AJP - Endo Add DOIs to your references at manuscript stage!
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Am J Physiol Endocrinol Metab 292: E485-E493, 2007. First published September 26, 2006; doi:10.1152/ajpendo.00080.2006
0193-1849/07 $8.00
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
292/2/E485    most recent
00080.2006v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (8)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Todd, M. K.
Right arrow Articles by Turcotte, L. P.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Todd, M. K.
Right arrow Articles by Turcotte, L. P.

Thiazolidinediones enhance skeletal muscle triacylglycerol synthesis while protecting against fatty acid-induced inflammation and insulin resistance

Mark K. Todd,1 Matthew J. Watt,3 Jamie Le,4 Andrea L. Hevener,4,* and Lorraine P. Turcotte1,2,*

1Department of Kinesiology and 2Department of Biological Sciences, University of Southern California, Los Angeles, California; 3St. Vincent's Institute of Medical Research and the Department of Medicine, The University of Melbourne, Fitzroy, Australia; and 4Department of Medicine, University of California, San Diego, La Jolla, California

Submitted 16 February 2006 ; accepted in final form 6 September 2006


   ABSTRACT
TOP
 ABSTRACT
EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
In the present investigation, we studied the effects of thiazolidinedione (TZD) treatment on insulin-stimulated fatty acid (FA) and glucose kinetics in perfused muscle from high-fat (HF)-fed rats. We tested the hypothesis that TZDs prevent FA-induced insulin resistance by attenuating proinflammatory signaling independently of myocellular lipid levels. Male Wistar rats were assigned to one of three 3-wk dietary groups: control chow fed (CON), 65% HF diet (HFD), or TZD- (troglitazone or rosiglitazone) enriched HF diet (TZD + HFD). TZD treatment led to a significant increase in plasma membrane content of CD36 protein in muscle (red: P = 0.01, and white: P = 0.001) that correlated with increased FA uptake (45%, P = 0.002) and triacylglycerol (TG) synthesis (46%, P = 0.03) during the perfusion. Importantly, whereas HF feeding caused increased basal TG (P = 0.047), diacylglycerol (P = 0.002), and ceramide (P = 0.01) levels, TZD treatment only prevented the increase in muscle ceramide. In contrast, all of the muscle inflammatory markers altered by HF feeding ({uparrow}NIK protein content, P = 0.009; {uparrow}IKKbeta activity, P = 0.006; {downarrow}I{kappa}B-{alpha} protein, P = 0.03; and {uparrow}JNK phosphorylation, P = 0.003) were completely normalized by TZD treatment. Consistent with this, HFD-induced decrements in insulin action were also prevented by TZD treatment. Thus our findings support the notion that TZD treatment causes increased FA uptake and TG accumulation in skeletal muscle under insulin-stimulated conditions. Despite this, TZDs suppress the inflammatory response to dietary lipid overload, and it is this mechanism that correlates strongly with insulin sensitivity.

insulin action; lipid oversupply; nuclear factor-{kappa}B inflammatory pathway; lipotoxicity

INSULIN RESISTANCE, A HALLMARK FEATURE of type 2 diabetes and the metabolic syndrome, is characterized by impairments in long-chain fatty acid (FA) and glucose metabolism (6, 7, 19, 27, 36). Although elevated plasma FA and tissue lipids are often found clinically in humans with insulin resistance or type 2 diabetes (1, 19, 21, 36, 37) and are believed to be involved in the pathogenesis of skeletal muscle insulin resistance (1, 4, 19, 21, 27), the mechanisms by which chronically elevated lipids impact upon skeletal muscle metabolism remain poorly defined.

Recently, a causal link between lipid oversupply, inflammation, and impairments in insulin action has been proposed (3, 9, 16, 39). A role for proinflammatory serine kinases in the diminution of proximal insulin signal transduction is well supported (2, 3, 12, 15, 17, 24). The insulin receptor substrate (IRS)-1 has been targeted as a critical site for inflammatory impingement on insulin signaling since IRS-1 serine phosphorylation reduces activation of phosphatidylinositol (PI) 3-kinase and subsequent downstream signaling (2, 10, 22, 24). A host of serine kinases, including c-Jun NH2-terminal kinase (JNK), I{kappa}B kinase (IKK)beta, and PKC{theta}, have been identified as mediators of these deleterious events (2, 10, 12, 22, 24). Although the details and circumstances relating to specific kinase activation remain poorly described, accumulation of proinflammatory lipid intermediates, including ceramide, diacylglycerol (DAG), and long-chain fatty acyl CoAs, are believed to stimulate serine kinase activation, and it is thought that this mechanism underlies impaired insulin action (1, 8, 13, 17, 43).

In addition to impinging directly on insulin signal transduction, the activation of proinflammatory/stress signaling pathways, including nuclear factor (NF)-{kappa}B, alters insulin action by modulating gene transcription. Upon stimulation, IKKbeta is phosphorylated, which leads to subsequent phosphorylation of the inhibitory subunit, I{kappa}B{alpha}, of the NF-{kappa}B complex (18). I{kappa}B{alpha} is then targeted for proteasomal degradation thus allowing for NF-{kappa}B translocation to the nucleus and transcriptional activation of proinflammatory target genes (e.g., TNF-{alpha}, IL-6, IL-1beta) (18, 30, 54). Given its pivotal role in activating the NF-{kappa}B inflammatory cascade, IKKbeta has been targeted clinically for pharmacological intervention as well as experimentally by genetic manipulation in mice. It is well known that salicylates ameliorate diabetic complications (16), and these clinical findings are supported experimentally in that similar treatment of high-fat (HF)-fed or genetically obese rodents causes reductions in IKKbeta activity and improved insulin action (52, 53). Moreover, mice heterozygous for the null IKKbeta mutation as well as IKKbeta tissue-specific knockouts are also protected from insulin resistance induced by HF feeding or leptin deficiency (3, 53). Confirmation of a central role for this inflammatory pathway in regulating skeletal muscle insulin action was achieved in vitro in L6 myotubes whereby pharmacological inhibition of NF-{kappa}B nuclear translocation was shown to prevent FA-induced insulin resistance (41). Together, this work indicates that FA oversupply is associated with activation of the NF-{kappa}B inflammatory pathway and that reduced IKKbeta activity or inhibition of NF-{kappa}B nuclear translocation prevents FA-induced decrements in insulin action (3, 41, 53).

To date, there is only modest in vivo data addressing the effect of clinically utilized insulin-sensitizing agents, such as TZDs, on skeletal muscle lipid metabolism and inflammation. Although it is believed by some (35, 40) that TZDs cause insulin sensitization in skeletal muscle via reductions in myocellular lipid levels, additional studies (14, 23, 26, 29) have shown that TZDs can prevent lipid-induced insulin resistance independently of a reduced lipid content. Given this, in the present investigation we sought to address the impact of TZD treatment on skeletal muscle lipid accumulation, inflammation, and insulin action following HF feeding. Here, we report that TZD treatment caused elevated rates of FA uptake and triacylglycerol (TG) synthesis in skeletal muscle from HF-fed rats perfused under insulin-stimulated conditions. Despite this, TZD treatment prevented the activation of IKKbeta and JNK as well as the development of insulin resistance.


