Exercise training enhances white adipose tissue metabolism in rats selectively bred for low- or high-endurance running capacity.

Impaired visceral white adipose tissue (WAT) metabolism has been implicated in the pathogenesis of several lifestyle-related disease states, with diminished expression of several WAT mitochondrial genes reported in both insulin-resistant humans and rodents. We have used rat models selectively bred for low- (LCR) or high-intrinsic running capacity (HCR) that present simultaneously with divergent metabolic phenotypes to test the hypothesis that oxidative enzyme expression is reduced in epididymal WAT from LCR animals. Based on this assumption, we further hypothesized that short-term exercise training (6 wk of treadmill running) would ameliorate this deficit. Approximately 22-wk-old rats (generation 22) were studied. In untrained rats, the abundance of mitochondrial respiratory complexes I-V, citrate synthase (CS), and PGC-1 was similar for both phenotypes, although CS activity was greater than 50% in HCR (P = 0.09). Exercise training increased CS activity in both phenotypes but did not alter mitochondrial protein content. Training increased the expression and phosphorylation of proteins with roles in ß-adrenergic signaling, including ß3-adrenergic receptor (16% increase in LCR; P < 0.05), NOR1 (24% decrease in LCR, 21% decrease in HCR; P < 0.05), phospho-ATGL (25% increase in HCR; P < 0.05), perilipin (25% increase in HCR; P < 0.05), CGI-58 (15% increase in LCR; P < 0.05), and GLUT4 (16% increase in HCR; P < 0.0001). A training effect was also observed for phospho-p38 MAPK (12% decrease in LCR, 20% decrease in HCR; P < 0.05) and phospho-JNK (29% increase in LCR, 20% increase in HCR; P < 0.05). We conclude that in the LCR-HCR model system, mitochondrial protein expression in WAT is not affected by intrinsic running capacity or exercise training. However, training does induce alterations in the activity and expression of several proteins that are essential to the intracellular regulation of WAT metabolism.

Insulin receptor and IRS-1 co-immunoprecipitation with SOCS-3, and IKKα/ß phosphorylation are increased in obese Zucker rat skeletal muscle.

AIMS: We evaluated if selected pro-inflammatory cytokines and/or the protein suppressor of cytokine signaling 3 (SOCS-3) could account for decreased insulin-stimulated phosphatidylinositol 3-kinase (PI3-K) activity in the skeletal muscle of the obese Zucker rat. MAIN METHODS: Eight lean and eight obese Zucker rats ~4weeks of age were obtained and allowed to feed ad libitum for 4weeks before undergoing hind limb perfusion in the presence of 500µU/ml insulin. KEY FINDINGS: Insulin-stimulated skeletal muscle PI3-K activity and 3-O-methylglucose transport rates were reduced (P<0.05) in obese compared to lean animals. IRS-1 concentration remained unchanged although IRS-1 tyrosine phosphorylation was decreased (P<0.05), and IRS-1 serine phosphorylation (pS) was increased (P<0.05) in obese animals compared to lean animals. IKKα/ß pS and JNK theronine/tyrosine phosphorylation was increased (P<0.05) in the obese animals. IκBα concentration was decreased (P<0.05) and IκBα pS was increased (P<0.05) in the obese compared to lean Zucker animals. SOCS-3 concentration and SOCS-3 co-immunoprecipitation with both insulin receptor ß-subunit (IR-ß) and IRS-1 were elevated (P<0.05) in obese compared to lean animals. IRS-1 co-immunoprecipitation with IR-ß was reduced 56% in the obese animals. SIGNIFICANCE: Increased IKKα/ß and JNK serine phosphorylation may contribute to increasing IRS-1 serine phosphorylation, while concurrent co-localization of SOCS-3 with both IR-ß and IRS-1 may prevent IRS-1 from interacting with IR-ß. These two mechanisms thusly may independently contribute to impairing insulin-stimulated PI3-K activation in the skeletal muscle of the obese Zucker rat.

Low intrinsic running capacity is associated with reduced skeletal muscle substrate oxidation and lower mitochondrial content in white skeletal muscle.

