucose homeostasis. As we advance from a conceptual understanding to molecular discoveries, a century-old story of inflammation and insulin resistance is re-born with new ideas.

Interleukin-10 prevents diet-induced insulin resistance by attenuating macrophage and cytokine response in skeletal muscle

OBJECTIVE

Insulin resistance is a major characteristic of type 2 diabetes and is causally associated with obesity. Inflammation plays an important role in obesity-associated insulin resistance, but the underlying mechanism remains unclear. Interleukin (IL)-10 is an anti-inflammatory cytokine with lower circulating levels in obese subjects, and acute treatment with IL-10 prevents lipid-induced insulin resistance. We examined the role of IL-10 in glucose homeostasis using transgenic mice with muscle-specific overexpression of IL-10 (MCK-IL10).

RESEARCH DESIGN AND METHODS

MCK-IL10 and wild-type mice were fed a high-fat diet (HFD) for 3 weeks, and insulin sensitivity was determined using hyperinsulinemic-euglycemic clamps in conscious mice. Biochemical and molecular analyses were performed in muscle to assess glucose metabolism, insulin signaling, and inflammatory responses.

RESULTS

MCK-IL10 mice developed with no obvious anomaly and showed increased whole-body insulin sensitivity. After 3 weeks of HFD, MCK-IL10 mice developed comparable obesity to wild-type littermates but remained insulin sensitive in skeletal muscle. This was mostly due to significant increases in glucose metabolism, insulin receptor substrate-1, and Akt activity in muscle. HFD increased macrophage-specific CD68 and F4/80 levels in wild-type muscle that was associated with marked increases in tumor necrosis factor-alpha, IL-6, and C-C motif chemokine receptor-2 levels. In contrast, MCK-IL10 mice were protected from diet-induced inflammatory response in muscle.

CONCLUSIONS

These results demonstrate that IL-10 increases insulin sensitivity and protects skeletal muscle from obesity-associated macrophage infiltration, increases in inflammatory cytokines, and their deleterious effects on insulin signaling and glucose metabolism. Our findings provide novel insights into the role of anti-inflammatory cytokine in the treatment of type 2 diabetes.

Pancreatin therapy in patients with insulin-treated diabetes mellitus and exocrine pancreatic insufficiency according to low fecal elastase 1 concentrations. Results of a prospective multi-centre trial

BACKGROUND

Recently, high prevalence of exocrine dysfunction in diabetic populations has been reported. Patients with fecal elastase 1 concentration (FEC) <100 microg/g have also been demonstrated to suffer from steatorrhea in about 60% of cases, indicating the need of pancreatic enzyme replacement therapy. Until now, there have only been a few reports on the use of enzyme replacement therapy in diabetic patients with exocrine pancreatic insufficiency. This investigation was designed to evaluate the impact of enzyme-replacement therapy on glucose metabolism and diabetes treatment in a prospective study of insulin-treated patients with diabetes mellitus.

METHODS

A total of 546 patients with diabetes mellitus requiring insulin treatment were screened for exocrine dysfunction by FEC measurements. One hundred and fifteen patients (21.1%) had FEC <100 microg/g (normal >200 microg/g). Of these, 95 patients entered the study and 80 patients were randomized to receive either pancreatin (Creon) (39 patients) or placebo (41 patients) in a double-blind manner. Parameters of glucose metabolism, diabetes therapy and clinical symptoms were recorded in standardized protocols for 16 weeks.

RESULTS

During the observation phase of 16 weeks, there were no significant differences between both groups concerning HbA(1c), fasting glucose levels, 2-h pp glucose levels, clinical parameters and safety parameters. A reduction in mild and moderate hypoglycemia was observed in the pancreatin group at the end of the study.

CONCLUSIONS

Pancreatin therapy can be used safely in patients with diabetes mellitus and exocrine dysfunction. Parameters of glucose metabolism were not improved by enzyme replacement therapy.

