Globular adiponectin resistance develops independently of impaired insulin-stimulated glucose transport in soleus muscle from high-fat-fed rats

High-fat (HF) diets induce insulin resistance and alter lipid metabolism, although controversy exists regarding the impact of saturated vs. polyunsaturated fats. Adiponectin (Ad) stimulates fatty acid (FA) oxidation and improves insulin sensitivity in humans and rodents, due in part to the activation of AMP-activated protein kinase (AMPK) and subsequent deactivation of acetyl coenzyme A carboxylase (ACC). In genetically obese, diabetic mice, this acute stimulatory effect on AMPK in muscle is lost. The ability of a HF diet to induce skeletal muscle Ad resistance has not been examined. The purpose of this study was to determine whether Ad's effects on FA oxidation and AMPK/ACC would be reduced following different HF diets, and if this coincided with the development of impaired maximal insulin-stimulated glucose transport. Rats were fed a control (10% kcal fat, CON), high unsaturated fat (60% kcal safflower oil, SAFF), or high saturated fat diet (60% kcal lard, LARD) for 4 wk. Following the dietary intervention, glucose transport, lipid metabolism, and AMPK/ACC phosphorylation were measured in the presence and absence of globular Ad (gAd, 2.5 microg/ml) in isolated soleus muscle. LARD rats showed reduced rates of maximal insulin-stimulated glucose transport compared with CON and SAFF (+68 vs. +172 and +184%, P < or = 0.001). gAd increased pACC (+25%, P < or = 0.01) and FA oxidation (+28%, P < or = 0.05) in CON rats, but not in either HF group. Thus 4 wk of HF feeding results in the loss of gAd stimulatory effect on ACC phosphorylation and muscle FA oxidation, and this can occur independently of impaired maximal insulin-stimulated glucose transport.     

Metformin and exercise reduce muscle FAT/CD36 and lipid accumulation and blunt the progression of high-fat diet-induced hyperglycemia

Derangements in skeletal muscle fatty acid (FA) metabolism associated with insulin resistance in obesity appear to involve decreased FA oxidation and increased accumulation of lipids such as ceramides and diacylglycerol (DAG). We investigated potential lipid-related mechanisms of metformin (Met) and/or exercise for blunting the progression of hyperglycemia/hyperinsulinemia and skeletal muscle insulin resistance in female Zucker diabetic fatty rats (ZDF), a high-fat (HF) diet-induced model of diabetes. Lean and ZDF rats consumed control or HF diet (48 kcal %fat) alone or with Met (500 mg/kg), with treadmill exercise, or with both exercise and Met interventions for 8 wk. HF-fed ZDF rats developed hyperglycemia (mean: 24.4 +/- 2.1 mM), impairments in muscle insulin-stimulated glucose transport, increases in the FA transporter FAT/CD36, and increases in total ceramide and DAG content. The development of hyperglycemia was significantly attenuated with all interventions, as was skeletal muscle FAT/CD36 abundance and ceramide and DAG content. Interestingly, improvements in insulin-stimulated glucose transport and increased GLUT4 transporter expression in isolated muscle were seen only in conditions that included exercise training. Reduced FA oxidation and increased triacylglycerol synthesis in isolated muscle were observed with all ZDF rats compared with lean rats (P < 0.01) and were unaltered by therapeutic intervention. However, exercise did induce modest increases in peroxisome proliferator-activated receptor-gamma coactivator-1alpha, citrate synthase, and beta-hydroxyacyl-CoA dehydrogenase activity. Thus reduction of skeletal muscle FAT/CD36 and content of ceramide and DAG may be important mechanisms by which exercise training blunts the progression of diet-induced insulin resistance in skeletal muscle.     