Chronic metabolic diseases develop from the complex interaction of environmental and genetic factors, although the extent to which each contributes to these disorders is unknown. Here, we test the hypothesis that artificial selection for low intrinsic aerobic running capacity is associated with reduced skeletal muscle metabolism and impaired metabolic health. Rat models for low- (LCR) and high- (HCR) intrinsic running capacity were derived from genetically heterogeneous N:NIH stock for 20 generations. Artificial selection produced a 530% difference in running capacity between LCR/HCR, which was associated with significant functional differences in glucose and lipid handling by skeletal muscle, as assessed by hindlimb perfusion. LCR had reduced rates of skeletal muscle glucose uptake (∼30%; P = 0.04), glucose oxidation (∼50%; P = 0.04), and lipid oxidation (∼40%; P = 0.02). Artificial selection for low aerobic capacity was also linked with reduced molecular signaling, decreased muscle glycogen, and triglyceride storage, and a lower mitochondrial content in skeletal muscle, with the most profound changes to these parameters evident in white rather than red muscle. We show that a low intrinsic aerobic running capacity confers reduced insulin sensitivity in skeletal muscle and is associated with impaired markers of metabolic health compared with high intrinsic running capacity. Furthermore, selection for high running capacity, in the absence of exercise training, endows increased skeletal muscle insulin sensitivity and oxidative capacity in specifically white muscle rather than red muscle. These data provide evidence that differences in white muscle may have a role in the divergent aerobic capacity observed in this generation of LCR/HCR.

Exercise training reverses impaired skeletal muscle metabolism induced by artificial selection for low aerobic capacity.

We have used a novel model of genetically imparted endurance exercise capacity and metabolic health to study the genetic and environmental contributions to skeletal muscle glucose and lipid metabolism. We hypothesized that metabolic abnormalities associated with low intrinsic running capacity would be ameliorated by exercise training. Selective breeding for 22 generations resulted in rat models with a fivefold difference in intrinsic aerobic capacity. Low (LCR)- and high (HCR)-capacity runners remained sedentary (SED) or underwent 6 wk of exercise training (EXT). Insulin-stimulated glucose transport, insulin signal transduction, and rates of palmitate oxidation were lower in LCR SED vs. HCR SED (P < 0.05). Decreases in glucose and lipid metabolism were associated with decreased ß2-adrenergic receptor (ß2-AR), and reduced expression of Nur77 target proteins that are critical regulators of muscle glucose and lipid metabolism [uncoupling protein-3 (UCP3), fatty acid transporter (FAT)/CD36; P < 0.01 and P < 0.05, respectively]. EXT reversed the impairments to glucose and lipid metabolism observed in the skeletal muscle of LCR, while increasing the expression of ß2-AR, Nur77, GLUT4, UCP3, and FAT/CD36 (P < 0.05) in this tissue. However, no metabolic improvements were observed following exercise training in HCR. Our results demonstrate that metabolic impairments resulting from genetic factors (low intrinsic aerobic capacity) can be overcome by an environmental intervention (exercise training). Furthermore, we identify Nur77 as a potential mechanism for improved skeletal muscle metabolism in response to EXT.

Neuroprotective effects of moderate aerobic exercise on the spastic Han-Wistar rat, a model of ataxia.

Research has shown that physical exercise may reduce degeneration in certain brain regions experiencing ataxia. Our laboratory utilized mutant spastic Han-Wistar rats (sHW) that display developmental abnormalities, including spastic paresis, fore limb tremors, hind limb rigidity, and a reduced life span (60-65 days of age). Concomitant neurodegeneration has been observed in the cerebellum (Purkinje cells). The purpose of this study was to investigate if moderate, aerobic exercise could reduce Purkinje cell neurodegeneration and improve the motor ability and survival of the mutant sHW rat. Mutant male littermates at the ages of 20 (n=11 pairs) and 30 (n=13 pairs) days old were divided into running groups and non-running groups. Mutant rats were run on a motorized treadmill at the rate of 15 m/min with a 10% slope. The "running" group ran for 30 min per day, 5 days a week; the "non-runners" remained nearby in the training facility. These conditions were held constant until the mutant runners could no longer run due to disease progression. Moderate exercise increased the lifespan of running mutant rats in both the 20-day start group (14% increase) and 30-day start group (13% increase). The rats exhibited improved motor function as open-field tests showed higher activity scores for runners after 50 days. Histological examination of the cerebellum revealed a 62% increase in Purkinje cell survival of the runners. These results suggest that aerobic exercise ameliorates, at least partially, cerebellar dysfunction in the sHW rat, an excellent model of ataxia.

