Saturated, but not n-6 polyunsaturated, fatty acids induce insulin resistance: role of intramuscular accumulation of lipid metabolites.

Consumption of a Western diet rich in saturated fats is associated with obesity and insulin resistance. In some insulin-resistant phenotypes this is associated with accumulation of skeletal muscle fatty acids. We examined the effects of diets high in saturated fatty acids (Sat) or n-6 polyunsaturated fatty acids (PUFA) on skeletal muscle fatty acid metabolite accumulation and whole-body insulin sensitivity. Male Sprague-Dawley rats were fed a chow diet (16% calories from fat, Con) or a diet high (53%) in Sat or PUFA for 8 wk. Insulin sensitivity was assessed by fasting plasma glucose and insulin and glucose tolerance via an oral glucose tolerance test. Muscle ceramide and diacylglycerol (DAG) levels and triacylglycerol (TAG) fatty acids were also measured. Both high-fat diets increased plasma free fatty acid levels by 30%. Compared with Con, Sat-fed rats were insulin resistant, whereas PUFA-treated rats showed improved insulin sensitivity. Sat caused a 125% increase in muscle DAG and a small increase in TAG. Although PUFA also resulted in a small increase in DAG, the excess fatty acids were primarily directed toward TAG storage (105% above Con). Ceramide content was unaffected by either high-fat diet. To examine the effects of fatty acids on cellular lipid storage and glucose uptake in vitro, rat L6 myotubes were incubated for 5 h with saturated and polyunsaturated fatty acids. After treatment of L6 myotubes with palmitate (C16:0), the ceramide and DAG content were increased by two- and fivefold, respectively, concomitant with reduced insulin-stimulated glucose uptake. In contrast, treatment of these cells with linoleate (C18:2) did not alter DAG, ceramide levels, and glucose uptake compared with controls (no added fatty acids). Both 16:0 and 18:2 treatments increased myotube TAG levels (C18:2 vs. C16:0, P < 0.05). These results indicate that increasing dietary Sat induces insulin resistance with concomitant increases in muscle DAG. Diets rich in n-6 PUFA appear to prevent insulin resistance by directing fat into TAG, rather than other lipid metabolites.     

Muscle-specific deletion of Prkaa1 enhances skeletal muscle lipid accumulation in mice fed a high-fat diet

Excessive intramyocellular triacylglycerols (IMTGs, muscle lipids) are associated with the abnormal energy metabolism and insulin resistance of skeletal muscle. AMP-activated protein kinase (AMPK), a crucial cellular energy sensor, consists of α, β and γ subunits. Researchers have not clearly determined whether Prkaa1 (also known as AMPKα1) affects IMTG accumulation in skeletal muscle. Here, we show an important role of Prkaa1 in skeletal muscle lipid metabolism. Deletion of muscle Prkaa1 leads to the delayed development of skeletal muscles but does not affect glucose tolerance or insulin sensitivity in animals fed a normal diet. Notably, when animals are fed a high-fat diet, the skeletal muscle of muscle-specific Prkaa1 knockout mice accumulates more lipids than the skeletal muscle of wild-type (WT) mice, with concomitant upregulation of adipogenic gene expressions and downregulation of the expression of genes associated with mitochondrial oxidation. Muscle-specific Prkaa1 ablation also results in hyperlipidemia, which may contribute to the increased IMTG levels. Furthermore, Prkaa1 deletion activates skeletal muscle mTOR signalling, which has a central role in lipid metabolism and mitochondrial oxidation. Collectively, our study provides new insights into the role of Prkaa1 in skeletal muscle. This knowledge may contribute to the treatment of related metabolic diseases.     