Effects of rexinoids on glucose transport and insulin-mediated signaling in skeletal muscles of diabetic (db/db) mice

Rexinoids and thiazolidinediones (TZDs) are two classes of nuclear receptor ligands that induce insulin sensitization in diabetic rodents. TZDs are peroxisome proliferator-activated receptor gamma (PPARgamma) activators, whereas rexinoids are selective ligands for the retinoid X receptors (RXRs). Activation of both the insulin receptor substrates (IRSs)/Akt and the c-Cbl-associated protein (CAP)/c-Cbl pathways are important in regulating insulin-stimulated glucose transport. We have compared the effects of a rexinoid (LG268) and a TZD (rosiglitazone) on these two signal pathways in skeletal muscle of diabetic (db/db) mice. The results we have obtained show that treatment of db/db mice with either LG268 or rosiglitazone for 2 weeks results in a significant increase in insulin-stimulated glucose transport activity in skeletal muscle. Treatment with LG268 increases insulin-stimulated IRS-1 tyrosine phosphorylation and Akt phosphorylation in skeletal muscle without affecting the activity of the CAP/c-Cbl pathway. In contrast, rosiglitazone increases the levels of CAP expression and insulin-stimulated c-Cbl phosphorylation without affecting the IRS-1/Akt pathway. The effects of LG268 on the IRS-1/Akt pathway were associated with a decrease in the level of IRS-1 Ser(307) phosphorylation. Taken together, these data suggest that rexinoids improve insulin sensitivity via changes in skeletal muscle metabolism that are distinct from those induced by TZDs. Rexinoids represent a novel class of insulin sensitizers with potential applications in the treatment of insulin resistance.     

Muscle-specific Pparg deletion causes insulin resistance

Thiazolidinediones (TZDs) are insulin-sensitizing drugs and are potent agonists of the nuclear peroxisome proliferator-activated receptor-gamma (PPAR-gamma). Although muscle is the major organ responsible for insulin-stimulated glucose disposal, PPAR-gamma is more highly expressed in adipose tissue than in muscle. To address this issue, we used the Cre-loxP system to knock out Pparg, the gene encoding PPAR-gamma, in mouse skeletal muscle. As early as 4 months of age, mice with targeted disruption of PPAR-gamma in muscle showed glucose intolerance and progressive insulin resistance. Using the hyperinsulinemic-euglycemic clamp technique, the in vivo insulin-stimulated glucose disposal rate (IS-GDR) was reduced by approximately 80% and was unchanged by 3 weeks of TZD treatment. These effects reveal a crucial role for muscle PPAR-gamma in the maintenance of skeletal muscle insulin action, the etiology of insulin resistance and the action of TZDs.     

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.     

A Thiazolidinedione Improves In Vivo Insulin Action on Skeletal Muscle Glycogen Synthase in Insulin-Resistant Monkeys

Thiazolidinediones (TZD) have been shown to have anti-diabetic effects including the ability to decrease fasting hyperglycemia and hyperinsulinemia, increase insulin-mediated glucose disposal rate (M) and decrease hepatic glucose production, but the mechanisms of action are not well established. To determine whether a TZD (R-102380, Sankyo Company Ltd., Tokyo, Japan) could improve insulin action on skeletal muscle glycogen synthase (GS), the rate-limiting enzyme in glycogen synthesis, 4 insulin-resistant obese monkeys were given I mg/kg/ day R-102380 p.o. for a 6-week period. Skeletal muscle GS activity and glucose 6-phosphate (G6P) content were compared between pre-dosing and dosing periods before and during the maximal insulin-stimulation of a euglycemic hyperinsulinemic clamp.Compared to pre-dosing, insulin-stimulated GS activity and G6P content were increased by this TZD: GS independent activity (p = 0.02), GS total activity (p = 0.005), GS fractional activity (p = 0.06) and G6P content (p = 0.02). The change in GS activity induced by in vivo insulin (insulin-stimulated minus basal) was also increased by this TZD: GS independent activity (p = 0.03) and GS fractional activity (p = 0.04).We conclude that the TZD R-102380 improves insulin action at the skeletal muscle in part by increasing the activity of glycogen synthase. This improvement in insulin sensitivity may be a key factor in the anti-diabetic effect of the thiazolidinedione class of agents.     

A single prior bout of exercise protects against palmitate-induced insulin resistance despite an increase in total ceramide content

