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We aimed to determine the combined effects of overexpressing plasma membrane fatty acid binding protein (FABPpm) and fatty acid translocase (CD36) on skeletal muscle fatty acid transport to establish if these transport proteins function collaboratively. Electrotransfection with either FABPpm or CD36 increased their protein content at the plasma membrane (+75% and +64%), increased fatty acid transport rates by +24% for FABPpm and +62% for CD36, resulting in a calculated transport efficiency of (0.019 and (0.053 per unit protein change for FABPpm and CD36, respectively. We subsequently used these data to determine if increasing both proteins additively or synergistically increased fatty acid transport. Cotransfection of FABPpm and CD36 simultaneously increased protein content in whole muscle (FABPpm, +46%; CD36, +45%) and at the sarcolemma (FABPpm, +41%; CD36, +42%), as well as fatty acid transport rates (+50%). Since the relative effects of changing FABPpm and CD36 content had been independently determined, we were able to a predict a change in fatty acid transport based on the overexpression of plasmalemmal transporters in the cotransfection experiments. This prediction yielded an increase in fatty acid transport of +0.984 and +1.722 pmol/mg prot/15 s for FABPpm and CD36, respectively, for a total increase of +2.96 pmol/mg prot/15 s. This calculated determination was remarkably consistent with the measured change in transport, namely +2.89 pmol/mg prot/15 s. Altogether, these data indicate that increasing CD36 and FABPpm alters fatty acid transport rates additively, but not synergistically, suggesting an independent mechanism of action within muscle for each transporter. This conclusion was further supported by the observation that plasmalemmal CD36 and FABPpm did not coimmunoprecipitate. [ABSTRACT FROM AUTHOR]

Copyright of Applied Physiology, Nutrition & Metabolism is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Leptin stimulates fatty acid oxidation in muscle and heart; but, the mechanism by which these tissues provide additional intracellular fatty acids for their oxidation remains unknown. We examined, in isolated muscle and cardiac myocytes, whether leptin, via AMP-activated protein kinase (AMPK) activation, stimulated fatty acid translocase (FAT/ CD36)-mediated fatty acid uptake to enhance fatty acid oxidation. In both mouse skeletal muscle and rat cardiomyocytes, leptin increased fatty acid oxidation, an effect that was blocked when AMPK phosphorylation was inhibited by adenine 9-β-D-arabinofuranoside or Compound C. In wild-type mice, leptin induced the translocation of FAT/CD36 to the plasma membrane and increased fatty acid uptake into giant sarcolemmal vesicles and into cardiomyocytes. In muscles of FAT/CD36-KO mice, and in cardiomyocytes in which cell surface FAT/CD36 action was blocked by sulfo-N-succinimidyl oleate, the leptin-stimulated influx of fatty acids was inhibited; concomitantly, the normal leptin-stimulated increase in fatty acid oxidation was also prevented, despite the normal leptin-induced increase in AMPK phosphorylation. Conversely, in muscle of AMPK kinase-dead mice, leptin failed to induce the translocation of FAT/CD36, along with a failure to stimulate fatty acid uptake and oxidation. Similarly, when siRNA was used to reduce AMPK in HL-1 cardiomyocytes, leptin failed to induce the translocation of FAT/CD36. Our studies have revealed a novel mechanism of leptin-induced fatty acid oxidation in muscle tissue; namely, this process is dependent on the activation of AMPK to induce the translocation of FAT/CD36 to the plasma membrane, thereby stimulating fatty acid uptake. Without increasing this leptin-stimulated, FAT/CD36-dependent fatty acid uptake process, leptinstimulated AMPK phosphorylation does not enhance fatty acid oxidation. [ABSTRACT FROM AUTHOR]

Aims/hypothesis: The mechanisms for diet-induced intramyocellular lipid accumulation and its association with insulin resistance remain contentious. In a detailed time-course study in rats, we examined whether a high-fat diet increased intramyocellular lipid accumulation via alterations in fatty acid translocase (FAT/CD36)-mediated fatty acid transport, selected enzymes and/or fatty acid oxidation, and whether intramyocellular lipid accretion coincided with the onset of insulin resistance. Methods: We measured, daily (on days 1-7) and/or weekly (for 6 weeks), the diet-induced changes in circulating substrates, insulin, sarcolemmal substrate transporters and transport, selected enzymes, intramyocellular lipids, mitochondrial fatty acid oxidation and basal and insulin-stimulated sarcolemmal GLUT4 and glucose transport. We also examined whether upregulating fatty acid oxidation improved glucose transport in insulin-resistant muscles. Finally, in Cd36-knockout mice, we examined the role of FAT/CD36 in intramyocellular lipid accumulation, insulin sensitivity and diet-induced glucose intolerance. Results: Within 2-3 days, diet-induced increases occurred in insulin, sarcolemmal FAT/CD36 (but not fatty acid binding protein [FABPpm] or fatty acid transporter [FATP]1 or 4), fatty acid transport and intramyocellular triacylglycerol, diacylglycerol and ceramide, independent of enzymatic changes or muscle fatty acid oxidation. Diet-induced increases in mitochondria and mitochondrial fatty acid oxidation and impairments in insulin-stimulated glucose transport and GLUT4 translocation occurred much later (≥21 days). FAT/CD36 ablation impaired insulin-stimulated fatty acid transport and lipid accumulation, improved insulin sensitivity and prevented diet-induced glucose intolerance. Increasing fatty acid oxidation in insulin-resistant muscles improved glucose transport. Conclusions/interpretations: High-fat feeding rapidly increases intramyocellular lipids (in 2-3 days) via insulin-mediated upregulation of sarcolemmal FAT/CD36 and fatty acid transport. The 16-19 day delay in the onset of insulin resistance suggests that additional mechanisms besides intramyocellular lipids contribute to this pathology. [ABSTRACT FROM AUTHOR]

Muscle contains various fatty acid transporters (CD36, FABPpm, FATP1, FATP4). Physiological stimuli (insulin, contraction) induce the translocation of all four transporters to the sarcolemma to enhance fatty acid uptake similarly to glucose uptake stimulation via glucose transporter-4 (GLUT4) translocation. Akt2 mediates insulin-induced, but not contraction-induced, GLUT4 translocation, but its role in muscle fatty acid transporter translocation is unknown. In muscle from Akt2-knockout mice, we observed that Akt2 is critically involved in both insulin-induced and contraction-induced fatty acid transport and translocation of fatty acid translocase/CD36 (CD36) and FATP1, but not of translocation of fatty acid-binding protein (FABPpm) and FATP4. Instead, Akt2 mediates intracellular retention of both latter transporters. Collectively, our observations reveal novel complexities in signaling mechanisms regulating the translocation of fatty acid transporters in muscle. [ABSTRACT FROM AUTHOR]

Exercise has been shown to induce the translocation of fatty acid translocase (FAT/CD36), a fatty acid transport protein, to both plasma and mitochondrial membranes. While previous studies have examined signals involved in the induction of FAT/CD36 translocation to sarcolemmal membranes, to date the signaling events responsible for FAT/CD36 accumulation on mitochondrial membranes have not been investigated. In the current study muscle contraction rapidly increased FAT/CD36 on plasma membranes (7.5 minutes), while in contrast, FAT/CD36 only increased on mitochondrial membranes after 22.5 minutes of muscle contraction, a response that was exercise-intensity dependent. Considering that previous research has shown that AMP activated protein kinase (AMPK) α2 is not required for FAT/CD36 translocation to the plasma membrane, we investigated whether AMPK α2 signaling is necessary for mitochondrial FAT/CD36 accumulation. Administration of 5-Aminoimidazole-4-carboxamide ribonucleotide (AICAR) induced AMPK phosphorylation, and resulted in FAT/CD36 accumulation on SS mitochondria, suggesting AMPK signaling may mediate this response. However, SS mitochondrial FAT/CD36 increased following acute treadmill running in both wild-type (WT) and AMPKα 2 kinase dead (KD) mice. These data suggest that AMPK signaling is not required for SS mitochondrial FAT/CD36 accumulation. The current data also implicates alternative signaling pathways that are exercise-intensity dependent, as IMF mitochondrial FAT/CD36 content only occurred at a higher power output. Taken altogether the current data suggests that activation of AMPK signaling is sufficient but not required for exercise-induced accumulation in mitochondrial FAT/CD36. [ABSTRACT FROM AUTHOR]