Aerobic training reverses high-fat diet-induced pro-inflammatory signalling in rat skeletal muscle.

High-fat feeding activates components of the pro-inflammatory pathway and increases co-immunoprecipitation of suppressor of cytokine signalling (SOCS)-3 with both the insulin receptor (IR)-ß subunit and IRS-1, which together contribute to keeping PI-3 kinase from being fully activated. However, whether aerobic training reverses these impairments is unknown. Sprague-Dawley rats were fed a chow (CON, n = 8) or saturated high-fat (n = 16) diets for 4 weeks. High-fat-fed rats were then allocated (n = 8/group) to either sedentary (HF) or aerobic exercise training (HFX) for an additional 4 weeks after which all animals underwent hind limb perfusions. Insulin-stimulated red quadriceps 3-O-methylglucose transport rates and PI-3 kinase activity were greater (p < 0.05) in CON and HFX compared to HF. IRS-1 tyrosine phosphorylation was increased (p < 0.05) and IRS-1 serine 307 phosphorylation was decreased (p < 0.05) in HFX compared to HF. IR-ß subunit co-immunoprecipitation with IRS-1 was increased in HFX compared to HF. SOCS-3 co-immunoprecipitation with both the IR-ß subunit and IRS-1 was decreased (p < 0.05) in HFX compared to HF. IKKα/ß serine phosphorylation, and IκBα serine phosphorylation were decreased (p < 0.05) while IκBα protein concentration was increased in HFX compared to HF. By decreasing the association of SOCS-3 with both the IR-ß subunit and IRS-1 the interaction between IRS-1 and the IR-ß subunit was normalized in the HFX, and may have contributed to skeletal muscle PI-3 kinase being fully activated by insulin. Additionally, the reduction in IKKα/ß serine phosphorylation in HFX may have contributed to decreasing IRS-1 serine phosphorylation, and in turn, promoted the normalization of insulin-stimulated activation of PI-3 kinase.

Contraction-induced changes in TNFalpha and Akt-mediated signalling are associated with increased myofibrillar protein in rat skeletal muscle.

Resistance training results in skeletal muscle hypertrophy, but the molecular signalling mechanisms responsible for this altered phenotype are incompletely understood. We used a resistance training (RT) protocol consisting of three sessions [day 1 (d1), day 3 (d3), day 5 (d5)] separated by 48 h recovery (squat exercise, 4 sets x 10 repetitions, 3 min recovery) to determine early signalling responses to RT in rodent skeletal muscle. Six animals per group were killed 3 h after each resistance training session and 24 and 48 h after the last training session (d5). There was a robust increase in TNFalpha protein expression, and IKK(Ser180/181) and p38MAPK(Thr180/Tyr182) phosphorylation on d1 (P < 0.05), which abated with subsequent RT, returning to control levels by d5 for TNFalpha and IKK(Ser180/181). There was a trend for a decrease in MuRF-1 protein expression, 48 h following d5 of training (P = 0.08). Notably, muscle myofibrillar protein concentration was elevated compared to control 24 and 48 h following RT (P < 0.05). Akt(Ser473) and mTOR(Ser2448) phosphorylation were unchanged throughout RT. Phosphorylation of p70S6k(Thr389) increased 3 h post-exercise on d1, d3 and d5 (P < 0.05), whilst phosphorylation of S6(Ser235/236) increased on d1 and d3 (P < 0.05). Our results show a rapid attenuation of inflammatory signalling with repeated bouts of resistance exercise, concomitant with summation in translation initiation signalling in skeletal muscle. Indeed, the cumulative effect of these signalling events was associated with myofibrillar protein accretion, which likely contributes to the early adaptations in response to resistance training overload in the skeletal muscle.

Lipid-induced mTOR activation in rat skeletal muscle reversed by exercise and 5'-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside.