Ceramide accumulation has been implicated in the impairment of insulin-stimulated glucose transport in skeletal muscle following saturated fatty acid (FA) exposure. Importantly, a single bout of exercise can protect against acute lipid-induced insulin resistance. The mechanism by which exercise protects against lipid-induced insulin resistance is not completely known but may occur through a redirection of FA toward triacylglycerol (TAG) and away from ceramide and diacylglycerol (DAG). Therefore, in the current study, an in vitro preparation was used to examine whether a prior bout of exercise could confer protection against palmitate-induced insulin resistance and whether the pharmacological [50 μM fumonisin B(1) (FB1)] inhibition of ceramide synthesis in the presence of palmitate could mimic the protective effect of exercise. Soleus muscle of sedentary (SED), exercised (EX), and SED in the presence of FB1 (SED+FB1) were incubated with or without 2 mM palmitate for 4 h. This 2-mM palmitate exposure impaired insulin-stimulated glucose transport (-28%, P < 0.01) and significantly increased ceramide, DAG, and TAG accumulation in the SED group (P < 0.05). A single prior bout of exercise prevented the detrimental effects of palmitate on insulin signaling and caused a partial redistribution of FA toward TAG (P < 0.05). However, the net increase in ceramide content in response to palmitate exposure in the EX group was not different compared with SED, despite the maintenance of insulin sensitivity. The incubation of soleus from SED rats with FB1 (SED+FB1) prevented the detrimental effects of palmitate and caused a redirection of FA toward TAG accumulation (P < 0.05). Therefore, this research suggests that although inhibiting ceramide accumulation can prevent the detrimental effects of palmitate, a single prior bout of exercise appears to protect against palmitate-induced insulin resistance, which may be independent of changes in ceramide content.     

Down-regulation of lipoprotein lipase increases glucose uptake in L6 muscle cells

Thiazolidinediones (TZDs) are synthetic hypoglycemic agents used to treat type 2 diabetes. TZDs target the peroxisome proliferator activated receptor-gamma (PPAR-γ) and improve systemic insulin sensitivity. The contributions of specific tissues to TZD action, or the downstream effects of PPAR-γ activation, are not very clear. We have used a rat skeletal muscle cell line (L6 cells) to demonstrate that TZDs directly target PPAR-γ in muscle cells. TZD treatment resulted in a significant repression of lipoprotein lipase (LPL) expression in L6 cells. This repression correlated with an increase in glucose uptake. Down-regulation of LPL message and protein levels using siRNA resulted in a similar increase in insulin-dependent glucose uptake. Thus, LPL down-regulation improved insulin sensitivity independent of TZDs. This finding provides a novel method for the management of insulin resistance.     

Effect of heterozygous PPARgamma deficiency and TZD treatment on insulin resistance associated with age and high-fat feeding

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is the target receptor for thiazolidinedione (TZD) compounds, which are a class of insulin-sensitizing drugs used in the treatment of type 2 diabetes. Paradoxically, however, mice deficient in PPARgamma (PPARgamma(+/-)) are more insulin sensitive than their wild-type (WT) littermates, not less, as would be predicted. To determine whether PPARgamma deficiency could prevent the development of the insulin resistance associated with increasing age or high-fat (HF) feeding, insulin sensitivity was assessed in PPARgamma(+/-) and WT mice at 2, 4, and 8 mo of age and in animals fed an HF diet. Because TZDs elicit their effect through PPARgamma receptor, we also examined the effect of troglitazone (a TZD) in these mice. Glucose metabolism was assessed by hyperinsulinemic euglycemic clamp and oral glucose tolerance test. Insulin sensitivity declined with age for both groups. However, the decline in the PPARgamma(+/-) animals was substantially less than that of the WT animals, such that, by 8 mo of age, the PPARgamma(+/-) mice were markedly more insulin sensitive than the WT mice. This greater sensitivity in PPARgamma(+/-) mice was lost with TZD treatment. HF feeding led to marked adipocyte hypertrophy and peripheral tissue and hepatic insulin resistance in WT mice but also in PPARgamma(+/-) mice. Treatment of these mice with troglitazone completely prevented the adipocyte hypertrophy and normalized insulin action. In conclusion, PPARgamma deficiency partially protects against age-related insulin resistance but does not protect against HF diet-induced insulin resistance.     