Nonacute effects of H-FABP deficiency on skeletal muscle glucose uptake in vitro

Heart-type fatty acid-binding protein (H-FABP) is required for high rates of skeletal muscle long-chain fatty acid (LCFA) oxidation and esterification. Here we assessed whether H-FABP affects soleus muscle glucose uptake when measured in vitro in the absence of LCFA. Wild-type and H-FABP null mice were fed a standard chow or high-fat diet before muscle isolation. With the chow, the mutation increased insulin-dependent deoxyglucose uptake by 141% (P < 0.01) at 0.02 mU/ml of insulin but did not cause a significant effect at 2 mU/ml of insulin; skeletal muscle triglyceride and long-chain acyl-CoA (LCA-CoA) levels remained normal. With the high-fat diet, the mutation increased insulin-dependent deoxyglucose uptake by 190% (P < 0.01) at 2 mU/ml of insulin, thus partially preventing insulin resistance, and it completely prevented the threefold (P < 0.001) diet-induced increase of muscle triglyceride levels; however, muscle LCA-CoA levels showed little or no reduction. With both diets, the mutation reduced the basal (insulin-independent) soleus muscle deoxyglucose uptake by 28% (P < 0.05). These results establish a close relation between FABP-dependent lipid pools and insulin sensitivity and indicate the existence of a nonacute, antagonistic, and H-FABP-dependent fatty acid regulation of basal and insulin-dependent muscle glucose uptake.     

Dibenzoylmethane Exerts Metabolic Activity through Regulation of AMP-Activated Protein Kinase (AMPK)-Mediated Glucose Uptake and Adipogenesis Pathways

Dibenzoylmethane (DBM) has been shown to exert a variety of beneficial effects on human health. However, the mechanism of action is poorly understood. In this study, DBM increased phosphorylation of AMP-activated protein kinase (AMPK) and stimulated glucose uptake in a skeletal muscle cell line. Both knockdown of AMPK with siRNA and inhibition with AMPK inhibitor blocked DBM-induced glucose uptake. DBM increased the concentration of intracellular calcium and glucose uptake due to DBM was abolished by STO-609 (a calcium/calmodulin-dependent protein kinase inhibitor). DBM stimulated phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK), which was blocked by pretreatment with compound C, an AMPK inhibitor. The expression of glucose transporter type 4 (GLUT4) was increased by DBM. The translocation of GLUT4 to the plasma membrane was also increased by DBM in AMPK dependently. In addition, DBM suppressed weight gain and prevented fat accumulation in the liver and abdomen in mice fed a high-fat diet. In pre-adipocyte cells, DBM decreased the activity of acetyl-CoA carboxylase (ACC), the rate-limiting enzyme of fatty acid synthesis. Expression of the adipogenic gene, fatty acid synthase (FAS), was suppressed by DBM in an AMPK-dependent manner. These results showed that the beneficial metabolic effects of DBM might be due to regulation of glucose uptake via AMPK in skeletal muscle and inhibition of adipogenesis in pre-adipocytes.     

Prevention of high-fat diet-induced muscular lipid accumulation in rats by �� lipoic acid is not mediated by AMPK activation

Skeletal muscle triglyceride accumulation is associated with insulin resistance in obesity. Recently, it has been suggested that α lipoic acid (ALA) improves insulin sensitivity by lowering triglyceride accumulation in nonadipose tissues via activation of skeletal muscle AMP-activated protein kinase (AMPK). We examined whether chronic ALA supplementation prevents muscular lipid accumulation that is associated with high-fat diets via activation of AMPK. In addition, we tested if ALA supplementation was able to improve insulin sensitivity in rats fed low- and high-fat diets (LFD, HFD). Supplementing male Wistar rats with 0.5% ALA for 8 weeks significantly reduced body weight, both on LFD and HFD (−24% LFD+ALA vs. LFD, P < 0.01, and −29% HFD+ALA vs. HFD, P < 0.001). Oil red O lipid staining revealed a 3-fold higher lipid content in skeletal muscle after HFD compared with LFD and ALA-supplemented groups (P < 0.05). ALA improved whole body glucose tolerance (∼20% lower total area under the curve (AUC) in ALA supplemented groups vs. controls, P < 0.05). These effects were not mediated by increased muscular AMPK activation or ALA-induced improvement of muscular insulin sensitivity. To conclude, the prevention of HFD-induced muscular lipid accumulation and the improved whole body glucose tolerance are likely secondary effects due to the anorexic nature of ALA.     

