Characterization of free and glyceride-esterified long chain fatty acids in different skeletal muscle types of the rat

Metabolic fingerprinting of cell types in mouse skeletal muscle by combining TOF-SIMS with immunofluorescence staining

Skeletal muscle tissue is composed of various muscle cell types which differ in physiological functions. Changes in cell type composition of skeletal muscle are associated with the development of metabolic diseases. Skeletal muscle cell types are currently distinguished by immunofluorescence (IF) staining based on myosin heavy chain (MHC) isoform difference. However, it remains a challenge to provide metabolic fingerprints of different muscle cell types by IF staining. Therefore, in this study, we proposed a method to examine metabolite distribution within different cell types by time-of-flight secondary ion mass spectrometry (TOF-SIMS) with high spatial resolution. Skeletal muscle samples from C57/BL6 mice were obtained by slicing. Cell types in TOF-SIMS images were labelled corresponding to IF images from the same region of serially cut sections. Mass spectra corresponding to individual muscle cells were extracted to compare metabolic fingerprints among cell types. Skeletal muscle cells were classified into two clusters based on the mass spectra of individual cells. Unsaturated diacylglycerol (DG) and fatty acid (FA) species were found to be distributed in a cell-type dependent manner. Moreover, relative quantification showed that the content of unsaturated DGs, oleic acid and linoleic acid was higher in type I and type IIA cells than in type IIB cells. TOF-SIMS in combination with IF enables us to directly visualize metabolite distribution in different cell types, to find potential biomarkers for cell type classification. TOF-SIMS imaging coupled with IF staining has been proved to be a promising tool for metabolic fingerprinting of different skeletal muscle cell types.

Oleic and linoleic fatty acids downregulate Slc2a4/GLUT4 expression via NFKB and SREBP1 in skeletal muscle cells

Oleic (OA) and linoleic (LA) fatty acids may be important regulators of Slc2a4 gene (GLUT4 protein) in skeletal muscle, thus participating in insulin resistance. We investigated the effect of OA and LA on the Slc2a4/GLUT4 expression in L6 muscle cells; as well as potential transcriptional regulators. OA and LA (50-400 µM) decreased the Slc2a4/GLUT4 expression in a dose-dependent way (maximum of ~50%, P < 0.001). OA and LA did not alter the Slc2a4-binding activity of oxysterols-receptor-LXR-alpha and peroxisome-proliferator-activated-receptor-gamma; but decreased the Slc2a4-binding activity of the sterol-regulatory-element-binding-protein-1 (SREBP1) enhancer (50%, P < 0.001), and increased (~30%, P < 0.001) the nuclear proteins binding into the Slc2a4-nuclear-factor-NF-kappa-B-binding site (repressor), and the phosphorylation of the inhibitors of nuclear-factor-kappa-B-kinase alpha/beta (150-300%, P < 0.001). In sum, OA and LA are potent inhibitors of the Slc2a4/GLUT4 expression in muscle cells; an effect involving reduced SREBP1 and increased NFKB transcriptional activity. These regulations may participate in the fatty acid-related pathophysiology of insulin resistance.

High-fat feeding does not induce an autophagic or apoptotic phenotype in female rat skeletal muscle

Apoptosis and autophagy are critical in normal skeletal muscle homeostasis; however, dysregulation can lead to muscle atrophy and dysfunction. Lipotoxicity and/or lipid accumulation may promote apoptosis, as well as directly or indirectly influence autophagic signaling. Therefore, the purpose of this study was to examine the effect of a 16-week high-fat diet on morphological, apoptotic, and autophagic indices in oxidative and glycolytic skeletal muscle of female rats. High-fat feeding resulted in increased fat pad mass, altered glucose tolerance, and lower muscle pAKT levels, as well as lipid accumulation and reactive oxygen species generation in soleus muscle; however, muscle weights, fiber type-specific cross-sectional area, and fiber type distribution were not affected. Moreover, DNA fragmentation and LC3 lipidation as well as several apoptotic (ARC, Bax, Bid, tBid, Hsp70, pBcl-2) and autophagic (ATG7, ATG4B, Beclin 1, BNIP3, p70 s6k, cathepsin activity) indices were not altered in soleus or plantaris following high-fat diet. Interestingly, soleus muscle displayed small increases in caspase-3, caspase-8, and caspase-9 activity, as well as higher ATG12-5 and p62 protein, while both soleus and plantaris muscle showed dramatically reduced Bcl-2 and X-linked inhibitor of apoptosis protein (XIAP) levels. In conclusion, this work demonstrates that 16 weeks of high-fat feeding does not affect tissue morphology or induce a global autophagic or apoptotic phenotype in skeletal muscle of female rats. However, high-fat feeding selectively influenced a number of apoptotic and autophagic indices which could have implications during periods of enhanced muscle stress.

Effects of chronic administration of arachidonic acid on lipid profiles and morphology in the skeletal muscles of aged rats

Arachidonic acid (20:4n-6, ARA) is a major component of the cell membrane, whereas ARA-derived eicosanoids are formed when cells are damaged. Aging is associated with an accretion of oxidative stress in skeletal muscles. In this study, we examined the effects of chronic administration (13 weeks) of ARA (240 mg/kg/day) on fatty acid composition, antioxidative status, and morphology of slow (soleus muscles) and fast (extensor digitorum longus muscles; EDL)-twitch muscles in aged rats (21 months old). The level of reactive oxygen species was higher in the EDL of ARA-administered rats than in that of control rats. ARA administration decreased the muscle cell volumes and increased the number of slow myosin heavy chain (MHC)-positive cells in the EDL. The relative content of MHC2X was increased whereas the relative content of MHC2B was decreased in the EDL of ARA-administered rats. These results suggest that ARA deposition in the fast-twitch muscle of aged rats reduced cell volume with an increase in oxidative stress.

Induction of miR-29a by saturated fatty acids impairs insulin signaling and glucose uptake through translational repression of IRS-1 in myocytes

MicroRNAs have been shown to play an important role in insulin signaling but their biological function in insulin resistance induced by saturated fatty acids (SFA) remains largely unknown. Here, we report that SFA palmitate and high fat diet (HFD) significantly increase expression of miR-29a in myocytes. miR-29a targets IRS-1 3'UTR directly and represses IRS-1 expression at the translational level. Furthermore, the ectopic expression of miR-29a impairs insulin signaling and glucose uptake in myocytes through a substantial decrease in IRS-1. These findings suggest that the up-regulation of miR-29a by SFA is causally related to the development of insulin resistance in myocytes.

