Liver Insulin Sensitivity Research Articles

Paradoxical effects of increased expression of PGC-1α on muscle mitochondrial function and insulin-stimulated muscle glucose metabolism

Peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1alpha has been shown to play critical roles in regulating mitochondria biogenesis, respiration, and muscle oxidative phenotype. Furthermore, reductions in the expression of PGC-1alpha in muscle have been implicated in the pathogenesis of type 2 diabetes. To determine the effect of increased muscle-specific PGC-1alpha expression on muscle mitochondrial function and glucose and lipid metabolism in vivo, we examined body composition, energy balance, and liver and muscle insulin sensitivity by hyperinsulinemic-euglycemic clamp studies and muscle energetics by using (31)P magnetic resonance spectroscopy in transgenic mice. Increased expression of PGC-1alpha in muscle resulted in a 2.4-fold increase in mitochondrial density, which was associated with an approximately 60% increase in the unidirectional rate of ATP synthesis. Surprisingly, there was no effect of increased muscle PGC-1alpha expression on whole-body energy expenditure, and PGC-1alpha transgenic mice were more prone to fat-induced insulin resistance because of decreased insulin-stimulated muscle glucose uptake. The reduced insulin-stimulated muscle glucose uptake could most likely be attributed to a relative increase in fatty acid delivery/triglyceride reesterfication, as reflected by increased expression of CD36, acyl-CoA:diacylglycerol acyltransferase1, and mitochondrial acyl-CoA:glycerol-sn-3-phosphate acyltransferase, that may have exceeded mitochondrial fatty acid oxidation, resulting in increased intracellular lipid accumulation and an increase in the membrane to cytosol diacylglycerol content. This, in turn, caused activation of PKC, decreased insulin signaling at the level of insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation, and skeletal muscle insulin resistance.

Amino acid supplementation decreases plasma and liver triacylglycerols in elderly

Objective Hypertriglyceridemia is a risk factor for coronary heart disease. The aim of this study was to determine the effect of amino acid (AA) supplementation on plasma, liver, and muscle lipid concentrations and insulin sensitivity in the elderly. Methods Twelve impaired glucose tolerant elderly (mean ± SD 67.0 ± 5.6 y of age, seven women and five men) ingested 11 g of essential AAs plus arginine twice a day for 16 wk, after a 7-wk control run-in. Diet and activity were not otherwise modified. Plasma lipid concentrations and oral glucose tolerance were measured every fourth week and tissue lipid concentrations (magnetic resonance spectroscopy) every eighth week. Results No changes in plasma lipids were observed during the control run-in. AA supplementation lowered plasma triacylglycerol (TG; P < 0.001), total cholesterol ( P = 0.048), and very low-density lipoprotein cholesterol ( P < 0.001) concentrations. Plasma TG decreased ∼20% from the initial value of 1.45 ± 0.18 mmol/L (mean ± SE, 128 ± 16 mg/dL), with the greatest decrease in the subjects starting out with the highest concentrations ( r = −0.83). Similarly, liver fat content (liver TG/Intralipid standard) decreased ∼50% from the initial value of 0.34 ± 0.06 ( P = 0.021, n = 8), with the greatest decrease in the subjects who initially had the highest values ( r = −0.86). Intramuscular fat content and insulin sensitivity did not change. Conclusion Diet supplementation with AAs lowers plasma TG, total cholesterol, and very low-density lipoprotein cholesterol concentrations and liver lipid content in impaired glucose tolerant elderly. AA supplementation may have a potential role in the treatment of hypertriglyceridemia or hepatic steatosis.

Interactions of the advanced glycation end product inhibitor pyridoxamine and the antioxidant α-lipoic acid on insulin resistance in the obese Zucker rat