有氧运动对糖尿病大鼠PPARα信号通路的影响及其与PPARγ关系*

目的: 探讨4周有氧运动对糖尿病大鼠血糖血脂的改善作用及其与PPARα和PPARγ的调控关系。方法: 6周龄雄性SD大鼠8周高脂饮食喂养后一次性腹腔注射链脲佐菌素以建立糖尿病模型大鼠。除普通膳食对照组(Con)外,建模成功的糖尿病大鼠随机分成糖尿病对照组(DM)、糖尿病运动组(TDM)、糖尿病运动加PPARγ激动剂吡格列酮组(TDP)和糖尿病运动加PPARγ抑制剂GW9662组(TDG),每组8只。TDP组和TDG组大鼠在运动前分别补充吡格列酮和GW9662,TDM、TDP和TDG组大鼠进行4周中等强度递增负荷跑台运动(第1周15 m/min,30 min;第2周15 m/min,60 min;第3周20 m/min,60 min;第4周20 m/min,90 min),每周运动6 d,每天1次。运动期间所有大鼠给予普通饲料。最后一次运动结束后36 h,大鼠麻醉、取血,然后处死大鼠、收集肝和腓肠肌。检测空腹血糖血脂指标(血糖、血胰岛素和血脂四项)。Western blot方法检测肝和腓肠肌PPARα、PPARα上游分子腺苷酸活化蛋白激酶(AMPK)和下游分子肉碱棕榈酰转移酶-1(CPT1)的蛋白水平。结果: ①与Con组大鼠比较,DM大鼠FBG(＞11.1 mmol/L)和血清TC、TG、LDL水平显著升高(P均＜0.01),表明DM造模成功。②与DM大鼠比较,TDM大鼠血糖血脂改善的同时,肝和腓肠肌PPARα、AMPK和CPT1的蛋白水平均显著升高(P均＜0.01)。③与TDM大鼠比较,TDG大鼠肝和腓肠肌的PPARα和CPT1,以及肝AMPK的蛋白水平无显著变化(仅腓肠肌AMPK显著降低(P＜0.05));而TDP大鼠的肝和腓肠肌PPARα、AMPK和CPT1蛋白水平均显著提高(P均＜0.01)。结论: 有氧运动对DM大鼠血糖血脂的改善作用-与运动激活肝和腓肠肌的AMPK-PPARα-CPT1通路有关。运动对DM大鼠PPARα通路的激活与PPARγ无关,但PPARγ激活可进一步增强运动对AMPK-PPARα-CPT1蛋白水平上调的作用。     

The Effects of Long‐ or Medium‐Chain Fat Diets on Glucose Tolerance and Myocellular Content of Lipid Intermediates in Rats

Accumulation of triacylglycerols (TAGs) and acylcarnitines in skeletal muscle upon high‐fat (HF) feeding is the resultant of fatty acid uptake and oxidation and is associated with insulin resistance. As medium‐chain fatty acids (MCFAs) are preferentially β‐oxidized over long‐chain fatty acids, we examined the effects of medium‐chain TAGs (MCTs) and long‐chain TAGs (LCTs) on muscle lipid storage and whole‐body glucose tolerance. Rats fed a low‐fat (LF), HFLCT, or an isocaloric HFMCT diet displayed a similar body weight gain over 8 weeks of treatment. Only HFLCT increased myocellular TAG (42.3 ± 4.9, 71.9 ± 6.7, and 48.5 ± 6.5 µmol/g for LF, HFLCT, and HFMCT, respectively, P < 0.05) and long‐chain acylcarnitine content (P < 0.05). Neither HF diet increased myocellular diacylglycerol (DAG) content. Intraperitoneal (IP) glucose tolerance tests (1.5 g/kg) revealed a significantly decreased glucose tolerance in the HFMCT compared to the HFLCT‐fed rats (802 ± 40, 772 ± 18, and 886 ± 18 area under the curve for LF, HFLCT, and HFMCT, respectively, P < 0.05). Finally, no differences in myocellular insulin signaling after bolus insulin injection (10 U/kg) were observed between LF, HFLCT, or HFMCT‐fed rats. These results show that accumulation of TAGs and acylcarnitines in skeletal muscle in the absence of body weight gain do not impede myocellular insulin signaling or whole‐body glucose intolerance.     