Oleate prevents saturated-fatty-acid-induced ER stress, inflammation and insulin resistance in skeletal muscle cells through an AMPK-dependent mechanism

AIMS/HYPOTHESIS

Although the substitution of saturated fatty acids with oleate has been recommended in the management of type 2 diabetes mellitus, the mechanisms by which oleate improves insulin resistance in skeletal muscle cells are not completely known. Here, we examined whether oleate, through activation of AMP-activated protein kinase (AMPK), prevented palmitate-induced endoplasmic reticulum (ER) stress, which is involved in the link between lipid-induced inflammation and insulin resistance.

METHODS

Studies were conducted in mouse C2C12 myotubes and in the human myogenic cell line LHCN-M2. To analyse the involvement of AMPK, activators and inhibitors of this kinase and overexpression of a dominant negative AMPK construct (K45R) were used.

RESULTS

Palmitate increased the levels of ER stress markers, whereas oleate did not. In palmitate-exposed cells incubated with a lower concentration of oleate, the effects of palmitate were prevented. The induction of ER stress markers by palmitate was prevented by the presence of the AMPK activators AICAR and A-769662. Moreover, the ability of oleate to prevent palmitate-induced ER stress and inflammation (nuclear factor-kappa B [NF-κB] DNA-binding activity and expression and secretion of IL6) as well as insulin-stimulated Akt phosphorylation and 2-deoxyglucose uptake was reversed in the presence of the AMPK inhibitor compound C or by overexpression of a dominant negative AMPK construct. Finally, palmitate reduced phospho-AMPK levels, whereas this was not observed in oleate-exposed cells or in palmitate-exposed cells supplemented with oleate.

CONCLUSIONS/INTERPRETATION

Overall, these findings indicate that oleate prevents ER stress, inflammation and insulin resistance in palmitate-exposed skeletal muscle cells by activating AMPK.

Not only accumulation, but also saturation status of intramuscular lipids is significantly affected by PPARγ activation

AIM

Intramuscular lipid accumulation has been associated with insulin resistance, and after thiazolidinediones (TZD) treatment, it was shown to be reduced in some, but not all, studies. This work was undertaken to investigate the relationships between intramuscular lipids [free fatty acids (FFA), diacylglycerols (DAG), triacylglycerol (TAG) and phospholipids] and plasmalemmal expression of fatty acid (FA) transporter [FAT/CD36 and FABPpm] in the muscles of varying oxidative capacity, after peroxisome proliferator-activated receptors gamma (PPARγ) activation (rosiglitazone) in an animal model of high-fat-diet-induced insulin resistance. Endurance training was also included to further explore the differences in these relationships.

METHODS

We have used gas liquid chromatography to estimate FA content and composition in each lipid fraction. For sarcolemmal expression of FA transporters, subfractionation of skeletal muscles with subsequent western blot technique was applied.

RESULTS

High-fat diet induced intramuscular accumulation of FFA, DAG and TAG, irrespective of muscle's fibre composition. PPARγ activation (rosiglitazone) and, to a lesser extent, endurance training further increased TAG accumulation, while it reduced DAG in oxidative muscles (soleus and red gastrocnemius). Aforementioned interventions increased also sarcolemmal FAT/CD36 and FABPpm expressions in particular muscles. Irrespective of diet, rosiglitazone and exercise decreased significantly FA saturation status favouring proportionate enhancement in monounsaturated FA (rosiglitazone) or polyunsaturated FAs (endurance training).

CONCLUSION

These findings support the conclusion that not only the change in total lipid content (DAG and TAG), but also FA composition is affected by rosiglitazone in an animal model of high-fat-diet-induced insulin resistance.

Fat-induced membrane cholesterol accrual provokes cortical filamentous actin destabilisation and glucose transport dysfunction in skeletal muscle

AIMS/HYPOTHESIS

Diminished cortical filamentous actin (F-actin) has been implicated in skeletal muscle insulin resistance, yet the mechanism(s) is unknown. Here we tested the hypothesis that changes in membrane cholesterol could be a causative factor, as organised F-actin structure emanates from cholesterol-enriched raft microdomains at the plasma membrane.

METHODS

Skeletal muscle samples from high-fat-fed animals and insulin-sensitive and insulin-resistant human participants were evaluated. The study also used L6 myotubes to directly determine the impact of fatty acids (FAs) on membrane/cytoskeletal variables and insulin action.

RESULTS

High-fat-fed insulin-resistant animals displayed elevated levels of membrane cholesterol and reduced F-actin structure compared with normal chow-fed animals. Moreover, human muscle biopsies revealed an inverse correlation between membrane cholesterol and whole-body glucose disposal. Palmitate-induced insulin-resistant myotubes displayed membrane cholesterol accrual and F-actin loss. Cholesterol lowering protected against the palmitate-induced defects, whereas characteristically measured defects in insulin signalling were not corrected. Conversely, cholesterol loading of L6 myotube membranes provoked a palmitate-like cytoskeletal/GLUT4 derangement. Mechanistically, we observed a palmitate-induced increase in O-linked glycosylation, an end-product of the hexosamine biosynthesis pathway (HBP). Consistent with HBP activity affecting the transcription of various genes, we observed an increase in Hmgcr, a gene that encodes 3-hydroxy-3-methyl-glutaryl coenzyme A reductase, the rate-limiting enzyme in cholesterol synthesis. In line with increased HBP activity transcriptionally provoking a membrane cholesterol-based insulin-resistant state, HBP inhibition attenuated Hmgcr expression and prevented membrane cholesterol accrual, F-actin loss and GLUT4/glucose transport dysfunction.

CONCLUSIONS/INTERPRETATION

Our results suggest a novel cholesterolgenic-based mechanism of FA-induced membrane/cytoskeletal disorder and insulin resistance.