   EXPERIMENTAL PROCEDURES
TOP
 ABSTRACT
 EXPERIMENTAL PROCEDURES
RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Animals

Male Wistar rats were randomly assigned to one of three 3-wk treatment groups distinguished by dietary composition. Animals assigned to the HF diet (HFD; n = 16) consumed 65% fat, 22% protein, and 13% carbohydrate (Dyets, Bethlehem, PA), whereas the TZD- (troglitazone or rosiglitazone) treated animals were fed the same HFD with the addition of troglitazone (TZD + HFD, 1.6%, n = 8) or rosiglitazone (TZD + HFD, 3 mg·kg–1·day–1, n = 8) as a food admixture. The control (CON) animals (n = 16) were fed normal chow containing 4% fat, 24% protein, and 72% carbohydrate (Harlan-Teklad, Madison, WI). All animals were given ad libitum access to food and water and were housed on a 12:12-h light-dark cycle. Animals were fasted for 8 h prior to basal blood or tissue sampling and prior to the start of the perfusion. All procedures were performed in accordance with the Guide for Care and Use of Laboratory Animals of the National Institutes of Health (NIH) and were approved by the University of Southern California and the University of California, San Diego, Animal Subjects Committees.

Perfusion

Following the 8-h fast, blood from a tail vein was collected for baseline blood analysis. Hindquarters were surgically isolated. Quickly, the left superficial portion of the vastus lateralis muscle [white quadriceps (WQ)], the left superficial medial portion of the gastrocnemius muscle [white gastrocnemius (WG)] consisting mainly of fast-twitch glycolytic fibers, the deep portion of the vastus lateralis [red quadriceps (RQ)], and the deep medial portion of the gastrocnemius muscle [red gastrocnemius (RG)] consisting mainly of fast-twitch oxidative fibers were excised. Muscles were immediately visualized for fiber type distribution, sectioned, trimmed of connective tissue, blotted, and freeze-clamped with aluminum clamps precooled in liquid N2 (45, 46, 48). Tissues were stored in liquid N2 until subsequent analysis. The left iliac vessels were then tied off, and a clamp was fixed tightly around the proximal part of the leg to prevent bleeding.

Meanwhile, the right leg was perfused (7 ml/min) for an initial 20-min equilibration period followed by a 40-min experimental period with washed bovine erythrocytes containing [1-14C]palmitate (8 µCi), 600 µM palmitate, 6 mM glucose, and 100 µU/ml insulin as previously described by our laboratory (46–48). Arterial and venous perfusate samples for the analysis of [14C]FA and 14CO2, as well as for FA and glucose concentrations, were taken at 10, 20, 30, and 40 min during the experimental period. Arterial and venous perfusate samples for determinations of PCO2, PO2, and pH were taken at 10 and 30 min. At the end of the 40-min experimental perfusion period, quadriceps (Q) and gastrocnemius (G) muscles of the right leg were immediately freeze-clamped in situ to preserve the insulin-induced activation state of the enzymes (45, 46, 48).

Blood and Perfusate Analyses

Plasma FA levels were measured enzymatically with a commercial kit (NEFA C; Wako Chemicals, Richmond, VA), glucose concentration was determined using the glucose oxidase method (YSI, Yellow Springs, OH), and Acrp30 and insulin were measured by radioimmunoassay kit (Linco, St. Louis, MO). Determination of PCO2, PO2, and pH of the perfusate samples was measured with an ABL5 analyzer (Radiometer America, Westlake, OH), and the collection of 14CO2 liberated from the perfusate was performed as previously described (46–48). Perfusate concentration of [14C]palmitate was measured by liquid scintillation with a Hewlett-Packard scintillation counter (46–48).

Skeletal Muscle Analyses

Muscles were harvested and assessed under basal or insulin stimulated conditions. Analyses performed on basal muscle, including membrane fraction CD36 and PKC{theta} content (RG, 400 mg; WG, 400 mg), were conducted on tissue harvested from the three groups of animals (CON, n = 8; HFD, n = 8; TZD + HFD, n = 8 troglitazone treated) prior to the perfusion. Insulin-stimulated analyses, including TG synthesis (mixed G, 400 mg), malonyl-CoA levels (mixed G, 200 mg), enzyme activities (mixed Q, 600 mg), and citrate levels (mixed Q, 100 mg), as well as insulin-signaling protein phosphorylation (mixed G, 250 mg), were conducted on tissues harvested from the three groups (CON, n = 8; HFD, n = 8; TZD + HFD, n = 8 troglitazone treated) following the perfusion. TG levels were assessed in basal muscle tissue from CON, n = 8; HFD, n = 8; and TZD + HFD, n = 4 troglitazone treated and n = 6 rosiglitazone treated, whereas lipid intermediates were assessed in basal muscle tissue from CON, n = 8; HFD, n = 8; TZD + HFD, n = 8 rosiglitazone treated. IKKbeta activity was assessed in insulin-stimulated mixed Q muscle (100 mg) harvested after the perfusion and correlated with the rate of insulin-stimulated glucose uptake during the perfusion. Assessment of inflammatory marker protein and phosphorylation content was performed in mixed muscle tissue (Q, 100 mg) harvested from each group (CON, n = 8; HFD, n = 8; TZD + HFD, n = 8 troglitazone treated) in the basal state. For simplicity, data from both TZD-treated groups (TZD + HFD, n = 8 troglitazone treated and n = 8 rosiglitazone treated) were collapsed for JNK phosphorylation since no differences were observed between the drug treatment groups.

Enzymatic Activity Measurements

Spectrophotometric analysis of beta-hydroxyacyl-CoA dehydrogenase (beta-HAD) activity was conducted to determine the potential for beta-oxidation of FA (20). Briefly, mixed Q muscle was homogenized in ice-cold 100 mM potassium phosphate buffer (pH 7.2) containing 5 mM EDTA. The homogenate was centrifuged for 15 min, the supernatant was added to the reaction mixture (100 mM triethanolamine-HCl, 5 mM EDTA, 0.28 mM NADH, 0.25 mM acetoacetyl-CoA, pH 7.0), and absorption was followed at 340 nm for 5 min. Citrate synthase (CS) (42) activity was assayed for indication of TCA cycle flux using the mixed muscle supernatant described above. As previously described, acetyl-CoA carboxylase (ACC) activity was measured as the incorporation of [14C] into malonyl-CoA following ammonium sulfate precipitation of mixed Q muscle homogenates (34, 50). AMP-activated protein kinase (AMPK) activity was measured as the incorporation of [32P] from [32P]ATP into SAMS peptide following ammonium sulfate precipitation of mixed Q muscle homogenates (34, 50).

