Fatty acids and insulin resistance in muscle and liver

Free fatty acids (FFAs) circulate round the body and represent important nutrients and the key oxidative fuel for the heart and resting skeletal muscle. In addition, FFAs are thought to be potent signalling molecules. Growing evidence indicates that FFAs may be involved in type 2 diabetes mellitus and obesity by mediating insulin resistance. In 1963, it was postulated that accumulated glucose-6-phosphate as a result of increased FFA oxidation leads to decreased glucose uptake. An alternative hypothesis is that increased concentrations of plasma FFA induce insulin resistance in humans through inhibition of glucose transport activity, which appears to be a consequence of decreased insulin receptor substrate-1-associated phosphatidyl inositol 3 kinase activity. Moreover, FFAs can arise locally, and increased intramyocellular and hepatocellular lipids have been shown to be associated with insulin resistance. This paper reviews the main aspects of FFA metabolism in the development of insulin resistance in skeletal muscle and liver, as well as the role of ectopic lipid deposits as a local source of FFAs. Finally, the role of thiazolidinediones as modulators of FFA-induced insulin resistance will be discussed.     

Insulin receptor tyrosine-kinase activity is altered in both muscle and adipose tissue from non-obese normoglycaemic insulin-resistant subjects

Summary We performed i. v. insulin tolerance test in 30 non-obese (BMI < 30 male and < 28 female) non-diabetic (by oral glucose tolerance test) subjects and subdivided them into three groups of 10 subjects each, according to their insulin sensitivity (K_itt values). Then we compared the tyrosine-kinase activity of immunopurified insulin receptors (using ³²P-ATP and poly-glu-tyr (4 : 1) from both muscle and adipose tissue in 7 of the most insulin-sensitive and 7 of the most insulin-resistant subjects. No difference was observed between the two groups in the basal (no insulin) receptor tyrosine-kinase activity from both tissues. In contrast, tyrosine-kinase activity response to insulin was significantly higher (p < 0.05 by 2-way ANOVA test) in receptors from both tissues of insulin-sensitive subjects. In addition, a decreased tyrosine-kinase sensitivity to insulin was observed in muscle, but not adipose, tissue of insulin-resistant subjects (insulin ED₅₀ being 0.87 ± 0.05 nmol/l vs 2.03 ± 0.07, p < 0.05 in insulin-sensitive and -resistant subjects). Insulin ED₅₀ of muscle receptor tyrosine-kinase significantly (p = 0.001) correlated to both K_itt values (r = −0.79) and plasma insulin values at 120 min during OGTT (r = +0.80). Insulin receptor content, as assessed by radioimmunoassay, was similar in both muscle (7.9 ± 1.3 and 9.2 ± 1.9 ng/mg protein) and adipose tissue (8.2 ± 1.3 and 7.5 ± 1.4) of insulin-sensitive and -resistant subjects. Exon 11+ isoform of insulin receptor was similarly represented in muscle specimens from six insulin-sensitive (80 ± 8 % of total receptor content) and six resistant (78 ± 6 %) subjects. In conclusion, a defective insulin stimulation of receptor tyrosine-kinase activity is present in both muscle and adipose tissue of euglycaemic non-obese insulin-resistant subjects. This defect is, therefore, an early event in the development of insulin resistance. [Diabetologia (1995) 38: 55–61] Received: 14 January 1994 and in revised form: 14 June 1994     

骨骼肌线粒体异常在胰岛素抵抗形成中的作用

胰岛素抵抗及其相关疾病与机体的各种代谢异常密切相关.线粒体是调控代谢通路的重要细胞器,其功能的改变可对机体代谢产生极大影响.骨骼肌是胰岛素刺激葡萄糖利用的主要器官,同时也是富含线粒体和依赖氧化磷酸化供能的组织.因此,骨骼肌线粒体异常可能与胰岛素抵抗高度相关,其机制有待阐明.     

Curcumin improves insulin resistance in skeletal muscle of rats

Background and aims

Curcumin has been reported to lower plasma lipids and glucose in diabetic rats, and to decrease body weight in obese rats, which may partly be due to increased fatty acid oxidation and utilization in skeletal muscle.