Hyperglycemia, maturity-onset obesity, and insulin resistance in NONcNZO10/LtJ males, a new mouse model of type 2 diabetes

As a new mouse model of obesity-induced diabetes generated by combining quantitative trait loci from New Zealand Obese (NZO/HlLt) and Nonobese Nondiabetic (NON/LtJ) mice, NONcNZO10/LtJ (RCS10) male mice developed type 2 diabetes characterized by maturity onset obesity, hyperglycemia, and insulin resistance. To metabolically profile the progression to diabetes in preobese and obese states, a 2-h hyperinsulinemic euglycemic clamp was performed and organ-specific changes in insulin action were assessed in awake RCS10 and NON/LtJ (control) males at 8 and 13 wk of age. Prior to development of obesity and attendant increases in hepatic lipid content, 8-wk-old RCS10 mice developed insulin resistance in liver and skeletal muscle due to significant decreases in insulin-stimulated glucose uptake and GLUT4 expression in muscle. Transition to an obese and hyperglycemic state by 13 wk of age exacerbated insulin resistance in skeletal muscle, liver, and heart associated with organ-specific increases in lipid content. Thus, this polygenic mouse model of type 2 diabetes, wherein plasma insulin is only modestly elevated and obesity develops with maturity yet insulin action and glucose metabolism in skeletal muscle and liver are reduced at an early prediabetic age, should provide new insights into the etiology of type 2 diabetes.

Intermuscular adipose tissue rivals visceral adipose tissue in independent associations with cardiovascular risk

BACKGROUND

The metabolic implications of intermuscular adipose tissue (IMAT) are poorly understood compared to those of visceral adipose tissue (VAT) even though the absolute quantities of both depots are similar in many individuals.

OBJECTIVE

The aim was to determine the independent relationship between whole-body IMAT and cardiovascular risk factor parameters.

DESIGN

Whole body magnetic resonance imaging (MRI) was used to quantify total skeletal muscle (SM), total adipose tissue (TAT) of which IMAT, defined as the AT visible by MRI within the boundary of the muscle fascia, is a sub-component. Fasting serum measures (n=262) of glucose, total cholesterol (T-Chol), high-density lipoprotein cholesterol (HDL-Chol), triglycerides (TG), protein bound glucose (PBG, n=206) and insulin (n=119) were acquired in healthy African-American (AA, n=78) and Caucasian (Ca, n=109) women (body mass index (BMI) 26.5+/-5.7 kg/m(2); 44.4+/-16.4 years) and men (39 AA, 62 Ca; BMI 25.6+/-3.5 kg/m(2); 45.6+/-17.4 years). General linear models identified the independent effects of IMAT after covarying for SM, VAT, TAT, race, sex and two-way interactions.

RESULTS

Significant independent associations were observed for IMAT with glucose (P<0.001), PBG (P<0.001) and T-Chol (P<0.05). The association of IMAT with cholesterol differed by race in such a manner that for a unit increase in IMAT, T-Chol increased more rapidly in Ca compared to AA (P<0.05). TG, HDL-Chol and insulin had no independent association with IMAT.

CONCLUSION

The strong independent associations of IMAT with fasting glucose and PBG suggest that IMAT may be related to glucose metabolism; however, IMAT is also associated with T-Chol in Ca.

Synthesis and evaluation of (S)-2-ethoxy-3-phenylpropanoic acid derivatives as insulin-sensitizing agents

A series of (S)-2-ethoxy-3-phenylpropanoic acid derivatives were synthesized and their insulin-sensitizing activities were evaluated in 3T3-L1 cells. Compounds 1b and 1d exhibited more potent insulin-sensitizing activity than rosiglitazone.

Antilipolytic Activity of a Novel Partial A1Adenosine Receptor Agonist Devoid of Cardiovascular Effects: Comparison with Nicotinic Acid