Activation of AMP-activated protein kinase (AMPK) in cardiomyocytes induces translocation of glucose transporter GLUT4 and long-chain fatty acid (LCFA) transporter CD36 from endosomal stores to the sarcolemma to enhance glucose and LCFA uptake, respectively. Ca2+/calmodulinactivated kinase kinase-β (CaMKKβ) has been positioned directly upstream of AMPK. However, it is unknown whether acute increases in [Ca2+]i stimulate translocation of GLUT4 and CD36 and uptake of glucose and LCFA or whether Ca2+ signaling converges with AMPK signaling to exert these actions. Therefore, we studied the interplay between Ca2+ and AMPK signaling in regulation of cardiomyocyte substrate uptake. Exposure of primary cardiomyocytes to inhibitors or activators of Ca2+ signaling affected neither AMPK-Thr172 phosphorylation nor basal and AMPK-mediated glucose and LCFA uptake. Despite their lack of an effect on substrate uptake, Ca2+ signaling activators induced GLUT4 and CD36 translocation. In contrast, AMPK activators stimulated GLUT4/CD36 translocation as well as glucose/LCFA uptake. When cardiomyocytes were cotreated with Ca2+ signaling and AMPK activators, Ca2+ signaling activators further enhanced AMPK-induced glucose/LCFA uptake. In conclusion, Ca2+ signaling shows no involvement in AMPK-induced GLUT4/CD36 translocation and substrate uptake but elicits transporter translocation via a separate pathway requiring CaMKKβ/ CaMKs. Ca2+-induced transporter translocation by itself appears to be ineffective to increase substrate uptake but requires additional AMPK activation to effectuate transporter translocation into increased substrate uptake. Ca2+-induced transporter translocation might be crucial under excessive cardiac stress conditions that require supraphysiological energy demands. Alternatively, Ca2+ signaling might prepare the heart for substrate uptake during physiological contraction by inducing transporter translocation. [ABSTRACT FROM AUTHOR]

Endurance exercise relies on transsarcolemmal flux of substrates in order to avoid depletion of intramuscular reserves. Previous studies of endurance trained sled dogs have shown a remarkable capacity of these dogs to adapt rapidly to endurance exercise by decreasing the utilization of intramuscular reserves. The current study tested the hypothesis that the dogs' glycogen-sparing phenotype is due to increased sarcolemmal transport of glucose and fatty acids. Basal and exercise-induced transport of glucose and fatty acids into sarcolemmal vesicles was evaluated in racing sled dogs prior to and after 7 months of exercise conditioning. Sarcolemmal substrate transport capacity was measured using sarcolemmal vesicles and radiolabelled substrates, and transporter abundance was measured using Western blot quantification in whole muscle homogenates and the sarcolemmal vesicle preparations. Conditioning resulted in increased basal and exercise-induced transport of both glucose and palmitate. Neither acute exercise nor conditioning resulted in changes in muscle content of GLUT4 or FAT/CD36, but conditioning did result in decreased abundance of both transporters in the sarcolemmal vesicles used for the basal transport assays, and this decrease was further amplified in the vesicles used for the exercise-induced transport assays. These results demonstrate conditioning-induced increases in sarcolemmal transport of oxidizable substrates, as well as increased gain of exercise-induced sarcolemmal transport of these substrates. These results further indicate that increased sarcolemmal transport of oxidizable substrates may be due to either an increased intrinsic capacity of the existing transporters or to a different population of transporters from those investigated. [ABSTRACT FROM AUTHOR]

Calcium/calmodulin-dependent protein kinase (CaMK) activation induces mitochondrial biogenesis in response to increasing cytosolic calcium concentrations. Calcium leak from the ryanodine receptor (RyR) is regulated by reactive oxygen species (ROS), which is increased with high-fat feeding. We examined whether ROS-induced CaMKII-mediated signaling induced skeletal muscle mitochondrial biogenesis in selected models of lipid oversupply. In obese Zucker rats and high-fat-fed rodents, in which muscle mitochondrial content was upregulated, CaMKII phosphorylation was increased independent of changes in calcium uptake because sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) protein expression or activity was not altered, implicating altered sarcoplasmic reticulum (SR) calcium leak in the activation of CaMKII. In support of this, we found that high-fat feeding increased mitochondrial ROS emission and S-nitrosylation of the RyR, whereas hydrogen peroxide induced SR calcium leak from the RyR and activation of CaMKII. Moreover, administration of a mitochondrial-specific antioxidant, SkQ, prevented high-fat diet-induced phosphorylation of CaMKII and the induction of mitochondrial biogenesis. Altogether, these data suggest that increased mitochondrial ROS emission is required for the induction of SR calcium leak, activation of CaMKII, and induction of mitochondrial biogenesis in response to excess lipid availability. [ABSTRACT FROM AUTHOR]

Calcium/calmodulin-dependent protein kinase (CaMK) activation induces mitochondrial biogenesis in response to increasing cytosolic calcium concentrations. Calcium leak from the ryanodine receptor (RyR) is regulated by reactive oxygen species (ROS), which is increased with high-fat feeding. We examined whether ROS-induced CaMKII-mediated signaling induced skeletal muscle mitochondrial biogenesis in selected models of lipid oversupply. In obese Zucker rats and high-fat–fed rodents, in which muscle mitochondrial content was upregulated, CaMKII phosphorylation was increased independent of changes in calcium uptake because sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) protein expression or activity was not altered, implicating altered sarcoplasmic reticulum (SR) calcium leak in the activation of CaMKII. In support of this, we found that high-fat feeding increased mitochondrial ROS emission and S-nitrosylation of the RyR, whereas hydrogen peroxide induced SR calcium leak from the RyR and activation of CaMKII. Moreover, administration of a mitochondrial-specific antioxidant, SkQ, prevented high-fat diet–induced phosphorylation of CaMKII and the induction of mitochondrial biogenesis. Altogether, these data suggest that increased mitochondrial ROS emission is required for the induction of SR calcium leak, activation of CaMKII, and induction of mitochondrial biogenesis in response to excess lipid availability. [ABSTRACT FROM AUTHOR]

Aims/hypothesis: Although insulin resistance has been associated with accumulations of specific intramuscular fatty acids and altered subcellular localisation of lipid droplets, these concepts remain controversial. Therefore, we aimed to identify specific intramuscular fatty acids and subcellular lipid localisations associated with improved insulin sensitivity following chronic muscle contraction. Methods: In lean and insulin-resistant obese Zucker rats the tibialis anterior muscle was stimulated (6 h/day for 6 days). Thereafter, muscles were examined for insulin sensitivity, intramuscular lipid droplet localisation and triacylglycerol (TAG), diacylglycerol (DAG) and ceramide fatty acid composition. Results: In lean and obese animals, regardless of muscle type, chronic muscle contraction improved muscle insulin sensitivity and increased intramuscular levels of total and most C14-C22 TAG fatty acids ( p < 0.05). Therefore, accumulation in subcellular lipid droplet compartments reflected the oversupply of lipids within muscle. In contrast, improvements in insulin sensitivity induced by muscle contraction were associated with reductions in specific DAG and ceramide species that were not uniform in red and white muscle of obese rats. However, these reductions were insufficient to fully normalise insulin sensitivity, indicating that other mechanisms are involved. Conclusions/interpretation: Reductions in 18 C length DAG and ceramide species were the most consistent in red and white muscle and therefore may represent therapeutic targets for improving insulin sensitivity. [ABSTRACT FROM AUTHOR]

Regulation of skeletal muscle fatty acid oxidation (FAO) and adaptation to exercise training have long been thought to depend on delivery of fatty acids (FAs) to muscle, their diffusion into muscle, and muscle mitochondrial content and biochemical machinery. However, FA entry into muscle occurs via a regulatable, protein-mediated mechanism, involving several transport proteins. Among these CD36 is key. Muscle contraction and pharmacological agents induce CD36 to translocate to the cell surface, a response that regulates FA transport, and hence FAO. In exercising CD36 KO mice, exercise duration (−44%), and FA transport (−41%) and oxidation (−37%) are comparably impaired, while carbohydrate metabolism is augmented. In trained CD36 KO mice, training-induced upregulation of FAO is not observed, despite normal training-induced increases in mitochondrial density and enzymes. Transfecting CD36 into sedentary WT muscle (+41%), comparable to training-induced CD36 increases (+44%) in WT muscle, markedly upregulates FAO to rates observed in trained WT mice, but without any changes in mitochondrial density and enzymes. Evidently, in vivo CD36-mediated FA transport is key for muscle fuel selection and training-induced FAO upregulation, independent of mitochondrial adaptations. This CD36 molecular mechanism challenges the view that skeletal muscle FAO is solely regulated by muscle mitochondrial content and machinery. [ABSTRACT FROM AUTHOR]