The IKK activity assay was performed following IKK complex immunoprecipitation of mixed Q muscle homogenate using an anti-IKKbeta antibody (Santa Cruz Biotechnology, Santa Cruz, CA) with protein A + G (1:1) sepharose beads (Upstate Biotechnology, Lake Placid, NY) as previously described (28). The beads were washed three times in 1) lysis buffer, 2) lysis buffer plus 2 M urea, and 3) kinase buffer (20 mM HEPES, pH 7.5, 10 MgCl2). The beads were harvested, and the kinase assay was performed for an incubation period of 30 min at 30°C using 20 mM HEPES (pH 7.5), 10 mM MgCl2, 20 mM beta-glycerophosphate, 10 mM p-nitrophenyl phosphate, 1 mM dithiothreitol, 50 µM sodium vanadate, 20 µM cold ATP, 0.5 µM [32P]ATP, and 2 µg glutathione-S-transferase-I{kappa}B{alpha} (1–317, Santa Cruz-4094). The reaction was terminated by adding SDS-PAGE loading buffer and by heating the samples to a boiling temperature. The samples were then run on a 10% polyacrylamide gel, transferred to nitrocellulose membranes, and exposed to X-ray film. Band intensities were quantified by densitometry on a Hewlett-Packard ScanJet II using NIH Image 1.6 software. To guarantee equal protein loading per lane, the nitrocellulose membranes were blotted with an anti-IKKbeta antibody (Santa Cruz Biotechnology), and total protein was expressed in arbitrary densitometry units on the basis of internal comparison to lysates prepared from control rats.

Muscle Metabolite Measurements

Malonyl-CoA levels were determined using neutralized perchloric acid extracts prepared from freeze-clamped mixed G muscle sampled after the perfusion as previously described (34). To measure the incorporation of [14C]palmitate into muscle TG, lipids from postperfusion mixed G were extracted and separated and lipids from the extracted organic layer were separated by liquid chromatography as previously described (45).

Total citrate concentration was determined spectrophotometrically at 340 nm (33). Briefly, postperfusion Q muscle was homogenized 1:2 in 0.6 N perchloric acid and centrifuged for 5 min. The supernatant was neutralized with 2 N potassium hydroxide and incubated on ice for 15 min. The sample was centrifuged again, and the supernatant was added to the reaction mixture containing 25 mM Tris·HCl, pH 7.6, 0.2 mM NADH, 0.04 mM ZnSO4, and 0.3 U/ml malate dehydrogenase (Roche, Indianapolis, IN). Citrate concentration was calculated after the addition of 0.12 U/ml citrate lyase (Roche).

To measure muscle TG, DAG, and ceramide contents, lipids were extracted by the Folch method from basal Q muscle dissected free of visible connective tissue and blood (23). TG was saponified in an ethanol-KOH solution at 60°C, and glycerol content was determined fluorometrically. DAG and ceramide were extracted and quantified as previously described (23). Lipids were extracted from freeze-dried powdered muscle using chloroform-methanol-PBS + 0.2% SDS (1:2:0.8). DAG kinase and [32P]ATP (15 mCi/mmol cold ATP) were added to lysates preincubated with cardiolipin/octylglucoside, and the reaction was stopped after 2 h by the addition of chloroform-methanol (2:1). Samples were spotted onto TLC plates and developed. 32P-labeled phosphatidic acid and ceramide 1-phosphate ceramide were identified, dried, scraped from the TLC plate, and counted in a liquid scintillation counter (Tri-Carb 2500TR; Packard, Canberra, Australia).

Western Blot Analysis

Plasma membrane fractions from preperfusion RG and WG were isolated from continuous sucrose gradient centrifugation for determination of CD36 transporter protein expression and translocation of PKC{theta} (Santa Cruz Biotechnology) as previously described (46). For the measurement of total content and/or activation state of the insulin transduction and inflammatory pathways, frozen samples from pre- and postperfusion muscle were homogenized in liquid N2 and treated with lysis buffer containing phosphatase and protease inhibitors. After a 10-min incubation, lysates were clarified by centrifugation (10,000 g at 4°C) and supernatants analyzed for total protein content or phosphorylation state (Bio-Rad, Hercules, CA). Proteins were separated by SDS-PAGE on 7.5, 10, or 12% resolving polyacrylamide gels. Equal amounts of protein from each muscle sample were loaded onto each gel.

Following electrophoresis, proteins were transferred to polyvinylidene difluoride membranes (Immobilon-P; Millipore, Bedford, MA) and incubated with the following antibodies according to the manufacturer's instructions: anti-CD36 mouse monoclonal antibody (Cascade Biosciences, Winchester, MA), anti-{alpha}-actin (Sigma, St. Louis, MO), anti-IRS-1 (Upstate Biotechnology), mouse monoclonal IRS-1 anti-phosphotyrosine antibody (PY-20; Transduction Laboratories, Lexington, KY), IRS-1 anti-phosphoserine 307 antibody (Cell Signaling), anti-Akt1/2 antibody (Santa Cruz Biotechnology), anti-phospho- (Ser473) Akt (Cell Signaling), anti-NIK (Santa Cruz), anti-IKKbeta antibody (Cell Signaling), anti-I{kappa}B{alpha} antibody (Cell Signaling), anti-JNK antibody (Cell Signaling), anti-phospho-JNK antibody (Thr183/Tyr185; Cell Signaling), and anti-NF-{kappa}B antibody (p65; Santa Cruz Biotechnology). Protein content was normalized to {alpha}-actin (found to be identical between the 3 groups, P = 0.67), and phospho-blots were normalized to the content of each respective protein. All phospho-blots were stripped and reprobed for protein content to confirm molecular weight. Band intensities were quantified by densitometry on a Hewlett-Packard ScanJet II using NIH Image 1.6 software. Figures generated from Western analyses contain representative blots and graphs of mean values ± SE expressed in arbitrary units obtained from densitometric analysis.

Calculations and Statistical Analyses

Palmitate kinetic parameters in blood and muscle were measured as previously described (46–47). Because there were no significant differences in values measured after 20, 30, and 40 min of perfusion, average values were used for each animal. Palmitate oxidation was corrected for label fixation using a previously determined acetate correction factor of 1.6 for all groups as previously described (48). Statistical significance was determined using ANOVA with Newman-Keuls test for post hoc comparisons where appropriate, with the level of significance set a priori at P < 0.05.


   RESULTS
TOP
 ABSTRACT
 EXPERIMENTAL PROCEDURES
 RESULTS
DISCUSSION
 GRANTS
 REFERENCES
 
Baseline Circulating Blood Parameters and Muscle Lipids

There were no detectable differences in animal body weights between the groups prior to or following the dietary intervention with or without the addition of TZD (P = 0.1, Table 1). Compared with CON, 3 wk of HF feeding led to a significant increase in both fasting plasma insulin (55% increase, P = 0.023) and FA levels (96%, P = 0.014), both of which were normalized by TZD treatment (P = 0.49 and P = 0.99 vs. CON, respectively; Table 1). Fasting plasma Acrp30 levels were significantly reduced by HF feeding (29% decrease, P = 0.026) but were significantly elevated above CON (75%, P = 0.003) following TZD treatment.