Oxidative stress and protein glycation can contribute to the development of insulin resistance and complications associated with type 2 diabetes mellitus. The antioxidant α-lipoic acid (ALA) reduces oxidative stress and the formation of advanced glycation end products (AGEs) and improves insulin sensitivity in skeletal muscle and liver. The AGE inhibitor pyridoxamine (PM) prevents irreversible protein glycation, thereby reducing various diabetic complications. The potential interactive effects of ALA and PM in the treatment of whole-body and skeletal muscle insulin resistance have not been investigated. Therefore, this study was designed to determine the effects of combined ALA and PM treatments on reducing muscle oxidative stress and ameliorating insulin resistance in prediabetic obese Zucker rats. Obese Zucker rats were assigned to either a control group or to a treatment group receiving daily injections of the R-(+)-enantiomer of ALA (R-ALA, 92 mg/kg) or PM (60 mg/kg), individually or in combination, for 6 weeks. The individual and combined treatments with R-ALA and PM were effective in significantly ( P < .05) reducing plantaris muscle protein carbonyls (33%-40%) and urine-conjugated dienes (22%-38%), markers of oxidative stress. The R-ALA and PM in combination resulted in the largest reductions of fasting plasma glucose (23%), insulin (16%), and free fatty acids (24%) and of muscle triglycerides (45%) compared with alterations elicited by individual treatment with R-ALA or PM. Moreover, the combination of R-ALA and PM elicited the greatest enhancement of whole-body insulin sensitivity both in the fasted state and during an oral glucose tolerance test. Finally, combined R-ALA/PM treatments maintained the 44% enhancement of in vitro insulin-mediated glucose transport activity in soleus muscle of obese Zucker rats treated with R-ALA alone. Collectively, these results document a beneficial interaction of the antioxidant R-ALA and the AGE inhibitor PM in the treatment of whole-body and skeletal muscle insulin resistance in obese Zucker rats.

Fat Talks, Liver and Muscle Listen

Fat metabolism and glucose homeostasis are processes that are highly interconnected. Cao et al. (2008) now take this concept a step further by identifying a fatty acid metabolite generated in adipose tissue that regulates insulin sensitivity in liver and muscle.

Effects of metformin on peripheral insulin sensitivity and intracellular lipid contents in muscle and liver of overweight Japanese subjects

Recent studies suggest that insulin resistance is associated with increased intrahepatic lipid (IHL) and intramyocellular lipid (IMCL) contents. While metformin improves insulin resistance mainly in liver, its effects on IHL and IMCL have not been clarified yet. The aim of this study was to investigate the effects of low-dose metformin (750 mg/day) on peripheral insulin sensitivity, IHL and IMCL. Before and 3 months after low-dose metformin therapy, eight overweight/obese Japanese subjects [body mass index (BMI) >25 kg/m2] were studied with blood sampling, measurement of IHL and IMCL by 1H magnetic resonance spectroscopy and glucose infusion rate (GIR) during euglycaemic-hyperinsulinaemic clamp as an index of peripheral insulin sensitivity. A 3-month low-dose metformin therapy did not alter body weight, total body fat, fat distribution or physical activity level but increased GIR by 31% (from 6.24 +/- 0.86 to 7.82 +/- 0.82 mg/kg/min, p < 0.01). Although metformin treatment did not alter IMCL (from 4.1 +/- 1.0 to 4.2 +/- 0.9, not significant), it decreased IHL by 21% (from 15.9 +/- 2.8 to 11.8 +/- 2.2%, p < 0.05). A 3-month low-dose metformin treatment improved peripheral insulin sensitivity and reduced IHL, without significantly changing BMI, adiposity or IMCL.

Nonalcoholic fatty liver disease is associated with hepatic and skeletal muscle insulin resistance in overweight adolescents

