Obesity and insulin resistance

Regulatory effects of HMG CoA reductase inhibitor and fish oils on apolipoprotein B-100 kinetics in insulin-resistant obese male subjects with dyslipidemia

Hepatic accumulation of lipid substrates perturbs apolipoproteinB-100 (apoB) metabolism in insulin-resistant, obese subjects and may account for increased risk of cardiovascular disease. In a placebo-controlled trial, we examined the independent and combined effects of decreasing cholesterol synthesis with atorvastatin (40 mg/day) and triglyceride synthesis with fish oils (4 g/day) on apoB kinetics. The subjects were 48 viscerally obese, insulin-resistant men with dyslipidemia who were studied in a fasted state. We found that atorvastatin significantly decreased plasma apoB-containing lipoproteins (P < 0.001, main effect) through increases in the fractional catabolic rate (FCR) of VLDL-, IDL-, and LDL-apoB (P < 0.01). Fish oils significantly decreased plasma levels of triglycerides and VLDL-apoB (P < 0.001), decreased the VLDL-apoB secretion rate (P < 0.01), but increased the conversion of VLDL to LDL (P < 0.001). Compared with placebo, combined treatment with atorvastatin and fish oils decreased VLDL-apoB secretion (P < 0.03) and increased the FCR of apoB in each lipoprotein fraction (P < 0.03) and the percent conversion of VLDL to LDL (P < 0.05). None of the treatments altered insulin resistance. In conclusion, in visceral obesity, atorvastatin increased hepatic clearance of all apoB-containing lipoproteins, whereas fish oils decreased hepatic secretion of VLDL-apoB. The differential effects of atorvastatin and fish oils on apoB kinetics support their combined use in correcting defective apoB metabolism in obese, insulin-resistant subjects.

Analysis of paradoxical observations on the association between leptin and insulin resistance

Obesity is commonly associated with the development of insulin resistance and diabetes in humans and rodents. Insulin resistance and diabetes are observed in lipoatrophic individuals or rodent models of lipoatrophy. Here we focus on the role of leptin, the product of the obesity (ob) gene, in the development of insulin resistance and diabetes associated with obesity and lipoatrophy. We review the reported effects of leptin on whole body glucose metabolism and compare and contrast these with direct effects on skeletal muscle, fat and liver. This summary of paradoxical observations on the effects of leptin on glucose homeostasis and the ability of leptin to induce or improve insulin resistance suggests that a complex interplay exists between direct peripheral and centrally mediated effects of the hormone. Evidence suggesting that leptin acts as a mediator of insulin release from pancreatic beta cells is reviewed. Finally, intracellular signaling mechanisms stimulated by both leptin and insulin are discussed, with potential points of cross-talk suggested.

Inhibition of gastric inhibitory polypeptide signaling prevents obesity

Secretion of gastric inhibitory polypeptide (GIP), a duodenal hormone, is primarily induced by absorption of ingested fat. Here we describe a novel pathway of obesity promotion via GIP. Wild-type mice fed a high-fat diet exhibited both hypersecretion of GIP and extreme visceral and subcutaneous fat deposition with insulin resistance. In contrast, mice lacking the GIP receptor (Gipr(-/-)) fed a high-fat diet were clearly protected from both the obesity and the insulin resistance. Moreover, double-homozygous mice (Gipr(-/-), Lep(ob)/Lep(ob)) generated by crossbreeding Gipr(-/-) and obese ob/ob (Lep(ob)/Lep(ob)) mice gained less weight and had lower adiposity than Lep(ob)/Lep(ob) mice. The Gipr(-/-) mice had a lower respiratory quotient and used fat as the preferred energy substrate, and were thus resistant to obesity. Therefore, GIP directly links overnutrition to obesity and it is a potential target for anti-obesity drugs.

Exacerbation of insulin resistance and postprandial triglyceride response in newly diagnosed hypertensive patients with hypertriglyceridaemia

The purpose of the study is to examine the differences in insulin resistance and postprandial triglyceride (TG) response between hypertensive patients with or without hypertriglyceridaemia. The study is a comparative cohort study with matching. Thirty-one newly diagnosed hypertensive patients without any medication were recruited from a health survey. The participants were further divided into two groups: those with fasting TG <2.26 mmol/L, and those with TG between 2.26 and 5.65 mmol/L. Both groups were matched in age, sex, body mass index and waist circumference. Each patient received a 75-g oral glucose tolerance test, an insulin suppression test, and a 1000 kcal high fat mixed meal test. The hypertriglyceridaemic hypertensive patients had significantly higher fasting insulin, 2-h plasma glucose, 2-h insulin, and steady-state plasma glucose (SSPG) (13.16 +/- 1.87 vs 9.76 +/- 3.18 mmol/L). They also had a greater postprandial TG response to the challenge of mixed meal (DeltaAUC 20.76 +/- 10.06 vs 7.97 +/- 3.18 mmol 8 h/L). The postprandial TG response was closely correlated (r = 0.72-0.95, P < 0.0001) with fasting TG in all hypertensive patients. Both fasting TG levels and postprandial TG response were significantly (P < 0.05) correlated with SSPG. In conclusion, the hypertensive patients with hypertriglyceridaemia were more insulin resistant than those without it. Exacerbation of postprandial hypertriglyceridaemia was identified in these patients. The TG response to the challenge of high fat meal was significantly correlated with fasting TG and insulin resistant in them. The results provide a rationale for the alleviation of insulin resistance and hypertriglyceridaemia in these atherosclerosis-prone hypertensive patients.

Improved insulin sensitivity is associated with restricted intake of dietary glycoxidation products in the db/db mouse

Advanced glycation end products (AGEs), known promoters of diabetic complications, form abundantly in heated foods and are ingested in bioreactive forms. To test whether dietary AGEs play a role in the progression of insulin resistance, C57/BL/KsJ db/db mice were randomly placed for 20 weeks on a diet with either a low AGE content (LAD) or a 3.4-fold higher content of AGE (high AGE diet [HAD]), including (epsilon)N-carboxymethyllysine (CML) and methylglyoxal (MG). LAD-fed mice showed lower fasting plasma insulin levels throughout the study (P = 0.01). Body weight was reduced by approximately 13% compared with HAD-fed mice (P = 0.04) despite equal food intake. LAD-fed mice exhibited significantly improved responses to both glucose (at 40 min, P = 0.003) and insulin (at 60 min, P = 0.007) tolerance tests, which correlated with a twofold higher glucose uptake by adipose tissue (P = 0.02). Compared with the severe hypertrophy and morphological disorganization of islets from HAD-fed mice, LAD-fed mice presented a better-preserved structure of the islets. LAD-fed mice demonstrated significantly increased plasma HDL concentrations (P < 0.0001). Consistent with these observations, LAD-fed mice exhibited twofold lower serum CML and MG concentrations compared with HAD-fed mice (P = 0.02). These results demonstrate that reduced AGE intake leads to lower levels of circulating AGE and to improved insulin sensitivity in db/db mice.

Diet-induced insulin resistance in mice lacking adiponectin/ACRP30

Here we investigated the biological functions of adiponectin/ACRP30, a fat-derived hormone, by disrupting the gene that encodes it in mice. Adiponectin/ACRP30-knockout (KO) mice showed delayed clearance of free fatty acid in plasma, low levels of fatty-acid transport protein 1 (FATP-1) mRNA in muscle, high levels of tumor necrosis factor-alpha (TNF-alpha) mRNA in adipose tissue and high plasma TNF-alpha concentrations. The KO mice exhibited severe diet-induced insulin resistance with reduced insulin-receptor substrate 1 (IRS-1)-associated phosphatidylinositol 3 kinase (PI3-kinase) activity in muscle. Viral mediated adiponectin/ACRP30 expression in KO mice reversed the reduction of FATP-1 mRNA, the increase of adipose TNF-alpha mRNA and the diet-induced insulin resistance. In cultured myocytes, TNF-alpha decreased FATP-1 mRNA, IRS-1-associated PI3-kinase activity and glucose uptake, whereas adiponectin increased these parameters. Our results indicate that adiponectin/ACRP30 deficiency and high TNF-alpha levels in KO mice reduced muscle FATP-1 mRNA and IRS-1-mediated insulin signaling, resulting in severe diet-induced 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.