The serine/threonine protein kinase, mammalian target of rapamycin (mTOR) is regulated by insulin and nutrient availability and has been proposed to play a central role as a nutrient sensor in skeletal muscle. mTOR associates with its binding partners, raptor and rictor, to form two structurally and functionally distinct complexes, mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2) respectively. We have investigated the assembly of mTORC1/2 and the activation of their downstream substrates (i.e. Akt, S6K1) in response to known effectors of mTOR, excess lipid availability and AMP-activated protein kinase (AMPK) activation/exercise training in rat skeletal muscle. The in vivo formation of mTORC1 and 2 and the activation of their respective downstream substrates were increased in response to chronic (8 weeks) consumption of a high-fat diet. Diet-induced mTORC activation and skeletal muscle insulin resistance were reversed by 4 weeks of exercise training, which was associated with enhanced muscle AMPK activation. In order to determine whether AMPK activation reverses lipid-induced mTOR activation, L6 myotubes were exposed to 0.4 mM palmitate to activate mTORC1/2 in the absence or presence of 5'-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR). Palmitate exposure (4 h) increased insulin-stimulated S6K1 Thr389 phosphorylation by 60%, indicating activation of mTORC1. AMPK activation with 1 mM AICAR abolished lipid-induced mTOR activation in vitro. Our data implicates reductions in mTOR complex activation with the reversal of lipid-induced skeletal muscle insulin resistance in response to exercise training or AICAR and identifies mTOR as a potential target for the treatment of insulin resistance.

High-fat feeding increases insulin receptor and IRS-1 coimmunoprecipitation with SOCS-3, IKKalpha/beta phosphorylation and decreases PI-3 kinase activity in muscle.

Suppressor of cytokine signaling (SOCS) proteins and/or activation of the proinflammatory pathway have been postulated as possible mechanisms that may contribute to skeletal muscle insulin resistance. Thus, the aims of the present investigation were to determine in high-fat-fed skeletal muscle: 1) whether SOCS-3 protein concentration is increased, 2) whether coimmunoprecipitation of SOCS-3 with the insulin receptor-beta subunit and/or IRS-1 is increased, and 3) whether select components of the proinflammatory pathway are altered. Thirty-two male Sprague-Dawley rats were assigned to either control (CON, n = 16) or high-fat-fed (HF, n = 16) dietary groups for 12 wk and then subjected to hind limb perfusions in the presence (n = 8/group) or absence (n = 8/group) of insulin. Insulin-stimulated skeletal muscle 3-MG transport rates and PI-3 kinase activity were greater (P < 0.05) in CON. IRS-1 tyrosine phosphorylation was decreased (P < 0.05), and IRS-1 serine 307 phosphorylation was increased (P < 0.05) in HF. Insulin receptor-beta (IR-beta) subunit coimmunoprecipitation with IRS-1 was reduced in HF. SOCS-3 protein concentration and SOCS-3 coimmunoprecipitation with both the IR-beta subunit and IRS-1 was increased (P < 0.05) in HF. IKKalpha/beta serine phosphorylation was increased (P < 0.05), IkappaBalpha protein concentration was decreased (P < 0.05) and IkappaBalpha serine phosphorylation was increased (P < 0.05) in HF. Increased colocalization of SOCS-3 with both the IR-beta subunit and IRS-1 may provide steric hindrance that prevents IRS-1 from interacting with IR-beta, while increased IKKbeta serine phosphorylation may contribute to increasing IRS-1 serine phosphorylation, both of which independently can have deleterious effects on insulin-stimulated PI-3 kinase activation in high-fat-fed rodent skeletal muscle.

Glycogen storage and muscle glucose transporters (GLUT-4) of mice selectively bred for high voluntary wheel running.

To examine the evolution of endurance-exercise behaviour, we have selectively bred four replicate lines of laboratory mice (Mus domesticus) for high voluntary wheel running (;high runner' or HR lines), while also maintaining four non-selected control (C) lines. By generation 16, HR mice ran approximately 2.7-fold more than C mice, mainly by running faster (especially in females), a differential maintained through subsequent generations, suggesting an evolutionary limit of unknown origin. We hypothesized that HR mice would have higher glycogen levels before nightly running, show greater depletion of those depots during their more intense wheel running, and have increased glycogen synthase activity and GLUT-4 protein in skeletal muscle. We sampled females from generation 35 at three times (photophase 07:00 h-19:00 h) during days 5-6 of wheel access, as in the routine selection protocol: Group 1, day 5, 16:00 h-17:30 h, wheels blocked from 13:00 h; Group 2, day 6, 02:00 h-03:30 h (immediately after peak running); and Group 3, day 6, 07:00 h-08:30 h. An additional Group 4, sampled 16:00 h-17:30 h, never had wheels. HR individuals with the mini-muscle phenotype (50% reduced hindlimb muscle mass) were distinguished for statistical analyses comparing C, HR normal, and HR mini. HR mini ran more than HR normal, and at higher speeds, which might explain why they have been favored by the selective-breeding protocol. Plasma glucose was higher in Group 1 than in Group 4, indicating a training effect (phenotypic plasticity). Without wheels, no differences in gastrocnemius GLUT-4 were observed. After 5 days with wheels, all mice showed elevated GLUT-4, but HR normal and mini were 2.5-fold higher than C. At all times and irrespective of wheel access, HR mini showed approximately three-fold higher [glycogen] in gastrocnemius and altered glycogen synthase activity. HR mini also showed elevated glycogen in soleus when sampled during peak running. All mice showed some glycogen depletion during nightly wheel running, in muscles and/or liver, but the magnitude of this depletion was not large and hence does not seem to be limiting to the evolution of even-higher wheel running.