View this table:
[in this window]
[in a new window]

  		Table 1. Preperfusion characteristics of fasted animals in the basal state, parameters during the perfusion (100 µU/ml insulin), and postperfusion muscle characteristics

 
In a subset of animals not participating in perfusion experiments, Q muscle was harvested in the basal state and analyzed for lipid content. Three weeks of HF feeding led to a significant increase in content of muscle TG (86%, P = 0.047), DAG (45%, P = 0.002), and ceramide (70%, P = 0.01) compared with CON rats (Fig. 1, A, B, and C, respectively). The addition of a TZD to the HFD did not reduce muscle TG (P = 0.36) or DAG (P = 0.32) content compared with HFD alone. However, the TZD-enriched diet did cause a significant reduction in muscle ceramide content to a level below that measured for HFD and CON (P = 0.0013; Fig. 1C).


Figure 1
View larger version (10K):
[in this window]
[in a new window]

  		Fig. 1. Effect of high-fat diet (HFD) alone or HFD enriched with a thiazolidinedione (TZD + HFD) on preperfusion triacylglycerol (A), diacylglycerol (B), and ceramide (C) content in mixed muscle. Values are expressed as means ± SE. Triacylglycerol measurements were made on control (CON) normal chow-fed (n = 8), HFD (n = 8), and TZD + HFD (n = 4 and n = 6, troglitazone- and rosiglitazone-treated, respectively) rats. Diacylglycerol and ceramide measurements were made on CON (n = 8), HFD (n = 8), and TZD + HFD (n = 8, rosiglitazone-treated only) rats. *Significant difference vs. CON (P < 0.05); #significant difference HFD vs. TZD + HFD (P < 0.05).

 
Hindlimb Perfusion Parameters

Perfusate FA (P = 0.39) and glucose (P = 0.41) concentrations and FA delivery to the muscle (P = 0.87), as well as muscle oxygen uptake (P = 0.67), were not different between the groups (Table 1).

Transport Capacity and Substrate Metabolism

Palmitate metabolism. The protein level for the FA transporter CD36 was unchanged following HF feeding; however, TZD treatment caused a significant increase in CD36 protein expression in plasma membrane fractions isolated from RG (60% increase, P = 0.01) and WG (100% increase, P = 0.001) muscle (Fig. 2A). Consistent with this, whereas the total uptake of palmitate was not affected by 3 wk of HF feeding, the addition of a TZD to the diet significantly increased muscle palmitate uptake by 45% (P = 0.002) during perfusion with insulin (Fig. 2B).


Figure 2
View larger version (19K):
[in this window]
[in a new window]

  		Fig. 2. Effect of HFD alone or TZD + HFD on CD36 protein content in red and white muscle (A), palmitate uptake (B), palmitate oxidation (C), and rate of triacylglycerol synthesis (D). Values are expressed as means ± SE. Analyses were performed on muscle from CON (n = 8), HFD (n = 8), and TZD + HFD (n = 8, troglitazone-treated) animals. Palmitate uptake and oxidation as well as the rate of triacylglycerol synthesis were measured in insulin-stimulated muscle (100 µU/ml) during the perfusion. CD36 protein level was analyzed by Western analysis of plasma membrane fractions isolated from red and white gastrocnemius muscle prior to perfusion. *Significant difference vs. CON (P < 0.05); #significant difference, HFD vs. TZD + HFD (P < 0.05).

 
Compared with CON, palmitate oxidation, when expressed as total oxidation or a percentage of palmitate uptake, was increased in both HFD (65%, P = 0.002; and 63%, P = 0.03, respectively) and TZD + HFD (53%, P = 0.0007; and 105%, P = 0.001, respectively; Fig. 2C). Palmitate oxidation averaged 12.0 ± 0.8% of the total palmitate uptake in HFD and HFD + TZD and was not different between these two groups. Consistent with the role of malonyl-CoA in the regulation of FA oxidation, postperfusion malonyl-CoA concentration was negatively correlated with FA oxidation (R2= –0.42, P = 0.008; data not shown). Whereas malonyl-CoA levels were not different between the three groups, citrate levels increased almost identically by ~80% for HFD and TZD + HFD (P = 0.0001; Table 1) vs. CON.

To assess whether HFD-induced changes in key oxidative enzymes in muscle perfused under insulin-stimulated conditions, we measured the activities of beta-HAD and CS, marker enzymes for the pathways of beta-oxidation and the Krebs cycle, respectively. We observed a significant (51–64%) increase in beta-HAD activity for HFD (P = 0.004) and TZD + HFD (P = 0.03; Table 1) compared with CON. CS activity was elevated only in HFD animals (P = 0.008) compared with CON. AMPK and ACC activities, shown to be important in the regulation of FA oxidation in skeletal muscle, were also assessed. However, no differences in AMPK and ACC activity or the Km for citrate activation of ACC were detected between the groups (Table 1) during insulin stimulation.

Despite elevated lipid levels in muscle prior to perfusion, we observed a significant increase in palmitate uptake in TZD + HFD muscle, and this was associated with a 57 and 46% increase in the incorporation of [14C]palmitate into muscle TG relative to CON (P = 0.014) and HFD (P = 0.03) alone (Fig. 2D). Collectively, these data indicate that TZD treatment during HF feeding leads to an upregulation of FA transport capacity, translating into greater FA uptake as well as oxidative and nonoxidative metabolism.

Glucose uptake and insulin action. HF feeding was associated with a 47% (P = 0.001) reduction in skeletal muscle insulin-stimulated glucose uptake during the perfusion (Fig. 3A). In contrast, muscle glucose uptake in the HF-fed animals treated with TZD was not different from that measured in the control group (P = 0.1).


Figure 3
View larger version (19K):
[in this window]
[in a new window]

  		Fig. 3. Effect of HFD alone or TZD + HFD on glucose uptake (A) and insulin receptor substrate (IRS)-1 (B) and Akt phosphorylation following insulin stimulation. Values are expressed as means ± SE. Glucose uptake was measured during the perfusion (insulin = 100 µU/ml), and tissue insulin action was assessed by Western analysis of homogenate samples prepared from postperfusion mixed gastrocnemius muscle from the CON (n = 8), HFD (n = 8), and TZD + HFD (n = 8, troglitazone-treated) groups. *Significant difference vs. CON (P < 0.05); #significant difference, HFD vs. TZD + HFD (P < 0.05).

 
To assess the impact of HF diet and TZD treatment on skeletal muscle insulin signaling, muscle protein content and phosphorylation levels were analyzed using Western analysis on crude homogenates of the hindlimb. Total IRS-1 protein expression was not different between the groups (P = 0.36; data not shown). The HF diet caused a fourfold (P = 0.001; Fig. 3B) elevation in IRS-1 serine phosphorylation and was associated with impaired IRS-1 tyrosine phosphorylation ({downarrow}45%, P = 0.001; Fig. 3B) compared with CON. The addition of a TZD to the HF diet prevented serine phosphorylation and restored tyrosine phosphorylation of IRS-1 to CON levels.

Downstream of IRS-1 and PI 3-kinase, Akt protein levels were found to be identical between groups (P = 0.58; data not shown). Akt serine (Ser473) phosphorylation was significantly reduced (35%, P = 0.01; Fig. 3B) by HF feeding, and, consistent with the upstream signaling events, TZD enrichment of the HF diet restored insulin-stimulated Akt phosphorylation to CON levels.