The antipsychotic drug sulpiride does not affect bodyweight in male rats. Is insulin resistance involved?

Previous studies have shown that prolonged administration of antipsychotic drugs induces obesity in female but not in male rats. To explore the mechanisms involved in this sex-dependent effect, we administered the dopamine antagonist sulpiride (20 mg/kg i.p.) or vehicle (0.1 N HCl) to adult male rats during 21 days and daily assessed bodyweight and food intake. Then, we evaluated the glucose tolerance and the serum levels of insulin, leptin, total testosterone, dehydroepiandrosterone-sulfate (DHEA-S), thyroid hormones and blood lipids. In another experiment, food intake and water intake were assessed after acute injections of sulpiride or vehicle into the perifornical lateral hypothalamus. Lastly, the dopamine metabolites dihydroxyphenylacetic acid (DOPAC) and homovanilic acid (HVA) in the lateral hypothalamus were assessed by in vivo microdialysis after acute systemic injections of sulpiride and vehicle. Chronic sulpiride administration did not affect bodyweight gain and food intake. However, prolactin levels and the area under the glucose and insulin curves were significantly elevated. Acute sulpiride significantly increased food intake, water intake, DOPAC and HVA levels. The acute effects of sulpiride show that this drug is active at the perifornical lateral hypothalamus, which is a brain area where blockade of dopamine receptors stimulates feeding. However, after prolonged administration, sulpiride did not affect body weight. This lack of effect may be related to the impairment of insulin sensitivity, which may prevent body weight gain, and counteract other effects of sulpiride that promote adiposity such as hyperprolactinemia. These findings noticeably contrast with those observed in sulpiride-treated female rats that appear to display enhanced insulin sensitivity. The changes in insulin sensitivity do not appear related to a decrease in androgenic activity, because testosterone and DHEA-S levels were not affected by sulpiride. However, these results should be considered as preliminary because other relevant endocrine variables such as free testosterone, steroid binding globulin and pituitary gonadotrophin levels were not evaluated. Since the same sex-dependent effect on body weight and food intake in rats has been observed during administration of risperidone, which has a different pharmacological profile than sulpiride, future studies must evaluate other neurotransmitters involved in food intake regulation such as serotonin, noradrenaline and histamine.

Early acarbose treatment ameliorates resistance of insulin-regulated GLUT4 trafficking in obese Zucker rats

Genetically (fa/fa) obese Zucker rats represent an established model of impaired glucose tolerance, with profound insulin resistance. Acarbose, an inhibitor of alpha-glucosidases, attenuates postprandial blood glucose peaks, and improves glucose tolerance in these animals. In the present study, we have tested the hypothesis that the effect of acarbose is associated with improved glucose transporter isoform 4 (GLUT4) trafficking in muscle tissue. Acarbose was administered to Zucker rats as a dietary admix (40 mg/100 g diet) for 12 weeks starting at the age of 6 weeks. Serum insulin and leptin were reduced by acarbose from 44 to 19 and 144 to 62 ng/ml, respectively. Glucose tolerance test was performed by i.v. injection of glucose (1 g/kg) and determination of serum glucose up to 60 min. Marked impaired glucose tolerance was observed in obese animals with a profound correction of this defect in acarbose-treated rats. Insulin-regulated translocation of GLUT4 to the plasma membrane in soleus muscle was increased twofold in lean animals, with a totally blunted response in obese rats. Acarbose feeding restored a 1.6-fold effect of insulin on GLUT4 translocation. The exocytotic GLUT4 storage pool in cardiac muscle was completely insulin-insensitive in obese animals, with a largely improved response after acarbose feeding. Activation of Akt, an insulin signaling event upstream of GLUT4, was completely normalized in acarbose-treated rats. In conclusion, we show here that early application of acarbose to obese Zucker rats can prevent the development of impaired glucose tolerance and obesity-associated insulin resistance at the level of the muscle cell, as reflected by an amelioration of defective GLUT4 trafficking in both cardiac and skeletal muscles.

Changes in FAT/CD36, UCP2, UCP3 and GLUT4 gene expression during lipid infusion in rat skeletal and heart muscle

OBJECTIVE

It has been reported that an increased availability of free fatty acids (NEFA) not only interferes with glucose utilization in insulin-dependent tissues, but may also result in an uncoupling effect of heart metabolism. We aimed therefore to investigate the effect of an increased availability of NEFA on gene expression of proteins involved in transmembrane fatty acid (FAT/CD36) and glucose (GLUT4) transport and of the uncoupling proteins UCP2 and 3 at the heart and skeletal muscle level.

STUDY DESIGN

Euglycemic hyperinsulinemic clamp was performed after 24 h Intralipid(R) plus heparin or saline infusion in lean Zucker rats. Skeletal and heart muscle glucose utilization was calculated by 2-deoxy-[1-(3)H]-D-glucose technique. Quantification of FAT/CD36, GLUT4, UCP2 and UCP3 mRNAs was obtained by Northern blot analysis or RT-PCR.

RESULTS

In Intralipid(R) plus heparin infused animals a significant decrease in insulin-mediated glucose uptake was observed both in the heart (22.62+/-2.04 vs 10.37+/-2.33 ng/mg/min; P<0.01) and in soleus muscle (13.46+/-1.53 vs 6.84+/-2.58 ng/mg/min; P<0.05). FAT/CD36 mRNA was significantly increased in skeletal muscle tissue (+117.4+/-16.3%, P<0.05), while no differences were found at the heart level in respect to saline infused rats. A clear decrease of GLUT4 mRNA was observed in both tissues. The 24 h infusion of fat emulsion resulted in a clear enhancement of UCP2 and UCP3 mRNA levels in the heart (99.5+/-15.3 and 80+/-4%) and in the skeletal muscle (291.5+/-24.7 and 146.9+/-12.7%).

CONCLUSIONS

As a result of the increased availability of NEFA, FAT/CD36 gene expression increases in skeletal muscle, but not at the heart level. The augmented lipid fuel supply is responsible for the depression of insulin-mediated glucose transport and for the increase of UCP2 and 3 gene expression in both skeletal and heart muscle.

Gene expression profile in skeletal muscle of type 2 diabetes and the effect of insulin treatment

Type 2 diabetes is characterized by muscle insulin resistance. Nondiabetic first-degree relatives of type 2 diabetic patients have also been reported to have insulin resistance. A polygenic basis for pathogenesis of type 2 diabetes has been proposed. A gene expression profile was evaluated in the skeletal muscle of patients with type 2 diabetes while not on treatment for 2 weeks and after 10 days of intensive insulin treatment. Comparison of gene expression pattern with age-, sex-, and BMI-matched people with no family history of diabetes was performed using a microarray technique (Hu6800 arrays; Affymetrix, Santa Clara, CA). Only those gene transcripts showing > or =1.9-fold changes and an average difference in fluorescence intensity of > or =1,000 in all subjects are reported. Insulin sensitivity (SI) was measured using an intravenous glucose tolerance test. Of 6,451 genes surveyed, transcriptional patterns of 85 genes showed alterations in the diabetic patients after withdrawal of treatment, when compared with patterns in the nondiabetic control subjects. Insulin treatment reduced the difference in patterns between diabetic and nondiabetic control subjects (improved) in all but 11 gene transcripts, which included genes involved in structural and contractile functions, growth and tissue development, stress response, and energy metabolism. These improved transcripts included genes involved in insulin signaling, transcription factors, and mitochondrial maintenance. However, insulin treatment altered the transcription of 29 additional genes involved in signal transduction; structural and contractile functions; growth and tissue development; and protein, fat, and energy metabolism. Type 2 diabetic patients had elevated circulating insulin during the insulin-treated phase, although their blood glucose levels (98.8 +/- 6.4 vs. 90.0 +/- 2.9 mg/dl for diabetic vs. control) were similar to those of the control subjects. In contrast, after withdrawal of treatment, the diabetic patients had reduced SI and elevated blood glucose (224.0 +/- 26.2 mg/dl), although their insulin levels were similar to those of the nondiabetic control subjects. This study identified several candidate genes for muscle insulin resistance, complications associated with poor glycemic control, and effects of insulin treatment in people with type 2 diabetes.