Human cyclooxygenase-2 is a sequence homodimer that functions as a conformational heterodimer

Prostaglandin endoperoxide H synthases 1 and 2, also known as cyclooxygenases (COXs) 1 and 2, convert arachidonic acid (AA) to prostaglandin endoperoxide H(2). Prostaglandin endoperoxide H synthases are targets of nonspecific nonsteroidal anti-inflammatory drugs and COX-2-specific inhibitors called coxibs. PGHS-2 is a sequence homodimer. Each monomer has a peroxidase and a COX active site. We find that human PGHS-2 functions as a conformational heterodimer having a catalytic monomer (E(cat)) and an allosteric monomer (E(allo)). Heme binds tightly only to the peroxidase site of E(cat), whereas substrates, as well as certain inhibitors (e.g. celecoxib), bind the COX site of E(cat). E(cat) is regulated by E(allo) in a manner dependent on what ligand is bound to E(allo). Substrate and nonsubstrate fatty acids (FAs) and some COX inhibitors (e.g. naproxen) preferentially bind to the COX site of E(allo). AA can bind to E(cat) and E(allo), but the affinity of AA for E(allo) is 25 times that for E(cat). Palmitic acid, an efficacious stimulator of human PGHS-2, binds only E(allo) in palmitic acid/murine PGHS-2 co-crystals. Nonsubstrate FAs can potentiate or attenuate actions of COX inhibitors depending on the FA and whether the inhibitor binds E(cat) or E(allo). Our studies suggest that the concentration and composition of the free FA pool in the environment in which PGHS-2 functions in cells, the FA tone, is a key factor regulating PGHS-2 activity and its responses to COX inhibitors. We suggest that differences in FA tone occurring with different diets will likely affect both base-line prostanoid synthesis and responses to COX inhibitors.

The effect of dietary fat content on phospholipid fatty acid profile is muscle fiber type dependent

A high-saturated-fat diet (HFD) induces obesity and insulin resistance (IR). IR has been linked to alterations and increased saturation in the phospholipid composition of skeletal muscles. We aimed to determine whether HFD feeding affects fatty acid (FA) membrane profile in a muscle fiber type-specific manner. We measured phospholipid FAs and expression of FA synthesis genes in oxidative soleus (SOL) and glycolytic extensor digitorum longus (EDL) muscles from rats fed either standard chow (standard laboratory diet, SLD) or a HFD. The HFD increased fat mass, plasma insulin, and leptin levels. Compared with EDL, SOL muscles preferentially accumulated C18 over C16 FAs and n-6 over n-3 polyunsaturated FAs (PUFAs) on either diet. With the HFD, SOL muscles contained more n-9 monounsaturated FAs (MUFAs) and n-6 PUFAs and less n-7 MUFAs and n-3 PUFAs than EDL muscles and had lower unsaturation index, a pattern known to be associated with IR. Stearoyl-CoA desaturase-1 expression was approximately 13-fold greater in EDL than in SOL muscles but did not change with the HFD in either muscle. The expression of Elongase-5 was higher, and that of Elongase-6 (Elovl6) was lower in EDL compared with SOL muscles with both diets. In EDL muscles, the expression of Elovl6 was lower in the HFD than in the SLD. The pattern of FA uptake, expression, and diet-induced changes in FA desaturating and elongating enzymes maintained higher FA unsaturation in EDL muscles. Accordingly, the fiber type composition of skeletal muscles and their distribution may be important in the development and progression of obesity and IR.

Cyclooxygenase 2 inhibition exacerbates palmitate-induced inflammation and insulin resistance in skeletal muscle cells

Palmitate-induced inflammation is involved in the development of insulin resistance in skeletal muscle cells. Here we evaluated the effect of the saturated fatty acid palmitate and the monounsaturated fatty acid oleate on Toll-like receptors (TLR)-2 and -4 and cyclooxygenase 2 (COX-2) expression and examined whether the inhibition of this enzyme modulates fatty acid-induced inflammation. Skeletal muscle cells exposed to palmitate showed enhanced TLR-2 and COX-2 mRNA levels, whereas oleate did not modify their expression. Palmitate-induced expression of these genes was dependent on nuclear factor (NF)-kappaB activation, because expression was reduced in the presence of the NF-kappaB inhibitor parthenolide. Coincubation of palmitate-exposed cells with oleate also prevented the increase in the expression of TLR-2 and COX-2, through a mechanism that may involve activation of peroxisome proliferator-activated receptor-alpha (PPAR alpha) by this monounsaturated fatty acid. COX-2 inhibition by NS-398 enhanced IL-6 and TNF-alpha expression and IL-6 protein secretion induced by palmitate. NF-kappaB binding activity and TNF-alpha mRNA levels were enhanced in palmitate-exposed cells in the absence or in the presence of NS-398, whereas coincubation of palmitate-exposed cells with NS-398 and prostaglandin E(2) (PGE(2)) prevented these changes. In contrast, 12-lypoxygenase and cytochrome P450 hydroxylase pathways were not involved in these changes. Similarly, COX-2 inhibition impaired insulin-stimulated Akt phosphorylation and 2-deoxy-D-[(14)C]glucose uptake in palmitate-exposed skeletal muscle cells, and this effect was abolished in the presence of PGE(2). These findings indicate that COX-2 activity, through the production of PGE(2), attenuates the fatty acid-induced inflammatory process and insulin resistance.

Membrane Fatty Acid Transporters as Regulators of Lipid Metabolism: Implications for Metabolic Disease

Long-chain fatty acids and lipids serve a wide variety of functions in mammalian homeostasis, particularly in the formation and dynamic properties of biological membranes and as fuels for energy production in tissues such as heart and skeletal muscle. On the other hand, long-chain fatty acid metabolites may exert toxic effects on cellular functions and cause cell injury. Therefore, fatty acid uptake into the cell and intracellular handling need to be carefully controlled. In the last few years, our knowledge of the regulation of cellular fatty acid uptake has dramatically increased. Notably, fatty acid uptake was found to occur by a mechanism that resembles that of cellular glucose uptake. Thus, following an acute stimulus, particularly insulin or muscle contraction, specific fatty acid transporters translocate from intracellular stores to the plasma membrane to facilitate fatty acid uptake, just as these same stimuli recruit glucose transporters to increase glucose uptake. This regulatory mechanism is important to clear lipids from the circulation postprandially and to rapidly facilitate substrate provision when the metabolic demands of heart and muscle are increased by contractile activity. Studies in both humans and animal models have implicated fatty acid transporters in the pathogenesis of diseases such as the progression of obesity to insulin resistance and type 2 diabetes. As a result, membrane fatty acid transporters are now being regarded as a promising therapeutic target to redirect lipid fluxes in the body in an organ-specific fashion.