BackgroundNonalcoholic fatty liver disease (NAFLD) and insulin resistance are common in overweight adolescents. ObjectiveThe purpose of this study was to determine the relation between NAFLD and insulin sensitivity in liver and skeletal muscle by studying overweight adolescents with a normal or high intrahepatic triglyceride (IHTG) content, who were matched for age, sex, body mass index (BMI; in kg/m2), and Tanner stage. DesignStable-isotope-labeled tracer infusion and the hyperinsulinemic-euglycemic clamp procedure were used to assess skeletal muscle and hepatic insulin sensitivity, and magnetic resonance spectroscopy was used to assess the IHTG content in 10 overweight (BMI = 35.9 ± 1.3) adolescents with NAFLD (IHTG = 28.4 ± 3.4%) and 10 overweight (BMI = 36.6 ± 1.5) adolescents with a normal IHTG content (3.3 ± 0.5%). ResultsThe baseline plasma glucose concentration and the rate of appearance of glucose in plasma were the same in subjects with a normal (87.1 ± 1.2 mg/dL, 16.2 ± 1.1 μmol · kg fat-free mass−1 · min−1) or high (89.2 ± 2.5 mg/dL, 16.3 ± 1.2 μmol · kg fat-free mass−1 · min−1) IHTG content. However, compared with subjects who had a normal IHTG content, subjects with NAFLD had a lower hepatic insulin sensitivity index, based on baseline glucose kinetics and insulin concentrations (4.0 ± 0.5 compared with 2.4 ± 0.4; P < 0.05) and an impaired increase in glucose uptake during insulin infusion (169 ± 28.1% compared with 67 ± 9.6% above baseline; P < 0.01). In addition, the plasma triglyceride concentration was greater and the plasma HDL-cholesterol concentration was lower in subjects with NAFLD than in those with a normal IHTG content. ConclusionAn elevated IHTG content in overweight adolescents is associated with dyslipidemia and with insulin-resistant glucose metabolism in both liver and skeletal muscle.

Reduction of JNK1 expression with antisense oligonucleotide improves adiposity in obese mice

To investigate the role of JNK1 in metabolism, male ob/ob and diet-induced obese mice were treated with a JNK1-specific antisense oligonucleotide (ASO) or control ASO at 25 mg/kg or saline twice/wk for 6 and 7 wk, respectively. JNK1 ASO reduced JNK1 mRNA and activity by 65-95% in liver and fat tissues in both models. Compared with controls, treatment with JNK1 ASO did not change food intake but lowered body weight, fat pad weight, and whole body fat content. The treatment increased metabolic rate. In addition, the treatment markedly reduced plasma cholesterol levels and improved liver steatosis and insulin sensitivity. These positive observations were accompanied by the following changes: 1) increased mRNA levels of AR-beta(3) and UCP1 by >60% in BAT, 2) reduced mRNA levels of ACC1, ACC2, FAS, SCD1, DGAT1, DGAT2, and RBP4 by 30-60% in WAT, and 3) reduced mRNA levels of ACC1, FAS, G-6-Pase, and PKCepsilon by 40-70% and increased levels of UCP2 and PPARalpha by more than twofold in liver. JNK1 ASO-treated mice demonstrated reduced levels of pIRS-1 Ser(302) and pIRS-1 Ser(307) and increased levels of pAkt Ser(473) in liver and fat in response to insulin. JNK1 ASO-transfected mouse hepatocytes showed decreased rates of de novo sterol and fatty acid synthesis and an increased rate of fatty acid oxidation. These results indicate that inhibition of JNK1 expression in major peripheral tissues can improve adiposity via increasing fuel combustion and decreasing lipogenesis and could therefore provide clinical benefit for the treatment of obesity and related metabolic abnormalities.

The lowering of hepatic fatty acid uptake improves liver function and insulin sensitivity without affecting hepatic fat content in humans

Lipolysis may regulate liver free fatty acid (FFA) uptake and triglyceride accumulation; both are potential causes of insulin resistance and liver damage. We evaluated whether 1) systemic FFA release is the major determinant of liver FFA uptake in fasting humans in vivo and 2) the beneficial metabolic effects of FFA lowering can be explained by a reduction in liver triglyceride content. Sixteen healthy subjects were subdivided in two groups of similar characteristics to undergo positron emission tomography with [(11)C]acetate and [(11)C]palmitate to quantify liver FFA metabolism (n = 8), or magnetic resonance spectroscopy (MRS) to measure hepatic fat content (n = 8), before and after the acute lowering of circulating FFAs by using the antilipolytic agent acipimox. MRS was again repeated after a 1-wk treatment period. Acipimox suppressed FFA levels while stimulating hepatic fractional extraction of FFAs (P < 0.05). As a result, fasting liver FFA uptake was decreased by 79% (P = 0.0002) in tight association with lipolysis (r = 0.996, P < 0.0001). The 1-wk treatment induced a significant improvement in systemic (+30%) and liver (+70%) insulin sensitivity (P < 0.05) and decreased circulating triglycerides (-20%, P = 0.06) and liver enzymes (ALT -20%, P = 0.03). No change in liver fat content was observed after either acute or sustained FFA suppression. We conclude that acute and sustained inhibitions of lipolysis and liver FFA uptake fail to deplete liver fat in healthy human subjects. Liver FFA uptake was decreased in proportion to FFA delivery. As a consequence, liver and systemic insulin sensitivity were improved, together with liver function, independently of changes in hepatic triglyceride accumulation.