Methods and results

Diabetic rats induced by high-fat diet plus streptozotocin (STZ, 30 mg/kg BW) were fed a diet containing 50, 150, or 250 mg/kg BW curcumin for 7 wk. Curcumin dose-dependently decreased plasma lipids and glucose and the dose 150 mg/kg BW appeared to be adequate to produce a significant effect. Curcumin supplementation reduced glucose and insulin tolerance measured as areas under the curve. L6 myotubes were treated with palmitate (0.25 mmol/L) in the presence of different levels of curcumin for 24 h in our in vitro experiment. Curcumin at 10 μmol/L was adequate to cause a significant increase in 2-deoxy-[³H]d-glucose uptake by L6 myotubes. Curcumin up-regulated expression of phosphorylated AMP-activated protein kinase (AMPK), CD36, and carnitine palmitoyl transferase 1, but down-regulated expression of pyruvate dehydrogenase 4 and phosphorylated glycogen synthase (GS) in both in vivo and in vitro studies. Moreover, curcumin increased phosphorylated acetyl COA carboxylase in L6 myotubes. The effects of curcumin on these enzymes except for GS were suppressed by AMPK inhibitor, Compound C. LKB1, an upstream kinase of AMPK, was activated by curcumin and inhibited by radicicol, an LKB1 destabilizer.

Conclusion

Curcumin improves muscular insulin resistance by increasing oxidation of fatty acid and glucose, which is, at least in part, mediated through LKB1-AMPK pathway.     

低出生体重大鼠发育期骨骼肌形态结构变化及胰岛素抵抗情况

观察低出生体重大鼠不同发育时期骨骼肌形态结构的变化和胰岛素抵抗情况.采用孕期低蛋白饮食法建立低出生体重仔鼠模型.于出生后7 d、21 d、2月龄测定仔鼠空腹血糖、血清胰岛素值,并检测骨骼肌形态结构.自7 d至2月龄,低出生体重仔鼠骨骼肌纤维均有明显萎缩、排列稀疏紊乱;2月龄时超微结构明显异常.2月龄内低出生体重仔鼠血糖和血清胰岛素与对照组比较无差异.     

Focal adhesion kinase regulates insulin resistance in skeletal muscle

Aims/hypothesis On the basis of our previous studies, we investigated the possible role of focal adhesion kinase (FAK) in the development of insulin resistance in skeletal muscle, a major organ responsible for insulin-stimulated glucose uptake. Materials and methods Insulin-resistant C2C12 skeletal muscle cells were transfected with FAK wild-type or FAK mutant plasmids, knocked down using small interfering RNA (siRNA), and their effects on the levels and activities of insulin-signalling molecules and on glucose uptake were determined. Results A significant decrease in tyrosine phosphorylation of FAK in insulin-resistant C2C12 cells was observed. A similar decrease was observed in skeletal muscle obtained from insulin-resistant Sprague–Dawley rats fed a high-fat diet. Increased levels of FAK in insulin-resistant C2C12 skeletal muscle cells increased insulin sensitivity and glucose uptake. These effects were reversed by an increase in the level of kinase activity mutant FAK or suppression of endogenous FAK by siRNA. FAK was also found to interact downstream with insulin receptor substrate-1, phosphatidylinositol 3-kinase and protein kinase C and glycogen synthase kinase 3β, leading to translocation of glucose transporter 4 and resulting in the regulation of glucose uptake. Conclusions/interpretation The present study provides strong evidence that the modulation of FAK level regulates the insulin sensitivity of skeletal muscle cells. The results demonstrate a direct role of FAK in insulin-resistant skeletal muscle cells for the first time. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorised users. This contains details of the authors’ contributions to the study. Niper communication no. 389     

Measurement of muscle insulin sensitivity in obese men

AimsIn 2007, a novel estimate of skeletal muscle insulin sensitivity was derived from the oral glucose tolerance test (OGTT). The aim of this investigation is to assess whether and to what extent the proposed index of skeletal muscle insulin sensitivity derived from the OGTT was associated with muscle insulin sensitivity measured using the hyperinsulinemic-euglycaemic clamp technique.MethodsForty-six middle-aged, abdominally obese men (age 44 ± 8 years, waist circumference 107.4 ± 6.2) were studied. Each participant participated in a 2-hour, 75-g OGTT and a 3-hour hyperinsulinemic-euglycaemic clamp protocol.ResultsThe OGTT-derived index of muscle insulin sensitivity correlated with muscle insulin sensitivity measured with the insulin clamp (r = 0.55, P < 0.01), however, the standard error of estimate (SEE) when predicting muscle insulin sensitivity by the OGTT-derived index was 5.3 (50%).ConclusionOur findings suggest that despite a statistically significant association between the two methods, the OGTT approach lacks precision and is not a useful method for estimating skeletal muscle insulin sensitivity in abdominally obese men.     