Skeletal Muscle Markers of Inflammation

To address the lipotoxic effects of HF feeding on the stress-activated NF-{kappa}B inflammatory pathway, we measured the relative protein expression, the phosphorylation states, and the activity level of key signaling molecules within this pathway. NF-{kappa}B-inducing kinase (NIK), the purported upstream kinase involved in the activation of the IKK complex (44), was shown to be elevated almost twofold (P = 0.009; Fig. 4A) following HF feeding. In agreement with this, IKKbeta activity was also increased approximately twofold (P = 0.006; Fig. 4B) in muscle from HF-fed animals. The activation of IKKbeta was significantly correlated with insulin-stimulated glucose uptake (R2 = –0.67, P = 0.005; Fig. 4C).


Figure 4
View larger version (24K):
[in this window]
[in a new window]

  		Fig. 4. Effect of HFD or TZD + HFD on inflammation. A: nuclear factor (NF)-{kappa}B-inducing kinase (NIK), I{kappa}B kinase (IKK)beta, I{kappa}B{alpha}, and NF-{kappa}B (p65) total protein. B: IKKbeta activity. C: correlation between IKKbeta activity and glucose uptake. D: JNK protein and phosphorylation. Values are expressed as means ± SE. Western analysis of inflammatory proteins was performed on basal mixed quadriceps muscle harvested from animals in the CON (n = 8), HFD (n = 8), and TZD + HFD (n = 8, rosiglitazone-treated) groups. IKKbeta activity assay was performed on postperfusion mixed quadriceps muscle (n = 5/group) so that a correlation with glucose uptake could be established. JNK protein and phosphorylation levels were assessed in basal mixed quadriceps muscle harvested from animals in the CON (n = 8), HFD (n = 8), and TZD + HFD (n = 8 and n = 8, troglitazone-treated and rosiglitazone-treated, respectively) groups *Significant difference vs. CON (P < 0.05); #significant difference, HFD vs. TZD + HFD (P < 0.05).

 
Conventionally, IKK phosphorylates the inhibitory subunit of the NF-{kappa}B complex, I{kappa}B{alpha}, targeting this inhibitory molecule for rapid ubiquitin-proteasome degradation (18, 28, 52, 54). The release of this inhibitory subunit (I{kappa}B{alpha}) allows for NF-{kappa}B nuclear translocation and modulation of target gene transcription. Consistent with this paradigm, the protein content of I{kappa}B{alpha} was diminished by ~25% (P = 0.03; Fig. 4A) in HFD muscle. HF feeding also increased NF-{kappa}B p65 total protein by 30% vs. CON (P = 0.05; Fig. 4A), and this increase was prevented by TZD treatment. Whereas HF feeding led to a marked increase in the protein level and activity of key NF-{kappa}B signaling molecules, the addition of TZD to the HFD completely suppressed FA-induced activation of this pathway.

Additionally, it is reported that other serine kinases are activated by HF feeding (15, 40). PKC{theta} is thought to be involved in FA-induced insulin resistance by phosphorylating IRS-1 at serine 1101 (24). Following HF feeding and muscle perfusion, we were unable to detect activation of PKC{theta}, as reflected by identical protein content levels in the total homogenate, plasma membrane, and particulate fractions between the three groups (P = 0.26; data not shown). This observation is consistent with that of Lessard et al. (23).

In contrast with observations made for PKC{theta}, HF feeding was associated with a 1.7-fold increase (P = 0.003; Fig. 4D) in JNK phosphorylation, a molecule also known to serine phosphorylate IRS-1 (15, 22). HFD-induced phosphorylation of JNK was completely reversed by dietary TZD enrichment. Total JNK protein levels were not different between the groups (P = 0.11; Fig. 4D).


   DISCUSSION
TOP
 ABSTRACT
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
GRANTS
 REFERENCES
 
Several studies have tested whether TZD treatment improves insulin action by reducing TG and lipid metabolites in muscle. However, to date, the findings are equivocal (21, 23, 26, 29, 35). Studies employing 1H nuclear magnetic resonance spectroscopy (26) or quantitative biochemical lipid analyses (23, 29) show that TZD-induced insulin sensitization in muscle occurs independently of changes in muscle lipid content. Due to these inconsistencies in the literature, we sought to test an alternative hypothesis that TZDs maintain insulin action during lipid oversupply by suppressing inflammation.

Our data confirm that HF feeding leads to the accumulation of lipid and lipid intermediates (including DAG, ceramide, and TG) in skeletal muscle, and this is associated with increased activation of specific serine kinases (IKKbeta and JNK) and reduced insulin action. Consistent with our hypothesis, skeletal muscle insulin sensitivity is maintained in rats fed a HF diet enriched with a TZD, and this is accompanied by the suppression of proinflammatory signaling and reduced muscle ceramide content. TZD-induced protection of insulin action in skeletal muscle occurred despite elevated rates of FA uptake and TG synthesis that resulted in increased TG accumulation. Thus these data contrast with reports suggesting that TZDs enhance insulin action by reducing FA storage in skeletal muscle (35, 40). To the contrary, we provide novel findings showing that the maintenance of insulin action occurs at least in part via suppression of proinflammatory signaling that is not associated with reduced lipid transport capacity or lipid accumulation.

Elevated circulating FA and TG, as well as excessive lipid storage within insulin-sensitive tissues, are associated with insulin resistance and are often found in type 2 diabetic patients or during experimental lipid oversupply (4, 7, 19, 32, 37). Although there is compelling evidence to suggest that excess cellular lipids play an etiological role in the development of insulin resistance in skeletal muscle (13, 17, 32), it is also known, but sometimes overlooked, that, under certain metabolic conditions (e.g., chronic exercise), high cellular lipids may be present without the presence of insulin resistance (5, 11, 38). This suggests that there must be some other cellular event that mediates FA-induced insulin resistance. In the present investigation, we sought to address this question (i.e., the lipid paradox) in TZD-treated HF-fed rats and investigate the role of the NF-{kappa}B proinflammatory signaling cascade as a direct mediator of FA-induced insulin resistance in skeletal muscle.

The notion that TZDs cause insulin sensitization due to reductions in skeletal muscle lipid accumulation would suggest that TZDs diminish FA uptake by muscle and/or elevate fatty acid oxidation so that less lipid is stored in muscle. If the primary mechanism for TZD-induced skeletal muscle insulin sensitization is enhanced FA partitioning into adipose tissue, thus sparing skeletal muscle form the lipotoxic effects of FA oversupply, a downregulation of skeletal muscle FA transport capacity should logically follow. Consistent with the published findings by several other groups (14, 31, 49), here we report that FA transport capacity, as measured by plasma membrane CD36 protein content, is increased in both red and white muscle following 3 wk of TZD treatment. Because plasma membrane CD36 was also measured following insulin stimulation, our results suggest that, compared with HFD alone, TZD treatment either increased total CD36 protein expression, restored muscle's sensitivity to the effects of insulin on CD36 translocation (25), or both. The TZD-induced increase in plasma membrane CD36 content was associated with a 45% increase in FA uptake into skeletal muscle, and this was paralleled by a 43% increase in TG synthesis. These data indicate that the additional FA taken up during the perfusion by muscle from TZD-treated HF-fed rats vs. HFD alone were partitioned to storage. These data parallel recent work showing increased FA transport capacity, uptake, and/or TG accumulation in muscle following TZD treatment in rodents and obese or type 2 diabetic humans (23, 26, 29, 49).