Adipose Fibroblast Growth Factor 21 Is Up-Regulated by Peroxisome Proliferator-Activated Receptor γ and Altered Metabolic States

Adipose tissue is a metabolically responsive endocrine organ that secretes a myriad of adipokines. Antidiabetic drugs such as peroxisome proliferator-activated receptor (PPAR) gamma agonists target adipose tissue gene expression and correct hyperglycemia via whole-body insulin sensitization. The mechanism by which altered gene expression in adipose tissue affects liver and muscle insulin sensitivity (and thus glucose homeostasis) is not fully understood. One possible mechanism involves the alteration in adipokine secretion, in particular the up-regulation of secreted factors that increase whole-body insulin sensitivity. Here, we report the use of transcriptional profiling to identify genes encoding for secreted proteins the expression of which is regulated by PPARgamma agonists. Of the 379 genes robustly regulated by two structurally distinct PPARgamma agonists in the epididymal white adipose tissue (EWAT) of db/db mice, 33 encoded for known secreted proteins, one of which was FGF21. Although FGF21 was recently reported to be up-regulated in cultured adipocytes by PPARgamma agonists and in liver by PPARalpha agonists and induction of ketotic states, we demonstrate that the protein is transcriptionally up-regulated in adipose tissue in vivo by PPARgamma agonist treatment and under a variety of physiological conditions, including fasting and high fat diet feeding. In addition, we found that circulating levels of FGF21 protein were increased upon treatment with PPARgamma agonists and under ketogenic states. These results suggest a role for FGF21 in mediating the antidiabetic activities of PPARgamma agonists.

Lipid Mobilization Following Roux-en-Y Gastric Bypass Examined by Magnetic Resonance Imaging and Spectroscopy

Recent developments of magnetic resonance imaging (MRI) and spectroscopy have made it possible to quantify lipid deposited in different tissues. To what extent an improvement of glucose tolerance shortly after Roux-en-Y gastric bypass surgery (RYGBP) is reflected in lipid levels in liver and skeletal muscle, markers of insulin resistance, has not been clarified. Whole-body MRI and MR spectroscopy (MRS) of liver and muscle and measurements of biochemical markers of glucose and lipid metabolism were performed at baseline and 1, 6, and 12 months following surgery in seven morbidly obese women. Volumes of adipose tissue depots and liver and muscle lipids were assessed from the MRI/MRS data. At 1 month postoperatively, body mass index and visceral and subcutaneous adipose tissues were reduced by 9%, 26%, and 10%, respectively, whereas no reductions in intrahepatocellular or skeletal intramyocellular lipid concentrations were found. Free fatty acid and beta-hydroxybutyrate levels were elevated two- and sixfold, respectively; glucose and insulin levels were lowered, indicating increased insulin sensitivity. Further weight loss up to 1 year was associated with reductions in all investigated lipid depots investigated, with the exception of the intramyocellular compartment. RYGBP causes rapid lipid mobilization from visceral and subcutaneous adipose depots and enhanced free fatty acid flux to the liver. An exceptional disconnection between liver fat and insulin sensitivity occurs in the early dynamic phase after surgery. However, in the late phase, the energy restriction imposed by the surgical procedure also reduces the liver lipids, but not the intramyocellular lipids.

Role of stearoyl-CoA desaturase in human metabolic disease

Stearoyl-CoA desaturase (SCD) is a highly conserved Δ-9 desaturase that converts saturated fatty acids into monounsaturated fatty acids. Mice lacking the Scd1 gene are characterized by decreased adiposity and increased lipid oxidation and insulin sensitivity in liver and skeletal muscle. Two isoforms of SCD have also been identified in humans, and there appears to be an association between SCD expression and adiposity. This review will provide a brief update of recent progress in understanding the role of SCD in mediating the progression of disease in human populations, with a special emphasis on metabolic disease.