Humoral regulation of resistin expression in 3T3-L1 and mouse adipose cells

Resistin is a hormone secreted by adipocytes that acts on skeletal muscle myocytes, hepatocytes, and adipocytes themselves, reducing their sensitivity to insulin. In the present study, we investigated how the expression of resistin is affected by glucose and by mediators known to affect insulin sensitivity, including insulin, dexamethasone, tumor necrosis factor-alpha (TNF-alpha), epinephrine, and somatropin. We found that resistin expression in 3T3-L1 adipocytes was significantly upregulated by high glucose concentrations and was suppressed by insulin. Dexamethasone increased expression of both resistin mRNA and protein 2.5- to 3.5-fold in 3T3-L1 adipocytes and by approximately 70% in white adipose tissue from mice. In contrast, treatment with troglitazone, a thiazolidinedione antihyperglycemic agent, or TNF-alpha suppressed resistin expression by approximately 80%. Epinephrine and somatropin were both moderately inhibitory, reducing expression of both the transcript and the protein by 30-50% in 3T3-L1 adipocytes. Taken together, these data make it clear that resistin expression is regulated by a variety of hormones and that cytokines are related to glucose metabolism. Furthermore, they suggest that these factors affect insulin sensitivity and fat tissue mass in part by altering the expression and eventual secretion of resistin from adipose cells.

Effect of BRX-220 against peripheral neuropathy and insulin resistance in diabetic rat models

Bimoclomol (BML), a symptomatic antidiabetic agent, has been developed by Biorex R & D Co. to treat diabetic neuropathy and retinopathy. BRX-220, an orally active member of the BRX family, has been developed to treat diabetic complications and insulin resistance (IR) as a follow-up compound. The effect of BRX-220 on peripheral neuropathy was examined in rats with diabetes (type 1) induced by administration of a beta-cell toxin, streptozotocin (STZ, 45 mg/kg iv). Nerve functions were evaluated by electrophysiological measurements of muscle motor and sensory nerve conduction velocities (MNCV and SNCV, respectively). MNCV and SNCV decreased in diabetic rats by 25% (p < 0.001). A 1-month preventive treatment with BRX-220 (2.5, 5, 10, and 20 mg/kg po) dose-dependently improved diabetes-related deficits in MNCV (51.3%, 71.3%, 86.1%, and 91.3%) and SNCV (48.9%, 68.5%, 86.1%, and 93.2%). Insulin sensitivity was measured using the insulin tolerance test (ITT), both in STZ diabetic and in Zucker diabetic fatty (ZDF) rats (model of type 2 diabetes). Severe IR was detected in STZ diabetic and ZDF rats. This resistance was significantly (p < 0.05) reduced by BRX-220 treatment.

Plasma adiponectin concentration is associated with skeletal muscle insulin receptor tyrosine phosphorylation, and low plasma concentration precedes a decrease in whole-body insulin sensitivity in humans

Adiponectin, the most abundant adipose-specific protein, has been found to be negatively associated with degree of adiposity and positively associated with insulin sensitivity in Pima Indians and other populations. Moreover, adiponectin administration to rodents has been shown to increase insulin-induced tyrosine phosphorylation of the insulin receptor (IR) and also increase whole-body insulin sensitivity. To further characterize the relationship between plasma adiponectin concentration and insulin sensitivity in humans, we examined 1) the cross-sectional association between plasma adiponectin concentration and skeletal muscle IR tyrosine phosphorylation and 2) the prospective effect of plasma adiponectin concentration at baseline on change in insulin sensitivity. Fasting plasma adiponectin concentration, body composition (hydrodensitometry or dual energy X-ray absorptiometry), insulin sensitivity (insulin-stimulated glucose disposal, hyperinsulinemic clamp), and glucose tolerance (75-g oral glucose tolerance test) were measured in 55 Pima Indians (47 men and 8 women, aged 31 +/- 8 years, body fat 29 +/- 8% [mean +/- SD]; 50 with normal glucose tolerance, 3 with impaired glucose tolerance, and 2 with diabetes). Group 1 (19 subjects) underwent skeletal muscle biopsies for the measurement of basal and insulin-stimulated tyrosine phosphorylation of the IR (stimulated by 100 nmol/l insulin). The fold increase after insulin stimulation was calculated as the ratio between maximal and basal phosphorylation. Group 2 (38 subjects) had follow-up measurements of insulin-stimulated glucose disposal. Cross-sectionally, plasma adiponectin concentration was positively associated with insulin-stimulated glucose disposal (r = 0.58, P < 0.0001) and negatively associated with percent body fat (r = -0.62, P < 0.0001) in the whole group. In group 1 plasma adiponectin was negatively associated with the basal (r = -0.65, P = 0.003) and positively associated with the fold increase in IR tyrosine phosphorylation (r = 0.69, P = 0.001) before and after the adjustment for percent body fat (r = -0.58, P = 0.01 and r = 0.54, P = 0.02, respectively). Longitudinally, after adjustment for age, sex, and percent body fat, low plasma adiponectin concentration at baseline was associated with a decrease in insulin sensitivity (P = 0.04). In conclusion, our cross-sectional data suggest a role of physiological concentration of fasting plasma adiponectin in the regulation of skeletal muscle IR tyrosine phosphorylation. Prospectively, low plasma adiponectin concentration at baseline precedes a decrease in insulin sensitivity. Our data indicate that adiponectin plays an important role in regulation of insulin sensitivity in humans.

Identification of novel putative membrane proteins selectively expressed during adipose conversion of 3T3-L1 cells

Fat tissue plays a critical role in the regulation of energy metabolism. Here we report the proteomic identification of a novel _fa_t tissue-specific _l_ow molecular weight _p_rotein (Falp) which responds to insulin. Falp is preferentially expressed in adipocytes but not in preadipocytes, as shown by two-dimensional gel electrophoresis. Northern blot analysis shows that the Falp gene is predominantly expressed in brown and white fat tissues, but not in any other tissues examined. Human homologs of mouse Falp are found to exist as two alternatively spliced isoforms, which share the same N-terminus but have different C-termini. Both human and mouse Falp contain a conserved putative transmembrane domain. Immunofluorescent analyses of 3T3-L1 adipocytes show that Falp protein strictly localizes at a compact perinuclear membrane compartment. Treatment of cells with insulin induces the redistribution of Falp into numerous discrete spotty structures spreading throughout the cytoplasm. Whereas the function of Falp is currently unclear, its tissue specific expression and the responsiveness to insulin suggest that Falp might be involved in a process specifically restricted to adipose tissue function, such as vesicular transport and protein secretion.

Energy from fat is associated with obesity in U.S. men: results from the Prostate Cancer Prevention Trial

BACKGROUND

Diets high in fat have been proposed as one cause of obesity, primarily because fat is more energy-dense than other macronutrients. However, the literature on fat consumption and human obesity is inconclusive. This research examines associations between dietary fat intake and obesity in men participating in the Prostate Cancer Prevention Trial.

METHODS

Data in this cross-sectional study are from 15,266 men (55-79 years) who completed questionnaires on usual diet, physical activity, and health-related characteristics. Height and weight were collected by clinic personnel. Obesity was defined as body mass index (BMI) greater than or equal to 30 kg/m2.

RESULTS

In this healthy cohort, 23.3% were obese. Younger age, a sedentary lifestyle, lower education, and black race were positively associated with obesity (all P < 0.001). Using two statistical approaches, both total energy and energy from fat, but not total energy from other macronutrients, increased linearly and significantly with increasing BMI. Mean fat intake increased from 691 kcal (31.4% energy) among normal-BMI men to 797 kcal (34.3% energy) among the obese (P for trend <0.001). After controlling for demographic and health-related characteristics in regression models, BMI increased by 0.53 and 0.14 kg/m(2) for every 500 kcal of fat and total energy consumed, respectively. Energy underreporting, based on estimated basal metabolic rate and physical activity, was fourfold higher among obese compared to normal-weight men.

CONCLUSIONS

In this large cohort of healthy older men, energy from fat was associated with obesity, suggesting that high-fat dietary patterns are contributing to the high rates of obesity in U.S. men.

