UCP3 gene expression does not correlate with muscle oxidation rates in troglitazone-treated Zucker fatty rats

Uncoupling protein-3 (UCP3), a mitochondrial carrier protein predominantly expressed in muscle, has been suggested to release stored energy as heat. The insulin-sensitizing thiazolidinediones enhance glucose disposal in skeletal muscle and have been reported to increase the expression of uncoupling proteins in various experimental systems. We therefore studied the effect of troglitazone treatment on UCP3 gene expression in muscles from lean and obese Zucker rats. In comparison with obese littermates, basal UCP3 mRNA levels in lean Zucker rats tended to be higher in white and red gastrocnemius muscles, but were lower in soleus (P<0.001) muscle and heart (P<0.01). In lean rats, troglitazone significantly increased UCP3 gene expression in white and red gastrocnemius and heart muscles (all P<0.01). In contrast, the drug reduced UCP3 mRNA expression in red gastrocnemius and soleus muscles of obese littermates (all P<0.001). The troglitazone-dependent decrease in UCP3 gene expression was accompanied by an increased weight gain in obese rats, while no such effect was observed in lean rats. In obese rats, improvement of insulin resistance by troglitazone was associated with increased rates of basal and insulin-stimulated CO(2) production from glucose measured in soleus muscle. These studies demonstrate that effects of troglitazone on UCP3 gene expression depend on the phenotype of Zucker rats and that troglitazone-induced metabolic improvements are not related to increased uncoupling resulting from upregulation of UCP3 mRNA expression in muscle.     

Overexpression of GLUT4 in mice causes up-regulation of UCP3 mRNA in skeletal muscle

Mitochondrial uncoupling protein 3 (UCP3) is expressed in skeletal muscles. We have hypothesized that increased glucose flux in skeletal muscles may lead to increased UCP3 expression. Male transgenic mice harboring insulin-responsive glucose transporter (GLUT4) minigenes with differing lengths of 5'-flanking sequence (-3237, -2000, -1000 and -442 bp) express different levels of GLUT4 protein in various skeletal muscles. Expression of the GLUT4 transgenes caused an increase in UCP3 mRNA that paralleled the increase of GLUT4 protein in gastrocnemius muscle. The effects of increased intracellular GLUT4 level on the expression of UCP1, UCP2 and UCP3 were compared in several tissues of male 4 month-old mice harboring the -1000 GLUT4 minigene transgene. In the -1000 GLUT4 transgenic mice, expression of GLUT4 mRNA and protein in skeletal muscles, brown adipose tissue (BAT), and white adipose tissue (WAT) was increased by 1.4 to 4.0-fold. Compared with non-transgenic littermates, the -1000 GLUT4 mice exhibited about 4- and 1.8-fold increases of UCP3 mRNA in skeletal muscle and WAT, respectively, and a 38% decrease of UCP1 mRNA in BAT. The transgenic mice had a 16% increase in oxygen consumption and a 14% decrease in blood glucose and a 68% increase in blood lactate, but no change in FFA or beta-OHB levels. T3 and leptin concentrations were decreased in transgenic mice. Expression of UCP1 in BAT of the -442 GLUT4 mice, which did not overexpress GLUT4 in this tissue, was not altered. These findings indicate that overexpression of GLUT4 up-regulates UCP3 expression in skeletal muscle and down-regulates UCP1 expression in BAT, possibly by increasing the rate of glucose uptake into these tissues.     

Effects of aging and denervation on the expression of uncoupling proteins in slow- and fast-twitch muscles of rats

We investigated the effects of aging and denervation on the gene expression of uncoupling proteins (UCPs) in slow-twitch soleus and fast-twitch gastrocnemius muscles. In a comparison between the control limbs of 6- and 24-month-old rats, the mRNA levels of UCP3, heart-type fatty acid binding protein (HFABP), and glucose transporter-4 (GLUT4) were considerably lower in the gastrocnemius muscles of the older rats, whereas no significant differences in the mRNA levels of those genes as well as UCP2 and cytochrome oxidase subunit IV (COX-IV) were observed in the soleus muscles of young and old rats. The UCP3 and COX-IV protein levels were also reduced considerably in the aged gastrocnemius muscles with atrophy. Denervation of the sciatic nerve caused an increase in UCP3 mRNA levels in both muscles, but the regulation of other genes contrasted between the two types of skeletal muscles. In spite of the increased mRNA level, a remarkable reduction in UCP3 protein was found in the denervated gastrocnemius muscles. These results indicate that the effects of aging and denervation on the gene expression of UCPs, HFABP, GLUT4, and COX-IV are different between the muscle types. The reduction in the mitochondrial UCP3 and COX proteins in aged fast-twitch muscles may have a negative effect on energy metabolism and thermogenesis in old animals.     