Concurrent with these events, our findings show that TZDs suppress the proinflammatory response to FA oversupply, and this suppression presumably allows for normal insulin signaling to proceed. It is well known that the production of lipid intermediates, including DAG and ceramide, is associated with the activation of serine kinases (i.e., JNK and IKKbeta) shown to diminish proximal insulin signaling in vitro (2, 10, 43). In vivo, systemic or tissue specific knockouts of these kinases have confirmed their role in mediating diet-induced insulin resistance (3, 15, 53). Here we report that HF feeding causes increased muscle accumulation of ceramide and DAG, which is paralleled by increased NIK expression, IKKbeta activation, and JNK phosphorylation. Although the exact circumstances leading to serine kinase activation are unknown, both IKKbeta and JNK have been shown to impair proximal insulin signaling via serine phosphorylation of IRS-1, a finding confirmed in the present investigation (22, 41). We also report that TZD treatment during HF feeding diminishes NIK protein expression and reduces the activation states of IKKbeta and JNK. Our findings for TZD-treated HF-fed animals are consistent with those of the serine kinase knockout mice, where functional or pharmacological inactivation of IKKbeta or JNK prevents HF diet-induced insulin resistance (3, 15, 53).

In conclusion, there is strong evidence that chronic inflammation is a major cause of insulin resistance. In the present investigation, we demonstrate that TZD treatment during HF feeding promotes insulin-stimulated skeletal muscle FA uptake and TG synthesis without activation of IKKbeta and JNK or impairments in insulin action. We propose the notion that TZD treatment in the presence of dietary lipid overload protects muscle from FA-induced insulin resistance by suppression of inflammation that is independent of myocellular lipid levels.


   GRANTS
TOP
 ABSTRACT
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 GRANTS
REFERENCES
 
This research was supported by National Institutes of Health grants to A. L. Hevener (DK-60484) and L. P. Turcotte (AR-45168). M. J. Watt is supported by a Peter Doherty postdoctoral fellowship from the National Health and Medical Research Council of Australia.


   ACKNOWLEDGMENTS
 
We are grateful to Drs. Malek Arkan (Technische Universität München) and Michael Karin (Department of Pharmacology, University of California, San Diego) for their technical assistance and helpful discussions regarding the IKKbeta activity assay and the impact of the NF-{kappa}B inflammatory signaling pathway on insulin action. We also appreciate Brian Folian for assistance with rodent handling, Dr. Mark Febbraio for contribution of resources for the lipid analyses, and the generous support and constructive criticism from Dr. Jerrold Olefsky, Department of Medicine, University of California, San Diego.


   FOOTNOTES
 

Address for reprint requests and other correspondence: A. L. Hevener, University of California, San Diego, Department of Medicine, Division of Endocrinology & Metabolism, 9500 Gilman Drive, MC 0673, La Jolla, CA 92093–0673 (e-mail: ahevener{at}ucsd.edu)