Copyright of Applied Physiology, Nutrition & Metabolism is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Applied Physiology, Nutrition & Metabolism is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Disruptions of ovarian function in women are associated with increased risk of metabolic disease due to dysregulation of peripheral glucose homeostasis in skeletal muscle. Our previous evidence suggests that alterations in skeletal muscle lipid metabolism coupled with altered mitochondrial function may also develop. The objective of this study was to use an integrative metabolic approach to identify potential areas of dysfunction that develop in skeletal muscle from ovariectomized (OVX) female mice compared with age-matched ovary-intact adult female mice (sham). The OVX mice exhibited significant increases in body weight, visceral, and inguinal fat mass compared with sham mice. OVX mice also had significant increases in skeletal muscle intramyocellular lipids (IMCL) compared with the sham animals, which corresponded to significant increases in the protein content of the fatty acid transporters CD36/FAT and FABPpm. A targeted metabolic profiling approach identified significantly lower levels of specific acyl carnitine species and various amino acids in skeletal muscle from OVX mice compared with the sham animals, suggesting a potential dysfunction in lipid and amino acid metabolism, respectively. Basal and maximal mitochondrial oxygen consumption rates were significantly impaired in skeletal muscle fibers from OVX mice compared with sham animals. Collectively, these data indicate that loss of ovarian function results in increased IMCL storage that is coupled with alterations in mitochondrial function and changes in the skeletal muscle metabolic profile. [ABSTRACT FROM AUTHOR]

Aim: The signaling pathways involved in the regulation of cardiac GLUT4 translocation/glucose uptake and CD36 translocation/long-chain fatty acid uptake are not fully understood. We compared in heart/muscle-specific PKC-λ knockout mice the roles of atypical PKCs (PKC-ζ and PKC-λ) in regulating cardiac glucose and fatty acid uptake. Results: Neither insulin-stimulated nor AMPK-mediated glucose and fatty acid uptake were inhibited upon genetic PKC-λ ablation in cardiomyocytes. In contrast, myristoylated PKC-ζ pseudosubstrate inhibited both insulin-stimulated and AMPK-mediated glucose and fatty acid uptake by >80% in both wild-type and PKC-λ-knockout cardiomyocytes. In PKC-λ knockout cardiomyocytes, PKC-ζ is the sole remaining atypical PKC isoform, and its expression level is not different from wild-type cardiomyocytes, in which it contributes to 29% and 17% of total atypical PKC expression and phosphorylation, respectively. Conclusion: Taken together, atypical PKCs are necessary for insulin-stimulated and AMPK-mediated glucose uptake into the heart, as well as for insulin-stimulated and AMPK-mediated fatty acid uptake. However, the residual PKC-ζ activity in PKC-λ-knockout cardiomyocytes is sufficient to allow optimal stimulation of glucose and fatty acid uptake, indicating that atypical PKCs are necessary but not rate-limiting in the regulation of cardiac substrate uptake and that PKC-λ and PKC-ζ have interchangeable functions in these processes. [ABSTRACT FROM AUTHOR]

Aim: The signaling pathways involved in the regulation of cardiac GLUT4 translocation/glucose uptake and CD36 translocation/ long-chain fatty acid uptake are not fully understood. We compared in heart/musclespecific PKC-λ knockout mice the roles of atypical PKCs (PKC-ζ and PKC-λ) in regulating cardiac glucose and fatty acid uptake. Results: Neither insulin-stimulated nor AMPK-mediated glucose and fatty acid uptake were inhibited upon genetic PKC-λ ablation in cardiomyocytes. In contrast, myristoylated PKC-ζ pseudosubstrate inhibited both insulin-stimulated and AMPK-mediated glucose and fatty acid uptake by >80% in both wild-type and PKC-λ-knockout cardiomyocytes. In PKC-λ knockout cardiomyocytes, PKC-ζ is the sole remaining atypical PKC isoform, and its expression level is not different from wild-type cardiomyocytes, in which it contributes to 29% and 17% of total atypical PKC expression and phosphorylation, respectively. Conclusion: Taken together, atypical PKCs are necessary for insulinstimulated and AMPK-mediated glucose uptake into the heart, as well as for insulin-stimulated and AMPKmediated fatty acid uptake. However, the residual PKC-ζ activity in PKC-λ-knockout cardiomyocytes is sufficient to allow optimal stimulation of glucose and fatty acid uptake, indicating that atypical PKCs are necessary but not rate-limiting in the regulation of cardiac substrate uptake and that PKC-λ and PKC-ζ have interchangeable functions in these processes. [ABSTRACT FROM AUTHOR]

Abstract: Insulin-, and contraction-induced GLUT4 and fatty acid (FA) transporter translocation may share common trafficking mechanisms. Our objective was to examine the effects of partial Munc18c ablation on muscle glucose and FA transport, FA oxidation, GLUT4 and FA transporter (FAT/CD36, FABPpm, FATP1, FATP4) trafficking to the sarcolemma, and FAT/CD36 to mitochondria. In Munc18c−/+ mice, insulin-stimulated glucose transport and GLUT4 sarcolemmal appearance were impaired, but were unaffected by contraction. Insulin- and contraction-stimulated FA transport, sarcolemmal FA transporter appearance, and contraction-mediated mitochondrial FAT/CD36 were increased normally in Munc18c−/+ mice. Hence, Munc18c provides stimulus-specific regulation of GLUT4 trafficking, but not FA transporter trafficking. [Copyright &y& Elsevier]

Chronically elevated glucocorticoids (GCs) and a high-fat diet (HFD) independently induce insulin resistance, abdominal obesity, and nonalcoholic fatty liver disease (NAFLD). GCs have been linked to increased food intake, particularly energy-dense "comfort" foods. Thus we examined the synergistic actions of GCs and HFD on hepatic disease development in a new rodent model of chronically elevated GCs. Six-week-old male Sprague-Dawley rats received exogenous GCs, via subcutaneous implantation of four 100-mg corticosterone (Cort) pellets, to elevate basal GC levels for 16 days (n = 8-10 per group). Another subset of animals received wax pellets (placebo) to serve as controls. Animals from each group were randomly assigned to receive a 60% HFD or a standard high-carbohydrate (13% fat and 60% carbohydrate) diet. Cort + HFD resulted in central obesity, despite a relative weight loss, a 4-fold increase in hepatic lipid content, hepatic fibrosis, and a 2.8-fold increase in plasma alanine aminotransferase levels compared with placebo + chow controls. Hepatic injury developed independent of inflammation, as plasma haptoglobin levels were reduced with Cort treatment. Insulin resistance and hepatic steatosis occurred with Cort alone; these outcomes were further exacerbated by the HFD in the presence of elevated Cort. In addition to fatty liver, the Cort + HFD group also developed severe insulin resistance, hyperinsulinemia, hyperglycemia, and hypertriglyceridemia, which were not evident with HFD or Cort alone. Thus a HFD dramatically exacerbates the development of NAFLD and characteristics of the metabolic syndrome in conditions of chronically elevated Cort. [ABSTRACT FROM AUTHOR]