Evidence that alpha-calcitonin gene-related peptide is a neurohormone that controls systemic lipid availability and utilization

Alpha-calcitonin gene-related peptide (alphaCGRP) is released mainly from sensory and motor nerves in response to physiological stimuli. Despite well-documented pharmacological effects, its primary physiological role has thus far remained obscure. Increased lipid content, particularly in skeletal muscle and liver, is strongly implicated in the pathogenesis of insulin resistance, but the physiological regulation of organ lipid is imperfectly understood. Here we report our systematic investigations of the effects of alphaCGRP on in vitro and in vivo indices of lipid metabolism. In rodents, levels of alphaCGRP similar to those in the blood markedly stimulated fatty acid beta-oxidation and evoked concomitant mobilization of muscle lipid via receptor-mediated activation of muscle lipolysis. alphaCGRP exerted potent in vivo effects on lipid metabolism in muscle, liver, and the blood via receptor-mediated pathways. Studies with receptor antagonists were consistent with tonic regulation of lipid metabolism by an endogenous CGRP agonist. These data reveal that alphaCGRP is a newly recognized regulator of lipid availability and utilization in key tissues and that it may elevate the availability of intramyocellular free fatty acids to meet muscle energy requirements generated by contraction by evoking their release from endogenous triglyceride.

Docosahexaenoic acid and n-6 docosapentaenoic acid supplementation alter rat skeletal muscle fatty acid composition

Background

Docosahexaenoic acid (22:6n-3, DHA) and n-6 docosapentaenoic acid (22:5n-6, DPAn-6) are highly unsaturated fatty acids (HUFA, ≥ 20 carbons, ≥ 3 double bonds) that differ by a single carbon-carbon double bond at the Δ19 position. Membrane 22:6n-3 may support skeletal muscle function through optimal ion pump activity of sarcoplasmic reticulum and electron transport in the mitochondria. Typically n-3 fatty acid deficient feeding trials utilize linoleic acid (18:2n-6, LA) as a comparison group, possibly introducing a lower level of HUFA in addition to n-3 fatty acid deficiency. The use of 22:5n-6 as a dietary control is ideal for determining specific requirements for 22:6n-3 in various physiological processes. The incorporation of dietary 22:5n-6 into rat skeletal muscles has not been demonstrated previously. A one generation, artificial rearing model was utilized to supply 22:6n-3 and/or 22:5n-6 to rats from d2 after birth to adulthood. An n-3 fatty acid deficient, artificial milk with 18:2n-6 was supplemented with 22:6n-3 and/or 22:5n-6 resulting in four artificially reared (AR) dietary groups; AR-LA, AR-DHA, AR-DPAn-6, AR-DHA+DPAn-6. A dam reared group (DAM) was included as an additional control. Animals were sacrificed at 15 wks and soleus, white gastrocnemius and red gastrocnemius muscles were collected for fatty acid analyses.

Results

In all muscles of the DAM group, the concentration of 22:5n-6 was significantly lower than 22:6n-3 concentrations. While 22:5n-6 was elevated in the AR-LA group and the AR-DPAn-6 group, 20:4n-6 tended to be higher in the AR-LA muscles and not in the AR-DPAn-6 muscles. The AR-DHA+DPAn-6 had a slight, but non-significant increase in 22:5n-6 content. In the red gastrocnemius of the AR-DPAn-6 group, 22:5n-6 levels (8.1 ± 2.8 wt. %) did not reciprocally replace the 22:6n-3 levels observed in AR-DHA reared rats (12.2 ± 2.3 wt. %) suggesting a specific preference/requirement for 22:6n-3 in red gastrocnemius.

Conclusion

Dietary 22:5n-6 is incorporated into skeletal muscles and appears to largely compete with 22:6n-3 for incorporation into lipids. In contrast, 18:2n-6 feeding tends to result in elevations of 20:4n-6 and restrained increases of 22:5n-6. As such, 22:5n-6 dietary comparison groups may be useful in elucidating specific requirements for 22:6n-3 to support optimal health and disease prevention.

Palmitate induced mitochondrial deoxyribonucleic acid damage and apoptosis in l6 rat skeletal muscle cells

A major characteristic of type 2 diabetes mellitus (T2DM) is insulin resistance in skeletal muscle. A growing body of evidence indicates that oxidative stress that results from increased production of reactive oxygen species and/or reactive nitrogen species leads to insulin resistance, tissue damage, and other complications observed in T2DM. It has been suggested that muscular free fatty acid accumulation might be responsible for the mitochondrial dysfunction and insulin resistance seen in T2DM, although the mechanisms by which increased levels of free fatty acid lead to insulin resistance are not well understood. To help resolve this situation, we report that saturated fatty acid palmitate stimulated the expression of inducible nitric oxide (NO) synthase and the production of reactive oxygen species and NO in L6 myotubes. Additionally, palmitate caused a significant dose-dependent increase in mitochondrial DNA (mtDNA) damage and a subsequent decrease in L6 myotube viability and ATP levels at concentrations as low as 0.5 mM. Furthermore, palmitate induced apoptosis, which was detected by DNA fragmentation, caspase-3 cleavage, and cytochrome c release. N-acetyl cysteine, a precursor compound for glutathione formation, aminoguanidine, an inducible NO synthase inhibitor, and 5,10,15,20-tetrakis(4-sulphonatophenyl) porphyrinato iron (III), a peroxynitrite inhibitor, all prevented palmitate-induced mtDNA damage and diminished palmitate-induced cytotoxicity. We conclude that exposure of L6 myotubes to palmitate induced mtDNA damage and triggered mitochondrial dysfunction, which caused apoptosis. Additionally, our findings indicate that palmitate-induced mtDNA damage and cytotoxicity in skeletal muscle cells were caused by overproduction of peroxynitrite.

A Lupinoside prevented fatty acid induced inhibition of insulin sensitivity in 3T3 L1 adipocytes

The decrease in insulin sensitivity to target tissues or insulin resistance leads to type 2 diabetes mellitus, an insidious disease threatening global health. Numerous evidences made free fatty acids (FFAs) responsible for insulin resistance and type 2 diabetes. We demonstrate here that the damage of insulin acitivity by a free fatty acid, palmitate could be prevented by a lupinoside. An incubation of 3T3 L1 adipocytes with a FFA i.e. palmitate inhibited insulin stimulated uptake of (3)H-2 deoxyglucose (2 DOG) significantly. Addition of a lupinoside purified from Pueraria tuberosa, lupinoside PA(4) (LPA(4)) strongly prevented this inhibition. We then examined insulin signaling pathway where palmitate significantly inhibited insulin stimulated phosphorylation of Insulin receptor tyrosine kinase, IRS 1and PI3 kinase, PDK1 and Akt/PKB. LPA(4) rescued this inhibition of signaling molecule by palmitate. Insulin mediated translocation of Glut4, the glucose transporter in insulin target cells, was effectively blocked by palmitate while, LPA(4) waived this block. Administration of LPA(4) to nutritionally induced diabetic rats significantly reduced the increase in plasma glucose. All these indicate LPA(4) to be a potentially therapeutic agent for insulin resistance and type 2 diabetes.