Activation of atypical protein kinase Czeta toward TC10 is regulated by high-fat diet and aerobic exercise in skeletal muscle.

We determined whether sustained aerobic exercise reverses high-fat diet-induced impairments in the c-Cbl associated protein (CAP)/Casitas b-lineage lymphoma (c-Cbl) signaling cascade in rodent skeletal muscle. Sprague-Dawley rats were placed into either control (n = 16) or high-fat-fed (n = 32) diet groups for 4 weeks. During a subsequent 4-week experimental period, 16 high-fat-fed rats remained sedentary, 16 high-fat-fed rats completed 4 weeks of exercise training, and control animals were sedentary and remained on the control diet. After the intervention period, animals were subjected to hind limb perfusions in the presence (n = 8 per group) or absence (n = 8 per group) of insulin. In the plasma membrane fractions, neither high-fat feeding nor exercise training altered adaptor protein with PH and SH2 domains, (APS), c-Cbl, or TC10 protein concentrations. In contrast, CAP protein concentration and insulin-stimulated plasma membrane c-Cbl tyrosine phosphorylation were reduced by high-fat feeding; but exercise training reversed these impairments. Of note was that insulin-stimulated atypical protein kinase Czeta kinase activity toward TC10 was reduced by high-fat feeding but normalized by exercise training. We conclude that sustained (4 weeks) exercise training can reverse high-fat diet-induced impairments on the CAP/c-Cbl pathway in high-fat-fed rodent skeletal muscle. We also provide the first evidence that the CAP/c-Cbl insulin signaling cascade in skeletal muscle may directly interact with components of the classic (phosphoinositide 3-kinase dependent) insulin signaling cascade.

Exercise training increases components of the c-Cbl-associated protein/c-Cbl signaling cascade in muscle of obese Zucker rats.

The purpose of this investigation was to determine whether alterations in the c-Cbl-associated protein/c-Cbl pathway and/or p38-mitogen-activated protein kinase (p38 MAP kinase) were associated with improved skeletal muscle insulin responsiveness in exercise-trained obese Zucker rats. Obese Zucker rats ran 5 d/wk on a motorized treadmill for 90 minutes over a 7-week period. Age-matched obese Zucker rats (OB-SED) and their lean littermates (LN-SED) were obtained to serve as nontrained controls. Twenty-four (OB-EX-24 h) or 48 hours (OB-EX-48 h) after the last exercise bout, the trained rats were studied via the hind limb perfusion technique in the presence of insulin. Insulin-stimulated glucose uptake was significantly decreased across the skeletal muscle of OB-SED rats compared with LN-SED, but was normalized in the obese rats by 7 weeks of training. The insulin-stimulated plasma membrane protein concentrations of TC10 and glucose transporter 4 were reduced in the sedentary Zuckers, but both proteins were increased by the training protocol. Training did not increase insulin-stimulated p38 MAP kinase protein concentration, nor did it have an effect on insulin-stimulated p38 MAP kinase phosphorylation at the plasma membrane. These results suggest that skeletal muscle insulin resistance is associated with reduced expression of TC10 and that this deficiency can be corrected with exercise training.

Effect of high-frequency resistance exercise on adaptive responses in skeletal muscle.