Elevated lipolysis and circulating free fatty acid (FFA) levels have been linked to the pathogenesis of insulin resistance. A1 adenosine receptor agonists are potent inhibitors of lipolysis. Several A1 agonists have been tested as potential antilipolytic agents; however, their effect on the cardiovascular system remains a potential problem for development of these agents as drugs. In the present study, we report that CVT-3619 [(2-{6-[((1R,2R)-2-hydroxycyclopentyl) amino] purin9-yl} (4S,5 S,2R,3R)5-[(2fluorophenylthio) methyl] oxolane-3,4-diol)], a novel partial A1 receptor agonist, significantly reduces circulating FFA levels without any effect on heart rate and blood pressure in awake rats. Rats were implanted with indwelling arterial and venous cannulas to obtain serial blood samples, record arterial pressure, and administer drug. CVT-3619 decreased FFA levels in a dose-dependent manner at doses from 1 up to 10 mg/kg. The FFA-lowering effect was blocked by the A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine. Triglyceride (TG) levels were also significantly reduced by CVT-3619 treatment in the absence and presence of Triton. Tachyphylaxis of the antilipolytic effect of CVT-3619 (1 mg/kg i.v. bolus) was not observed with three consecutive treatments. An acute reduction of FFA by CVT-3619 was not followed by a rebound increase of FFA as seen with nicotinic acid. The potency of insulin to decrease lipolysis was increased 4-fold (p < 0.01) in the presence of CVT-3619 (0.5 mg/kg). In summary, CVT-3619 is an orally bioavailable A1 agonist that lowers circulating FFA and TG levels by inhibiting lipolysis. CVT-3619 has antilipolytic effects at doses that do not elicit cardiovascular effects.

Effect of ezetimibe on insulin sensitivity and lipid profile in obese and dyslipidaemic patients

AIM

To evaluate the effect of ezetimibe on insulin sensitivity and lipid profile in obese and dyslipidaemic patients.

METHODS

A randomized, double-blind, placebo-controlled clinical trial was carried out in 12 obese, dyslipidaemic patients, independently of their basal insulin sensitivity. At the beginning of the study, a metabolic profile was measured, and insulin sensitivity estimated using the euglycaemic-hyperinsulinaemic clamp technique. The volunteers were randomly assigned to receive ezetimibe (10 mg/day in the morning) or placebo for a period of 90 days. After intervention, a similar metabolic profile was measured and a second clamp study was performed.

RESULTS

Ezetimibe administration for 90 days decreased total (6.0 +/- 0.5 vs. 4.2 +/- 0.9 mmol/L, p = 0.011) and low-density lipoprotein (4.0 +/- 0.7 vs. 2.2 +/- 0.8 mmol/L, p=0.003) cholesterol concentrations without modification of insulin sensitivity (3.0 +/- 0.6 vs. 2.9 +/- 0.7 mg/kg/min, p = 0.345).

CONCLUSIONS

Ezetimibe significantly decreased total cholesterol and low-density lipoprotein cholesterol concentrations without affecting insulin sensitivity in obese and dyslipidaemic patients.

Leptin levels in serum depending on Body Mass Index in patients with endometrial hyperplasia and cancer

Leptin levels in serum depending on Body Mass Index (BMI) in patients with endometrial hyperplasia and cancer.

OBJECTIVES

Concentrations of leptin, a hormone secreted by white adipose tissue, correlate strongly with body mass. Leptin interacts with several other hormones, modifies the activities of some enzymes and proinflammatory cytokines, participates in hematopoiesis, thermogenesis, and angiogenesis, and is involved in the control of carbohydrate and lipid metabolism. This study was undertaken to determine whether serum concentrations of leptin in obese patients with endometrial hyperplasia and cancer deviate from values in patients with normal endometrium.

STUDY DESIGN

We enrolled 86 obese postmenopausal women, including 40 with endometrial cancer and hyperplasia and 46 with normal endometrium. Depending on BMI, three subgroups were formed: I<30; II = 30-40; III > 40. Leptin concentrations were measured with immunoenzymatic test kits from IBL. Statistical comparison was done with the chi square (chi(2)) test and Statistica software package.

RESULTS

Mean serum concentration of leptin in endometrial cancer and hyperplasia was 16737.1 pg/ml as opposed to 9048.7 pg/ml in patients without endometrial pathology (p<0.0001). Significantly, higher concentrations of leptin were noted in every BMI subgroup of patients with endometrial pathology in comparison to controls (p<0.005).

CONCLUSIONS

Leptin appears to participate in proliferative processes of the endometrium. Obesity is an important risk factor in endometrial cancer.