Palmitate induces tumor necrosis factor-alpha expression in C2C12 skeletal muscle cells by a mechanism involving protein kinase C and nuclear factor-kappaB activation

The mechanisms responsible for increased expression of TNF-alpha in skeletal muscle cells in diabetic states are not well understood. We examined the effects of the saturated acid palmitate on TNF-alpha expression. Exposure of C2C12 skeletal muscle cells to 0.75 mm palmitate enhanced mRNA (25-fold induction, P < 0.001) and protein (2.5-fold induction) expression of the proinflammatory cytokine TNF-alpha. This induction was inversely correlated with a fall in GLUT4 mRNA levels (57% reduction, P < 0.001) and glucose uptake (34% reduction, P < 0.001). PD98059 and U0126, inhibitors of the ERK-MAPK cascade, partially prevented the palmitate-induced TNF-alpha expression. Palmitate increased nuclear factor (NF)-kappaB activation and incubation of the cells with the NF-kappaB inhibitors pyrrolidine dithiocarbamate and parthenolide partially prevented TNF-alpha expression. Incubation of palmitate-treated cells with calphostin C, a strong and specific inhibitor of protein kinase C (PKC), abolished palmitate-induced TNF-alpha expression, and restored GLUT4 mRNA levels. Palmitate treatment enhanced the expression of phospho-PKCtheta, suggesting that this PKC isoform was involved in the changes reported, and coincubation of palmitate-treated cells with the PKC inhibitor chelerythrine prevented the palmitate-induced reduction in the expression of IkappaBalpha and insulin-stimulated Akt activation. These findings suggest that enhanced TNF-alpha expression and GLUT4 down-regulation caused by palmitate are mediated through the PKC activation, confirming that this enzyme may be a target for either the prevention or the treatment of fatty acid-induced insulin resistance.

Palmitate-induced interleukin 6 production is mediated by protein kinase C and nuclear-factor kappaB activation and leads to glucose transporter 4 down-regulation in skeletal muscle cells

The mechanisms by which elevated levels of free fatty acids cause insulin resistance are not well understood. In addition, accumulating evidence suggests a link between inflammation and type 2 diabetes. Here, we report that exposure of C2C12 skeletal muscle cells to 0.5 mm palmitate results in increased mRNA levels (3.5-fold induction; P < 0.05) and secretion (control 375 +/- 57 vs. palmitate 1129 +/- 177 pg/ml; P < 0.001) of the proinflammatory cytokine IL-6. Palmitate increased nuclear factor-kappaB activation and coincubation of the cells with palmitate and the nuclear factor-kappaB inhibitor pyrrolidine dithiocarbamate prevented both IL-6 expression and secretion. Furthermore, incubation of palmitate-treated cells with calphostin C, a strong and specific inhibitor of protein kinase C, and phorbol myristate acetate, that down-regulates protein kinase C in long-term incubations, abolished induction of IL-6 production. Finally, exposure of skeletal muscle cells to palmitate caused a fall in the mRNA levels of glucose transporter 4 and insulin-stimulated glucose uptake, whereas in the presence of anti-IL-6 antibody, which neutralizes the biological activity of mouse IL-6 in cell culture, these reductions were prevented. These findings suggest that IL-6 may mediate several of the prodiabetic effects of palmitate.

Agonist-induced activation releases peroxisome proliferator-activated receptor β/δ from its inhibition by palmitate-induced nuclear factor-κB in skeletal muscle cells

The mechanisms by which elevated levels of free fatty acids cause insulin resistance are not well understood, but there is a strong correlation between insulin resistance and intramyocellular lipid accumulation in skeletal muscle. In addition, accumulating evidence suggests a link between inflammation and type 2 diabetes. The aim of this work was to study whether the exposure of skeletal muscle cells to palmitate affected peroxisome proliferator-activated receptor (PPAR) beta/delta activity. Here, we report that exposure of C2C12 skeletal muscle cells to 0.75 mM palmitate reduced (74%, P<0.01) the mRNA levels of the PPARbeta/delta-target gene pyruvatedehydrogenase kinase 4 (PDK-4), which is involved in fatty acid utilization. This reduction was not observed in the presence of the PPARbeta/delta agonist L-165041. This drug prevented palmitate-induced nuclear factor (NF)-kappaB activation. Increased NF-kappaB activity after palmitate exposure was associated with enhanced protein-protein interaction between PPARbeta/delta and p65. Interestingly, treatment with the PPARbeta/delta agonist L-165041 completely abolished this interaction. These results indicate that palmitate may reduce fatty acid utilization in skeletal muscle cells by reducing PPARbeta/delta signaling through increased NF-kappaB activity.

Effects of exercise on the fatty-acid composition of blood and tissue lipids

This article reviews the effects of acute and chronic exercise on the fatty-acid composition of animal and human tissues (plasma, skeletal muscle, heart, adipose tissue, liver, artery and erythrocytes), as reported in 68 studies spanning four decades. The most consistently observed effect has been an increase in the relative amount of unsaturated, especially monounsaturated, non-esterified fatty acids in plasma of both animals and humans after acute exercise. Chronic exercise seems to increase the proportion of polyunsaturated fatty acids and omega6 fatty acids, while decreasing the proportion of monounsaturated fatty acids in animal and human adipose tissue. Additionally, chronic exercise seems to decrease the relative amount of unsaturated fatty acids in liver lipids of animals and humans. There is no consensus regarding the effect of exercise on the fatty-acid composition of lipids in any other tissue. In general, the effects of exercise are independent of nutrition and, regarding skeletal muscle, muscle fibre type. The available literature shows that, in addition to modifying the concentrations of animal and human tissue lipids, exercise also changes their fatty-acid profile. Unfortunately, the available studies are so much divided among exercise models, species and biological samples that a cohesive picture of the plasticity of the fatty-acid pattern of most tissues toward exercise has not emerged. Future studies should focus on determining the fatty-acid profile of separate lipid classes (rather than total lipids) in separate subcellular fractions (rather than whole tissues), examining tissues and organs on which no data are available and exploring the mechanisms of the exercise-induced changes in fatty-acid composition.