* These authors contributed equally to this work. Back


   REFERENCES
TOP
 ABSTRACT
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 

  1. Adams JM, Prantipanawatr T, Berria R, Wang E, DeFronzo RA, Sullards MC, Mandarino LJ. Ceramide content is increased in skeletal muscle from obese insulin-resistant humans. Diabetes 53: 25–31, 2004.[Abstract/Free Full Text]
  2. Aguirre V, Uchida T, Yenush L, Davis R, White MF. The c-Jun NH(2)-terminal kinase promotes insulin resistance during association with insulin receptor substrate-1 and phosphorylation of Ser(307). J Biol Chem 275: 9047–9054, 2000.[Abstract/Free Full Text]
  3. Arkan M, Hevener AL, Greten FR, Maeda S, Li ZW, Long JM, Wynshaw-Boris A, Poli G, Olefsky JM, Karin M. IKKbeta links inflammation to obesity-induced insulin resistance. Nat Med 11: 191–198, 2005.[CrossRef][Web of Science][Medline]
  4. Belfort R, Mandarino L, Kashyap S, Wirfel K, Pratipanawatr T, Berria R, DeFronzo RA, Cusi K. Dose-response effect of elevated plasma free fatty acid on insulin signaling. Diabetes 54: 1640–1648, 2005.[Abstract/Free Full Text]
  5. Bruce CR, Kriketos AD, Cooney GJ, Hawley JA. Disassociation of muscle triglyceride content and insulin sensitivity after exercise training in patients with type 2 diabetes. Diabetologia 47: 23–30, 2004.[CrossRef][Web of Science][Medline]
  6. DeFronzo RA. Lilly lecture 1987. The triumvirate: beta-cell, muscle, liver. A collusion responsible for NIDDM. Diabetes 37: 667–687, 1988.[Web of Science][Medline]
  7. DeFronzo RA, Bonadonna RC, Ferrannini E. Pathogenesis of NIDDM: a balanced overview. Diabetes Care 15: 318–368, 1992.[Abstract]
  8. Ellis BA, Poynten A, Lowy AJ, Furler SM, Chisholm DJ, Kraegen EW, Cooney GJ. Long-chain acyl-CoA esters as indicators of lipid metabolism and insulin sensitivity in rat and human muscle. Am J Physiol Endocrinol Metab 279: E554–E560, 2000.[Abstract/Free Full Text]
  9. Garg R, Tripathy D, Dandona P. Insulin resistance as a proinflammatory state: mechanisms, mediators, and therapeutic interventions. Curr Drug Targets 4: 487–492, 2003.[CrossRef][Web of Science][Medline]
  10. Gao Z, Hwang D, Bataille F, Lefevre M, York D, Quon MJ, Ye J. Serine phosphorylation of insulin receptor substrate 1 by inhibitor {kappa}B kinase complex. J Biol Chem 277: 48115–48121, 2002.[Abstract/Free Full Text]
  11. Goodpaster BH, He J, Watkins S, Kelley DE. Skeletal muscle lipid content and insulin resistance: evidence for a paradox in endurance-trained athletes. J Clin Endocrinol Metab 86: 5755–5761, 2001.[Abstract/Free Full Text]
  12. Griffin ME, Marcucci MJ, Cline GW, Bell K, Barucci N, Goodyear LJ, Kraegen EW, White MF, Shulman GI. Free fatty acid-induced insulin resistance is associated with activation of protein kinase C theta and alterations in the insulin signaling cascade. Diabetes 48: 1270–1274, 1999.[Abstract]
  13. Hajduch E, Balendran A, Batty IH, Litherland GJ, Blair AS, Downes CP, Hundal HS. Ceramide impairs the insulin-dependent membrane recruitment of protein kinase B leading to a loss in downstream signalling in L6 skeletal muscle cells. Diabetologia 44: 173–183, 2001.[CrossRef][Web of Science][Medline]
  14. Hevener AL, Reichart D, Janez A, Olefsky J. Thiazolidinedione treatment prevents free fatty acid-induced insulin resistance in male Wistar rats. Diabetes 50: 2316–2322, 2001.[Abstract/Free Full Text]
  15. Hirosumi J, Tuncman G, Chang L, Gorgun CZ, Uysal KT, Maeda K, Karin M, Hotamisligil GS. A central role for JNK in obesity and insulin resistance. Nature 420: 333–336, 2002.[CrossRef][Medline]
  16. Hundal RS, Petersen KF, Mayerson AB, Randhawa PS, Inzucchi S, Shoelson SE, Shulman GI. Mechanisms by which high-dose aspirin improves glucose metabolism in type 2 diabetes. J Clin Invest 109: 1321–1326, 2002.[CrossRef][Web of Science][Medline]
  17. Itani SI, Ruderman NB, Schmieder F, Boden G. Lipid-induced insulin resistance in human muscle is associated with changes in diacylglycerol, protein kinase C and I{kappa}B-{alpha}. Diabetes 51: 2005–2011, 2002.[Abstract/Free Full Text]
  18. Karin M, Ben-Neriah Y. Phosphorylation meets ubiquitination: the control of NF-[kappa]B activity. Annu Rev Immunol 18: 621–663, 2000.[CrossRef][Web of Science][Medline]
  19. Kelley DE, Goodpaster B, Wing RR, Simoneau JA. Skeletal muscle fatty acid metabolism in association with insulin resistance, obesity, and weight loss. Am J Physiol Endocrinol Metab 277: E1130–E1141, 1999.[Abstract/Free Full Text]
  20. Kennedy SL, Stanley WC, Panchal AR, Mazzeo RS. Alterations in enzymes involved in fat metabolism after acute and chronic altitude exposure. J Appl Physiol 90: 17–22, 2001.[Abstract/Free Full Text]
  21. Krssak M, Falk Petersen K, Dresner A, DiPietro L, Vogel SM, Rothman DL, Roden M, Shulman GI. Intramyocellular lipid concentrations are correlated with insulin sensitivity in humans: a 1H NMR spectroscopy study. Diabetologia 42: 113–116, 1999.[CrossRef][Web of Science][Medline]
  22. Lee YH, Giraud J, Davis RJ, White MF. c-Jun N-terminal kinase (JNK) mediates feedback inhibition of the insulin signaling cascade. J Biol Chem 278: 2896–2902, 2003.[Abstract/Free Full Text]
  23. Lessard SJ, Giudice SL, Lau W, Reid JJ, Turner N, Febbraio MA, Hawley JA, Watt MJ. Rosiglitazone enhances glucose tolerance by mechanisms other than reduction of fatty acid accumulation within skeletal muscle. Endocrinology 145: 5665–5670, 2004.[Abstract/Free Full Text]
  24. Li Y, Soos TJ, Li X, Wu J, DeGennaro M, Sun X, Littman DR, Birnbaum MJ, Polakiewicz RD. Protein kinase C Theta inhibits insulin signaling by phosphorylating IRS1 at Ser(1101). J Biol Chem 279: 45304–45307, 2004.[Abstract/Free Full Text]
  25. Luiken JJ, Koonen DP, Willems J, Zorzano A, Becker C, Fischer Y, Tandon NN, Van Der Vusse GJ, Bonen A, Glatz JF. Insulin stimulates long-chain fatty acid utilization by rat cardiac myocytes through cellular redistribution of FAT/CD36. Diabetes 51: 3113–3119, 2002.[Abstract/Free Full Text]
  26. Mayerson AB, Hundal RS, Dufour S, Lebon V, Befroy D, Cline GW, Enocksson S, Inzucchi SE, Shulman GI, Petersen KF. The effects of rosiglitazone on insulin sensitivity, lipolysis, and hepatic and skeletal muscle triglyceride content in patients with type 2 diabetes. Diabetes 51: 797–802, 2002.[Abstract/Free Full Text]
  27. McGarry JD. Banting lecture 2001: dysregulation of fatty acid metabolism in the etiology of type 2 diabetes. Diabetes 51: 7–18, 2002.[Free Full Text]
  28. Mercurio F, Zhu H, Murray BW, Shevchenko A, Bennett BL, Li JW, Young DB, Barbosa M, Mann M, Manning A, Rao A. IKK-1 and IKK-2: cytokine-activated I{kappa}B kinases essential for NF-{kappa}B activation. Science 278: 860–866, 1997.[Abstract/Free Full Text]
  29. Muurling M, Mensink RP, Pijl H, Romijn JA, Havekes LM, Voshol PJ. Rosiglitazone improves muscle insulin sensitivity, irrespective of increased triglyceride content, in ob/ob mice. Metabolism 52: 1078–7083, 2003.[CrossRef][Web of Science][Medline]
  30. Nakano H, Shindo M, Sakon S, Nishinaka S, Mihara M, Yagita H, Okumura K. Differential regulation of I{kappa}B kinase alpha and beta by two upstream kinases, NF-kappaB-inducing kinase and mitogen-activated protein kinase/ERK kinase kinase-1. Proc Natl Acad Sci USA 95: 3537–3542, 1998.[Abstract/Free Full Text]
  31. Oakes N, Thalen P, Wettesten M, Bamberg K, Jung B. Evidence that PPAR{gamma} agonist effects on free fatty acid metabolism in vivo depend on CD36 (Abstract). Diabetes 52, Suppl 2: A9, 2003.
  32. Pan DA, Lillioja S, Kriketos AD, Milner MR, Baur LA, Bogardus C, Jenkins AB, Storlien LH. Skeletal muscle triglyceride levels are inversely related to insulin action. Diabetes 46: 983–988, 1997.[Abstract]
  33. Passonneau JV, Lowry OH. Enzymatic Analysis: A Practical Guide. Totowa, NJ: Humana, 1993.
  34. Raney MA, Yee AJ, Todd MK, Turcotte LP. AMPK activation is not critical in the regulation of muscle FA uptake and oxidation during low-intensity muscle contraction. Am J Physiol Endocrinol Metab 288: E592–E598, 2005.[Abstract/Free Full Text]
  35. Rasouli N, Raue U, Miles LM, Lu T, Di Gregorio GB, Elbein SC, Kern PA. Pioglitazone improves insulin sensitivity through reduction in muscle lipid and redistribution of lipid into adipose tissue. Am J Physiol Endocrinol Metab 288: E930–E934, 2005.[Abstract/Free Full Text]
  36. Reaven GM. Role of insulin resistance in human disease. Diabetes 37: 1595–1607, 1988.[Abstract]
  37. Reaven GM, Hollenbeck C, Jeng CY, Wu MS, Chen YD. Measurement of plasma glucose, free fatty acid, lactate and insulin for 24 hours in patients with NIDDM. Diabetes 37: 1020–1024, 1998.
  38. Russell AP, Gastaldi G, Bobbioni-Harsch E, Arboit P, Gobelet C, Deriaz O, Golay A, Witztum JL, Giacobino JP. Lipid peroxidation in skeletal muscle of obese as compared to endurance-trained humans: a case of good vs. bad lipids? FEBS Lett 551: 104–106, 2003.[CrossRef][Web of Science][Medline]
  39. Schmidt MI, Duncan BB. Diabesity: an inflammatory metabolic condition. Clin Chem Lab Med 41: 1120–1130, 2003.[CrossRef][Web of Science][Medline]
  40. Schmitz-Peiffer C, Oakes ND, Browne CL, Kraegen EW, Biden TJ. Reversal of chronic alterations of skeletal muscle protein kinase C from fat-fed rats by BRL-49653. Am J Physiol Endocrinol Metab 273: E915–E921, 1997.[Abstract/Free Full Text]
  41. Sinha S, Perdomo G, Brown NF, O'Doherty RM. Fatty acid-induced insulin resistance in L6 myotubes is prevented by inhibition of activation and nuclear localization of nuclear factor {kappa}B. J Biol Chem 279: 41294–41301, 2004.[Abstract/Free Full Text]
  42. Srere PA. Citrate synthase. Methods Enzymol 12: 3–11, 1969.
  43. Stratford S, Hoehn KL, Liu F, Summers SA. Regulation of insulin action by ceramide: dual mechanisms linking ceramide accumulation to the inhibition of AKT/protein kinase B. J Biol Chem 279: 36608–36615, 2004.[Abstract/Free Full Text]
  44. Suzawa M, Takada I, Yanagisawa J, Ohtake F, Ogawa S, Yamauchi T, Kadowaki T, Takeuchi Y, Shibuya H, Gotoh Y, Matsumoto K, Kato S. Cytokines suppress adipogenesis and PPAR{gamma}-function through the TAK1/TAB1/NIK cascade. Nat Cell Biol 5: 224–230, 2003.[CrossRef][Web of Science][Medline]
  45. Tucker MZ, Turcotte LP. Brief food restriction increases FA oxidation and glycogen synthesis under insulin-stimulated conditions. Am J Physiol Regul Integr Comp Physiol 282: R1210–R1218, 2002.[Abstract/Free Full Text]
  46. Turcotte LP, Swenberger JR, Tucker MZ, Yee AJ. Training-induced elevation in FABP(PM) is associated with increased palmitate use in contracting muscle. J Appl Physiol 87: 285–293, 1999.[Abstract/Free Full Text]
  47. Turcotte LP, Swenberger JR, Tucker MZ, Yee AJ. Increased fatty acid uptake and altered fatty acid metabolism in insulin-resistant muscle of obese Zucker rats. Diabetes 50: 1389–1396, 2001.[Abstract/Free Full Text]
  48. Turcotte LP, Swenberger JR, Yee AJ. High carbohydrate availability increases LCFA uptake and decreases LCFA oxidation in perfused muscle. Am J Physiol Endocrinol Metab 282: E177–E183, 2002.[Abstract/Free Full Text]
  49. Wilmsen HM, Ciaraldi TP, Carter L, Reehman N, Mudaliar SR, Henry RR. Thiazolidinediones upregulate impaired fatty acid uptake in skeletal muscle of type 2 diabetic subjects. Am J Physiol Endocrinol Metab 285: E354–E362, 2003.[Abstract/Free Full Text]
  50. Winder WW, Holmes BF. Insulin stimulation of glucose uptake fails to decrease palmitate oxidation in muscle if AMPK is activated. J Appl Physiol 89: 2430–2437, 2000.[Abstract/Free Full Text]
  51. Wojtaszewski JFP, Lynge J, Jakobsen AB, Goodyear LJ, Richter EA. Differential regulation of MAP kinase by contraction and insulin in skeletal muscle: metabolic implications. Am J Physiol Endocrinol Metab 277: E724–E732, 1999.[Abstract/Free Full Text]
  52. Yin MJ, Yamamoto Y, Gaynor RB. The anti-inflammatory agents aspirin and salicylate inhibit the activity of I{kappa}B kinase-beta. Nature 396: 77–80, 1998.[CrossRef][Medline]
  53. Yuan M, Konstantopoulos N, Lee J, Hansen L, Li ZW, Karin M, Shoelson SE. Reversal of obesity- and diet-induced insulin resistance with salicylates or targeted disruption of IKKbeta. Science 293: 1673–1677, 2001.[Abstract/Free Full Text]
  54. Zandi E, Rothwarf DM, Delhasse M, Hayakawa M, Karin M. The I{kappa}B kinase complex (IKK) contains two kinase subunits, IKK{alpha} and IKKbeta, necessary for I{kappa}B phosphorylation and NF-{kappa}B activation. Cell 91: 243–252, 1997.[CrossRef][Web of Science][Medline]