Mechanism of glucose intolerance in mice with dominant negative mutation of CEACAM1

Mice with liver-specific overexpression of dominant negative phosphorylation-defective S503A-CEACAM1 mutant (L-SACC1) developed chronic hyperinsulinemia resulting from blunted hepatic clearance of insulin, visceral obesity, and glucose intolerance. To determine the underlying mechanism of altered glucose homeostasis, a 2-h hyperinsulinemic euglycemic clamp was performed, and tissue-specific glucose and lipid metabolism was assessed in awake L-SACC1 and wild-type mice. Inactivation of carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) caused insulin resistance in liver that was mostly due to increased expression of fatty acid synthase and lipid metabolism, resulting in elevated intrahepatic levels of triglyceride and long-chain acyl-CoAs. Whole body insulin resistance in the L-SACC1 mice was further attributed to defects in insulin-stimulated glucose uptake in skeletal muscle and adipose tissue. Insulin resistance in peripheral tissues was associated with significantly elevated intramuscular fat contents that may be secondary to increased whole body adiposity (assessed by (1)H-MRS) in the L-SACC1 mice. Overall, these results demonstrate that L-SACC1 is a mouse model in which chronic hyperinsulinemia acts as a cause, and not a consequence, of insulin resistance. Our findings further indicate the important role of CEACAM1 and hepatic insulin clearance in the pathogenesis of obesity and insulin resistance.

Synthesis and insulin-sensitizing activity of (S)-2-ethoxy-3-phenylpropanoic acid derivatives

A series of (S)-2-ethoxy-3-phenylpropanoic acid derivatives were synthesized and their insulin-sensitizing activities were evaluated in 3T3-L1 cells. Compounds 1b (EC30 = 9.43 × 10–3 µmol/L), 1d (EC30 = 7.45 × 10–3 µmol/L), 1e (EC30 = 6.22 × 10–3 µmol/L), and 1f (EC30 = 7.76 × 10–3 µmol/L) exhibited more potent insulin-sensitizing activity than rosiglitazone (EC30 = 2.06 × 10–2 µmol/L).Key words: (S)-2-ethoxy-3-phenylpropanoic acid derivatives, type 2 diabetes, insulin-sensitizing agents.

A polygenic model of the metabolic syndrome with reduced circulating and intra-adipose glucocorticoid action

Despite major advances in understanding monogenic causes of morbid obesity, the complex genetic and environmental etiology of idiopathic metabolic syndrome remains poorly understood. One hypothesis suggests that similarities between the metabolic disease of plasma glucocorticoid excess (Cushing's syndrome) and idiopathic metabolic syndrome results from increased glucocorticoid reamplification within adipose tissue by 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD-1). Indeed, 11beta-HSD-1 is now a major therapeutic target. Because much supporting evidence for a role of adipose 11beta-HSD-1 comes from transgenic or obese rodents with single-gene mutations, we investigated whether the predicted traits of metabolic syndrome and glucocorticoid metabolism were coassociated in a unique polygenic model of obesity developed by long-term selection for divergent fat mass (Fat and Lean mice with 23 vs. 4% fat as body weight, respectively). Fat mice exhibited an insulin-resistant metabolic syndrome including fatty liver and hypertension. Unexpectedly, Fat mice had a marked intra-adipose (11beta-HSD-1) and plasma glucocorticoid deficiency but higher liver glucocorticoid action. Furthermore, metabolic disease was exacerbated only in Fat mice when challenged with exogenous glucocorticoids or a high-fat diet. Our data suggest that idiopathic metabolic syndrome might associate with such a novel pattern of glucocorticoid action and sensitivity in humans, with implications for tissue-specific therapeutic targeting of 11beta-HSD-1.

Hormone-sensitive lipase knockout mice have increased hepatic insulin sensitivity and are protected from short-term diet-induced insulin resistance in skeletal muscle and heart