Brain PPAR-γ promotes obesity and is required for the insulin-sensitizing effect of thiazolidinediones

In adipose tissue, muscle, liver and macrophages, signaling by the nuclear receptor peroxisome proliferator-activated receptor-γ (PPAR-γ) is a determinant of insulin sensitivity and this receptor mediates the insulin-sensitizing effects of thiazolidinediones (TZDs). As PPAR-γ is also expressed in neurons, we generated mice with neuron-specific Pparg knockout (Pparg brain knockout (BKO)) to determine whether neuronal PPAR-γ signaling contributes to either weight gain or insulin sensitivity. During high-fat diet (HFD) feeding, food intake was reduced and energy expenditure increased in Pparg-BKO mice compared to Pparg(f/f) mice, resulting in reduced weight gain. Pparg-BKO mice also responded better to leptin administration than Pparg(f/f) mice. When treated with the TZD rosiglitazone, Pparg-BKO mice were resistant to rosiglitazone-induced hyperphagia and weight gain and, relative to rosiglitazone-treated Pparg(f/f) mice, experienced only a marginal improvement in glucose metabolism. Hyperinsulinemic euglycemic clamp studies showed that the increase in hepatic insulin sensitivity induced by rosiglitazone treatment during HFD feeding was completely abolished in Pparg-BKO mice, an effect associated with the failure of rosiglitazone to improve liver insulin receptor signal transduction. We conclude that excess weight gain induced by HFD feeding depends in part on the effect of neuronal PPAR-γ signaling to limit thermogenesis and increase food intake. Neuronal PPAR-γ signaling is also required for the hepatic insulin sensitizing effects of TZDs.     

Aging is associated with elevated muscle triglyceride content and increased insulin-stimulated fatty acid uptake

The purpose of the present study was to examine the utilization of fatty acids (FA) and muscle substrates by skeletal muscle in young, middle-aged, and old adult rats under hyperglycemic and hyperinsulinemic conditions. Male Fischer 344 x Brown Norway rats aged 5, 15, or 24 mo underwent hindlimb perfusion with a medium of 20 mM glucose, 1 mM palmitate, 1,000 microU/ml insulin, [1-14C]palmitate, and [3-3H]glucose. Glucose uptake and palmitate delivery were similar among age groups. Palmitate uptake and oxidation as well as muscle protein concentration of fatty acid translocase (FAT/CD36) and plasma membrane fatty acid-binding protein (FABPPM) were significantly increased (P < or = 0.05) in 24- vs. 5- and 15-mo-old animals. Compared with 5- and 15-mo-old animals, pre- and postperfusion muscle triglyceride (TG) levels were significantly (P < 0.05) elevated 72-145% in red and 112-129% in white muscles of 24-mo-old animals. Palmitate uptake was associated with total preperfusion TG concentration (r2 = 0.27, P < 0.05) and total TG synthesis rate (r2 = 0.68, P < 0.05). These results indicate that, under insulin-stimulated conditions, FA uptake is significantly increased in old animals, which is associated with increased rates of TG synthesis and may contribute to the accumulation of TG in muscle of old animals.     

High fat diet-induced changes in mouse muscle mitochondrial phospholipids do not impair mitochondrial respiration despite insulin resistance

Background

Type 2 diabetes mellitus and muscle insulin resistance have been associated with reduced capacity of skeletal muscle mitochondria, possibly as a result of increased intake of dietary fat. Here, we examined the hypothesis that a prolonged high-fat diet consumption (HFD) increases the saturation of muscle mitochondrial membrane phospholipids causing impaired mitochondrial oxidative capacity and possibly insulin resistance.

Methodology

C57BL/6J mice were fed an 8-week or 20-week low fat diet (10 kcal%; LFD) or HFD (45 kcal%). Skeletal muscle mitochondria were isolated and fatty acid (FA) composition of skeletal muscle mitochondrial phospholipids was analyzed by thin-layer chromatography followed by GC. High-resolution respirometry was used to assess oxidation of pyruvate and fatty acids by mitochondria. Insulin sensitivity was estimated by HOMA-IR.

Principal Findings

At 8 weeks, mono-unsaturated FA (16∶1n7, 18∶1n7 and 18∶1n9) were decreased (−4.0%, p<0.001), whereas saturated FA (16∶0) were increased (+3.2%, p<0.001) in phospholipids of HFD vs. LFD mitochondria. Interestingly, 20 weeks of HFD descreased mono-unsaturated FA while n-6 poly-unsaturated FA (18∶2n6, 20∶4n6, 22∶5n6) showed a pronounced increase (+4.0%, p<0.001). Despite increased saturation of muscle mitochondrial phospholipids after the 8-week HFD, mitochondrial oxidation of both pyruvate and fatty acids were similar between LFD and HFD mice. After 20 weeks of HFD, the increase in n-6 poly-unsaturated FA was accompanied by enhanced maximal capacity of the electron transport chain (+49%, p = 0.002) and a tendency for increased ADP-stimulated respiration, but only when fuelled by a lipid-derived substrate. Insulin sensitivity in HFD mice was reduced at both 8 and 20 weeks.