Insulin-sensitizing properties of adiponectin

Adiponectin administration improves glucose tolerance in rodents. This is due to both reductions in hepatic glucose production, and likely improved insulin stimulated glucose disposal in skeletal muscle. Adiponectin's effects in both liver and muscle are believed to be due in large part to AMP-activated protein kinase (AMPK) activation, resulting in a reduction in hepatic gluconeogenic enzymes and increased fatty acid oxidation and reduced ectopic lipid deposition in muscle. In addition, adiponectin can robustly stimulate mitochondrial biogenesis, at least in muscle, and this appears to be due to AMPK-independent mechanisms. Various treatments successful at improving insulin response (thiazolidinediones (TZDs), n-3 polyunsaturated fatty acid (PUFA) supplementation) also stimulate adiponectin production. Obesity and insulin resistance are often characterized by both a state of resistance to adiponectin (both liver and muscle), as well as a reduction in total circulating adiponectin concentrations. The mechanisms underlying the impaired response of muscle and liver to adiponectin have not been clearly elucidated. Surprisingly, the significance of adiponectin resistance, at least in muscle, is not entirely clear. While the development of adiponectin resistance precedes intramuscular lipid accumulation and impaired insulin response in high-fat fed rodents, the restoration of adiponectin response does not appear to be necessary in order to restore insulin response in muscle. Further research examining the cellular mechanisms underlying the development of adiponectin resistance, and the importance of treating this, needs to be conducted.     

Dehydrozingerone exerts beneficial metabolic effects in high‐fat diet‐induced obese mice via AMPK activation in skeletal muscle

Dehydrozingerone (DHZ) exerts beneficial effects on human health; however, its mechanism of action remains unclear. Here, we found that DHZ suppressed high‐fat diet‐induced weight gain, lipid accumulation and hyperglycaemia in C57BL/6 mice and increased AMP‐activated protein kinase (AMPK) phosphorylation and stimulated glucose uptake in C2C12 skeletal muscle cells. DHZ activated p38 mitogen‐activated protein kinase (MAPK) signalling in an AMPK‐dependent manner. Inhibiting AMPK or p38 MAPK blocked DHZ‐induced glucose uptake. DHZ increased GLUT4 (major transporter for glucose uptake) expression in skeletal muscle. Glucose clearance and insulin‐induced glucose uptake increased in DHZ‐fed animals, suggesting that DHZ increases systemic insulin sensitivity in vivo. Thus, the beneficial health effects of DHZ could possibly be explained by its ability to activate the AMPK pathway in skeletal muscle.     

Dietary supplementation with vitamin E and C attenuates dexamethasone-induced glucose intolerance in rats

Glucocorticoid excess induces marked insulin resistance and glucose intolerance. A recent study has shown that antioxidants prevent dexamethasone (DEX)-induced insulin resistance in cultured adipocytes. The purpose of this investigation was to examine the effects of dietary vitamin E and C (Vit E/C) supplementation on DEX-induced glucose intolerance in rats. We hypothesized that feeding rats a diet supplemented with Vit E/C would improve glucose tolerance and restore insulin signaling in skeletal muscle, adipose, and liver and prevent alterations in AMPK signaling in these tissues. Male Wistar rats received either a control or Vit E/C-supplemented diet (0.5 g/kg diet each of L-ascorbate and DL-all rac-alpha-tocopherol) for 9 days prior to, and during, 5 days of daily DEX treatment (subcutaneous injections 0.8 mg/g body wt). DEX treatment resulted in increases in the glucose and insulin area under the curve (AUC) during an intraperitoneal glucose tolerance test. The glucose, but not insulin, AUC was lowered with Vit E/C supplementation. Improvements in glucose tolerance occurred independent of a restoration of PKB phosphorylation in tissues of rats stimulated with an intraperitoneal injection of insulin but were associated with increases in AMPK signaling in muscle and reductions in AMPK signaling and the expression of fatty acid oxidation enzymes in liver. There were no differences in mitochondrial enzymes in triceps muscles between groups. This study is the first to report that dietary Vit E/C supplementation can partially prevent DEX-induced glucose intolerance in rats.     