Insulin resistance in skeletal muscle and heart plays a major role in the development of type 2 diabetes and diabetic heart failure and may be causally associated with altered lipid metabolism. Hormone-sensitive lipase (HSL) is a rate-determining enzyme in the hydrolysis of triglyceride in adipocytes, and HSL-deficient mice have reduced circulating fatty acids and are resistant to diet-induced obesity. To determine the metabolic role of HSL, we examined the changes in tissue-specific insulin action and glucose metabolism in vivo during hyperinsulinemic euglycemic clamps after 3 wk of high-fat or normal chow diet in awake, HSL-deficient (HSL-KO) mice. On normal diet, HSL-KO mice showed a twofold increase in hepatic insulin action but a 40% decrease in insulin-stimulated cardiac glucose uptake compared with wild-type littermates. High-fat feeding caused a similar increase in whole body fat mass in both groups of mice. Insulin-stimulated glucose uptake was reduced by 50-80% in skeletal muscle and heart of wild-type mice after high-fat feeding. In contrast, HSL-KO mice were protected from diet-induced insulin resistance in skeletal muscle and heart, and these effects were associated with reduced intramuscular triglyceride and fatty acyl-CoA levels in the fat-fed HSL-KO mice. Overall, these findings demonstrate the important role of HSL on skeletal muscle, heart, and liver glucose metabolism.

Adipocyte-specific overexpression of FOXC2 prevents diet-induced increases in intramuscular fatty acyl CoA and insulin resistance

Insulin resistance plays a major role in the development of type 2 diabetes and may be causally associated with increased intracellular fat content. Transgenic mice with adipocyte-specific overexpression of FOXC2 (forkhead transcription factor) have been generated and shown to be protected against diet-induced obesity and glucose intolerance. To understand the underlying mechanism, we examined the effects of chronic high-fat feeding on tissue-specific insulin action and glucose metabolism in the FOXC2 transgenic (Tg) mice. Whole-body fat mass were significantly reduced in the FOXC2 Tg mice fed normal diet or high-fat diet compared with the wild-type mice. Diet-induced insulin resistance in skeletal muscle of the wild-type mice was associated with defects in insulin signaling and significant increases in intramuscular fatty acyl CoA levels. In contrast, FOXC2 Tg mice were completely protected from diet-induced insulin resistance and intramuscular accumulation of fatty acyl CoA. High-fat feeding also blunted insulin-mediated suppression of hepatic glucose production in the wild-type mice, whereas FOXC2 Tg mice were protected from diet-induced hepatic insulin resistance. These findings demonstrate an important role of adipocyte-expressed FOXC2 on whole-body glucose metabolism and further suggest FOXC2 as a novel therapeutic target for the treatment of insulin resistance and type 2 diabetes.

Free fatty acids and insulin secretion in humans

Acute increases in plasma levels of long-chain fatty acids raise plasma insulin levels by stimulating insulin secretion or by decreasing insulin clearance. In normal subjects, long-term elevations of fatty acids also stimulate insulin secretion. In fact, they increase insulin precisely to the degree needed to compensate for the fatty acid-induced insulin resistance. In contrast, in individuals who are genetically predisposed to develop type 2 diabetes (prediabetic subjects), the free fatty acid (FFA) stimulation of insulin secretion is not sufficient to fully compensate for the FFA-induced insulin resistance. Therefore, obesity, if associated with elevated fatty acid levels, may lead to hyperglycemia in prediabetic but not in normal individuals.

Effects of oral fat load on insulin output and glucose tolerance in healthy control subjects and obese patients without diabetes

OBJECTIVE

Elevated plasma nonesterified fatty acid (NEFA) concentrations cause peripheral and hepatic insulin resistance and may play an important role in regulating glucose-induced insulin secretion. The aim of our study was to investigate the influence of physiologically elevated NEFA levels on glucose-stimulated insulin secretion in order to find evidence that NEFAs are a potential factor predisposing for type 2 diabetes and related metabolic disorders, which are known risk factors for cardiovascular disease.

RESEARCH DESIGN AND METHODS

We combined an orally administered fat emulsion with an intravenous glucose tolerance test and measured the time course of NEFA, insulin, and glucose. In order to find pathological conditions we applied the experiment to healthy and obese subjects.

RESULTS

The main findings are a significant increase in glucose-stimulated insulin secretion after oral fat load in both groups compared with the condition without preceding fat ingestion and a prolonged insulin secretion after fat load in obese patients compared with control subjects.

CONCLUSIONS

The results provide evidence that fat ingestion modulates beta-cell function and that NEFA is a plausible mediator that acts as a link between fat and glucose metabolism by modulating glucose-stimulated insulin secretion. Under the condition of elevated plasma levels of NEFA, this mechanism may be responsible for hyperinsulinemia in obese patients and a potential target of type 2 diabetes prevention strategies.