Acid ceramidase overexpression prevents the inhibitory effects of saturated fatty acids on insulin signaling

Recent studies indicate that insulin resistance and type 2 diabetes result from the accumulation of lipids in tissues not suited for fat storage, such as skeletal muscle and the liver. To elucidate the mechanisms linking exogenous fats to the inhibition of insulin action, we evaluated the effects of free fatty acids (FFAs) on insulin signal transduction in cultured C2C12 myotubes. As we described previously (Chavez, J. A., and Summers, S. A. (2003) Arch. Biochem. Biophys. 419, 101-109), long-chain saturated FFAs inhibited insulin stimulation of Akt/protein kinase B, a central regulator of glucose uptake and anabolic metabolism. Moreover, these FFAs stimulated the de novo synthesis of ceramide and sphingosine, two sphingolipids shown previously to inhibit insulin action. To determine the contribution of either sphingolipid in FFA-dependent inhibition of insulin action, we generated C2C12 myotubes that constitutively overexpress acid ceramidase (AC), an enzyme that catalyzes the lysosomal conversion of ceramide to sphingosine. AC overexpression negated the inhibitory effects of saturated FFAs on insulin signaling while blocking their stimulation of ceramide accumulation. By contrast, AC overexpression stimulated the accrual of sphingosine. These results support a role for aberrant accumulation of ceramide, but not sphingosine, in the inhibition of muscle insulin sensitivity by exogenous FFAs.

Stearoyl-CoA desaturase-1 deficiency reduces ceramide synthesis by downregulating serine palmitoyltransferase and increasing beta-oxidation in skeletal muscle

Stearoyl-CoA desaturase (SCD) has recently been shown to be a critical control point of lipid partitioning and body weight regulation. Lack of SCD1 function significantly increases insulin sensitivity in skeletal muscles and corrects the hypometabolic phenotype of leptin-deficient ob/ob mice, indicating the direct antilipotoxic action of SCD1 deficiency. The mechanism underlying the metabolic effects of SCD1 mutation is currently unknown. Here we show that SCD1 deficiency reduced the total ceramide content in oxidative skeletal muscles (soleus and red gastrocnemius) by approximately 40%. The mRNA levels and activity of serine palmitoyltransferase (SPT), a key enzyme in ceramide synthesis, as well as the incorporation of [14C]palmitate into ceramide were decreased by approximately 50% in red muscles of SCD1-/- mice. The content of fatty acyl-CoAs, which contribute to de novo ceramide synthesis, was also reduced. The activity and mRNA levels of carnitine palmitoyltransferase I (CPT I) and the rate of beta-oxidation were increased in oxidative muscles of SCD1-/- mice. Furthermore, SCD1 deficiency increased phosphorylation of AMP-activated protein kinase (AMPK), suggesting that AMPK activation may be partially responsible for the increased fatty acid oxidation and decreased ceramide synthesis in red muscles of SCD1-/- mice. SCD1 deficiency also reduced SPT activity and ceramide content and increased AMPK phosphorylation and CPT I activity in muscles of ob/ob mice. Taken together, these results indicate that SCD1 deficiency reduces ceramide synthesis by decreasing SPT expression and increasing the rate of beta-oxidation in oxidative muscles.

Fiber type- and fatty acid composition-dependent effects of high-fat diets on rat muscle triacylglyceride and fatty acid transporter protein-1 content

Intramuscular triacylglyceride (TAG) is considered an independent marker of insulin resistance in humans. Here, we examined the effect of high-fat diets, based on distinct fatty acid compositions (saturated, monounsaturated or n-6 polyunsaturated), on TAG levels and fatty acid transporter protein (FATP-1) expression in 2 rat muscles that differ in their fiber type, soleus, and gastrocnemius; the relationship to whole body glucose intolerance was also studied. Compared with carbohydrate-fed rats, the groups subjected to any one of the high-fat diets consistently exhibited enhanced body weight gain and adiposity, elevated plasma free fatty acids and TAG in the fed condition, hyperinsulinemia, and glucose intolerance. TAG content was consistently higher in soleus than in gastrocnemius, but was only significantly elevated by the n-6 polyunsaturated-based diet. FATP-1 levels in soleus were double those in gastrocnemius muscle in carbohydrate-fed animals. High-fat diets caused an elevation in FATP-1 protein content in soleus, but a reduction in gastrocnemius. In conclusion, the hyperinsulinemic hyperlipidemic condition upregulates FATP-1 expression in soleus and downregulates that of gastrocnemius. Hypercaloric saturated, monounsaturated, or n-6 polyunsaturated lipid diets cause equivalent whole body insulin resistance in rats, but only an n-6 polyunsaturated acid-based diet triggers intramuscular TAG accumulation.

Liver fatty acid-binding protein colocalizes with peroxisome proliferator activated receptor alpha and enhances ligand distribution to nuclei of living cells

Although it is hypothesized that long-chain fatty acyl CoAs (LCFA-CoAs) and long-chain fatty acids (LCFAs) regulate transcription in the nucleus, little is known regarding factors that determine the distribution of these ligands to nuclei of living cells. Immunofluorescence colocalization showed that liver fatty acid-binding protein (L-FABP; binds LCFA-CoA as well as LCFA) significantly colocalized with PPARalpha in nuclei of transfected L-cell fibroblasts. Colocalization with a DNA binding dye (SYTO59) revealed that, within the nucleus of control L-cells, the nonhydrolyzable fluorescent LCFA-CoA (BODIPY-C16-S-S-CoA) was distributed primarily in a punctate pattern throughout the nucleoplasm, while nonmetabolizable fluorescent LCFAs (BODIPY-C16 and BODIPY-C12) were localized primarily near the nuclear envelope membranes. L-FABP overexpression selectively increased the targeting of BODIPY-C16-S-S-CoA by 1.9- and 2.7-fold into the nuclear membrane and nucleoplasm, respectively. L-FABP also increased the targeting of fluorescent LCFAs (especially long-chain-length BODIPY-C16) by 1.7-fold to the nuclear membrane and 7.4-fold into the nucleoplasm. A cis-parinaric acid displacement assay showed that L-FABP bound BODIPY-C12 and BODIPY-C16 with K(i)s of 10.1 +/- 2.5 and 20.7 +/- 1.5 nM, respectively, in the same range as naturally occurring LCFAs. Finally, solid-phase extraction and HPLC analysis revealed that, depending on the fatty acid content of the culture medium, L-FABP expression also increased the cellular LCFA-CoA pool size and altered the LCFA-CoA acyl chain composition. Thus, L-FABP may function as a carrier for selectively enhancing the distribution of LCFA-CoA, as well as LCFA, to nuclei for potential interaction with nuclear receptors.