Conclusions/Interpretation

Our findings do not support the concept that prolonged HF feeding leads to increased saturation of skeletal muscle mitochondrial phospholipids resulting in a decrease in mitochondrial fat oxidative capacity and (muscle) insulin resistance.     

Recovered insulin response by 2 weeks of leptin administration in high-fat fed rats is associated with restored AS160 activation and decreased reactive lipid accumulation

Leptin is an adipokine that increases fatty acid (FA) oxidation, decreases intramuscular lipid stores, and improves insulin response in skeletal muscle. In an attempt to elucidate the underlying mechanisms by which these metabolic changes occur, we administered leptin (Lep) or saline (Sal) by miniosmotic pumps to rats during the final 2 wk of a 6-wk low-fat (LF) or high-fat (HF) diet. Insulin-stimulated glucose transport was impaired by the HF diet (HF-Sal) but was restored with leptin administration (HF-Lep). This improvement was associated with restored phosphorylation of Akt and AS160 and decreased in reactive lipid species (ceramide, diacylglycerol), known inhibitors of the insulin-signaling cascade. Total muscle citrate synthase (CS) activity was increased by both leptin and HF diet, but was not additive. Leptin increased subsarcolemmal (SS) and intramyofibrillar (IMF) mitochondria CS activity. Total muscle, sarcolemmal, and mitochondrial (SS and IMF) FA transporter (FAT/CD36) protein content was significantly increased with the HF diet, but not altered by leptin. Therefore, the decrease in reactive lipid stores and subsequent improvement in insulin response, secondary to leptin administration in rats fed a HF diet was not due to a decrease in FA transport protein content or altered cellular distribution.     

Insulin increases FA uptake and esterification but reduces lipid utilization in isolated contracting muscle

We examined the effect of insulin on the synthesis and degradation of muscle lipid pools [phospholipid (PL), diacylglycerol (DG), triacylglycerol (TG)] and palmitate oxidation in isolated resting and contracting (20 tetani/min) soleus muscles. Lipid metabolism was monitored using the previously defined pulse-chase procedure. At rest, insulin significantly increased total palmitate uptake into soleus muscle (+49%, P < 0.05), corresponding to enhanced DG (+60%, P < 0.05) and TG (+61%, P < 0.05) esterification, but blunted palmitate oxidation (-38%, P < 0.05) and TG hydrolysis (-34%, P < 0.05). During muscle contraction, when total palmitate uptake was increased, insulin further enhanced uptake (+21%, P < 0.05) and esterification of fatty acids (FA) to PL (+73%, P < 0.05), DG (+19%, P < 0.05), and TG (+161%, P < 0.01). Despite a profound shift in the relative partitioning of FA away from esterification and toward oxidation during contraction, the increase in palmitate oxidation and TG hydrolysis was significantly blunted by insulin [oxidation, -24% (P = 0.05); hydrolysis, -83% (P < 0.01)]. The effects of insulin on FA esterification (stimulation) and oxidation (inhibition) during contraction were reduced in the presence of the phosphatidylinositol 3-kinase inhibitor LY-294002. In summary, the effects of insulin and contraction on palmitate uptake and esterification are additive, while insulin opposes the stimulatory effect of contraction on FA oxidation and TG hydrolysis. Insulin's modulatory effects on muscle FA metabolism during contraction are mediated at least in part through phosphatidylinositol 3-kinase.     

Divergent effects of rosiglitazone on protein-mediated fatty acid uptake in adipose and in muscle tissues of Zucker rats