胰岛素对骨骼肌微血管的调节作用

骨骼肌血管系统有丰富的分支网状结构,近年来人们多采用测定1-甲基黄嘌呤(1-MX)的代谢情况、对比增强超声成像(CEU)、激光多普勒超声仪(LDF)和实时对比超声等方法检测骨骼肌微血管的灌注情况.研究发现,胰岛素在未改变骨骼肌大动脉血流量的情况下,就可以引起骨骼肌微循环灌注的增加,而且这一作用具有剂量依赖性;而存在胰岛素抵抗时,胰岛素对阻力动脉的调节作用受损.胰岛素对血管的调节作用与其介导的一氧化氮和内皮素-1的表达密切相关,胰岛素抵抗时二者的表达失衡.     

The role of skeletal muscle sphingolipids in the development of insulin resistance

Insulin resistance is an important risk factor for type 2 diabetes, obesity, cardiovascular disease, polycystic ovary syndrome and other diseases. The most important stage in the development of insulin resistance is impairment of insulin-stimulated skeletal muscle glucose uptake. There is evidence that intramyocellular lipids might be responsible for this process through inhibition of insulin signaling. One of the important intracellular lipid pools is associated with the sphingomyelin signaling pathway. The second messenger in this pathway is ceramide. In vitro data indicate that ceramide inhibits insulin signaling, mainly through inactivation of protein kinase B. In vivo data suggest that ceramide accumulation within muscle cells might be associated with the development of insulin resistance. In this review, we discuss both in vitro and in vivo evidence for the role of muscle ceramide in the impairment of insulin action with particular focus on the question whether findings from animal studies are applicable to humans. We describe problems that are unresolved so far and topics of potential interest for future research.     

线粒体功能在骨骼肌胰岛素抵抗中的作用

线粒体是提供细胞进行各种生命活动所需能量的细胞器,越来越多的证据表明,线粒体功能与骨骼肌胰岛素抵抗状态密切相关,这种机制可能因为线粒体功能损伤引发脂肪酸β-氧化功能障碍,最终影响胰岛素受体后信号转导通路而致胰岛素抵抗的发生;也可因为线粒体融合蛋白或基因调控受损造成线粒体动力学异常或膜电位下降,造成胰岛素抵抗.目前对于线...     

Influence of cell size on the effects of insulin and noradrenaline on human adipose tissue

Summary In the present study dose-response relationships of the effects of noradrenaline and insulin on fat cells of different sizes were performed. Adipose cells larger than 100 m were more responsive (expressed as absolute effects) to the lipolytic action of noradrenaline as well as to the antilipolytic effect of insulin. This suggests that in the larger cells thecapacity, i. e. the sum of factors contributing to the ability to stimulate or inhibit the metabolic rates, was greater than in the smaller ones. In contrast thesensitivity to these agents, i.e. the readiness to respond, was not different between small and large cells. It is shown that the concentrations of insulin needed to obtain an antilipolytic effect is far below that needed to stimulate glucose incorporation. This discrepancy in insulin concentrations required may be due to binding of insulin to receptors with different affinity.     

PC-1 content in skeletal muscle of non-obese,non-diabetic subjects: relationship to insulin receptor tyrosine kinase and whole body insulin sensitivity