Clenbuterol, a β2-adrenergic agonist, reduces mitochondrial content and enzyme activities in skeletal muscle, but the mechanism involved has yet to be identified. We examined whether clenbuterol-induced changes in the muscles' metabolic profile and the intrinsic capacity of mitochondria to oxidize substrates are associated with reductions in the nuclear receptor coactivator PGC-1 alpha and/or an increase in the nuclear corepressor RIP140. In rats, clenbuterol was provided in the drinking water (30 mg/l). In 3 wk, this increased body (8%) and muscle weights (12-17%). In red (R) and white (W) muscles, clenbuterol induced reductions in mitochondrial content (citrate synthase: R, 27%; W, 52%; cytochrome-c oxidase: R, 24%; W, 34%), proteins involved in fatty acid transport (fatty acid translocase/CD36: R, 36%; W, 35%) and oxidation [β-hydroxyacyl CoA dehydrogenase (β-HAD): R, 33%; W, 62%], glucose transport (GLUT4: R, 8%; W, 13%), lactate transport monocarboxylate transporter (MCT1: R, 61%; W, 37%), and pyruvate oxidation (PDHE1α, R, 18%; W, 12%). Concurrently, only red muscle lactate dehydrogenase activity (25%) and MCT4 (31%) were increased. Palmitate oxidation was reduced in subsarcolemmal (SS) (R, 30%; W, 52%) and intermyofibrillar (IMF) mitochondria (R, 17%; W, 44%) along with reductions in β-HAD activity (SS: R, 17%; W, 51%; IMF: R, 20%; W, 57%). Pyruvate oxidation was only reduced in SS mitochondria (R, 20%; W, 28%), but this was not attributable solely to PDHE1α, which was reduced in both SS (R, 21%; W, 20%) and IMF mitochondria (R, 15%; W, 43%). These extensive metabolic changes induced by clenbuterol were associated with reductions in PGC-1α (R, 37%; W, 32%) and increases in RIP140 (R, 23%; W, 21%). This is the first evidence that clenbuterol appears to exert its metabolic effects via simultaneous and reciprocal changes in the nuclear receptor coactivator PGC-1α and the nuclear corepressor RIP140. [ABSTRACT FROM AUTHOR]

Fatty acid transport proteins are present on the plasma membrane and are involved in the uptake of long-chain fatty acids into skeletal muscle. The present study determined whether acute endurance exercise increased the plasma membrane content of fatty acid transport proteins in rat and human skeletal muscle and whether the increase was accompanied by an increase in long-chain fatty acid transport in rat skeletal muscle. Sixteen subjects cycled for 120 min at ∼60 ± 2% VO2 peak. Two skeletal muscle biopsies were taken at rest and again following cycling. In a parallel study, eight Sprague-Dawley rats ran for 120 min at 20 m/min, whereas eight rats acted as nonrunning controls. Giant sarcolemmal vesicles were prepared, and protein content of FAT/CD36 and FABPpm was measured in human and rat vesicles and whole muscle homogenate. Palmitate uptake was measured in the rat vesicles. In human muscle, plasma membrane FAT/ CD36 and FABPpm protein contents increased 75 and 20%, respectively, following 120 min of exercise. In rat muscle, plasma membrane FAT/CD36 and FABPpm increased 20 and 30%, respectively, and correlated with a 30% increase in palmitate transport following 120 min of running. These data suggest that the translocation of FAT/CD36 and FABPpm to the plasma membrane in rat skeletal muscle is related to the increase in fatty acid transport and oxidation that occurs with endurance running. This study is also the first to demonstrate that endurance cycling induces an increase in plasma membrane FAT/CD36 and FABPpm content in human skeletal muscle, which is predicted to increase fatty acid transport. [ABSTRACT FROM AUTHOR]

Stroke is not only more prevalent but is also associated with more severe adverse functional outcomes among patients with sleep apnea. Monocarboxylate transporters (MCT) are important regulators of cellular bioenergetics, have been implicated in brain susceptibility to acute severe hypoxia (ASH), and could underlie the unfavorable prognosis of cerebrovascular accidents in sleep apnea patients. Rodents were exposed to either intermittent hypoxia (IH) during sleep, a characteristic feature of sleep apnea, or to sustained hypoxia (SH), and expression of MCT1 and MCT2 was assessed. In addition, the functional recovery to middle cerebral artery occlusion (MCAO) in rats and hMCT2 transgenic mice and of hippocampal slices subjected to ASH was assessed, as well as the effects of MCT blocker and MCT2 antisense oligonucleotides and siRNAs. IH, but not SH, induced significant reductions in MCT2 expression over time at both the mRNA and protein levels and in the functional recovery of hippocampal slices subjected to ASH. Similarly, MCAO-induced infarcts were significantly greater in IH-exposed rats and mice, and overexpression of hMCT2 in mice markedly attenuated the adverse effects of IH. Exogenous pyruvate treatment reduced infarct volumes in normoxic rats but not in IH-exposed rats. Administration of the MCT2 blocker 4CN, but not the MCT1 antagonist p-chloromercuribenzene sulfonate, increased infarct size. Thus, prolonged exposures to IH mimicking sleep apnea are associated with increased CNS vulnerability to ischemia that is mediated, at least in part, by concomitant decreases in the expression and function of MCT2. Efforts to develop agonists of MCT2 should provide opportunities to ameliorate the overall outcome of stroke. [ABSTRACT FROM AUTHOR]

Leptin is an adipokine that increases fatty acid (FA) oxidation, decreases intramuscular lipid stores, and improves insulin response in skeletal muscle. In an attempt to elucidate the underlying mechanisms by which these metabolic changes occur, we administered leptin (Lep) or saline (Sal) by miniosmotic pumps to rats during the final 2 wk of a 6-wk low-fat (LF) or high-fat (HF) diet. Insulin-stimulated glucose transport was impaired by the HF diet (HF-Sal) but was restored with leptin administration (HF-Lep). This improvement was associated with restored phosphorylation of Akt and AS 160 and decreased in reactive lipid species (ceramide, diacylglycerol), known inhibitors of the insulin-signaling cascade. Total muscle citrate synthase (CS) activity was increased by both leptin and HF diet, but was not additive. Leptin increased subsarcolemmal (SS) and intramyofibrillar (IMF) mitochondria CS activity. Total muscle, sarcolemmal, and mitochondrial (SS and IMF) FA transporter (FAT/CD36) protein content was significantly increased with the HF diet, but not altered by leptin. Therefore, the decrease in reactive lipid stores and subsequent improvement in insulin response, secondary to leptin administration in rats fed a HF diet was not due to a decrease in FA transport protein content or altered cellular distribution. [ABSTRACT FROM AUTHOR]

Silent mating type information regulator 2 homolog 1 (SIRT1)-mediated peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) deacetylation is potentially key for activating mitochondrial biogenesis. Yet, at the whole muscle level, SIRT1 is not associated with mitochondrial biogenesis (Gurd, BJ, Yoshida Y, Lally J, Holloway GP, Bonen A. J Physiol 587: 1817-1828, 2009). Therefore, we examined nuclear SIRT1 protein and activity in muscle with varied mitochondrial content and in response to acute exercise. We also measured these parameters after stimulating mitochondrial biogenesis with chronic muscle contraction and 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) administration in rodents and exercise training in humans. In skeletal and heart muscles, nuclear SIRT1 protein was negatively correlated with indices of mitochondrial density (citrate synthase activity, CS; cytochrome oxidase IV, COX IV), but SIRT1 activity was positively correlated with these parameters (r > 0.98). Acute exercise did not alter nuclear SIRT1 protein but did induce a time-dependent increase in nuclear SIRT1 activity. This increase in SIRT1 activity was temporally related to increases in mRNA expression of genes activated by PGC-1α. Both chronic muscle stimulation and AICAR increased mitochondrial biogenesis and muscle PGC-1α, but not nuclear PGC-1α. Concomitantly, muscle and nuclear SIRT1 protein contents were reduced, but nuclear SIRT1 activity was increased. In human muscle, training-induced mitochondrial biogenesis did not alter muscle or nuclear SIRT1 protein content, but it did increase muscle and nuclear PGC-1α and SIRT1 activity. Thus, nuclear SIRT1 activity, but not muscle or nuclear SIRT1 protein content, is associated with contraction-stimulated mitochondrial biogenesis in rat and human muscle, possibly via AMPK activation. [ABSTRACT FROM AUTHOR]

Copyright of Applied Physiology, Nutrition & Metabolism is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