This article has been cited by other articles:


Home page
 Am. J. Physiol. Regul. Integr. Comp. Physiol.Home page
M. Laplante, W. T. Festuccia, G. Soucy, P.-G. Blanchard, A. Renaud, J. P. Berger, G. Olivecrona, and Y. Deshaies
Tissue-specific postprandial clearance is the major determinant of PPAR{gamma}-induced triglyceride lowering in the rat
Am J Physiol Regulatory Integrative Comp Physiol, January 1, 2009; 296(1): R57 - R66.
[Abstract] [Full Text] [PDF]

Home page
 DiabetesHome page
P. M. Rogers, N. Mashtalir, M. A. Rathod, O. Dubuisson, Z. Wang, K. Dasuri, S. Babin, A. Gupta, N. Markward, W. T. Cefalu, et al.
Metabolically Favorable Remodeling of Human Adipose Tissue by Human Adenovirus Type 36
Diabetes, September 1, 2008; 57(9): 2321 - 2331.
[Abstract] [Full Text] [PDF]

Home page
 J. Physiol.Home page
C. R. Benton, G. P. Holloway, S. E. Campbell, Y. Yoshida, N. N. Tandon, J. F. C. Glatz, J. J. J. F. P. Luiken, L. L. Spriet, and A. Bonen
Rosiglitazone increases fatty acid oxidation and fatty acid translocase (FAT/CD36) but not carnitine palmitoyltransferase I in rat muscle mitochondria
J. Physiol., March 15, 2008; 586(6): 1755 - 1766.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Physiol. Endocrinol. Metab.Home page
B. B. Yaspelkis III, S. J. Lessard, D. W. Reeder, J. J. Limon, M. Saito, D. A. Rivas, I. Kvasha, and J. A. Hawley
Exercise reverses high-fat diet-induced impairments on compartmentalization and activation of components of the insulin-signaling cascade in skeletal muscle
Am J Physiol Endocrinol Metab, October 1, 2007; 293(4): E941 - E949.
[Abstract] [Full Text] [PDF]

Home page
 DiabetesHome page
S. J. Lessard, D. A. Rivas, Z.-P. Chen, A. Bonen, M. A. Febbraio, D. W. Reeder, B. E. Kemp, B. B. Yaspelkis III, and J. A. Hawley
Tissue-Specific Effects of Rosiglitazone and Exercise in the Treatment of Lipid-Induced Insulin Resistance
Diabetes, July 1, 2007; 56(7): 1856 - 1864.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
292/2/E485    most recent
00080.2006v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (8)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Todd, M. K.
Right arrow Articles by Turcotte, L. P.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Todd, M. K.
Right arrow Articles by Turcotte, L. P.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Visit Other APS Journals Online
Copyright © 2007 by the American Physiological Society.