Exercise alters the profile of phospholipid molecular species in rat skeletal muscle

We have determined the effect of two exercise-training intensities on the phospholipid profile of both glycolytic and oxidative muscle fibers of female Sprague-Dawley rats using electrospray-ionization mass spectrometry. Animals were randomly divided into three training groups: control, which performed no exercise training; low-intensity (8 m/min) treadmill running; or high-intensity (28 m/min) treadmill running. All exercise-trained rats ran 1,000 m/session for 4 days/wk for 4 wk and were killed 48 h after the last training bout. Exercise training was found to produce no novel phospholipid species but was associated with significant alterations in the relative abundance of a number of phospholipid molecular species. These changes were more prominent in glycolytic (white vastus lateralis) than in oxidative (red vastus lateralis) muscle fibers. The largest observed change was a decrease of approximately 20% in the abundance of 1-stearoyl-2-docosahexaenoyl-phosphatidylethanolamine [PE(18:0/22:6); P < 0.001] ions in both the low- and high-intensity training regimes in glycolytic fibers. Increases in the abundance of 1-oleoyl-2-linoleoyl phopshatidic acid [PA(18:1/18:2); P < 0.001] and 1-alkenylpalmitoyl-2-linoleoyl phosphatidylethanolamine [plasmenyl PE (16:0/18:2); P < 0.005] ions were also observed for both training regimes in glycolytic fibers. We conclude that exercise training results in a remodeling of phospholipids in rat skeletal muscle. Even though little is known about the physiological or pathophysiological role of specific phospholipid molecular species in skeletal muscle, it is likely that this remodeling will have an impact on a range of cellular functions.

Characterizing the effects of saturated fatty acids on insulin signaling and ceramide and diacylglycerol accumulation in 3T3-L1 adipocytes and C2C12 myotubes

A strong correlation between intramyocellular lipid concentrations and the severity of insulin resistance has fueled speculation that lipid oversupply to skeletal muscle, fat, or liver may desensitize these tissues to the anabolic effects of insulin. To identify free fatty acids (FFAs) capable of inhibiting insulin action, we treated 3T3-L1 adipocytes or C2C12 myotubes with either the saturated FFA palmitate (C16:0) or the monounsaturated FFA oleate (C18:1), which were shown previously to be the most prevalent FFAs in rat soleus and gastrocnemius muscles. In C2C12 myotubes, palmitate, but not oleate, inhibited insulin-stimulation of glycogen synthesis, as well as its activation of Akt/Protein Kinase B (PKB), an obligate intermediate in the regulation of anabolic metabolism. Palmitate also induced the accrual of ceramide and diacylglycerol (DAG), two lipid metabolites that have been shown to inhibit insulin signaling in cultured cells and to accumulate in insulin resistant tissues. Interestingly, in 3T3-L1 adipocytes, neither palmitate nor oleate inhibited glycogen synthesis or Akt/PKB activation, nor did they induce ceramide or DAG synthesis. Using myotubes, we also tested whether other saturated fatty acids blocked insulin signaling while promoting ceramide and DAG accumulation. The long-chain fatty acids stearate (18:0), arachidate (20:0), and lignocerate (24:0) reproduced palmitate's effects on these events, while saturated fatty acids with shorter hydrocarbon chains [i.e., laurate (12:0) and myristate (14:0)] failed to induce ceramide accumulation or inhibit Akt/PKB activation. Collectively these findings implicate excess delivery of long-chain fatty acids in the development of insulin resistance resulting from lipid oversupply to skeletal muscle.

A role for ceramide, but not diacylglycerol, in the antagonism of insulin signal transduction by saturated fatty acids

Multiple studies suggest that lipid oversupply to skeletal muscle contributes to the development of insulin resistance, perhaps by promoting the accumulation of lipid metabolites capable of inhibiting signal transduction. Herein we demonstrate that exposing muscle cells to particular saturated free fatty acids (FFAs), but not mono-unsaturated FFAs, inhibits insulin stimulation of Akt/protein kinase B, a serine/threonine kinase that is a central mediator of insulin-stimulated anabolic metabolism. These saturated FFAs concomitantly induced the accumulation of ceramide and diacylglycerol, two products of fatty acyl-CoA that have been shown to accumulate in insulin-resistant tissues and to inhibit early steps in insulin signaling. Preventing de novo ceramide synthesis negated the antagonistic effect of saturated FFAs toward Akt/protein kinase B. Moreover, inducing ceramide buildup recapitulated and augmented the inhibitory effect of saturated FFAs. By contrast, diacylglycerol proved dispensable for these FFA effects. Collectively these results identify ceramide as a necessary and sufficient intermediate linking saturated fats to the inhibition of insulin signaling.

Liver fatty acid-binding protein targets fatty acids to the nucleus. Real time confocal and multiphoton fluorescence imaging in living cells

Although unesterified long chain fatty acids interact with peroxisome proliferator-activated receptors to initiate transcription within the nucleus, almost nothing is known regarding factors regulating long chain fatty acid distribution to the nucleus of living cells. The possibility that the liver fatty acid-binding protein (L-FABP) may function in this role was addressed in transfected L-cell fibroblasts overexpressing L-FABP using a series of fluorescent fatty acids differing in chain length and unsaturation. After 30 min of incubation, oxidation of BODIPY-, NBD-, and cis-parinaric acids was undetectable in L-cells. Likewise, L-cells very poorly esterified these fluorescent fatty acids in the following order: 0% BODIPY-C5, NBD-C6 (short chain length) < 0-3% NBD-C18, BODIPY-C16, cis-parinaric acid (long chain length) < 11% BODIPY-C12 (medium chain length). Real time confocal and multiphoton laser scanning microscopy (CLSM and MPLSM) showed that these fluorescent fatty acids were generally taken up in the following order: long chain (BODIPY-C16, NBD-C18) > medium chain (BODIPY-C12) short chain (BODIPY-C5, NBD-C6). The fluorescent fatty acids were imaged in the nucleus, primarily associated with the nuclear envelope, at levels about 2-3-fold lower than outside the nucleus. CLSM and MPLSM showed that L-FABP expression enhanced by 2-4-fold the initial rate and/or average maximal uptake of the long and medium chain but not the short chain fluorescent fatty acids in living cells. Furthermore, L-FABP expression increased the targeting of long and medium but not short chain fluorescent fatty acids to the nucleus by 2.9-4.4-fold and increased the proportion (i.e. nuclear:cytoplasm ratio) of medium and long chain but not short chain fatty acids by 2-3.6-fold. In summary, these results showed for the first time the presence of unesterified fatty acids in the nucleus of living cells and demonstrated that expression of a fatty acid-binding protein, L-FABP, specifically enhanced uptake and intracellular targeting of long and medium chain fatty acids to the nucleus.