OBJECTIVE--The fatty acid translocase and scavenger receptor CD36 is important in the recognition and uptake of lipids. Accordingly, we hypothesized that it plays a role in saturated fatty acid-induced macrophage lipid accumulation and proinflammatory activation. RESEARCH DESIGN AND METHODS--In vitro, the effect of CD36 inhibition and deletion in lipid-induced macrophage inflammation was assessed using the putative CD36 inhibitor, sulfosuccinimidyl oleate (SSO), and bone marrow-derived macrophages from mice with (CD36KO) or without (wild-type) global deletion of CD36. To investigate whether deletion of macrophage CD36 would improve insulin sensitivity in vivo, wild-type mice were transplanted with bone marrow from CD36KO or wild-type mice and then fed a standard or high-fat diet (HFD) for 20 weeks. RESULTS--SSO treatment markedly reduced saturated fatty acid-induced lipid accumulation and inflammation in RAW264.7 macrophages. Mice harboring CD36-specific deletion in hematopoietic-derived cells (HSC CD36KO) fed an HFD displayed improved insulin signaling and reduced macrophage infiltration in adipose tissue compared with wild-type mice, but this did not translate into protection against HFD-induced whole-body insulin resistance. Contrary to our hypothesis and our results using SSO in RAW264.7 macrophages, neither saturated fatty acid-induced lipid accumulation nor inflammation was reduced when comparing CD36KO with wild-type bone marrow-derived macrophages. CONCLUSIONS--Although CD36 does not appear important in saturated fatty acid-induced macrophage lipid accumulation, our study uncovers a novel role for CD36 in the migration of proinflammatory phagocytes to adipose tissue in obesity, with a concomitant improvement in insulin action. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Physiology is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Exercise training induces mitochondrial biogenesis, but the time course of molecular sequelae that accompany repetitive training stimuli remains to be determined in human skeletal muscle. Therefore, throughout a seven-session, high-intensity interval training period that increased (12%), we examined the time course of responses of (a) mitochondrial biogenesis and fusion and fission proteins, and (b) selected transcriptional and mitochondrial mRNAs and proteins in human muscle. Muscle biopsies were obtained 4 and 24 h after the 1st, 3rd, 5th and 7th training session. PGC-1α mRNA was increased >10-fold 4 h after the 1st session and returned to control within 24 h. This 'saw-tooth' pattern continued until the 7th bout, with smaller increases after each bout. In contrast, PGC-1α protein was increased 24 h after the 1st bout (23%) and plateaued at +30-40% between the 3rd and 7th bout. Increases in PGC-1β mRNA and protein were more delayed and smaller, and did not persist. Distinct patterns of increases were observed in peroxisome proliferator-activated receptor (PPAR) α and γ protein (1 session), PPAR β/δ mRNA and protein (5 sessions) and nuclear respiratory factor-2 protein (3 sessions) while no changes occurred in mitochondrial transcription factor A protein. Citrate synthase (CS) and β-HAD mRNA were rapidly increased (1 session), followed 2 sessions later (session 3) by increases in CS and β-HAD activities, and mitochondrial DNA. Changes in COX-IV mRNA (session 3) and protein (session 5) were more delayed. Training also increased mitochondrial fission proteins (fission protein-1, >2-fold; dynamin-related protein-1, 47%) and the fusion protein mitofusin-1 (35%) but not mitofusin-2. This study has provided the following novel information: (a) the training-induced increases in transcriptional and mitochondrial proteins appear to result from the cumulative effects of transient bursts in their mRNAs, (b) training-induced mitochondrial biogenesis appears to involve re-modelling in addition to increased mitochondrial content, and (c) the 'transcriptional capacity' of human muscle is extremely sensitive, being activated by one training bout. It is believed that exercise training stimulates mitochondrial biogenesis by repeatedly increasing mRNA contents for mitochondrial proteins via activation of specific nuclear transcription proteins. These transcription proteins may also increase early during training. The timing of these responses may be important and occur in unique patterns but this has not been determined in human muscle. We show that mitochondrial biogenesis occurs very rapidly (3 exercise sessions) with steady increases thereafter. Furthermore, only one session increased the content of two important transcription factors (PGC-1α and PPARα) with others increasing after 3-5 sessions. Numerous mitochondrial and transcription protein increases appeared to result from repeated short bursts in their mRNA contents which occurred in very distinct patterns. These results show that mitochondrial biogenesis occurs rapidly and suggests the contribution of specific transcription proteins to this process may be time-dependent. This information may help our understanding of how exercise improves muscle metabolic capacities. [ABSTRACT FROM AUTHOR]

Long-chain fatty acid uptake has now been shown to occur via a highly regulated, protein-mediated mechanism involving plasma membrane fatty acid transporters. This process is especially important in skeletal muscle, a tissue with a highly variable metabolic rate that constitutes approximately 40% of body mass. We review the evidence that skeletal muscle fatty acid transport is acutely and chronically regulated by muscle contraction and insulin, largely by the fatty acid transporter CD36. We also examine recent data suggesting that CD36 may contribute to regulating fatty acid oxidation by mitochondria. In addition, we review evidence showing that skeletal muscle insulin resistance is associated with the dysregulation of CD36-mediated fatty acid transport, and that the insulin-sensitizing effects of proliferator-activated receptor-γ coactivator-1a may depend on limiting CD36 upregulation. Taken altogether, it is apparent that skeletal muscle fatty acid transport is central to the regulation of whole-body lipid metabolism in health and disease. [ABSTRACT FROM AUTHOR]

Thujone is thought to be the main constituent of medicinal herbs that have antidiabetic properties. Therefore, we examined whether thujone ameliorated palmitate-induced insulin resistance in skeletal muscle. Soleus muscles were incubated for ≤12 h without or with palmitate (2 mM). Thujone (0.01 mg/ml), in the presence of palmitate, was provided in the last 6 h of incubation. Palmitate oxidation, AMPK/ acetyl-CoA carboxylase (ACC) phosphorylation and insulin-stimulated glucose transport, plasmalemmal GLUT4, and AS 160 phosphorylation were examined at 0, 6, and 12 h. Palmitate treatment for 12 h reduced fatty acid oxidation (-47%), and insulin-stimulated glucose transport (-71%), GLUT4 translocation (-40%), and AS160 phosphorylation (-26%), but it increased AMPK (+51%) and ACC phosphorylations (+44%). Thujone (6-12 h) fully rescued palmitate oxidation and insulin-stimulated glucose transport, but only partially restored GLUT4 translocation and AS 160 phosphorylation, raising the possibility that an increased GLUT4 intrinsic activity may also have contributed to the restoration of glucose transport. Thujone also further increased AMPK phosphorylation but had no further effect on ACC phosphorylation. Inhibition of AMPK phosphorylation with adenine 9-β-o-arabinofuranoside (Ara) (2.5 mM) or compound C (50 μM) inhibited the thujone-induced improvement in insulin-stimulated glucose transport, GLUT4 translocation, and AS 160 phosphorylation. In contrast, the thujone-induced improvement in palmitate oxidation was only slightly inhibited (≤20%) by Ara or compound C. Thus, while thujone, a medicinal herb component, rescues palmitate-induced insulin resistance in muscle, the improvement in fatty acid oxidation cannot account for this thujone-mediated effect. Instead, the rescue of palmitate-induced insulin resistance appears to occur via an AMPK-dependent mechanism involving partial restoration of insulin- stimulated GLUT4 translocation. [ABSTRACT FROM AUTHOR]

Fatty acid oxidation is highly regulated in skeletal muscle and involves several sites of regulation, including the transport of fatty acids across both the plasma and mitochondrial membranes. Transport across these membranes is recognized to be primarily protein mediated, limited by the abundance of fatty acid transport proteins on the respective membranes. In recent years, evidence has shown that fatty acid transport proteins move in response to acute and chronic perturbations; however, in human skeletal muscle the localization of fatty acid transport proteins in response to training has not been examined. Therefore, we determined whether high-intensity interval training (HIIT) increased total skeletal muscle, sarcolemmal, and mitochondrial membrane fatty acid transport protein contents. Ten untrained females (22 ± 1 yr, 65 ± 2 kg; V̇o2peak: 2.8 ± 0.1 l/min) completed 6·wk of HIIT, and biopsies from the vastus lateralis muscle were taken before training, and following 2 and 6 wk of HIIT. Training significantly increased maximal oxygen uptake at 2 and 6 wk (3.1 ± 0.1, 3.3 ± 0.1 l/min). Training for 6 wk increased FAT/CD36 at the whole muscle (10%) and mitochondrial levels (51%) without alterations in sarcolemmal content. Whole muscle plasma membrane fatty acid binding protein (FABPpm) also increased (48%) after 6 wk of training, but in contrast to FAT/CD36, sarcolemmal FABPpm increased (23%). whereas mitochondrial FABPpm was unaltered. The changes on sarcolemmal and mitochondrial membranes occurred rapidly, since differences (≤2 wk) were not observed between 2 and 6 wk. This is the first study to demonstrate that exercise training increases fatty acid transport protein content in whole muscle (FAT/CD36 and FABPpm) and sarcolemmal (FABPpm) and mitochondrial (FAT/CD36) membranes in human skeletal muscle of females. These results suggest that increases in skeletal muscle fatty acid oxidation following training are related in part to changes in fatty acid transport protein content and localization. [ABSTRACT FROM AUTHOR]