Effects of acute and chronic administration of the melanocortin agonist MTII in mice with diet-induced obesity

High-fat diet-induced obesity (DIO) in rodents is associated with hyperleptinemia and resistance to leptin, but the response to agents acting downstream of leptin receptors remains unknown. We assessed the response of mice with DIO to treatment with MTII, an alpha-melanocyte-stimulating hormone analog. MTII delivered four times daily by intraperitoneal injection to C57BL/6J mice produced a dose-responsive effect on food intake, body weight, leptin, corticosterone, insulin, and free fatty acids. In DIO mice, administration of MTII 100 microg q.i.d. i.p. markedly suppressed feeding during the first 4 days of treatment, with food intake returning to control levels at day 5. Progressive weight loss also occurred over the first 4 days, after which weight plateaued at a level below control. After 8 days of treatment, MTII-treated DIO mice had major suppression of both leptin and insulin levels. Central administration of MTII for 4 days (10 nmol/day) in DIO mice significantly suppressed food intake, induced weight loss, and increased energy expenditure. These results indicate that 1) MTII administration to DIO mice causes suppression of food intake and body weight loss, and decreased food intake is primarily responsible for weight loss; 2) peripheral MTII improves insulin resistance in DIO mice; 3) "tachyphylaxis" to the effect of chronic MTII treatment on food intake occurs; and 4) at least some of the effects of MTII are exerted centrally. In conclusion, treatment with a melanocortin agonist is a promising therapeutic approach to DIO and associated insulin resistance.

Recurrent hypoglycaemic episodes in a patient with Type 2 diabetes under fibrate therapy

The causal relationship between dyslipidaemia and insulin resistance, two central components of the metabolic syndrome, is still poorly understood. We describe a 76-year-old patient with Type 2 diabetes mellitus and pronounced hypertriglyceridaemia. The patient required a total insulin dose of 148 IU/day for decent glycaemic control. The addition of the fibrate gemfibrozil to her medication resulted in a remarkable reduction in triglyceride levels (from 31.7 to 7.1 mmol/l). Consequently, the patient experienced recurrent hypoglycaemic episodes concurrent with a massive reduction in insulin requirements. Eventually, the insulin dosage was reduced by more than 65% to 48 IU/day, and HbA1c levels dropped from 9% to 6.5% over a period of 5 months without alterations in lifestyle, diet, body weight, or any other blood chemical values. Thus, this case report illustrates that treatment of hypertriglyceridaemia can reduce insulin requirements and harbours the risk of hypoglycaemia in patients with insulin-treated Type 2 diabetes mellitus.

Effects of free fatty acids on insulin secretion in obesity

The prevalence of obesity in Western society has reached epidemic proportions and its aetiological role in the development of type 2 diabetes has made finding an effective treatment for the condition of crucial importance. Of the many consequences of obesity, derangements in glucose metabolism present one of the greatest problems to health. While the role of obesity in causing insulin resistance has received much attention, the effect of obesity on beta-cell failure and the consequent development of type 2 diabetes requires re-emphasis. In this review, the current understanding of the effects of elevated free-fatty acids on beta-cell function will be examined, including a discussion of potential mechanisms. In particular, dysregulation of biochemical pathways and alterations in key enzymes, proteins and hormones will be considered as grounds for the progression to a diabetic phenotype.

Transgenic mice expressing human fibroblast growth factor-19 display increased metabolic rate and decreased adiposity

The fibroblast growth factors (FGFs), and the corresponding receptors, are implicated in more than just the regulation of epithelial cell proliferation and differentiation. Specifically, FGF23 is a regulator of serum inorganic phosphate levels, and mice deficient in FGF receptor-4 have altered cholesterol metabolism. The recently described FGF19 is unusual in that it is nonmitogenic and appears to interact only with FGF receptor-4. Here, we report that FGF19 transgenic mice had a significant and specific reduction in fat mass that resulted from an increase in energy expenditure. Further, the FGF19 transgenic mice did not become obese or diabetic on a high fat diet. The FGF19 transgenic mice had increased brown adipose tissue mass and decreased liver expression of acetyl coenzyme A carboxylase 2, providing two mechanisms by which FGF19 may increase energy expenditure. Consistent with the reduction in expression of acetyl CoA carboxylase 2, liver triglyceride levels were reduced.

ADAM 12 protease induces adipogenesis in transgenic mice

ADAM 12 (meltrin-alpha) is a member of the ADAM (a disintegrin and metalloprotease) family. ADAM 12 functions as an active metalloprotease, supports cell adhesion, and has been implicated in myoblast differentiation and fusion. Human ADAM 12 exists in two forms: the prototype membrane-anchored protein, ADAM 12-L, and a shorter secreted form, ADAM 12-S. Here we report the occurrence of adipocytes in the skeletal muscle of transgenic mice in which overexpression of either form is driven by the muscle creatine kinase promoter. Cells expressing a marker of early adipogenesis were apparent in the perivascular space in muscle tissue of 1- to 2-week-old transgenic mice whereas mature lipid-laden adipocytes were seen at 3 to 4 weeks. Moreover, female transgenics expressing ADAM 12-S exhibited increases in body weight, total body fat mass, abdominal fat mass, and herniation, but were normoglycemic and did not exhibit increased serum insulin, cholesterol, or triglycerides. Male transgenics were slightly overweight and also developed herniation but did not become obese. Transgenic mice expressing a truncated form of ADAM 12-S lacking the prodomain and the metalloprotease domain did not develop this adipogenic phenotype, suggesting a requirement for ADAM 12 protease activity. This is the first in vivo demonstration that an ADAM protease is involved in adipogenesis.

Early differential defects of insulin secretion and action in 19-year-old caucasian men who had low birth weight

Several studies have linked low birth weight (LBW) and type 2 diabetes. We investigated hepatic and peripheral insulin action including intracellular glucose metabolism in 40 19-year-old men (20 LBW, 20 matched control subjects), using the hyperinsulinemic-euglycemic clamp technique at two physiological insulin levels (10 and 40 mU/m(2) per min), indirect calorimetry, and [3-(3)H]glucose. Insulin secretion was examined during an oral and intravenous glucose tolerance test. Fasting p-glucose was higher in the LBW group (5.6 +/- 0.1 vs. 5.4 +/- 0.1; P < 0.05). Basal plasma glycerol concentrations were significantly lower in the LBW group. Insulin-stimulated glycolytic flux was significantly reduced, and suppression of endogenous glucose production was enhanced in the LBW group. Nevertheless, basal and insulin-stimulated rates of whole-body peripheral glucose disposal, glucose oxidation, lipid oxidation, exogenous glucose storage, and nonoxidative glucose metabolism were similar in the two groups. Insulin secretion was reduced by 30% in the LBW group, when expressed relative to insulin sensitivity (disposition index = insulin secretion x insulin action). We propose that reduced insulin-stimulated glycolysis precedes overt insulin resistance in LBW men. A lower insulin secretion may contribute to impaired glucose tolerance and ultimately lead to diabetes.

Disordered fat storage and mobilization in the pathogenesis of insulin resistance and type 2 diabetes

The primary genetic, environmental, and metabolic factors responsible for causing insulin resistance and pancreatic beta-cell failure and the precise sequence of events leading to the development of type 2 diabetes are not yet fully understood. Abnormalities of triglyceride storage and lipolysis in insulin-sensitive tissues are an early manifestation of conditions characterized by insulin resistance and are detectable before the development of postprandial or fasting hyperglycemia. Increased free fatty acid (FFA) flux from adipose tissue to nonadipose tissue, resulting from abnormalities of fat metabolism, participates in and amplifies many of the fundamental metabolic derangements that are characteristic of the insulin resistance syndrome and type 2 diabetes. It is also likely to play an important role in the progression from normal glucose tolerance to fasting hyperglycemia and conversion to frank type 2 diabetes in insulin resistant individuals. Adverse metabolic consequences of increased FFA flux, to be discussed in this review, are extremely wide ranging and include, but are not limited to: 1) dyslipidemia and hepatic steatosis, 2) impaired glucose metabolism and insulin sensitivity in muscle and liver, 3) diminished insulin clearance, aggravating peripheral tissue hyperinsulinemia, and 4) impaired pancreatic beta-cell function. The precise biochemical mechanisms whereby fatty acids and cytosolic triglycerides exert their effects remain poorly understood. Recent studies, however, suggest that the sequence of events may be the following: in states of positive net energy balance, triglyceride accumulation in "fat-buffering" adipose tissue is limited by the development of adipose tissue insulin resistance. This results in diversion of energy substrates to nonadipose tissue, which in turn leads to a complex array of metabolic abnormalities characteristic of insulin-resistant states and type 2 diabetes. Recent evidence suggests that some of the biochemical mechanisms whereby glucose and fat exert adverse effects in insulin-sensitive and insulin-producing tissues are shared, thus implicating a diabetogenic role for energy excess as a whole. Although there is now evidence that weight loss through reduction of caloric intake and increase in physical activity can prevent the development of diabetes, it remains an open question as to whether specific modulation of fat metabolism will result in improvement in some or all of the above metabolic derangements or will prevent progression from insulin resistance syndrome to type 2 diabetes.