Bioactives from bitter melon enhance insulin signaling and modulate acyl carnitine content in skeletal muscle in high-fat diet-fed mice

Bioactive components from bitter melon (BM) have been reported to improve glucose metabolism in vivo, but definitive studies on efficacy and mechanism of action are lacking. We sought to investigate the effects of BM bioactives on body weight, muscle lipid content and insulin signaling in mice fed a high-fat diet and on insulin signaling in L6 myotubes. Male C57BL/6J mice were randomly divided into low-fat diet control (LFD), high-fat diet (HFD) and HFD plus BM (BM) groups. Body weight, body composition, plasma glucose, leptin, insulin and muscle lipid profile were determined over 12 weeks. Insulin signaling was determined in the mouse muscle taken at end of study and in L6 myotubes exposed to the extract. Body weight, plasma glucose, insulin, leptin levels and HOMA-IR values were significantly lower in the BM-fed HFD group when compared to the HFD group. BM supplementation significantly increased IRS-2, IR β, PI 3K and GLUT4 protein abundance in skeletal muscle, as well as phosphorylation of IRS-1, Akt1 and Akt2 when compared with HFD (P<.05 and P<.01). BM also significantly reduced muscle lipid content in the HFD mice. BM extract greatly increased glucose uptake and enhanced insulin signaling in L6 myotubes. This study shows that BM bioactives reduced body weight, improved glucose metabolism and enhanced skeletal muscle insulin signaling. A contributing mechanism to the enhanced insulin signaling may be associated with the reduction in skeletal muscle lipid content. Nutritional supplementation with this extract, if validated for human studies, may offer an adjunctive therapy for diabetes.     

Topiramate treatment causes skeletal muscle insulin sensitization and increased Acrp30 secretion in high-fat-fed male Wistar rats

We show that Topiramate (TPM) treatment normalizes whole body insulin sensitivity in high-fat diet (HFD)-fed male Wistar rats. Thus drug treatment markedly lowered glucose and insulin levels during glucose tolerance tests and caused increased insulin sensitization in adipose and muscle tissues as assessed by euglycemic clamp studies. The insulin-stimulated glucose disposal rate increased twofold (indicating enhanced muscle insulin sensitivity), and suppression of circulating FFAs increased by 200 to 300%, consistent with increased adipose tissue insulin sensitivity. There were no effects of TPM on hepatic insulin sensitivity in these TPM-treated HFD-fed rats. In addition, TPM administration resulted in a three- to fourfold increase in circulating levels of total and high-molecular-weight (HMW) adiponectin (Acrp30). Western blot analysis revealed normal AMPK (Thr(172)) phosphorylation in liver with a twofold increased phospho-AMPK in skeletal muscle in TPM-treated rats. In conclusion, 1) TPM treatment prevents overall insulin resistance in HFD male Wistar rats; 2) drug treatment improved insulin sensitivity in skeletal muscle and adipose tissue associated with enhanced AMPK phosphorylation; and 3) the tissue "specific" effects are associated with increased serum levels of adiponectin, particularly the HMW component.     