The peroxisome proliferator-activated receptor-γ (PPARγ) coactivator-1α (PGC-1α) is a major regulator of exercise-induced phenotypic adaptation and substrate utilization. We provide an overview of 1) the role of PGC-1α in exercise-mediated muscle adaptation and 21 the possible insulin-sensitizing role of PGC-1α. To these ends, the following questions are addressed. 1) How is PGC-1α regulated, 2) what adaptations are indeed dependent on PGC-1α action, 3) is PGC-1α altered in insulin resistance, and 4) are PGC-1α knockout and -transgenic mice suitable models for examining therapeutic potential of this coactivator? In skeletal muscle, an orchestrated signaling network, including Ca2+-dependent pathways, reactive oxygen species (ROS), nitric oxide (NO), AMP-dependent protein kinase (AMPK), and p38 MAPK, is involved in the control of contractile protein expression, angiogenesis, mitochondrial biogenesis, and other adaptations. However, the p38γ MAPK/PGC-1α regulatory axis has been confirmed to be required for exercise-induced angiogenesis and mitochondrial biogenesis but not for fiber type transformation. With respect to a potential insulin-sensitizing role of PGC-1α, human studies on type 2 diabetes suggest that PGC-1α and its target genes are only modestly downregulated (≤34%). However, studies in PGC-1α-knockout or PGC-1α-transgenic mice have provided unexpected anomalies, which appear to suggest that PGC-1α does not have an insulin-sensitizing role. In contrast, a modest (~25%l upregulation of PGC-1α, within physiological limits, does improve mitochondrial biogenesis, fatty acid oxidation, and insulin sensitivity in healthy and insulin-resistant skeletal muscle. Taken altogether, there is substantial evidence that the p38γ MAPK-PGC-1α regulatory axis is critical for exercise-induced metabolic adaptations in skeletal muscle, and strategies that upregulate PGC-1α, within physiological limits, have revealed its insulin-sensitizing effects. [ABSTRACT FROM AUTHOR]

Agonists targeting the nuclear receptor peroxisome proliferator-activated receptors (PPAR)-δ may be potential therapeutic agents for insulin-resistant related conditions, as they may be able to stimulate fatty acid (FA) oxidation and attenuate the accumulation of harmful lipid species in skeletal muscle. Several reports have demonstrated that PPAR-δ agonists improve whole body insulin sensitivity. However, whether these agonists exert their direct effects on glucose and FA metabolism in skeletal muscle, and specifically with different fiber types, is unknown. This study was undertaken to determine the effects of oral treatment with the PPAR-δ agonist, OW 501516, in conjunction with the administration of a high-saturated-fat diet on insulin-stimulated glucose transport in isolated oxidative (soleus) and glycolytic (epitrochlearis) rodent skeletal muscle in vitro. High-fat feeding significantly decreased maximal insulin-stimulated glucose transport in soleus, but not epitrochlearis muscle, and was associated with increased skeletal muscle diacylglycerol and ceramide content. Unexpectedly, treatment with the PPAR-δ agonist significantly reduced insulin-stimulated glucose transport in both soleus and epitrochlearis muscles, regardless of dietary fat content. The reduction in insulin-stimulated glucose transport induced by the agonist was associated with large increases in total muscle fatty acid translocase (FAT)/CD36protein content, but not diacylglycerol or ceramide contents. Agonist treatment did not alter the protein content of PPAR-δ, GLUT4, or insulin-signaling proteins (IRS-I, p85 P13-K, Akt). Agonist treatment led to a small, but significant increase, in the oxidative capacity of glycolytic but not oxidative muscle. We propose that chronic treatment with the PPAR-δ agonist OW 501516 may induce or worsen insulin resistance in rodent skeletal muscle by increasing the capacity for FA transport across the sarcolemma without a sufficient compensatory increase in FA oxidation. However, an accumulation of diacylglycerol and ceramide, while associated with diet-induced insulin resistance, does not appear to be responsible for the agonist-induced reduction in insulin-stimulated glucose transport. [ABSTRACT FROM AUTHOR]

Insulin and contraction stimulate both cardiac glucose and long-chain fatty acid (LCFA) uptake via translocation of the substrate transporters GLUT4 and CD36, respectively, from intracellular compartments to the sarcolemma. Little is known about the role of vesicular trafficking elements in insulin- and contraction-stimulated glucose and LCFA uptake in the heart, especially whether certain trafficking elements are specifically involved in GLUT4 versus CD36 translocation. Therefore, we studied the role of coat proteins, actin- and microtubule-filaments and endosomal pH on glucose and LCFA uptake into primary cardiomyocytes under basal conditions and during stimulation with insulin or oligomycin (contraction-like AMP-activated protein kinase activator). Inhibition of coat protein targeting to Golgi/endosomes decreased insulin/oligomycin-stimulated glucose (-42%/-51%) and LCFA (-39%/-68%) uptake. Actin disruption decreased insulin/oligomycin-stimulated glucose uptake (-41%/75%), while not affecting LCFA uptake. Microtubule disruption did not affect substrate uptake under any conditions. Endosomal alkalinization increased basal sarcolemmal CD36 (2-fold), but not GLUT4, content, and concomitantly decreased basal intracellular membrane GLUT4 and CD36 content (-60% and -62%, respectively), indicating successful CD36 translocation and incomplete GLUT4 translocation. Additionally, endosomal alkalinization elevated basal LCFA uptake (1,4-fold) in a nonadditive manner to insulin/oligomycin, and decreased insulin/oligomycin-stimulated glucose uptake (-32%/-68%). In conclusion, 1) CD36 translocation, just like GLUT4 translocation, is a vesicle-mediated process depending on coat proteins, and 2) GLUT4 and CD36 trafficking are differentially dependent on endosomal pH and actin filaments. The latter conclusion suggests novel strategies to alter cardiac substrate preference as part of metabolic modulation therapy. [ABSTRACT FROM AUTHOR]

The effects of training on silent mating-type information regulator 2 homolog 1 (SIRT1) activity and protein in relationship to peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) and mitochondrial content were determined in human skeletal muscle. Six weeks of high-intensity interval training (~1 h of 10 × 4 min intervals at 90% peak oxygen consumption separated by 2 min rest, 3 days per week) increased maximal activities of mitochondrial enzymes in skeletal muscle by 28% to 36% (citrate synthase, β-hydroxyacyl-coenzyme A dehydrogenase, and cytochrome c oxidase subunit IV) and PGC-1α protein (16%) when measured 4 days after training. Interestingly, total muscle SIRT1 activity (31%) and activity per SIRT1 protein (58%) increased despite decreased SIRT1 protein (20%). The present data demonstrate that exercise-induced mitochondrial biogenesis is accompanied by elevated SIRT1 activity in human skeletal muscle. Dans cette étude, on analyse les effets de l’entraînement physique sur l’activité et les protéines de SIRT1 en relation avec le cofacteur PGC-1α et le contenu mitochondrial du muscle squelettique humain. Quatre jours après la fin d’un entraînement intensif par intervalles (10 × 4 min en ~1 h à une intensité exigeant 90 % du consommation d’oxygene de pointe avec des repos de 2 min entre chaque effort, et ce, 3 jours par semaine durant 6 semaines), on observe une augmentation de 28 à 36 % des activités maximales des enzymes mitochondriaux dans le muscle squelettique (citrate synthase, β-HAD et COX-IV) et une augmentation de 16 % du cofacteur protéique PGC-1α. Curieusement, on observe une augmentation de 31 % de l’activité de SIRT1 dans tout le muscle et de 58 % du ratio activité/protéines de SIRT1 malgré une diminution de 20 % des protéines de SIRT1. Ces observations révèlent que la biogenèse mitochondriale induite par l’exercice physique est accompagnée d’une augmentation de l’activité de SIRT1 dans le muscle squelettique. [ABSTRACT FROM AUTHOR]