Thiazolidinediones (TZDs) increase tissue insulin sensitivity in diabetes. Here, we hypothesize that, in adipose tissue, skeletal muscle, and heart, alterations in protein-mediated FA uptake are involved in the effect of TZDs. As a model, we used obese Zucker rats, orally treated for 16 days with 5 mg rosiglitazone (Rgz)/kg body mass/day. In adipose tissue from Rgz-treated rats, FA uptake capacity increased by 2.0-fold, coinciding with increased total contents of fatty acid translocase (FAT/CD36; 2.3-fold) and fatty acid transport protein 1 (1.7-fold) but not of plasmalemmal fatty acid binding protein, whereas only the plasmalemmal content of FAT/CD36 was changed (increase of 1.7-fold). The increase in FA uptake capacity of adipose tissue was associated with a decline in plasma FA and triacylglycerols (TAGs), suggesting that Rgz treatment enhanced plasma FA extraction by adipocytes. In obese hearts, Rgz treatment had no effect on the FA transport system, yet the total TAG content decreased, suggesting enhanced insulin sensitivity. Also, in skeletal muscle, the FA transport system was not changed. However, the TAG content remained unaltered in skeletal muscle, which coincided with increased cytoplasmic adipose-type FABP content, suggesting that increased extramyocellular TAGs mask the decline of intracellular TAG in muscle. In conclusion, our study implicates FAT/CD36 in the mechanism by which Rgz increases tissue insulin sensitivity.     

Metformin counters the insulin-induced suppression of fatty acid oxidation and stimulation of triacylglycerol storage in rodent skeletal muscle

The present study examined the acute effects of metformin on fatty acid (FA) metabolism in oxidative soleus (SOL) and glycolytic epitrochlearis (EPT) rodent muscle. SOL and EPT were incubated for either 30 or 180 min in the absence or presence of 2 mM metformin and with or without insulin (10 mU/ml). Metformin did not alter basal FA metabolism but countered the effects of insulin on FA oxidation and incorporation into triacylglyerol (TAG). Specifically, metformin prevented the insulin-induced suppression of FA oxidation in SOL but did not alter FA incorporation into lipid pools. In contrast, in EPT metformin blunted the incorporation of FA into TAG when insulin was present but did not alter FA oxidation. In SOL, metformin resulted in a 50% increase in AMP-activated protein kinase alpha2 activity and prevented the insulin-induced increase in malonyl-CoA content. In both fiber types, basal and insulin-stimulated glucose oxidation were not significantly altered by metformin. All effects were similar regardless of whether they were measured after 30 or 180 min. Because increased muscle lipid storage and impaired FA oxidation have been associated with insulin resistance in this tissue, the ability of metformin to reverse these abnormalities in muscle FA metabolism may be a part of the mechanism by which metformin improves glucose clearance and insulin sensitivity. The present data also suggest that increased glucose clearance is not due to its enhanced subsequent oxidation. Additional studies are warranted to determine whether chronic metformin treatment has similar effects on muscle FA metabolism.     

A futile metabolic cycle activated in adipocytes by antidiabetic agents

Thiazolidinediones (TZDs) are effective therapies for type 2 diabetes, which has reached epidemic proportions in industrialized societies. TZD treatment reduces circulating free fatty acids (FFAs), which oppose insulin actions in skeletal muscle and other insulin target tissues. Here we report that TZDs, acting as ligands for the nuclear receptor peroxisome proliferator-activated receptor (PPAR)-gamma, markedly induce adipocyte glycerol kinase (GyK) gene expression. This is surprising, as standard textbooks indicate that adipocytes lack GyK and thereby avoid futile cycles of triglyceride breakdown and resynthesis from glycerol and FFAs. By inducing GyK, TZDs markedly stimulate glycerol incorporation into triglyceride and reduce FFA secretion from adipocytes. The 'futile' fuel cycle resulting from expression of GyK in adipocytes is thus a novel mechanism contributing to reduced FFA levels and perhaps insulin sensitization by antidiabetic therapies.     

Endurance training in obese humans improves glucose tolerance and mitochondrial fatty acid oxidation and alters muscle lipid content