Fat content in individual muscle fibers of lean and obese subjects

OBJECTIVE

To examine skeletal muscle intracellular triglyceride concentration in different fiber types in relation to obesity.

DESIGN

Skeletal muscle fiber type distribution and intracellular lipid content were measured in vastus lateralis samples obtained by needle biopsy from lean and obese individuals.

SUBJECTS

Seven lean controls (body mass index (BMI) 23.0+/-3.3 kg/m(2); mean+/-s.d.) and 14 obese (BMI 33.7+/-2.7 kg/m(2)) individuals; both groups included comparable proportions of men and women.

MEASUREMENTS

Samples were histochemically stained for the identification of muscle fiber types (myosin ATPase) and intracellular lipid aggregates (oil red O dye). The number and size of fat aggregates as well as their concentration within type I, IIA and IIB muscle fiber types were measured. The cellular distribution of the lipid aggregates was also examined.

RESULTS

The size of fat aggregates was not affected by obesity but the number of lipid droplets within muscle fibers was twice as abundant in obese compared to lean individuals. This was seen in type I (298+/-135 vs 129+/-75; obese vs lean, P<0.05), IIA (132+/-67 vs 79+/-29; P<0.05), and IIB (103+/-63 vs 51+/-13; P<0.05) muscle fibers. A more central distribution of lipid droplets was observed in muscle fibers of obese compared to lean subjects (27.2+/-5.7 vs 19.7+/-6.4%; P<0.05).

CONCLUSION

The higher number of lipid aggregates and the disposition to a greater central distribution in all fiber types in obesity indicate important changes in lipid metabolism and/or storage that are fiber type-independent.

Signalling aspects of insulin resistance in skeletal muscle: mechanisms induced by lipid oversupply

A reduced capacity for insulin to elicit increases in glucose uptake and metabolism in target tissues such as skeletal muscle is a common feature of obesity and diabetes. The association between lipid oversupply and such insulin resistance is well established, and evidence for mechanisms through which lipids could play a causative role in the generation of muscle insulin resistance is reviewed. While the effects of lipids may in part be mediated by substrate competition through the glucose-fatty acid cycle, interference with insulin signal transduction by lipid-activated signalling pathways is also likely to play an important role. Thus, studies of insulin resistance in Type 2 diabetes, obesity, fat-fed animals and lipid-treated cells have identified defects both at the level of insulin receptor-mediated tyrosine phosphorylation and at downstream sites such as protein kinase B (PKB) activation. Lipid signalling molecules can be derived from free fatty acids, and include diacylglycerol, which activates isozymes of the protein kinase C (PKC) family, and ceramide, which has several effectors including PKCs and a protein phosphatase. In addition, elevated lipid availability can increase flux through the hexosamine biosynthesis pathway which can also lead to activation of PKC as well as protein glycosylation and modulation of gene expression. The mechanisms giving rise to decreased insulin signalling include serine/threonine phosphorylation of insulin receptor substrate-1, but also direct inhibition of components such as PKB. Thus lipids can inhibit glucose disposal by causing interference with insulin signal transduction, and most likely by more than one pathway depending on the prevalent species of fatty acids.

Long-chain acyl-CoA esters as indicators of lipid metabolism and insulin sensitivity in rat and human muscle

Long-chain acyl-CoAs (LCACoA) are an activated lipid species that are key metabolites in lipid metabolism; they also have a role in the regulation of other cellular processes. However, few studies have linked LCACoA content in rat and human muscle to changes in nutritional status and insulin action. Fasting rats for 18 h significantly elevated the three major LCACoA species in muscle (P < 0.001), whereas high-fat feeding of rats with a safflower oil (18:2) diet produced insulin resistance and increased total LCACoA content (P < 0.0001) by specifically increasing 18:2-CoA. The LCACoA content of red muscle from rats (4-8 nmol/g) was 4- to 10-fold higher than adipose tissue (0.4-0.9 nmol/g, P < 0.001), suggesting that any contamination of muscle samples with adipocytes would contribute little to the LCACoA content of muscle. In humans, the LCACoA content of muscle correlated significantly with a measure of whole body insulin action in 17 male subjects (r(2) = 0.34, P = 0.01), supporting a link between muscle lipid metabolism and insulin action. These results demonstrate that the LCACoA pool reflects lipid metabolism and nutritional state in muscle. We conclude that the LCACoA content of muscle provides a direct index of intracellular lipid metabolism and its links to insulin action, which, unlike triglyceride content, is not subject to contamination by closely associated adipose tissue.

Ceramide generation is sufficient to account for the inhibition of the insulin-stimulated PKB pathway in C2C12 skeletal muscle cells pretreated with palmitate

We have employed C2C12 myotubes to investigate lipid inhibition of insulin-stimulated signal transduction and glucose metabolism. Cells were preincubated for 18 h in the absence or presence of free fatty acids (FFAs) and stimulated with insulin, and the effects on glycogen synthesis and signaling intermediates were determined. While the unsaturated FFAs oleate and linoleate inhibited both basal and insulin-stimulated glycogen synthesis, the saturated FFA palmitate reduced only insulin-stimulated glycogen synthesis, and was found to inhibit insulin-stimulated phosphorylation of glycogen synthase kinase-3 and protein kinase B (PKB). However, no effect of palmitate was observed on tyrosine phosphorylation, p85 association, or phosphatidylinositol 3-kinase activity in IRS-1 immunoprecipitates. In contrast, palmitate promoted phosphorylation of mitogen-activated protein MAP) kinases. Ceramide, a derivative of palmitate, has recently been associated with similar inhibition of PKB, and here, ceramide levels were found to be elevated 2-fold in palmitate-treated C2C12 cells. Incubation of C2C12 cells with ceramide closely reproduced the effects of palmitate, leading to inhibition of glycogen synthesis and PKB and to stimulation of MAP kinase. We conclude that palmitate-induced insulin resistance occurs by a mechanism distinct from that of unsaturated FFAs, and involves elevation of ceramide by de novo synthesis, leading to PKB inhibition without affecting IRS-1 function.