Association between dietary fat intake, liver fat, and insulin sensitivity in sedentary, abdominally obese, older men

Whether dietary fat influences the interaction between liver fat, visceral adipose tissue (VAT), and metabolic risk is unclear. Thus, we sought to determine the independent associations between 1 and 10 d dietary fat intake, liver fat, and VAT on insulin sensitivity using a cross-sectional design. Liver fat score (LFS) was assessed by computed tomography and VAT by magnetic resonance imaging in 42 abdominally obese older men. Insulin sensitivity was measured by the hyperinsulinemic-euglycemic clamp technique. Diet composition was determined from self-recorded diet records for 1 and 10 d preceding the assessment of LFS. LFS was positively associated with the 10 d average fat and alcohol intake, but not with any 1 d average dietary variables. VAT (r = -0.36) and LFS (r = -0.32) were both negatively correlated with insulin sensitivity (p < 0.05). When LFS and VAT were entered in the same model, only VAT was an independent predictor of insulin sensitivity (p < 0.05). Control for the average 10 d alcohol consumption and fat intake improved the association between insulin sensitivity and LFS (from r = -0.32, p > 0.10 to r = -0.49, p < 0.05), but not VAT. In fact, after control for the 10 d dietary variables, both LFS and VAT were independent predictors of insulin sensitivity (p < 0.05). This was not true for any of the 1 d dietary intake variables. In conclusion, liver fat is associated with dietary fat intake and alcohol consumption over 10 d, but not 1 d. Furthermore, dietary habits may influence the relationship between liver fat and insulin sensitivity.

Metabolic Abnormalities Underlying the Different Prediabetic Phenotypes in Obese Adolescents

The aim of this study was to define the metabolic abnormalities underlying the prediabetic status of isolated impaired fasting glucose (IFG), isolated impaired glucose tolerance (IGT), and combined IFG/IGT in obese youth. We used state-of-the-art techniques (hyperinsulinemic-euglycemic and hyperglycemic clamps), applying a model of glucose-stimulated insulin secretion to the glucose and C-peptide concentration, in 40 normal glucose tolerance (NGT), 17 IFG, 23 IGT, and 11 IFG/IGT obese adolescents. Percent fat (by dual-energy x-ray absorptiometry), age, gender and ethnicity were comparable among groups. Peripheral insulin sensitivity was similar between the IFG and NGT groups. In contrast, the IGT and IFG/IGT groups showed marked reductions in peripheral insulin sensitivity (P < 0.002). Basal hepatic insulin resistance index (basal hepatic glucose production x fasting plasma insulin) was significantly increased in IFG, IGT, and IFG/IGT (P < 0.009) compared with NGT. Glucose sensitivity of first-phase insulin secretion was progressively lower in IFG, IGT, and IFG/IGT compared with NGT. Glucose sensitivity of second-phase secretion showed a statistically significant defect only in the IFG/IGT group. In a multivariate regression analysis, glucose sensitivity of first-phase secretion and basal insulin secretion rate were significant independent predictors of FPG (total r(2) = 25.9%). IFG, in obese adolescents, is linked primarily to alterations in glucose sensitivity of first-phase insulin secretion and liver insulin sensitivity. The IGT group is affected by a more severe degree of peripheral insulin resistance and reduction in first-phase secretion. IFG/IGT is hallmarked by a profound insulin resistance and by a new additional defect in second-phase insulin secretion.

Effect of rosmarinic acid on insulin sensitivity, glyoxalase system and oxidative events in liver of fructose-fed mice.

The study investigates the effects of rosmarinic acid (RA) on insulin sensitivity, protein glycation and oxidative events in fructose-fed mice, a model of insulin resistance (IR). Experiments were performed in four groups of animals administered either fructose diet or starch diet with and without RA administration. Insulin sensitivity indices were computed at the end of the treatment period. Redox homeostasis in liver was determined by assaying lipid peroxidative markers and antioxidants in the liver. Glyoxalase system and protein damage were assessed by assaying aldehydes, glyoxalase I and II, protein carbonyls, total thiols and nitrosothiols. Protein glycation was studied by measuring glycated hemoglobin, fructosamine and advanced glycation end products. Mitochondrial function was assessed by assaying succinate dehydrogenase and calcium ATPase. Fructose administration caused glycation of proteins, changes in metabolic parameters, inactivation of the glyoxalase system and depletion of antioxidants. Oxidative stress and reduced mitochondrial function were observed. Administration of RA to fructose-fed mice mitigated the above alterations. The data suggest that metabolic and redox disturbances in this dietary model of IR could be mitigated by RA. The antioxidant action of RA could be one of the contributing mechanisms for the improvement of insulin sensitivity.