Metabolic consequences of ENPP1 overexpression in adipose tissue

Ectonucleotide pyrophosphate phosphodiesterase (ENPP1) has been shown to negatively modulate insulin receptor and to induce cellular insulin resistance when overexpressed in various cell types. Systemic insulin resistance has also been observed when ENPP1 is overexpressed in multiple tissues of transgenic models and attributed largely to tissue insulin resistance induced in skeletal muscle and liver. Another key tissue in regulating glucose and lipid metabolism is adipose tissue (AT). Interestingly, obese patients with insulin resistance have been reported to have increased AT ENPP1 expression. However, the specific effects of ENPP1 in AT have not been studied. To better understand the specific role of AT ENPP1 on systemic metabolism, we have created a transgenic mouse model (C57/Bl6 background) with targeted overexpression of human ENPP1 in adipocytes, using aP2 promoter in the transgene construct (AdiposeENPP1-TG). Using either regular chow or pair-feeding protocol with 60% fat diet, we compared body fat content and distribution and insulin signaling in adipose, muscle, and liver tissues of AdiposeENPP1-TG and wild-type (WT) siblings. We also compared response to intraperitoneal glucose tolerance test (IPGTT) and insulin tolerance test (ITT). Our results show no changes in Adipose ENPP1-TG mice fed a regular chow diet. After high-fat diet with pair-feeding protocol, AdiposeENPP1-TG and WT mice had similar weights. However, AdiposeENPP1-TG mice developed fatty liver in association with changes in AT characterized by smaller adipocyte size and decreased phosphorylation of insulin receptor Tyr(1361) and Akt Ser(473). These changes in AT function and fat distribution were associated with systemic abnormalities of lipid and glucose metabolism, including increased plasma concentrations of fatty acid, triglyceride, plasma glucose, and insulin during IPGTT and decreased glucose suppression during ITT. Thus, our results show that, in the presence of a high-fat diet, ENPP1 overexpression in adipocytes induces fatty liver, hyperlipidemia, and dysglycemia, thus recapitulating key manifestations of the metabolic syndrome.     

Alpha-lipoic acid increases insulin sensitivity by activating AMPK in skeletal muscle

Triglyceride accumulation in skeletal muscle contributes to insulin resistance in obesity. We recently showed that alpha-lipoic acid (ALA) reduces body weight and prevents the development of diabetes in diabetes-prone obese rats by reducing triglyceride accumulation in non-adipose tissues. AMP-activated protein kinase (AMPK) is a major regulator of cellular energy metabolism. We examined whether ALA lowers triglyceride accumulation in skeletal muscle by activating AMPK. Alpha2-AMPK activity was decreased in obese rats compared to control rats. Administration of ALA to obese rats increased insulin-stimulated glucose disposal in whole body and in skeletal muscle. ALA also increased fatty acid oxidation and activated AMPK in skeletal muscle. Adenovirus-mediated administration of dominant negative AMPK into skeletal muscle prevented the ALA-induced increases in fatty acid oxidation and insulin-stimulated glucose uptake. These results suggest that ALA-induced improvement of insulin sensitivity is mediated by activation of AMPK and reduced triglyceride accumulation in skeletal muscle.     

Prevention and Reversal of Obesity and Glucose Intolerance in Mice by DHA Derivatives

The n‐3 polyunsaturated fatty acids, especially eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), exert hypolipidemic effects and prevent development of obesity and insulin resistance in animals fed high‐fat diets. We sought to determine the efficacy of α‐substituted DHA derivatives as lipid‐lowering, antiobesity, and antidiabetic agents. C57BL/6 mice were given a corn oil‐based high‐fat (35% weight/weight) diet (cHF), or cHF with 1.5% of lipids replaced with α‐methyl DHA ethyl ester (Substance 1), α‐ethyl DHA ethyl ester (Substance 2), α,α‐di‐methyl DHA ethyl ester (Substance 3), or α‐thioethyl DHA ethyl ester (Substance 4) for 4 months. Plasma markers of glucose and lipid metabolism, glucose tolerance, morphology, tissue lipid content, and gene regulation were characterized. The cHF induced obesity, hyperlipidemia, impairment of glucose homeostasis, and adipose tissue inflammation. Except for Substance 3, all other substances prevented weight gain and Substance 2 exerted the strongest effect (63% of cHF‐controls). Glucose intolerance was significantly prevented (～67% of cHF) by both Substance 1 and Substance 2. Moreover, Substance 2 lowered fasting glycemia, plasma insulin, triacylglycerols, and nonesterified fatty acids (73, 9, 47, and 81% of cHF‐controls, respectively). Substance 2 reduced accumulation of lipids in liver and skeletal muscle, as well as adipose tissue inflammation associated with obesity. Substance 2 also induced weight loss in dietary obese mice. In contrast to DHA administered either alone or as a component of the EPA/DHA concentrate (replacing 15% of dietary lipids), Substance 2 also reversed established glucose intolerance in obese mice. Thus, Substance 2 represents a novel compound with a promising potential in the treatment of obesity and associated metabolic disturbances.     