Role of adipocyte-derived factors in enhancing insulin signaling in skeletal muscle and white adipose tissue of mice lacking Acyl CoA:diacylglycerol acyltransferase 1

Mice that lack acyl CoA:diacylglycerol acyltransferase 1 (DGAT1), a key enzyme in mammalian triglyceride synthesis, have decreased adiposity and increased insulin sensitivity. Here we show that insulin-stimulated glucose transport is increased in the skeletal muscle and white adipose tissue (WAT) of chow-fed DGAT1-deficient mice. This increase in glucose transport correlated with enhanced insulin-stimulated activities of phosphatidylinositol 3-kinase, protein kinase B (or Akt), and protein kinase Clambda (PKC-lambda), three key molecules in the insulin-signaling pathway, and was associated with decreased levels of serine-phosphorylated insulin receptor substrate 1 (IRS-1), a molecule implicated in insulin resistance. Similar findings in insulin signaling were also observed in DGAT1-deficient mice fed a high-fat diet. Interestingly, the increased PKC-lambda activity and decreased serine phosphorylation of IRS-1 were observed in chow-fed wild-type mice transplanted with DGAT1-deficient WAT, consistent with our previous finding that transplantation of DGAT1-deficient WAT enhances glucose disposal in wild-type recipient mice. Our findings demonstrate that DGAT1 deficiency enhances insulin signaling in the skeletal muscle and WAT, in part through altered expression of adipocyte-derived factors that modulate insulin signaling in peripheral tissues.

Differential effects of interleukin-6 and -10 on skeletal muscle and liver insulin action in vivo

The circulating level of the inflammatory cytokine interleukin (IL)-6 is elevated in various insulin-resistant states including type 2 diabetes, obesity, cancer, and HIV-associated lipodystrophy. To determine the role of IL-6 in the development of insulin resistance, we examined the effects of IL-6 treatment on whole-body insulin action and glucose metabolism in vivo during hyperinsulinemic-euglycemic clamps in awake mice. Pretreatment of IL-6 blunted insulin's ability to suppress hepatic glucose production and insulin-stimulated insulin receptor substrate (IRS)-2-associated phosphatidylinositol (PI) 3-kinase activity in liver. Acute IL-6 treatment also reduced insulin-stimulated glucose uptake in skeletal muscle, and this was associated with defects in insulin-stimulated IRS-1-associated PI 3-kinase activity and increases in fatty acyl-CoA levels in skeletal muscle. In contrast, we found that co-treatment of IL-10, a predominantly anti-inflammatory cytokine, prevented IL-6-induced defects in hepatic insulin action and signaling activity. Additionally, IL-10 co-treatment protected skeletal muscle from IL-6 and lipid-induced defects in insulin action and signaling activity, and these effects were associated with decreases in intramuscular fatty acyl-CoA levels. This is the first study to demonstrate that inflammatory cytokines IL-6 and IL-10 alter hepatic and skeletal muscle insulin action in vivo, and the mechanism may involve cytokine-induced alteration in intracellular fat contents. These findings implicate an important role of inflammatory cytokines in the pathogenesis of insulin resistance.

Free fatty acids, insulin resistance, and pregnancy

Acute elevation of plasma free fatty acid (FFA) levels causes insulin resistance to rise dose dependently in pregnant and nonpregnant women. Plasma FFA levels are commonly elevated during late pregnancy, partly due to rising blood levels of lipolytic placental hormones, and are a likely cause for much of the increase in insulin resistance occurring at that time in all pregnant women. Plasma FFA levels are similar or higher and the insulin resistance is comparable or more severe in women with gestational diabetes mellitus (GDM) than in nondiabetic pregnant women. In contrast to healthy pregnant women, insulin secretion in women with GDM is defective and, therefore, is unable to rise adequately to compensate for the insulin resistance; the result is hyperglycemia. The mechanism by which elevated plasma FFA levels cause insulin resistance in skeletal muscle includes intramyocellular accumulation of diacylglycerol, which activates protein kinase C (the b II and d isoforms). This results in reduction of tyrosine phosphorylation of the insulin receptor substrate-1 and inhibits activation of phosphoinositol-3 kinase, an enzyme that is essential for normal insulin-stimulated glucose uptake.