Summary Insulin sensitivity varies widely in non-obese, non-diabetic subjects, and we have previously reported that in vivo insulin action correlates with in vitro insulin stimulated insulin receptor tyrosine-kinase activity in skeletal muscle. Plasma membrane glycoprotein PC-1 content is elevated in fibroblasts of insulin-resistant subjects, and expression of PC-1 cDNA in cultured cells reduces both insulin receptor tyrosine-kinase activity and the biological actions of insulin. In the present study we investigated non-obese, non-diabetic subjects and found a significant negative correlation between muscle PC-1 content and both in vivo insulin action as measured by the intravenous insulin tolerance test (r=−0.51,p=0.035) and the sensitivity (ED₅₀) of in vitro insulin stimulation of insulin receptor tyrosine-kinase activity (r=0.66,p=0.027). These studies indicate, therefore, that increased muscle PC-1 content is associated with reduced insulin action both in vivo and in vitro. Moreover, they suggest a possible role for PC-1 in regulating insulin receptor function in human skeletal muscle. recipient of a Juvenile Diabetes Foundation Post-Doctoral Fellowship     

The anti-leishmanial drug miltefosine causes insulin resistance in skeletal muscle cells in vitro

Aims/hypothesis Miltefosine, the first oral anti-leishmanial drug, is reported to inhibit phosphatidylinositol 3-kinase (PI3K)/Akt activity in carcinoma cell lines. Inhibition of the PI3K/Akt pathway is known to result in insulin resistance. Therefore, we investigated whether miltefosine has any deleterious effect(s) on insulin sensitivity in L6E9 skeletal muscle cells.Materials and methods L6E9 myotubes were treated with miltefosine and its effect was observed on insulin-signalling proteins such as Akt, PI3K, insulin receptor-β, IRS-1, c-Jun N-terminal kinase, p38 and glycogen synthase kinase β, as well as on glucose uptake.Results Miltefosine caused skeletal muscle insulin resistance in vitro by interfering with the insulin-signalling pathway and inhibiting insulin-stimulated glucose uptake.Conclusions/interpretation Miltefosine may contribute to the risk of type 2 diabetes and needs further clinical exploration.NIPER Communication no. 370     

The effect of oxytetracycline on the response to insulin of diaphragm muscle and on lipid synthesisin vivo andin vitro in the ob/ob mouse

Summary The insulin sensitivity of the peripheral tissues in ob/ob mice treated with oxytetracydine was assessed bothin vivo andin vitro by measuring the effect of insulin on the incorporation of glucose into glycogen in diaphragm muscle and on the incorporation of glucose and³H₂O into lipids in epididymal adipose tissue. The results indicated that OTC treatment improved the insulin sensitivity of muscle but not that of adipose tissue. The results presented also indicated that oxytetracycline treatment led to a decrease in the basal incorporation of lipids into the liverin vivo and the adipose tissuein vivo andin vitro. The results are discussed in view of the relative importance of liver and adipose tissue in the lipid metabolism of ob/ob and lean mice and in view of results obtained by other workers on the effects of B -cell cytotoxic agents.     

Melatonin prevents mitochondrial dysfunction and insulin resistance in rat skeletal muscle

Melatonin has a number of beneficial metabolic actions and reduced levels of melatonin may contribute to type 2 diabetes. The present study investigated the metabolic pathways involved in the effects of melatonin on mitochondrial function and insulin resistance in rat skeletal muscle. The effect of melatonin was tested both in vitro in isolated rats skeletal muscle cells and in vivo using pinealectomized rats (PNX). Insulin resistance was induced in vitro by treating primary rat skeletal muscle cells with palmitic acid for 24 hr. Insulin‐stimulated glucose uptake was reduced by palmitic acid followed by decreased phosphorylation of AKT which was prevented my melatonin. Palmitic acid reduced mitochondrial respiration, genes involved in mitochondrial biogenesis and the levels of tricarboxylic acid cycle intermediates whereas melatonin counteracted all these parameters in insulin‐resistant cells. Melatonin treatment increases CAMKII and p‐CREB but had no effect on p‐AMPK. Silencing of CREB protein by siRNA reduced mitochondrial respiration mimicking the effect of palmitic acid and prevented melatonin‐induced increase in p‐AKT in palmitic acid‐treated cells. PNX rats exhibited mild glucose intolerance, decreased energy expenditure and decreased p‐AKT, mitochondrial respiration, and p‐CREB and PGC‐1 alpha levels in skeletal muscle which were restored by melatonin treatment in PNX rats. In summary, we showed that melatonin could prevent mitochondrial dysfunction and insulin resistance via activation of CREB‐PGC‐1 alpha pathway. Thus, the present work shows that melatonin play an important role in skeletal muscle mitochondrial function which could explain some of the beneficial effects of melatonin in insulin resistance states.