Objective: We examined in insulin-resistant muscle if, in contrast to long-standing dogma, mitochondrial fatty acid oxidation is increased and whether this is attributed to an increased nuclear content of peroxisome proliferator-activated receptor (PPAR) gamma coactivator (PGC) 1alpha and the adaptations of specific mitochondrial subpopulations.Research Design and Methods: Skeletal muscles from male control and Zucker diabetic fatty (ZDF) rats were used to determine 1) intramuscular lipid distribution, 2) subsarcolemmal and intermyofibrillar mitochondrial morphology, 3) rates of palmitate oxidation in subsarcolemmal and intermyofibrillar mitochondria, and 4) the subcellular localization of PGC1alpha. Electotransfection of PGC1alpha cDNA into lean animals tested the notion that increased nuclear PGC1alpha preferentially targeted subsarcolemmal mitochondria.Results: Transmission electron microscope analysis revealed that in ZDF animals the number (+50%), width (+69%), and density (+57%) of subsarcolemmal mitochondria were increased (P < 0.05). In contrast, intermyofibrillar mitochondria remained largely unchanged. Rates of palmitate oxidation were approximately 40% higher (P < 0.05) in ZDF subsarcolemmal and intermyofibrillar mitochondria, potentially as a result of the increased PPAR-targeted proteins, carnitine palmitoyltransferase-I, and fatty acid translocase (FAT)/CD36. PGC1alpha mRNA and total protein were not altered in ZDF animals; however, a greater (approximately 70%; P < 0.05) amount of PGC1alpha was located in nuclei. Overexpression of PGC1alpha only increased subsarcolemmal mitochondrial oxidation rates.Conclusions: In ZDF animals, intramuscular lipids accumulate in the intermyofibrillar region (increased size and number), and this is primarily associated with increased oxidative capacity in subsarcolemmal mitochondria (number, size, density, and oxidation rates). These changes may result from an increased nuclear content of PGC1alpha, as under basal conditions, overexpression of PGC1alpha appears to target subsarcolemmal mitochondria. [ABSTRACT FROM AUTHOR]

OBJECTIVE--We examined in insulin-resistant muscle if, in contrast to long-standing dogma, mitochondrial fatty acid oxidation is increased and whether this is attributed to an increased nuclear content of peroxisome proliferator-activated receptor (PPAR) γ coactivator (PGC) 1α and the adaptations of specific mitochondrial subpopulations. RESEARCH DESIGN AND METHODS--Skeletal muscles from male control and Zucker diabetic fatty (ZDF) rats were used to determine 1) intramuscular lipid distribution, 2) subsarcolemmal and intermyofibrillar mitochondrial morphology, 3) rates of palmitate oxidation in subsarcolemmal and intermyofibrillar mitochondria, and 4) the subcellular localization of PGC1α. Electotransfection of PGC1α cDNA into lean animals tested the notion that increased nuclear PGC1α preferentially targeted subsarcolemmal mitochondria. RESULTS--Transmission electron microscope analysis revealed that in ZDF animals the number (+50%), width (+69%), and density (+57%) of subsarcolemmal mitochondria were increased (P < 0.05). In contrast, intermyofibrillar mitochondria remained largely unchanged. Rates of palmitate oxidation were ∼40% higher (P < 0.05) in ZDF subsarcolemmal and intermyofibrillar mitochondria, potentially as a result of the increased PPAR-targeted proteins, carnitine palmitoyltransferase-I, and fatty acid translocase (FAT)/CD36. PGC1α mRNA and total protein were not altered in ZDF animals; however, a greater (∼70%; P < 0.05) amount of PGC1α was located in nuclei. Overexpression of PGC1α only increased subsarcolemmal mitochondrial oxidation rates. CONCLUSIONS--In ZDF animals, intramuscular lipids accumulate in the intermyofibrillar region (increased size and number), and this is primarily associated with increased oxidative capacity in subsarcolemmal mitochondria (number, size, density, and oxidation rates). These changes may result from an increased nuclear content of PGC1α, as under basal conditions, overexpression of PGC1α appears to target subsarcolemmal mitochondria. Diabetes 59:819-828, 2010 [ABSTRACT FROM AUTHOR]

The article discusses a review on the role of membrane fatty acid transporters in cellular lipid metabolism. It states that specific fatty acid transporters transfer from intracellular stores to the plasma membrane to help in fatty acid uptake. Furthermore, the regulatory mechanism made by the fatty acid transporters is important to clear lipids from the circulation and helps in providing substrates when the metabolic demands of organs and tissues are increased by contractile activity.

We examined whether AICAR or leptin rapidly rescued skeletal muscle insulin resistance via increased palmitate oxidation, reductions in intramuscular lipids, and/or restoration of insulin-stimulated AS60 phosphorylation. Incubation with palmitate (2 mM, 0-18 h) induced insulin resistance in soleus muscle. From 12-18 h, palmitate was removed or AICAR or leptin was provided while 2 mM palmitate was maintained. Palmitate oxidation, intramuscular triacylglycerol, diacylglycerol, ceramide, AMPK phosphorylation, basal and insulinstimulated glucose transport, plasmalemmal GLUT4, and Akt and AS 160 phosphorylation were examined at 0, 6, 12, and 18 h. Palmitate treatment (12 h) increased intramuscular lipids (triacylglycerol +54%, diacylglycerol + 11%, total ceramide + 18%, C 16:0 ceramide +60%) and AMPK phosphorylation (+118%), whereas it reduced fatty acid oxidation (-60%) and insulin-stimulated glucose transport (-70%), GLUT4 translocation (-50%), and AS 160 phosphorylation (-40%). Palmitate removal did not rescue insulin resistance or associated parameters. The AICAR and leptin treatments did not consistently reduce intramuscular lipids, but they did rescue palmitate oxidation and insulin-stimulated glucose transport, GLUT4 translocation, and AS 160 phosphorylation. Increased AMPK phosphorylation was associated with these improvements only when AICAR and leptin were present. Hence, across all experiments, AMPK phosphorylation did not correlate with any parameters. In contrast, palmitate oxidation and insulin-stimulated AS 160 phosphorylation were highly correlated (r = 0.83). We speculate that AICAR and leptin activate both of these processes concomitantly, involving activation of unknown kinases in addition to AMPK. In conclusion, despite the maintenance of high concentrations of palmitate (2 mM), as well as increased concentrations of intramuscular lipids (triacylglycerol, diacylglycerol, and ceramide), the rapid AICARand leptin-mediated rescue of palmitate-induced insulin resistance is attributable to the restoration of insulinstimulated AS 160 phosphorylation and GLUT4 translocation. [ABSTRACT FROM AUTHOR]

We examined fatty acid transporters, transport, and metabolism in hearts and red and white muscles of lean and insulin-resistant (week 6) and type 2 diabetic (week 24) Zucker diabetic fatty (ZDF) rats. Cardiac fatty acid transport was similar in lean and ZDF hearts at week 6 but was reduced at week 24 (-40%) in lean but not ZDF hearts. Red muscle of ZDF rats exhibited an early susceptibility to upregulation (+66%) of fatty acid transport at week 6 that was increased by 50% in lean and ZDF rats at week 24 but remained 44% greater in red muscle of ZDF rats. In white muscle, no differences were observed in fatty acid transport between groups or from week 6 to week 24. In all tissues (heart and red and white muscle), FAT/CD36 protein and plasmalemmal content paralleled the changes in fatty acid transport. Triacyiglycerol content in red and white muscles, but not heart, in lean and ZDF rats correlated with fatty acid transport (r = 0.91) and sarcolemmal FAT/CD36 (r = 0.98). Red and white muscle fatty acid oxidation by isolated mitochondria was not impaired in ZDF rats but was reduced by 18-24% in red muscle of lean rats at week 24. Thus, in red, but not white, muscle of insulin-resistant and type 2 diabetic animals, a marked upregulation in fatty acid transport and intramuscular triacylglycerol was associated with increased levels of FAT/CD36 expression and plasmalemmal content. In heart, greater rates of fatty acid transport and FAT/CD36 in ZDF rats (week 24) were attributable to the inhibition of age-related reductions in these parameters. However, intramuscular triacylglycerol did not accumulate in hearts of ZDF rats. Thus insulin resistance and type 2 diabetes are accompanied by tissue-specific differences in FAT/CD36 and fatty acid transport and metabolism. Upregulation of fatty acid transport increased red muscle, but not cardiac, triacylglycerol accumulation. White muscle lipid metabolism dysregulation was not observed. [ABSTRACT FROM AUTHOR]