Effects of free fatty acids on glucose uptake and utilization in healthy women

To study effects of sex on free fatty acid (FFA)-induced insulin resistance, we have examined the effects of acute elevations of plasma FFA levels on insulin-stimulated total body glucose uptake in nine healthy young women. Euglycemic-hyperinsulinemic (approximately 500 pmol/l) clamps were performed for 4 h with coinfusion of either lipid/heparin (L/H) to acutely raise plasma FFA levels (from approximately 600 to approximately 1,200 micro mol/l) or saline/glycerol to lower fatty acids (from approximately 600 to approximately 50 micro mol/l). L/H infusion inhibited insulin-stimulated glucose uptake (determined with [3-(3)H]glucose) and glycogen synthesis by 31 and 40%, respectively (P < 0.01), almost completely abolished insulin suppression of endogenous glucose production (EGP) (13.6 vs. 10.0 micro mol x kg(-1) x min(-1), NS), prevented the insulin induced increase in carbohydrate oxidation (8.1 vs. 7.4 micro mol x kg(-1) x min(-1), NS), and stimulated fat oxidation (from 3.6 to 5.1 micro mol x kg(-1) x min(-1), P < 0.01). These data showed that acute increases in plasma FFA levels inhibited the actions of insulin on glucose uptake, glycogen synthesis, and EGP in women to a degree similar to that previously reported in men. We conclude that at insulin and FFA levels in the postprandial range, women and men were susceptible to FFA-induced peripheral and hepatic insulin resistance.

Dissociation of obesity and impaired glucose disposal in mice overexpressing acyl coenzyme a:diacylglycerol acyltransferase 1 in white adipose tissue

Acyl coenzyme A:diacylglycerol acyltransferase 1 (DGAT1) is one of two DGAT enzymes known to catalyze the final step in mammalian triglyceride synthesis. Mice deficient in DGAT1 are resistant to obesity and have enhanced insulin sensitivity. To understand better the relationship between triglyceride synthesis and energy and glucose metabolism, we generated transgenic (aP2-Dgat1) mice in which expression of murine DGAT1 in the white adipose tissue (WAT) was twofold higher than normal. aP2-Dgat1 mice that were fed a regular diet had larger adipocytes and greater total fat pad weight than wild-type (WT) mice. In response to a high-fat diet, aP2-Dgat1 mice became more obese ( approximately 20% greater body weight after 15 weeks) than WT mice. However, the increase in adiposity in aP2-Dgat1 mice was not associated with impaired glucose disposal, as demonstrated by glucose and insulin tolerance tests. Correlating with this finding, triglyceride deposition in the liver and skeletal muscle, two major target tissues of insulin, was similar in aP2-Dgat1 and WT mice. Thus, DGAT1 overexpression in murine WAT provides a model in which obesity does not impair glucose disposal. Our findings support the lipotoxicity hypothesis that the deposition of triglycerides in insulin-sensitive tissues other than adipocytes causes insulin resistance.

Fatty acids, triglycerides, and glucose metabolism: recent insights from knockout mice

PURPOSE OF REVIEW

Cellular lipid metabolism plays an important role in modulating glucose metabolism. Recent models of mice with disruptions in genes involved in cellular fatty acid and triglyceride metabolism have provided insight into the long recognized but incompletely understood relationship between fatty acid metabolism and glucose metabolism.

RECENT FINDINGS

Here we review findings from mice with deficiency in selected genes involved in the cellular uptake, storage, and hydrolysis of fatty acids. Our review is organized from the perspective of a fatty acid, as it makes its way from the circulation into the anabolic and then catabolic pathways in the cell. Although we focus primarily on the phenotypes of knockout mice, we also point out several transgenic models in which the overexpression phenotype provides complementary information.

SUMMARY

The inactivation of enzymes in the anabolic process of fatty acid uptake and storage is more likely to enhance tissue glucose disposal or insulin secretion, whereas disruptions in the catabolic process tend to impair insulin action or secretion. These findings suggest that pharmacological inhibition of fatty acid uptake or storage may be an effective strategy for treating insulin resistance and diabetes.