Restoration of skeletal muscle leptin response does not precede the exercise-induced recovery of insulin-stimulated glucose uptake in high-fat-fed rats

Leptin administration increases fatty acid (FA) oxidation rates and decreases lipid storage in oxidative skeletal muscle, thereby improving insulin response. We have previously shown high-fat (HF) diets to rapidly induce skeletal muscle leptin resistance, prior to the disruption of normal muscle FA metabolism (increase in FA transport; accumulation of triacylglycerol, diacylglycerol, ceramide) that occurs in advance of impaired insulin signaling and glucose transport. All of this occurs within a 4-wk period. Conversely, exercise can rapidly improve insulin response, in as little as one exercise bout. Thus, if the early development of leptin resistance is a contributor to HF diet-induced insulin resistance (IR) in skeletal muscle, then it is logical to predict that the rapid restoration of insulin response by exercise training would be preceded by the recovery of leptin response. In the current study, we sought to determine 1) whether 1, 2, or 4 wk of exercise training was sufficient to restore leptin response in isolated soleus muscle of rats already consuming a HF diet (60% kcal), and 2) whether this preceded the training-induced corrections in FA metabolism and improved insulin-stimulated glucose transport. In the low-fat (LF)-fed control group, insulin increased glucose transport by 153% and leptin increased AMPK and ACC phosphorylation and the rate of palmitate oxidation (+73%). These responses to insulin and leptin were either severely blunted or absent following 4 wk of HF feeding. Exercise intervention decreased muscle ceramide content (-28%) and restored insulin-stimulated glucose transport to control levels within 1 wk; muscle leptin response (AMPK and ACC phosphorylation, FA oxidation) was also restored, but not until the 2-wk time point. In conclusion, endurance exercise training is able to restore leptin response, but this does not appear to be a necessary precursor for the restoration of insulin response.     

Long-term fermented soybean paste improves metabolic parameters associated with non-alcoholic fatty liver disease and insulin resistance in high-fat diet-induced obese mice

Recently, Korean traditional fermented soybean paste, called Doenjang, has attracted attention for its protective effect against diet-related chronic diseases such as obesity and type 2 diabetes. Long-term fermented soybean pastes (LFSPs) are made by fermentation with naturally-occurring microorganisms for several months, whereas short-term fermented soybean pastes (SFSPs) are produced by shorter-time fermentation inoculated with a starter culture. Here, we demonstrate that administration of LFSP, but not SFSP, protects high-fat diet (HFD)-fed obese mice against non-alcohol fatty liver disease (NAFLD) and insulin resistance. LFSP suppressed body weight gain in parallel with reduction in fat accumulation in mesenteric adipose tissue (MAT) and the liver via modulation of MAT lipolysis and hepatic lipid uptake. LFSP-treated mice also had improved glucose tolerance and increased adiponectin levels concomitantly with enhanced AMPK activation in skeletal muscle and suppressed expression of pro-inflammatory cytokines in skeletal muscle and the liver. LFSP also attenuated HFD-induced gut permeability and lowered serum lipopolysaccharide level, providing an evidence for its probiotic effects, which was supported by the observation that treatment of a probiotic mixture of LFSP-originated Bacillus strains protected mice against HFD-induced adiposity and glucose intolerance. Our findings suggest that the intake of LFSP, but not SFSP, offers protection against NAFLD and insulin resistance, which is an effect of long-term fermentation resulting in elevated contents of active ingredients (especially flavonoids) and higher diversity and richness of Bacillus probiotic strains compared to SFSP.     