Increased insulin and leptin sensitivity in mice lacking acyl CoA:diacylglycerol acyltransferase 1

Acyl coenzyme A:diacylglycerol acyltransferase 1 (DGAT1) is one of two known DGAT enzymes that catalyze the final step in mammalian triglyceride synthesis. DGAT1-deficient mice are resistant to diet-induced obesity through a mechanism involving increased energy expenditure. Here we show that these mice have decreased levels of tissue triglycerides, as well as increased sensitivity to insulin and to leptin. Importantly, DGAT1 deficiency protects against insulin resistance and obesity in agouti yellow mice, a model of severe leptin resistance. In contrast, DGAT1 deficiency did not affect energy and glucose metabolism in leptin-deficient (ob/ob) mice, possibly due in part to a compensatory upregulation of DGAT2 expression in the absence of leptin. Our results suggest that inhibition of DGAT1 may be useful in treating insulin resistance and leptin resistance in human obesity.

Novel adipocyte lines from brown fat: a model system for the study of differentiation, energy metabolism, and insulin action

Adipose tissue has emerged as an important endocrine regulator of glucose metabolism and energy homeostasis. By virtue of the mitochondrial protein uncoupling protein-1 (UCP-1), brown fat additionally plays a unique role in thermoregulation. Interest has focused on this tissue not only as a target for pharmacotherapy of obesity and insulin resistance but also as an endocrine tissue with leptin secretion and high insulin sensitivity. Most studies of adipocytes have been limited either to primary cell culture or to a small number of established cell lines. Recently, we have generated immortalized brown adipocyte cell lines from single newborn mice of different knockout mouse models. These cell lines retain the main characteristics of primary cells including UCP-1 expression. They display sensitive and diverse metabolic responses to insulin and adrenergic stimulation and have proven to be useful in the characterization of UCP regulation and the role of key insulin signaling elements for insulin action. Here, we outline common approaches to the generation of adipose tissue cell lines. Furthermore, we propose that the novel technique of generating brown adipocyte lines from a single newborn mouse will be instrumental in gaining further insight into the role of a broad range of signaling molecules in adipose tissue biology and in the pathogenesis of insulin resistance.

Modulation of adipose tissue expression of murine matrix metalloproteinases and their tissue inhibitors with obesity

The potential role of the matrix metalloproteinase (MMP) system in the pathophysiology of the adipose tissue was investigated in a mouse model of nutritionally induced obesity. mRNA levels of 16 MMPs and 4 tissue inhibitors of MMPs (TIMPs) were measured by semiquantitative RT-PCR in adipose tissue isolated from mice maintained for 15 weeks on a standard or high-fat diet. In mice on standard diet, with the exception of MMP-8, all MMP and TIMP transcripts were detected in both gonadal and subcutaneous depots. In obese mice, the expression of MMP-3, -11, -12, -13, and -14 and TIMP-1 mRNAs was upregulated, whereas that of MMP-7, -9, -16, and -24 and TIMP-4 was downregulated. Most MMP and TIMP mRNAs were expressed at higher levels in stromal-vascular cells than in mature adipocytes. Analysis of adipose tissue by in situ fluorescent zymography revealed MMP-dependent proteolytic activities, demonstrating the presence of active MMPs in the intact tissue. In vitro conversion of adipogenic 3T3-F442A cells into mature adipocytes was associated with substantial modulations of MMP and TIMP expression. Moreover, this in vitro adipogenesis was reduced in the presence of a synthetic MMP inhibitor. Thus, the adipose tissue expresses a large array of MMPs and TIMPs, which modulate adipocyte differentiation.

Indinavir acutely inhibits insulin-stimulated glucose disposal in humans: a randomized, placebo-controlled study

BACKGROUND

Therapy with HIV protease inhibitors (PI) causes insulin resistance even in the absence of HIV infection, hyperlipidemia or changes in body composition. The mechanism of the effects on insulin action is unknown. In vitro studies suggest that PI selectively and rapidly inhibit the activity of the insulin-responsive glucose transporter GLUT-4. We hypothesized that a single dose of the PI indinavir resulting in therapeutic plasma concentrations would acutely decrease insulin-stimulated glucose disposal in healthy human volunteers.

METHODS

Randomized, double-blind, cross-over study comparing the effect of 1200 mg of orally administered indinavir and placebo on insulin-stimulated glucose disposal during a 180-min euglycemic, hyperinsulinemic clamp. Six healthy HIV-seronegative adult male volunteers were studied twice with 7 to 10 days between studies.

RESULTS

There were no significant differences in baseline fasting body weight, or plasma glucose, insulin, lipid and lipoprotein levels between placebo- and indinavir-treated subjects. During steady-state (t60-180 min) insulin reached comparable levels (394 +/- 13 versus 390 +/- 11 pmol/l) and glucose was clamped at approximately 4.4 mmol/l under both conditions. The average maximum concentration of indinavir was 9.4 +/- 2.2 microM and the 2-h area under the curve was 13.5 +/- 3.1 microM.h. Insulin-stimulated glucose disposal per unit of insulin (M/I) decreased in all subjects from 14.1 +/- 1.2 to 9.2 +/- 0.8 mg/kg.min per microUI/ml (95% confidence interval for change, 3.7-6.1; P < 0.001) on indinavir (average decrease, 34.1 +/- 9.2%). The non-oxidative component of total glucose disposal (storage) decreased from 3.9 +/- 1.8 to 1.9 +/- 0.9 mg/kg.min (P < 0.01). Free fatty acid levels were not significantly different at baseline and were suppressed equally with insulin administration during both studies.

CONCLUSIONS

A single dose of indinavir acutely decreases total and non-oxidative insulin-stimulated glucose disposal during a euglycemic, hyperinsulinemic clamp. Our data are compatible with the hypothesis that an acute effect of indinavir on glucose disposal in humans is mediated by a direct blockade of GLUT-4 transporters.

Transactivation of ErbB2 and ErbB3 by tumor necrosis factor-alpha and anisomycin leads to impaired insulin signaling through serine/threonine phosphorylation of IRS proteins

The cellular pathways involved in the impairment of insulin signaling by cellular stress, triggered by the inflammatory cytokine tumor necrosis factor-alpha (TNF) or by translational inhibitors like cycloheximide and anisomycin were studied. Similar to TNF, cycloheximide and anisomycin stimulated serine phosphorylation of IRS-1 and IRS-2, reduced their ability to interact with the insulin receptor, inhibited the insulin-induced tyrosine phosphorylation of IRS proteins, and diminished their association with phosphatidylinositol 3-kinase (PI3K). These defects were partially reversed by wortmannin and LY294002, indicating that a PI3K-regulated step is critical for the impairment of insulin signaling by cellular stress. Induction of cellular stress resulted in complex formation between PI3K and ErbB2/ErbB3 and enhanced PI3K activity, implicating ErbB proteins as downstream effectors of stress-induced insulin resistance. Indeed, stimulation of ErbB2/ErbB3 by NDFbeta1, the ErbB3 ligand, inhibited IRS protein tyrosine phosphorylation and recruitment of downstream effectors. Specific inhibitors of the ErbB2 tyrosine kinase abrogated the activation of ErbB2/ErbB3 and in parallel prevented the reduction in IRS protein functions. Taken together, our results suggest a novel mechanism by which cellular stress induces cross-talk between two different signaling pathways. Stress-dependent transactivation of ErbB2/ErbB3 receptors triggers a PI3K cascade that induces serine phosphorylation of IRS proteins culminating in insulin resistance.