The plasma membrane fatty acid transport protein FAT/CD36 is also present at the rnitochondria, where it may contribute to the regulation of fatty acid oxidation, although this has been challenged. Therefore, we have compared enzyme activities and rates of rnitochondrial palmitate oxidation in muscles of wild-type (WT) and FAT/CD36 knockout (KO) mice, at rest and after muscle contraction. In WT and KO mice, carnitine palmitoyltransferase-1, citrate synthase, and β-hydroxyacyl-CoA dehydrogenase activities did not differ in subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria of WT and FAT/CD36 KO mice. Basal palmitate oxidation rates were lower (P < 0.05) in KO mice (SS -18%; IMF -13%). Muscle contraction increased fatty acid oxidation (+ 18%) and mitochondrial FAT/CD36 protein (+ 16%) in WT IMF but not in WT SS, or in either mitochondrial subpopulation in KO mice. This revealed that the difference in IMF mitochondrial fatty acid oxidation between WT and KO mice can be increased -2.5-fold from 13% under basal conditions to 35% during muscle contraction. The FAT/CD36 inhibitor sulfo-N-succinimidyl oleate (SSO), inhibited palmitate transport across the plasma membrane in WT, but not in KO mice. In contrast, SSO bound to mitochondrial membranes and reduced palmitate oxidation rates to a similar extent in both WT and KO mitochondria (--80%; P < 0.05). In addition, SSO reduced state III respiration with succinate as a substrate, without altering mitochondrial coupling (PlO ratios). Thus, while SSO inhibits FATICD36-mediated palmitate transport at the plasma membrane, SSO has undefined effects on niitochondria. Nevertheless, the KO animals reveal that FAT/CD36 contributes to the regulation of mitochondrial fatty acid oxidation, which is especially important for meeting the increased metabolic demands during muscle contraction. [ABSTRACT FROM AUTHOR]

Endurance flights of birds, some known to last several days, can only be sustained by high rates of fatty acid uptake by flight muscles. Previous research in migratory shorebirds indicates that this is made possible in part by very high concentrations of cytosolic heart-type fatty acid binding protein (H-FABP), which is substantially upregulated during migratory seasons. We investigated if H-FABP and other components of muscle fatty acid transport also increase during these seasons in a passerine species, the white-throated sparrow (Zonotrichia albicollis). Fatty acid translocase (FAT/CD36) and plasma-membrane fatty acid binding protein (FABPpm) are well characterized mammalian proteins that facilitate transport of fatty acid through the muscle membrane, and in this study they were identified for the first time in birds. We used quantitative PCR to measure mRNA of FAT/CD36, FABPpm and H-FABP and immunoblotting to measure protein expression of FABPpm and H-FABP in the pectoralis muscles of sparrows captured in migratory (spring, fall) and non-migratory (winter) seasons. During migratory seasons, mRNA expression of these genes increased 70-1000% above wintering levels, while protein expression of H-FABP and FABPpm increased 43% and 110% above wintering levels. Activities of key metabolic enzymes, 3-hydroxyacyl-C0A-dehydrogenase (HOAD), carnitine palmitoyl transferase II (CPT II), and citrate synthase (CS) also increased (90-110%) in pectoralis muscles of migrant birds. These results support the hypothesis that enhanced protein-mediated transport of fatty acids from the circulation into muscle is a key component of the changes in muscle biochemistry required for migration in birds. [ABSTRACT FROM AUTHOR]

Abstract: This study reveals that the activation of either PPARα (WY 14643) or PPARβ (GW0742) each induce the translocation of FAT/CD36 from an intracellular pool(s) to the plasma membrane, while PPARβ also induces the subcellular redistribution of FABPpm(Got2) to the plasma membrane. In contrast, activation of PPARγ failed to induce the subcellular redistribution of FAT/CD36 and FABPpm. These PPARα-, and PPARβ-induced changes in the plasmalemmal content of these fatty acid transporters were associated with the concurrent upregulation of fatty acid triacylglycerol esterification (PPARβ) and oxidation (PPARα and PPARβ). Observed effects of chronic PPAR stimulation were not related to either AMPK or ERK1/2 activation. [Copyright &y& Elsevier]

Abstract: Insulin and muscle contraction increase fatty acid transport into muscle by inducing the translocation of FAT/CD36. We examined (a) whether these effects are additive, and (b) whether other fatty acid transporters (FABPpm, FATP1, FATP4, and FATP6) are also induced to translocate. Insulin and muscle contraction increased glucose transport and plasmalemmal GLUT4 independently and additively (positive control). Palmitate transport was also stimulated independently and additively by insulin and by muscle contraction. Insulin and muscle contraction increased plasmalemmal FAT/CD36, FABPpm, FATP1, and FATP4, but not FATP6. Only FAT/CD36 and FATP1 were stimulated in an additive manner by insulin and by muscle contraction. [Copyright &y& Elsevier]

In heart and skeletal muscle, enhanced contractile activity induces an increase in the uptake of glucose and long-chain fatty acids (LCFA) via an AMP-activated protein kinase (AMPK)-regulated mechanism. AMPK activation induces glucose uptake through translocation of glucose transporter 4 (GLUT4) from intracellular pools to the plasma membrane (PM). AMPK-mediated LCFA uptake has been suggested to be regulated by a similar translocation of the LCFA transporters CD36 and plasma membrane-associated fatty acid binding protein (FABPpm). In contrast to the well-characterized GLUT4 translocation, documentation of the proposed translocation of both LCFA transporters is rudimentary. Therefore, we adopted a cell culture system to investigate the localization of CD36 and FABPpm compared with GLUT4, in the absence and presence of AMPK activators oligomycin and AICAR. To this end, intact Chinese hamster ovary (CHO) cells stably expressing CD36 or myc-tagged GLUT4 (GLUT4myc) were used; FABPpm is endogenously expressed in CHO cells. Immuno-fluorescence microscopy revealed that CD36 PM localization resembled that of GLUT4, while FABPpm localized to other PM domains. Upon stimulation with oligomycin or AICAR, CD36 translocated (1.5-fold increase) to a PM location similar to that of GLUT4myc. In contrast, the PM FABPpm content did not change upon AMPK activation. Thus, for the first time in intact cells, we present evidence for AMPK-mediated translocation of CD36 from intracellular pools to the PM, similar to GLUT4, whereas FABPpm is not relocated. [ABSTRACT FROM AUTHOR]

Copyright of Applied Physiology, Nutrition & Metabolism is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Deacetylation of PGC-1α by SIRT1 is thought to be an important step in increasing PGC-1α transcriptional activity, since in muscle cell lines SIRT1 induces PGC-1α protein expression and mitochondrial biogenesis. We examined the relationship between SIRT1 protein and activity, PGC-1α and markers of mitochondrial density, (a) across a range of metabolically heterogeneous skeletal muscles and the heart, and when mitochondrial biogenesis was stimulated by (b) chronic muscle stimulation (7 days) and (c) AICAR administration (5 days), and finally, (d) we also examined the effects of SIRT1 overexpression on mitochondrial biogenesis and PGC-1α. SIRT1 protein and activity were correlated ( r= 0.97). There were negative correlations between SIRT1 protein and PGC-1α ( r=−0.95), COX IV ( r=−0.94) and citrate synthase ( r=−0.97). Chronic muscle stimulation and AICAR upregulated PGC-1α protein (22–159%) and oxidative capacity (COX IV, 20–69%); in each instance SIRT1 protein was downregulated by 20–40%, while SIRT1 intrinsic activity was increased. SIRT1 overexpression in rodent muscle increased SIRT1 protein (+240%) and doubled SIRT1 activity, but PGC-1α (−25%), mtTFA (−14%) and COX IV (−10%) proteins were downregulated. Taken altogether these experiments are not consistent with the notion that SIRT1 protein plays an obligatory regulatory role in the process of PGC-1α-mediated mitochondrial biogenesis in mammalian muscle. [ABSTRACT FROM AUTHOR]