PURPOSE: Regulation of skeletal muscle mass is highly dependent on contractile loading. The purpose of this study was to examine changes in growth factor and inflammatory pathways following high-frequency resistance training. METHODS: Using a novel design in which male Sprague-Dawley rats undertook a "stacked" resistance training protocol designed to generate a summation of transient exercise-induced signaling responses (four bouts of three sets x 10 repetitions of squat exercise, separated by 3 h of recovery), we determined the effects of high training frequency on signaling pathways and transcriptional activity regulating muscle mass. RESULTS: The stacked training regimen resulted in acute suppression of insulin-like growth factor 1 mRNA abundance (P < 0.05) and Akt phosphorylation (P < 0.05), an effect that persisted 48 h after the final training bout. Conversely, stacked training elicited a coordinated increase in the expression of tumor necrosis factor alpha, inhibitor kappa B kinase alpha/beta activity (P < 0.05), and p38 mitogen-activated protein kinase phosphorylation (P < 0.05) at 3 h after each training bout. In addition, the stacked series of resistance exercise bouts induced an increase in p70 S6 kinase phosphorylation 3 h after bouts x3 and x4, independent of the phosphorylation state of Akt. CONCLUSIONS: Our results indicate that high resistance training frequency extends the transient activation of inflammatory signaling cascades, concomitant with persistent suppression of key mediators of anabolic responses. We provide novel insights into the effects of the timing of exercise-induced overload and recovery on signal transduction pathways and transcriptional activity regulating skeletal muscle mass in vivo.

Exercise reverses high-fat diet-induced impairments on compartmentalization and activation of components of the insulin-signaling cascade in skeletal muscle.

The aims of this investigation were 1) to determine whether endurance exercise training could reverse impairments in insulin-stimulated compartmentalization and/or activation of aPKCzeta/lambda and Akt2 in skeletal muscle from high-fat-fed rodents and 2) to assess whether the PPARgamma agonist rosiglitazone could reverse impairments in skeletal muscle insulin signaling typically observed after high-fat feeding. Sprague-Dawley rats were placed on chow (NORCON, n = 16) or high-fat (n = 64) diets for 4 wk. During a subsequent 4-wk experimental period, high-fat-fed rats were allocated (n = 16/group) to either sedentary control (HFC), exercise training (HFX), rosiglitazone treatment (HFRSG), or a combination of both exercise training and rosiglitazone (HFRX). Following the 4-wk experimental period, animals underwent hindlimb perfusions. Insulin-stimulated plasma membrane-associated aPKCzeta and -lambda protein concentration, aPKCzeta/lambda activity, GLUT4 protein concentration, cytosolic Akt2, and aPKCzeta/lambda activities were reduced (P < 0.05) in HFC compared with NORCON. Cytosolic Akt2, aPKCzeta, and aPKClambda protein concentrations were not affected in HFC compared with NORCON. Exercise training reversed the deleterious effects of the high-fat diet such that insulin-stimulated compartmentalization and activation of components of the insulin-signaling cascade in HFX were normalized to NORCON. High-fat diet-induced impairments to skeletal muscle glucose metabolism were not reversed by rosiglitazone administration, nor did rosiglitazone augment the effect of exercise. Our findings indicate that chronic exercise training, but not rosiglitazone, reverses high-fat diet induced impairments in compartmentalization and activation of components of the insulin-signaling cascade in skeletal muscle.

Tissue-specific effects of rosiglitazone and exercise in the treatment of lipid-induced insulin resistance.

Both pharmacological intervention (i.e., thiazolidinediones [TZDs]) and lifestyle modification (i.e., exercise training) are clinically effective treatments for improving whole-body insulin sensitivity. However, the mechanism(s) by which these therapies reverse lipid-induced insulin resistance in skeletal muscle is unclear. We determined the effects of 4 weeks of rosiglitazone treatment and exercise training and their combined actions (rosiglitazone treatment and exercise training) on lipid and glucose metabolism in high-fat-fed rats. High-fat feeding resulted in decreased muscle insulin sensitivity, which was associated with increased rates of palmitate uptake and the accumulation of the fatty acid metabolites ceramide and diacylglycerol. Impairments in lipid metabolism were accompanied by defects in the Akt/AS160 signaling pathway. Exercise training, but not rosiglitazone treatment, reversed these impairments, resulting in improved insulin-stimulated glucose transport and increased rates of fatty acid oxidation in skeletal muscle. The improvements to glucose and lipid metabolism observed with exercise training were associated with increased AMP-activated protein kinase alpha1 activity; increased expression of Akt1, peroxisome proliferator-activated receptor gamma coactivator 1, and GLUT4; and a decrease in AS160 expression. In contrast, rosiglitazone treatment exacerbated lipid accumulation and decreased insulin-stimulated glucose transport in skeletal muscle. However, rosiglitazone, but not exercise training, increased adipose tissue GLUT4 and acetyl CoA carboxylase expression. Both exercise training and rosiglitazone decreased liver triacylglycerol content. Although both interventions can improve whole-body insulin sensitivity, our results show that they produce divergent effects on protein expression and triglyceride storage in different tissues. Accordingly, exercise training and rosiglitazone may act as complementary therapies for the treatment of insulin resistance.