Increased fatty acid re-esterification by PEPCK overexpression in adipose tissue leads to obesity without insulin resistance

Adipose tissue glyceroneogenesis generates glycerol 3-phosphate, which could be used for fatty acid esterification during starvation. To determine whether increased glyceroneogenesis leads to increased fat mass and to explore the role of obesity in the development of insulin resistance, we overexpressed PEPCK, a regulatory enzyme of glyceroneogenesis in adipose tissue. Transgenic mice showed a chronic increase in PEPCK activity, which led to increased glyceroneogenesis, re-esterification of free fatty acids (FFAs), increased adipocyte size and fat mass, and higher body weight. In spite of increased fat mass, transgenic mice showed decreased circulating FFAs and normal leptin levels. Moreover, glucose tolerance and whole-body insulin sensitivity were preserved. Skeletal muscle basal and insulin-stimulated glucose uptake and glycogen content were not affected, suggesting that skeletal muscle insulin sensitivity is normal in transgenic obese mice. Our results indicate the key role of PEPCK in the control of FFA re-esterification in adipose tissue and, thus, the contribution of glyceroneogenesis to fat accumulation. Moreover, they suggest that higher fat mass without increased circulating FFAs does not lead to insulin resistance or type 2 diabetes in these mice.

ACRP30/adiponectin: an adipokine regulating glucose and lipid metabolism

In recent years, we have learned that adipocytes are not merely inert storage depots for triglycerides but rather highly active cells with potent autocrine, paracrine and endocrine functions. Adipose tissue secretes a large number of physiologically active polypeptides. Although leptin remains one of the best-studied examples of an adipocyte-specific secretory factor, recent reports describe potent physiological activities for another adipocyte-specific secreted protein, adipocyte complement-related protein of 30 kDa (Acrp30). Full-length versions of Acrp30 or its proteolytic fragments decrease the postprandial rise of plasma free fatty acids and improve postabsorptive insulin-mediated suppression of hepatic glucose output. A strong correlation between plasma Acrp30 levels and systemic insulin sensitivity is well established and the protein has putative anti-atherogenic properties that are relevant for the prevention of formation of atherosclerotic plaques. The current challenge is to understand the molecular mechanisms through which the protein exerts its multiple functions.

Soluble leptin receptor and insulin resistance as determinant of sleep apnea

OBJECTIVE

To investigate the possible associations between sleep apnea syndrome, hyperinsulinemia/insulin resistance and hyperleptinemia in subjects with different degrees of body mass index.

DESIGN

To test for the presence or absence of sleep apnea syndrome in association with hyperinsulinemia/insulin resistance and hyperleptinemia.

SUBJECTS

Twenty subjects with different body mass index (mean BMI 30.9+/-4.2).

MEASUREMENTS

Insulin action and plasma soluble leptin receptor were measured by euglycemic hyperinsulinemic glucose clamp and by ELISA method, respectively. Occurrence of sleep apnea syndrome was assessed by clinical and nocturnal monitoring using a validated sleep apnea recorder.

RESULTS

The apnea/hypopnea index (AHI) was positively correlated with plasma soluble leptin receptor (0.76; P<0.001) and negatively with the degree of insulin-mediated glucose uptake (r=-0.73; P<0.001). In a multivariate analysis AHI was associated with plasma soluble leptin receptor and insulin mediated glucose uptake independently of age, gender, BMI, plasma leptin levels and PaCO(2).

CONCLUSION

Sleep apnea syndrome is associated with plasma soluble leptin receptor and insulin resistance independently of BMI.

Small molecule insulin mimetics reduce food intake and body weight and prevent development of obesity

Obesity and insulin resistance are major risk factors for a number of metabolic disorders, such as type 2 diabetes mellitus. Insulin has been suggested to function as one of the adiposity signals to the brain for modulation of energy balance. Administration of insulin into the brain reduces food intake and body weight, and mice with a genetic deletion of neuronal insulin receptors are hyperphagic and obese. However, insulin is also an anabolic factor; when administered systemically, pharmacological levels of insulin are associated with body weight gain in patients. In this study, we investigated the efficacy and feasibility of small molecule insulin mimetic compounds to regulate key parameters of energy homeostasis. Central intracerebroventricular (i.c.v.) administration of an insulin mimetic resulted in a dose-dependent reduction of food intake and body weight in rats, and altered the expression of hypothalamic genes known to regulate food intake and body weight. Oral administration of a mimetic in a mouse model of high-fat diet-induced obesity reduced body weight gain, adiposity and insulin resistance. Thus, insulin mimetics have a unique advantage over insulin in the control of body weight and hold potential as a novel anti-obesity treatment.

Hormonal regulation of adiponectin gene expression in 3T3-L1 adipocytes

Recently, it has been demonstrated that the fat-derived protein adiponectin is an important insulin-sensitizing adipocytokine which is downregulated in insulin resistance and obesity and replenishment of which in adiponectin-deficient states improves insulin sensitivity. To clarify the regulation of adiponectin gene expression, 3T3-L1 adipocytes were treated with various hormones known to induce insulin resistance in vivo and adiponectin mRNA was measured by quantitative real-time reverse transcription-polymerase chain reaction. Interestingly, treatment of 3T3-L1 cells with 100 nM insulin, 10 ng/ml tumor necrosis factor (TNF) alpha, or 100 nM dexamethasone for 16 h suppressed adiponectin gene expression by about 50 to 85% while angiotensin 2, growth hormone, and triiodothyronine did not have any effect. Furthermore, insulin reduced the level of adiponectin mRNA in a dose- and time-dependent fashion with inhibition detectable at concentrations as low as 10 nM insulin and as early as 4 h after effector addition. The inhibitory effect of insulin was partially reversed by pretreatment of 3T3-L1 cells with pharmacological inhibitors of p44/42 mitogen-activated protein (MAP) kinase, phosphatidylinositol (PI) 3-kinase, and p70S6 kinase. Moreover, the negative effects of insulin, TNFalpha, and dexamethasone on adiponectin gene expression could be completely reversed by withdrawal of the hormones for 24 h. Taken together, our results suggest that adiponectin gene expression is reversibly downregulated by insulin, TNFalpha, and dexamethasone. The data support the concept of adiponectin being an important selectively controlled modulator of insulin sensitivity.

Leptin stimulates fatty-acid oxidation by activating AMP-activated protein kinase

Leptin is a hormone secreted by adipocytes that plays a pivotal role in regulating food intake, energy expenditure and neuroendocrine function. Leptin stimulates the oxidation of fatty acids and the uptake of glucose, and prevents the accumulation of lipids in nonadipose tissues, which can lead to functional impairments known as "lipotoxicity". The signalling pathways that mediate the metabolic effects of leptin remain undefined. The 5'-AMP-activated protein kinase (AMPK) potently stimulates fatty-acid oxidation in muscle by inhibiting the activity of acetyl coenzyme A carboxylase (ACC). AMPK is a heterotrimeric enzyme that is conserved from yeast to humans and functions as a 'fuel gauge' to monitor the status of cellular energy. Here we show that leptin selectively stimulates phosphorylation and activation of the alpha2 catalytic subunit of AMPK (alpha2 AMPK) in skeletal muscle, thus establishing a previously unknown signalling pathway for leptin. Early activation of AMPK occurs by leptin acting directly on muscle, whereas later activation depends on leptin functioning through the hypothalamic-sympathetic nervous system axis. In parallel with its activation of AMPK, leptin suppresses the activity of ACC, thereby stimulating the oxidation of fatty acids in muscle. Blocking AMPK activation inhibits the phosphorylation of ACC stimulated by leptin. Our data identify AMPK as a principal mediator of the effects of leptin on fatty-acid metabolism in muscle.

Phosphorylation of Ser307 in insulin receptor substrate-1 blocks interactions with the insulin receptor and inhibits insulin action

Serine phosphorylation of insulin receptor substrate-1 (IRS-1) inhibits insulin signal transduction in a variety of cell backgrounds, which might contribute to peripheral insulin resistance. However, because of the large number of potential phosphorylation sites, the mechanism of inhibition has been difficult to determine. One serine residue located near the phosphotyrosine-binding (PTB) domain in IRS-1 (Ser(307) in rat IRS-1 or Ser(312) in human IRS-1) is phosphorylated via several mechanisms, including insulin-stimulated kinases or stress-activated kinases like JNK1. During a yeast tri-hybrid assay, phosphorylation of Ser(307) by JNK1 disrupted the interaction between the catalytic domain of the insulin receptor and the PTB domain of IRS-1. In 32D myeloid progenitor cells, phosphorylation of Ser(307) inhibited insulin stimulation of the phosphatidylinositol 3-kinase and MAPK cascades. These results suggest that inhibition of PTB domain function in IRS-1 by phosphorylation of Ser(307) (Ser(312) in human IRS-1) might be a general mechanism to regulate insulin signaling.