Muscle fatty acid (FA) metabolism is impaired in obesity and insulin resistance, reflected by reduced rates of FA oxidation and accumulation of lipids. It has been suggested that interventions that increase FA oxidation may enhance insulin action by reducing these lipid pools. Here, we examined the effect of endurance training on rates of mitochondrial FA oxidation, the activity of carnitine palmitoyltransferase I (CPT I), and the lipid content in muscle of obese individuals and related these to measures of glucose tolerance. Nine obese subjects completed 8 wk of moderate-intensity endurance training, and muscle biopsies were obtained before and after training. Training significantly improved glucose tolerance, with a reduction in the area under the curve for glucose (P < 0.05) and insulin (P = 0.01) during an oral glucose tolerance test. CPT I activity increased 250% (P = 0.001) with training and became less sensitive to inhibition by malonyl-CoA. This was associated with an increase in mitochondrial FA oxidation (+120%, P < 0.001). Training had no effect on muscle triacylglycerol content; however, there was a trend for training to reduce both the total diacylglcyerol (DAG) content (-15%, P = 0.06) and the saturated DAG-FA species (-27%, P = 0.06). Training reduced both total ceramide content (-42%, P = 0.01) and the saturated ceramide species (-32%, P < 0.05). These findings suggest that the improved capacity for mitochondrial FA uptake and oxidation leads not only to a reduction in muscle lipid content but also a to change in the saturation status of lipids, which may, at least in part, provide a mechanism for the enhanced insulin action observed with endurance training in obese individuals.     

Prolonged decrease of adipocyte size after rosiglitazone treatment in high- and low-fat-fed rats

Objective: The anti‐diabetic thiazolidinediones (TZDs) stimulate adipocyte differentiation and decrease mean adipocyte size. However, whether these smaller, more insulin‐sensitive adipocytes maintain their size after TZD therapy is discontinued has not been studied. Research Methods and Procedures: Adult female Sprague‐Dawley rats were fed a low‐fat (10% fat) diet or, to elevate body weight (BW), a high‐fat (HF) diet (45% fat) for 6 weeks. Rats were initially randomized to groups (n = 12) fed either low‐fat or HF diets, with or without the TZD rosiglitazone (ROSI; 5 mg/kg per day), for 6 weeks. ROSI was then discontinued, and all animals were fed HF for another 6 weeks before sacrifice. Retroperitoneal (RP) adipose tissue morphology was determined from tissue collected by serial biopsies before and after 6 weeks of ROSI treatment and at sacrifice. Results: Measures of BW and adiposity did not differ among groups 6 weeks after stopping ROSI treatment. However, during treatment, ROSI in both diets significantly decreased RP adipocyte size and increased RP DNA content, and these effects continued to be observed after discontinuing treatment. ROSI administration also decreased circulating insulin, leptin, and triglycerides and increased circulating adiponectin levels; however, these effects were reversed on stopping treatment. Discussion: These results demonstrated that TZD‐induced effects on adipocyte size and number were maintained after discontinuing treatment, even with consumption of an obesigenic diet. However, additional studies are needed to determine whether TZD‐treated animals eventually achieve an adipocyte size similar to that of untreated animals at the expense of a higher BW.     

Fatty acid-induced insulin resistance in L6 myotubes is prevented by inhibition of activation and nuclear localization of nuclear factor kappa B

Recent studies have implicated inhibitor of kappaB kinase (IKK) in mediating fatty acid (FA)-induced insulin resistance. How IKK causes these effects is unknown. The present study addressed the role of nuclear factor kappaB (NFkappaB), the distal target of IKK activity, in FA-induced insulin resistance in L6 myotubes, an in vitro skeletal muscle model. A 6-h exposure of myotubes to the saturated FA palmitate reduced insulin-stimulated glucose uptake by approximately 30%, phosphatidylinositol-3 kinase and protein kinase B phosphorylation by approximately 40%, and stimulated inhibitor of kappaBalpha degradation and the nuclear translocation of NFkappaB. On the other hand, the Omega-3 polyunsaturated FA linolenate neither induced insulin resistance nor promoted nuclear localization of NFkappaB. Supporting the hypothesis that IKK acts through NFkappaB to cause insulin resistance, the IKK inhibitors acetylsalicylate and parthenolide prevented FA-induced reductions in insulin-stimulated glucose uptake and NFkappaB nuclear translocation. Most importantly, NFkappaB SN50, a cell-permeable peptide that inhibits NFkappaB nuclear translocation downstream of IKK, was sufficient to prevent palmitate-induced reductions in insulin-stimulated glucose uptake. Acetylsalicylate, but not NFkappaB SN50, prevented FA effects on phosphatidylinositol-3 kinase activity and protein kinase B phosphorylation. We conclude that FAs induce insulin resistance and activates NFkappaB in L6 cells. Furthermore, inhibition of NFkappaB activation, indirectly by preventing IKK activation or directly by inhibiting NFkappaB nuclear translocation, prevents the detrimental effects of palmitate on the metabolic actions of insulin in L6 myotubes.