Resistance training improves insulin signaling and action in skeletal muscle.

Resistance training can improve glucose transport in both normal and insulin-resistant skeletal muscle by enhancing the activation of the insulin signaling cascade and increasing GLUT-4 protein concentration. These training-induced alterations improve the quality of the skeletal muscle and can occur independent of significant increases in skeletal muscle mass.

High-fat feeding effects on components of the CAP/Cbl signaling cascade in Sprague-Dawley rat skeletal muscle.

The aim of this investigation was to determine whether the CAP (Cbl-associated protein)/Cbl signaling cascade is present and responsive to insulin in skeletal muscle and if high-fat feeding impairs insulin-stimulated activation of this signaling cascade. Sprague-Dawley rats were assigned to either control (n = 16) or high fat-fed (n = 16) dietary groups. After a 12-week dietary period, animals were subjected to hind limb perfusions in the presence (n = 8 per group) or absence (n = 8 per group) of insulin. High-fat feeding reduced rates of insulin-stimulated skeletal muscle phosphatidylinositol 3-kinase activity and 3-O-methylglucose transport. In plasma membrane fractions, neither the high-fat diet nor insulin altered the insulin receptor beta subunit (IR-beta), APS (adaptor protein containing PH and SH2 domains), c-Cbl, or TC10 protein concentration, but high-fat feeding did decrease CAP protein concentration. APS, c-Cbl, CAP, and TC10 messenger RNA were present in the skeletal muscle and reflected the protein concentration of experimental groups. Despite insulin-stimulated plasma membrane IR-beta tyrosine phosphorylation being unaffected by high-fat feeding, c-Cbl tyrosine phosphorylation, the kinase activity of IR-beta toward APS, and glucose transporter 4 protein concentration were all significantly reduced in insulin-stimulated plasma membrane prepared from the skeletal muscle of high fat-fed animals. These findings suggest that the CAP/Cbl signaling cascade is present in skeletal muscle, activated by insulin, and impaired by high-fat feeding.

Short-term leptin treatment increases fatty acids uptake and oxidation in muscle of high fat-fed rats.

The purpose of this study was to measure the effects of short-term (10 days) leptin treatment on insulin sensitivity as it pertains to fatty acid (FA) uptake, oxidation, and muscle triglyceride (mTG) synthesis in animals that have been administered a high-fat (HF) diet for 3 months. Male Wistar rats were randomly assigned to 1 of 4 groups. One group was fed a control diet (CON) and 3 groups were fed a HF diet. The HF and HF-leptin (HF-LEP) groups were fed the HF diet ad libitum and the amount of food eaten by the HF-pair fed (HF-P) group was equal to that of the HF-LEP group. At the end of the dietary period, rats were injected daily either with saline (CON, HF, HF-P) or with leptin (HF-LEP; 10 mg.kg-1.d-1) for 10 days before hindlimb perfusion. The perfusate contained 600 micromol/L palmitate traced with [14C]palmitate, 9 mmol/L glucose, and 100 microU/mL insulin. As dictated by the protocol, energy expenditure was not significantly different (P>.05) between HF-LEP and HF-P. Palmitate uptake and oxidation as well as mTG synthesis were greater (P<.05) in HF (9.8+/-0.3, 2.0+/-0.1, and 1.9+/-0.2 nmol.min-1.g-1) than in CON (8.0+/-0.4, 1.4+/-0.1, and 1.1+/-0.1 nmol.min-1.g-1) and this was associated with higher levels of mTG in HF. Palmitate uptake and oxidation were higher (P<.05) in HF-LEP (10.3+/-0.6 and 2.0+/-0.1 nmol.min-1.g-1) than in HF-P (8.3+/-0.5 and 1.5+/-0.2 nmol.min-1.g-1, P<.05), but mTG synthesis and mTG levels were not changed significantly by leptin treatment (P>.05). High-fat feeding decreased glucose uptake by 41% when compared with CON (2.4+/-0.4 vs 4.1+/-0.4 micromol.h-1.g-1; P<.05) but pair feeding alone (4.7+/-0.4 micromol.h-1.g-1) or leptin treatment (3.8+/-0.3 micromol.h-1.g-1) similarly prevented the HF diet-induced decrease in glucose uptake. These data indicate that short-term leptin treatment in HF-fed rats alters muscle FA metabolism by increasing FA uptake and oxidation relative to pair feeding alone. This results in a decrease in the FA esterification-oxidation ratio.