Predictability of childhood adiposity and insulin for developing insulin resistance syndrome (syndrome X) in young adulthood: the Bogalusa Heart Study

The occurrence of insulin resistance syndrome (syndrome X) is common in the general population. However, information is scant on the childhood predictors of syndrome X. This study examined the relative contribution of childhood adiposity and insulin to the adulthood risk of developing syndrome X in a biracial (black-white) community-based longitudinal cohort (n = 745; baseline age, 8-17 years; mean +/- SD follow-up period, 11.6 +/- 3.4 years). The four criterion risk variables considered were the highest quartile (specific for age, race, sex, and study year) of 1) BMI, 2) fasting insulin, 3) systolic or mean arterial blood pressure, and 4) total cholesterol to HDL cholesterol ratio or triglycerides to HDL cholesterol ratio. Clustering was defined as the combination of all four risk variables. In addition to the criterion risk variables, clustered adults had adverse levels of total cholesterol, triglycerides, LDL cholesterol, HDL cholesterol, diastolic blood pressure, and glucose compared with those who did not cluster (P < 0.001). Childhood variables, except glucose, showed adverse trends with increasing number of criterion risk variables present in adulthood (P for trend, 0.01-0.0001). The proportion of individuals who developed clustering as adults increased across childhood BMI (P for trend <0.0001) and insulin (P for trend <0.01) quartiles. The relationship to childhood BMI remained significant even after adjusting for childhood insulin. In contrast, corresponding association with childhood insulin disappeared after adjusting for childhood BMI. In a logistic regression model, childhood BMI and insulin were significant predictors of adulthood clustering, with BMI being the strongest and showing a curvilinear relationship. Using an insulin resistance index instead of insulin did not change the above findings. These results indicate that childhood obesity is a powerful predictor of development of syndrome X and underscore the importance of weight control early in life.

Adipogenesis: forces that tip the scales

The balance of contradictory signals experienced by preadipocytes influences whether these cells undergo adipogenesis. In addition to the endocrine system, these signals originate from the preadipocytes themselves or operate as part of a feedback loop involving mature adipocytes. The factors that regulate adipogenesis either promote or block the cascade of transcription factors that coordinate the differentiation process. Some of the positive factors reviewed include insulin-like growth factor I, macrophage colony-stimulating factor, fatty acids, prostaglandins and glucocorticoids, and negative factors reviewed include Wnt, transforming growth factor beta, inflammatory cytokines and prostaglandin F(2alpha). Tipping the scales towards or away from adipogenesis has profound implications for human health. In this review, we describe recent contributions to the field and will focus on factors that probably play a role in vivo.

Phosphatidylinositol 3-kinase redistribution is associated with skeletal muscle insulin resistance in gestational diabetes mellitus

Insulin resistance during pregnancy provokes gestational diabetes mellitus (GDM); however, the cellular mechanisms for this type of insulin resistance are not well understood. We evaluated the mechanisms(s) for insulin resistance in skeletal muscle from an animal model of spontaneous GDM, the heterozygous C57BL/KsJ-(db/+) mouse. Pregnancy triggered a novel functional redistribution of the insulin-signaling environment in skeletal muscle in vivo. This environment preferentially increases a pool of phosphatidylinositol (PI) 3-kinase activity associated with the insulin receptor, away from insulin receptor substrate (IRS)-1. In conjunction with the redistribution of PI 3-kinase to the insulin receptor, there is a selective increase in activation of downstream serine kinases Akt and p70S6. Furthermore, we show that redistribution of PI 3-kinase to the insulin receptor increases insulin-stimulated IRS-1 serine phosphorylation, impairs IRS-1 expression and its tyrosine phosphorylation, and decreases the ability of IRS-1 to bind and activate PI 3-kinase in response to insulin. Thus, the pool of IRS-1-associated PI 3-kinase activity is reduced, resulting in the inability of insulin to stimulate GLUT4 translocation to the plasma membrane. These defects are unique to pregnancy and suggest that redistribution of PI 3-kinase to the insulin receptor may be a primary defect underlying insulin resistance in skeletal muscle during gestational diabetes.

Increasing fructose 2,6-bisphosphate overcomes hepatic insulin resistance of type 2 diabetes

Hepatic glucose production is increased as a metabolic consequence of insulin resistance in type 2 diabetes. Because fructose 2,6-bisphosphate is an important regulator of hepatic glucose production, we used adenovirus-mediated enzyme overexpression to increase hepatic fructose 2,6-bisphosphate to determine if the hyperglycemia in KK mice, polygenic models of type 2 diabetes, could be ameliorated by reduction of hepatic glucose production. Seven days after treatment with virus encoding a mutant 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase designed to increase fructose 2,6-bisphosphate levels, plasma glucose, lipids, and insulin were significantly reduced in KK/H1J and KK.Cg-A(y)/J mice. Moreover, high fructose 2,6-bisphosphate levels downregulated glucose-6-phosphatase and upregulated glucokinase gene expression, thereby reversing the insulin-resistant pattern of hepatic gene expression of these two key glucose-metabolic enzymes. The increased hepatic fructose 2,6-bisphosphate also reduced adiposity in both KK mice. These results clearly indicate that increasing hepatic fructose 2,6-bisphosphate overcomes the impairment of insulin in suppressing hepatic glucose production, and it provides a potential therapy for type 2 diabetes.

Resistin and obesity-associated insulin resistance

Obesity is a major risk factor for insulin resistance and type 2 diabetes mellitus. Adipocytes secrete numerous substances that might contribute to peripheral insulin sensitivity. These include leptin, tumor necrosis factor alpha, Acrp30/adiponectin/adipoQ and interleukin 6, the potential roles of which are briefly reviewed here. Thiazolidinedione (TZD) antidiabetic drugs regulate gene transcription by binding to peroxisome proliferator activated receptor gamma, a nuclear hormone receptor found at its highest levels in adipocytes. A search for genes that are downregulated by TZDs in mouse adipocytes led to the discovery of an adipose-specific secreted protein called resistin. Resistin circulates in the mouse, with increased levels in obesity, and has effects on glucose homeostasis that oppose those of insulin. Thus, resistin is a potential link between TZDs, obesity and insulin resistance in the mouse. Future studies must address the mechanism of action and biological role of resistin and related family members in mice and humans.

Resistin gene expression in human adipocytes is not related to insulin resistance

OBJECTIVES

Obesity is an important risk factor for the development of insulin resistance and type 2 diabetes. Recently, a newly described circulating hormone resistin, which is expressed primarily in adipocytes, has been shown to antagonize insulin action in mice. Resistin, therefore, has been suggested to play a role in the pathogenesis of insulin resistance.

RESEARCH METHODS AND PROCEDURES

We studied the expression of the resistin gene in primary cultured human adipocytes and preadipocytes. We also examined resistin gene expression in subcutaneous abdominal adipocytes in women (n = 24) over a wide range of body weight and insulin sensitivity.

RESULTS

Whereas resistin gene expression was barely detectable in mature adipocytes, it was highly expressed in preadipocytes. Adipogenic differentiation of preadipocytes was associated with a time-dependent down-regulation of resistin gene expression. There was no relationship between body weight, insulin sensitivity, or other metabolic parameters and adipocyte resistin gene expression in the clinical study.

DISCUSSION

Together these findings do not support an important role of adipose-tissue resistin gene expression in human insulin resistance.