Statin modulates insulin signaling and insulin resistance in liver and muscle of rats fed a high-fat diet

Recent studies have shown that statins might have relevant effects on insulin resistance in animal models and in humans. However, the molecular mechanisms that account for this improvement in insulin sensitivity are not well established. The aim of the present study was to investigate the effect of a statin on insulin sensitivity and insulin signaling in liver and muscle of rats fed on a high-fat diet (HFD) for 4 weeks, treated or not with lovastatin during the last week. Our data show that treatment with lovastatin results in a marked improvement in insulin sensitivity characterized by an increase in glucose disappearance rate during the insulin tolerance test. This increase in insulin sensitivity was associated with an increase in insulin-induced insulin receptor (IR) tyrosine phosphorylation and, in parallel, a decrease in IR serine phosphorylation and association with PTP1B. Our data also show that lovastatin treatment was associated with an increase in insulin-stimulated insulin receptor substrate (IRS) 1/phosphatidylinositol 3-kinase/Akt pathway in the liver and muscle of HFD-fed rats in parallel with a decrease in the inflammatory pathway (c-jun N-terminal kinase and I kappa β kinase (IKK β)/inhibitor of κB/nuclear factor κB) related to insulin resistance. In summary, statin treatment improves insulin sensitivity in HFD-fed rats by reversing the decrease in the insulin-stimulated IRS-1/phosphatidylinositol 3-kinase/Akt pathway in liver and muscle. The effect of statins on insulin action is further supported by our findings that HFD rats treated with statin show a reduction in IRS-1 serine phosphorylation, I kappa kinase (IKK)/inhibitor of κB/nuclear factor κB pathway, and c-jun N-terminal kinase activity, associated with an improvement in insulin action. Overall, these results provide important new insight into the mechanism of statin action in insulin sensitivity.

Effect Of Diet On Hormone-Induced Lipolysis

Increased accumulation of triglycerides in adipose tissue represents the diagnostic milestone of obesity. The rapidly increasing prevalence of obesity worldwide, in adults as well as in children, is recognized today as a major public healthcare problem owing to the increased risk of developing cardiovascular diseases, Type 2 diabetes, several types of cancer, arthritis and sleep apnea, as well as a wide range of psychological problems as a result of social exclusion, which is present as early as in children of preschool age. Numerous genetic as well as environmental factors act synergically in the etiopathogenesis of obesity. Among these, adipose tissue metabolism and, especially, the regulation of triglyceride hydrolysis and the release of free fatty acids (FFAs) into the circulation in a process of lipolysis plays a central role as it participates in the control of whole-body adipose tissue stores. Lipolysis regulation becomes crucial under conditions of exercise, fasting and during hypocaloric diets (weight reduction) when FFAs released from adipose tissue represent a major energy substrate for other tissues. Increased plasma FFA concentration is often present in obesity and is attributed to an increased basal lipolytic rate in adipose tissue. FFAs induce several negative effects in the human body, including a decrease in insulin sensitivity of muscle, liver and pancreatic β cells, thus, linking insulin resistance with obesity. Lipolysis is controlled by a complex network of hormonal regulations together with a vegetative neuronal activity in adipose tissue. The most potent regulators of hormone-sensitive lipase activity – representing the key enzyme in triglyceride hydrolysis – are catecholamines and insulin. While insulin is the most potent inhibitor of lipolysis, catecholamines, owing to their interaction with β1,2,3-adrenergic receptors (stimulating lipolysis) or α2-adrenergic receptors (inhibiting lipolysis), can either stimulate or inhibit lipolysis, depending on the actual number and binding to both types of receptors. Recently, a role of atrialnatriuretic peptide in lipolysis stimulation, independent of insulin and catecholamines, has been identified and its position has been verified under physiological circumstances. In addition, several other hormones (e.g., thyroid hormones, glucocorticoids, growth hormone, sex steroids, parathyroid hormone and glucagon) also influence lipolytic processes in humans; however, their contribution is minor compared with the previously mentioned substances.