Insulin-stimulated plasma membrane association and activation of Akt2, aPKC zeta and aPKC lambda in high fat fed rodent skeletal muscle.

Several recent reports using cell lines have suggested that both Akt and atypical protein kinase C (aPKC) zeta/lambda are translocated to the plasma membrane (PM) in response to insulin. However, it has yet to be determined in skeletal muscle whether: (1) insulin increases PM-associated Akt2, aPKC zeta and/or lambda protein concentration, (2) the activity of these kinases is altered by insulin at the PM, and (3) high fat feeding alters the insulin-stimulated PM concentration and/or activity of Akt2 and aPKC zeta/lambda. Sprague-Dawley rats were randomly assigned to either normal (n=16) or high fat (n=16) dietary groups. Following a 12 week dietary period, animals were subjected to hind limb perfusions in the presence (n=8 per group) or absence (n=8 per group) of insulin. In normal skeletal muscle, total PI3-kinase, Akt2 and aPKC zeta/lambda activities were increased by insulin. PM-associated aPKC zeta and lambda, and aPKC zeta/lambda activity, but not Akt2 or Akt2 activity, were increased by insulin in normal muscle. High fat feeding did not alter total skeletal muscle Akt2, aPKC zeta or aPKC lambda protein concentration. Insulin-stimulated total PI3-kinase, Akt2 and aPKC zeta/lambda activities were reduced in the high fat fed animals. Insulin-stimulated PM aPKC zeta, aPKC lambda, aPKC zeta/lambda activity and GLUT4 protein concentration were also reduced in high fat fed animals. These findings suggest that in skeletal muscle, insulin stimulates translocation of aPKC zeta and lambda, but not Akt2, to the PM. In addition, high fat feeding impairs insulin-stimulated activation of total aPKC zeta/lambda and Akt2, as well as PM association and activation of aPKC zeta and lambda.

Chronic leptin treatment enhances insulin-stimulated glucose disposal in skeletal muscle of high-fat fed rodents.

The aim of this investigation was to evaluate if chronic leptin administration corrects high fat diet-induced skeletal muscle insulin resistance, in part, by enhancing rates of glucose disposal and if the improvements are accounted for by alterations in components of the insulin-signaling cascade. Sprague-Dawley rats consumed normal (CON) or high fat diets for three months. After the dietary lead in, the high fat diet group was further subdivided into high fat (HF) and high fat, leptin treated (HF-LEP) animals. HF-LEP animals were injected twice daily with leptin (5 mg/100 g body weight) for 10 days, while the CON and HF animals were injected with vehicle. Following the treatment periods, all animals were prepared for and subjected to hind limb perfusion. The high fat diet decreased rates of insulin-stimulated skeletal muscle glucose uptake and glycogen synthesis in the red gastrocnemius (RG), but did not affect glycogen synthase activity, rates of glucose oxidation or nonoxidative disposal of glucose. Of interest, IRS-1-associated PI3-K activity and total GLUT4 protein concentration were reduced in the RG of the high fat-fed animals. Leptin treatment increased rates of insulin-stimulated glucose uptake and glucose oxidation, and normalized rates of glycogen synthesis. Leptin appeared to mediate these effects by normalizing insulin-stimulated PI3-K activation and GLUT4 protein concentration in the RG. Collectively, these data suggest that chronic leptin treatment reverses the effects of a high fat diet thereby allowing the insulin signaling cascade and glucose transport effector system to be fully activated which in turn affects the amount of glucose that is transported across the plasma membrane and made available for glycogen synthesis.