Mitochondrial dysfunction and inhibition of myoblast differentiation in mice with high-fat-diet-induced pre-diabetes

Pre-diabetes is characterized by impaired glucose tolerance (IGT) and/or impaired fasting glucose. Impairment of skeletal muscle function is closely associated with the progression of diabetes. However, the entire pathological characteristics and mechanisms of pre-diabetes in skeletal muscle remain fully unknown. Here, we established a mouse model of pre-diabetes, in which 6-week-old male C57BL6/J mice were fed either normal diet or high-fat diet (HFD) for 8 or 16 weeks. Both non-fasting and fasting glucose levels and the results of glucose and insulin tolerance tests showed that mice fed an 8-week HFD developed pre-diabetes with IGT; whereas mice fed a 16-week HFD presented with impaired fasting glucose and impaired glucose tolerance (IFG-IGT). Mice at both stages of pre-diabetes displayed decreased numbers of mitochondria in skeletal muscle. Moreover, IFG-IGT mice exhibited decreased mitochondrial membrane potential and ATP production in skeletal muscle and muscle degeneration characterized by a shift in muscle fibers from predominantly oxidative type I to glycolytic type II. Western blotting and histological analysis confirmed that myoblast differentiation was only inhibited in IFG-IGT mice. For primary skeletal muscle satellite cells, inhibition of differentiation was observed in palmitic acid-induced insulin resistance model. Moreover, enhanced myoblast differentiation increased glucose uptake and insulin sensitivity. These findings indicate that pre-diabetes result in mitochondrial dysfunction and inhibition of myoblast differentiation in skeletal muscle. Therefore, interventions that enhance myoblast differentiation may improve insulin resistance of diabetes at the earlier stage.     

Tocotrienol-rich fraction supplementation reduces hyperglycemia-induced skeletal muscle damage through regulation of insulin signaling and oxidative stress in type 2 diabetic mice

Chronic hyperglycemia induces impairment of muscle growth and development of diabetes mellitus (DM). Since skeletal muscle is the major site for disposal of ingested glucose, impaired glucose metabolism causes imbalance between protein synthesis and degradation which adversely affects physical mobility.In this study, we investigated the effect of tocotrienol-rich fraction (TRF) supplementation on skeletal muscle damage in diabetic mice. Diabetes was induced by a high-fat diet with streptozotocin (STZ) injection (100 mg/kg) in male C57BL/6J mice. After diabetes was induced (fasting blood glucose levels≥250 mg/dl), normal control (CON) and diabetic control (DMC) groups were administrated with olive oil, while TRF treatment groups were administrated with TRF (dissolved in olive oil) at low dose (100 mg/kg BW, LT) or high dose (300 mg/kg BW, HT) by oral gavage for 12 weeks.TRF supplementation ameliorated muscle atrophy, plasma insulin concentration and homeostatic model assessment estimated insulin resistance in diabetic mice. Moreover, TRF treatment up-regulated IRS-1 and Akt levels accompanied by increased translocation of GLUT4. Furthermore, TRF increased mitochondrial biogenesis by activating SIRT1, SIRT3 and AMPK in diabetic skeletal muscle. These changes were in part mechanistically explained by reduced levels of skeletal muscle proteins related to oxidative stress (4-hydroxynonenal, protein carbonyls, Nrf2 and HO-1), inflammation (NFkB, MCP-1, IL-6 and TNF-α), and apoptosis (Bax, Bcl₂ and caspase-3) in diabetic mice. Taken together, these results suggest that TRF might be useful as a beneficial nutraceutical to prevent skeletal muscle atrophy associated with diabetes by regulating insulin signaling via AMPK/SIRT1/PGC1α pathways in type 2 diabetic mice.