Small molecule activators of SIRT1 as therapeutics for the treatment of type 2 diabetes

Calorie restriction extends lifespan and produces a metabolic profile desirable for treating diseases of ageing such as type 2 diabetes. SIRT1, an NAD+-dependent deacetylase, is a principal modulator of pathways downstream of calorie restriction that produce beneficial effects on glucose homeostasis and insulin sensitivity. Resveratrol, a polyphenolic SIRT1 activator, mimics the anti-ageing effects of calorie restriction in lower organisms and in mice fed a high-fat diet ameliorates insulin resistance, increases mitochondrial content, and prolongs survival. Here we describe the identification and characterization of small molecule activators of SIRT1 that are structurally unrelated to, and 1,000-fold more potent than, resveratrol. These compounds bind to the SIRT1 enzyme-peptide substrate complex at an allosteric site amino-terminal to the catalytic domain and lower the Michaelis constant for acetylated substrates. In diet-induced obese and genetically obese mice, these compounds improve insulin sensitivity, lower plasma glucose, and increase mitochondrial capacity. In Zucker fa/fa rats, hyperinsulinaemic-euglycaemic clamp studies demonstrate that SIRT1 activators improve whole-body glucose homeostasis and insulin sensitivity in adipose tissue, skeletal muscle and liver. Thus, SIRT1 activation is a promising new therapeutic approach for treating diseases of ageing such as type 2 diabetes.

Effects of atazanavir/ritonavir and lopinavir/ritonavir on glucose uptake and insulin sensitivity

We read with interest the article by Noor et al. [1] examining the differential effects on glucose metabolism of the combinations of atazanavir/ritonavir versus lopinavir/ritonavir. Their paper deals with an important issue, having both theoretical interest and clinical impact. We think that some comments concerning aspects of the experimental procedure, data analysis and interpretation of the results could be helpful to other readers. The euglycemic hyperinsulinemic clamp is the golden standard for the assessment of insulin sensitivity. This experimental approach requires that the glucose concentration is maintained at a constant level throughout the study [2]. However, in the study by Noor et al.[1], the glucose concentration increased remarkably (up to 100 mg/dl) during the initial portion of the clamp (their fig. 3a), a defect which may influence the clamp-derived estimate of insulin sensitivity. During the clamp, it is assumed that endogenously secreted insulin is inhibited and only peripherally administered insulin contributes to insulin elevation. If the glycemic increase triggered an endogenous insulin response, this likely influenced the degree of inhibition of endogenous glucose production because the acute elevation of the portal insulin level has been shown to have a long-lasting effect on the liver [3]. As a result, the clamp-derived estimate of insulin sensitivity would also be affected. Additional data on the C-peptide would be welcome to verify the time course of endogenous insulin secretion, as a bias cannot otherwise be ruled out. In analyzing the oral glucose tolerance test (OGTT), Noor et al.[1] used plasma glucose and insulin levels to calculate insulin resistance for each time point of the OGTT by means of the homeostasis model assessment (HOMA) (their fig. 3c). That, however, is a misuse of the HOMA method, which has been developed to provide an index of insulin resistance from the concentrations of glucose and insulin measured at the basal steady state [4]. This method cannot be applied to nonsteady-state conditions when insulin and glucose concentrations continuously change. Thus, the authors had no methodological support for calculating the HOMA index at each time point during the OGTT and performing statistical comparisons among groups. In discussing their data obtained in vivo, Noor et al.[1] faced the problem of reconciling a discrepancy between the euglycemic, hyperinsulinemic clamp and OGTT results. Although the clamp revealed a significantly different reduction in insulin sensitivity between atazanavir/ritonavir and lopinavir/ritonavir, the OGTT did not (their table 2). The authors claimed that this discrepancy was due to the fact that the OGTT reflects both liver and peripheral insulin sensitivity, whereas the glucose clamp only reflects the latter component. This argument is questionable for the following reasons. First, a patent contradiction immediately emerges. If the OGTT reflects both liver and peripheral insulin sensitivity, whereas the clamp only reflects the latter, the OGTT would be expected to be more capable than the clamp of detecting differences in insulin sensitivity among the study groups. This contradicts the experimental evidence, which shows the opposite. Second, the claim that the clamp only reflects peripheral insulin sensitivity has no grounds. The index of clamp-based insulin sensitivity used by Noor et al.[1] is the ratio M/I, where M is the exogenous glucose infusion rate and I is the insulin concentration, both measured at the end of the clamp. It is worth emphasizing that the glucose infused during the clamp allows a constant plasma glucose concentration to be maintained because it balances exactly the increase in glucose disappearance (Rd) and the reduction in endogenous glucose production (EGP). Thus, the M-value measures the ability of insulin to stimulate peripheral glucose uptake, as well as to inhibit glucose production (M = ΔRd − ΔEGP, where Δ is the change from pretest level) [5]. In the study by Noor et al.[1], the inhibition of endogenous glucose production could not be assumed to be identical in the study groups because protease-inhibitors are known to affect liver insulin sensitivity [6]. The contribution of glucose uptake can only be isolated from that of endogenous glucose production if a glucose tracer is concurrently infused along with unlabeled glucose [5], but Noor et al.[1] did not use a tracer. It can be concluded that the true reason for the discrepancy between the clamp and OGTT results remains to be elucidated. In conclusion, we hope that the above considerations may prove useful to clinicians in the interpretation of studies assessing the effects of antiretroviral drugs on carbohydrate metabolism using methods such as the euglycemic hyperinsulinemic clamp, the HOMA, or the OGTT.