SOCS-3 inhibits insulin signaling and is up-regulated in response to tumor necrosis factor-alpha in the adipose tissue of obese mice

SOCS (suppressor of cytokine signaling) proteins are inhibitors of cytokine signaling involved in negative feedback loops. We have recently shown that insulin increases SOCS-3 mRNA expression in 3T3-L1 adipocytes. When expressed, SOCS-3 binds to phosphorylated Tyr(960) of the insulin receptor and prevents Stat 5B activation by insulin. Here we show that in COS-7 cells SOCS-3 decreases insulin-induced insulin receptor substrate 1 (IRS-1) tyrosine phosphorylation and its association with p85, a regulatory subunit of phosphatidylinositol-3 kinase. This mechanism points to a function of SOCS-3 in insulin resistance. Interestingly, SOCS-3 expression was found to be increased in the adipose tissue of obese mice, but not in the liver and muscle of these animals. Two polypeptides known to be elevated during obesity, insulin and tumor necrosis factor-alpha (TNF-alpha), induce SOCS-3 mRNA expression in mice. Insulin induces a transient expression of SOCS-3 in the liver, muscle, and the white adipose tissue (WAT). Strikingly, TNF-alpha induced a sustained SOCS-3 expression, essentially in the WAT. Moreover, transgenic ob/ob mice lacking both TNF receptors have a pronounced decrease in SOCS-3 expression in the WAT compared with ob/ob mice, providing genetic evidence for a function of this cytokine in obesity-induced SOCS-3 expression. As SOCS-3 appears as a TNF-alpha target gene that is elevated during obesity, and as SOCS-3 antagonizes insulin-induced IRS-1 tyrosine phosphorylation, we suggest that it is a player in the development of insulin resistance.

Endogenous glucose production is inhibited by the adipose-derived protein Acrp30

Intraperitoneal injection of purified recombinant Acrp30 lowers glucose levels in mice. To gain insight into the mechanism(s) of this hypoglycemic effect, purified recombinant Acrp30 was infused in conscious mice during a pancreatic euglycemic clamp. In the presence of physiological hyperinsulinemia, this treatment increased circulating Acrp30 levels by approximately twofold and stimulated glucose metabolism. The effect of Acrp30 on in vivo insulin action was completely accounted for by a 65% reduction in the rate of glucose production. Similarly, glucose flux through glucose-6-phosphatase (G6Pase) decreased with Acrp30, whereas the activity of the direct pathway of glucose-6-phosphate biosynthesis, an index of hepatic glucose phosphorylation, increased significantly. Acrp30 did not affect the rates of glucose uptake, glycolysis, or glycogen synthesis. These results indicate that an acute increase in circulating Acrp30 levels lowers hepatic glucose production without affecting peripheral glucose uptake. Hepatic expression of the gluconeogenic enzymes phosphoenolpyruvate carboxykinase and G6Pase mRNAs was reduced by more than 50% following Acrp30 infusion compared with vehicle infusion. Thus, a moderate rise in circulating levels of the adipose-derived protein Acrp30 inhibits both the expression of hepatic gluconeogenic enzymes and the rate of endogenous glucose production.

The Q121 PC-1 variant and obesity have additive and independent effects in causing insulin resistance

PC-1 is a membrane glycoprotein that impairs insulin receptor function. Its K121Q polymorphism is a genetic determinant of insulin resistance. We investigated whether the PC-1 gene modulates insulin sensitivity independently of weight status (i.e. both in nonobese and obese individuals). Nondiabetic subjects [164 males, 267 females; age, 37 +/- 0.6 yr, mean +/- SEM; body mass index (BMI), 32.7 +/- 0.5 kg/m(2)], who were subdivided into 220 nonobese (BMI < or = 29.9) and 211 obese, were studied. Although subjects were nondiabetic by selection criteria, plasma insulin concentrations during oral glucose tolerance test were higher (P < 0.05) in Q allele-carrying subjects (K121Q or Q121Q genotypes), compared with K121K individuals, in both the nonobese and obese groups. Insulin sensitivity, measured by euglycemic clamp in a representative subgroup of 131 of 431 randomly selected subjects, progressively decreased (P < 0.001) from nonobese K121K [n = 61; glucose disposal (M) = 34.9 +/- 1.1 micromol/kg/min] to nonobese Q (n = 21; M = 29.9 +/- 2.0), obese K121K (n = 31, M = 18.5 +/- 1.2), and obese Q (n = 18, M = 15.5 +/- 1.2) carriers. The K121Q polymorphism was correlated with insulin sensitivity independently (P < 0.05) of BMI, gender, age, and waist circumference. In conclusion, the Q121 PC-1 variant and obesity have independent and additive effects in causing insulin resistance.

Negative and independent influence of apolipoprotein E on C-reactive protein (CRP) concentration in obese adults. Potential anti-inflammatory role of apoE in vivo

BACKGROUND

Obesity is frequently associated with an increase in the early inflammation marker C-reactive protein (CRP), insulin resistance and changes in lipoprotein metabolism. Increased CRP is known as an independent cardiovascular risk factor. Since the apolipoproteins (apo) E and CIII components of HDL are associated with reduced cardiovascular risk and since apoE has in vitro anti-inflammatory effect, we have investigated the relationships between apoE, apoCIII (in apoB and non apoB containing lipoproteins) and CRP in obese adults.

METHODS

The following parameters from 34 healthy obese fasting women (age 22-64 y, body mass index (BMI) 28-68 kg/m2) were measured: (1) ApoE and apoCIII, in total plasma, in apoB- (E LpB, CIII LpB) and non-apoB-containing lipoproteins (E LpnonB, CIII LpnonB); (2) CRP and cytokine secreted by adipose tissue (TNF-alpha and its soluble receptor TNFR2); (3) triglyceride, HDL-cholesterol, systolic blood pressure, diastolic blood pressure, waist and hip circumferences, insulin, glucose. HOMA, a marker of insulin sensitivity, and the ratio E/CIII in LpB and LpnonB were calculated.

RESULTS

CRP was positively correlated with BMI (P<0.05), waist circumference (WC, P<0.05), triglyceride (P<0.05) and negatively correlated with apoE (P<0.01) and E LpnonB (P<0.05). Two multiple regression models including parameters related to CRP with a P<0.25 were run stepwise to assess their independent contribution to CRP concentration. In the first model (including BMI, WC, HOMA, insulin, triglyceride, apoE, E LpnonB), apoE was the best predictor of CRP (P=0.01) together with triglyceride (P=0.02) and BMI (P=0.08). The second model took into account E/CIII LpnonB ratio with the parameters included in the first model. In this second model, E/CIII LpnonB was the best predictor of CRP (P=0.007), explaining 39% of CRP variance.

CONCLUSION

ApoE is strongly correlated with CRP and could have an anti-inflammatory effect in vivo in obese subjects. This correlation could be limited to LpnonB lipoproteins, depending on their apoE and CIII relative content.

Enhanced glucose uptake into adipose tissue induced by early growth restriction augments excursions in plasma leptin response evoked by changes in insulin status

OBJECTIVE

The study used a rat model of moderate protein restriction exclusively during fetal and early neonatal life, which has been established to cause intrauterine early growth retardation, to investigate possible association between adipocyte glucose utilisation and leptin secretion in vivo.

DESIGN

These rats, termed early protein restricted, were transferred to a diet containing the standard amount of protein at weaning and remained on this diet til adulthood, at which time adipocyte glucose utilisation and leptin secretion was compared with that of age-matched controls. Insulin status was modulated by acute (2 h) insulin infusion at a constant rate (4.2 mU/min per kg) to elevate insulin to the high physiological range. Euglycaemia was maintained by variable glucose infusion.

MEASUREMENTS

Glucose utilisation was measured in vivo in conscious unrestrained rats using 2-deoxy[1-3H] glucose. Leptin concentrations (measured by radioimmunoassay) and whole-body glucose kinetics (measured using [3-3H] glucose) were studied in the postabsorptive state and after acute insulin stimulation.

RESULTS

Adipose-tissue glucose utilisation rates in vivo tended to be higher in the post-absorptive state and were consistently 1.8-3.0-fold higher after insulin stimulation in the early-protein-restricted group compared with the control group. Both the absolute increase in leptin concentration elicited by hyperinsulinaemia and the magnitude of the effect of insulin to elevate plasma leptin levels were greater in the early-protein-restricted group compared with the control group (by 2.2-fold and 1.6-fold, respectively). The effect of insulin to stimulate R(d) was much greater in the early-protein-restricted group (4.1-fold) than in the control group (2.2-fold) and the absolute increase in R(d) elicited by insulin was 43% higher in the early-protein-restricted group than in the control group.

CONCLUSIONS

It is concluded that poor early growth enhances the acute leptin response to changes in insulin status through programmed changes in adipocyte glucose handling.