Relationship of Liver Enzymes to Insulin Sensitivity and Intra-Abdominal Fat

The purpose of this study was to determine the relationship between plasma liver enzyme concentrations, insulin sensitivity, and intra-abdominal fat (IAF) distribution. Plasma gamma-glutamyl transferase (GGT), aspartate transaminase (AST), alanine transaminase (ALT) levels, insulin sensitivity (insulin sensitivity index [S(I)]), IAF area, and subcutaneous fat (SCF) area were measured in 177 nondiabetic subjects (75 men and 102 women, aged 31-75 years) with no history of liver disease. On the basis of BMI (< or > or = 27.5 kg/m2) and S(I) (< or > or = 7.0 x 10(-5) min/pmol) subjects were divided into lean insulin sensitive (LIS, n = 53), lean insulin resistant (LIR, n = 60), and obese insulin resistant (OIR, n = 56) groups. Levels of all three liver enzymes were higher in men than in women (P < 0.0001 for each). In men, GGT levels were higher in insulin-resistant than in insulin-sensitive subjects (P < 0.01). In women, GGT levels were higher in the OIR than in the LIS group (P < 0.01) but no different in the LIR group. There was no difference in ALT and AST levels among the LIS, LIR, and OIR groups. GGT was associated with S(I) (r = -0.26, P < 0.0001), IAF area (r = 0.22, P < 0.01), waist-to-hip ratio (WHR) (r = 0.25, P = 0.001), BMI (r = 0.17, P < 0.05), and SCF area (r = 0.16, P < 0.05) after adjustments for age and sex. In men, only S(I) (r = -0.29, P < 0.05) remained independently correlated with GGT in multiple regression analysis. In women, IAF area (r = 0.29, P < 0.01) and WHR (r = 0.29, P < 0.01) were independently associated with GGT, but S(I) was not. In nondiabetic men GGT but not AST or ALT levels, are inversely related to insulin sensitivity independent of IAF area. However in women, GGT is related to measures of central body fat rather than to insulin sensitivity.

Muscle and Liver Insulin Resistance Indexes Derived From the Oral Glucose Tolerance Test

It was recently reported (1) that it is important to assess both hepatic and muscle insulin resistance because doing so can lead to different treatment approaches according to the insulin resistance status of each organ. Currently, the hyperinsulinemic-euglycemic clamp is the gold standard for the measurement of liver and muscle insulin sensitivity (2). However, this method is unpractical in clinical practice. For this reason, Abdul-Ghani et al. (1) developed several formulas that were derived from an oral glucose tolerance test (OGTT) for the assessment of …