Tissue-specific expression and cold-induced mRNA levels of uncoupling proteins in the Djungarian hamster

The uncoupling protein 1 (UCP1), a mitochondrial transmembrane protein, is responsible for adaptive thermogenesis in brown adipose tissue (BAT). Two UCP1 homologues, UCP2 and UCP3, were recently discovered, but it is controversial whether they also play a role in energy homeostasis. Djungarian hamster UCPs were found to exhibit high similarity with homologues known in other species. UCP1 mRNA was restricted to BAT, UCP2 mRNA was expressed in multiple tissues, and UCP3 mRNA was detected mainly in BAT and skeletal muscles. We examined the cold-induced regulation of hamster UCP mRNA levels and tested their correlation with serum free fatty acid (FFA) concentrations. In BAT UCP1, UCP2, and UCP3 expression was upregulated in the cold, but the increase and time course of increase differed. In skeletal muscle, UCP2 and UCP3 mRNA levels were not altered. Cold-induced changes of serum FFA levels correlated with the stimulation of UCP1 mRNA in BAT but not with UCP2 and UCP3.     

13C/31P NMR assessment of mitochondrial energy coupling in skeletal muscle of awake fed and fasted rats. Relationship with uncoupling protein 3 expression

To examine the relationship between mitochondrial energy coupling in skeletal muscle and change in uncoupling protein 3 (UCP3) expression during the transition from the fed to fasted state, we used a novel noninvasive (31)P/(13)C NMR spectroscopic approach to measure the degree of mitochondrial energy coupling in the hind limb muscles of awake rats before and after a 48-h fast. Compared with fed levels, UCP3 mRNA and protein levels in the gastrocnemius increased 1.7- (p < 0.01) and 2.9-fold (p < 0.001), respectively, following a 48-h fast. Tricarboxylic acid cycle flux measured using (13)C NMR as an index of mitochondrial substrate oxidation was 212 +/- 23 and 173 +/- 25 nmol/g/min (p not significant) in the fed and 48-h fasted groups, respectively. Unidirectional ATP synthesis flux measured using (31)P NMR was 79 +/- 15 and 57 +/- 9 nmol/g/s (p not significant) in the fed and 48-h fasted groups, respectively. Mitochondrial energy coupling as expressed by the ratio of ATP synthesis to tricarboxylic acid cycle flux was not different between the fed and fasted states. To test the hypothesis that UCP3 may be involved in the translocation of long chain free fatty acids (FFA) into the mitochondrial matrix under conditions of elevated FFA availability, [U-(13)C]palmitate/albumin was administered in a separate group of rats with (+) or without (-) etomoxir (an inhibitor of carnitine palmitoyltransferase I). The ratio of glutamate enrichment ((+) etomoxir/(-) etomoxir) in the hind limb muscles was the same between groups, indicating that UCP3 does not appear to function as a translocator for long chain FFA in skeletal muscle following a 48-h fast. In summary, these data demonstrate that despite a 2-3-fold increase in UCP3 mRNA and protein expression in skeletal muscle during the transition from the fed to fasted state, mitochondrial energy coupling does not change. Furthermore, UCP3 does not appear to have a major role in FFA translocation into the mitochondria. The physiological role of UCP3 following a 48-h fast in skeletal muscle remains to be elucidated.     

Evidence for mitochondrial thioesterase 1 as a peroxisome proliferator-activated receptor-alpha-regulated gene in cardiac and skeletal muscle

The physiological role of mitochondrial thioesterase 1 (MTE1) is unknown. It was proposed that MTE1 promotes fatty acid (FA) oxidation (FAO) by acting in concert with uncoupling protein (UCP)3. We previously showed that ucp3 is a peroxisome proliferator-activated receptor-alpha (PPAR alpha)-regulated gene, allowing induction when FA availability increases. On the assumption that UCP3 and MTE1 act in partnership to increase FAO, we hypothesized that mte1 is also a PPAR alpha-regulated gene in cardiac and skeletal muscle. Using real-time RT-PCR, we characterized mte1 gene expression in rat heart and soleus muscles. Messenger RNA encoding for mte1 was 3.2-fold higher in heart than in soleus muscle. Cardiac mte1 mRNA exhibited modest diurnal variation, with 1.4-fold higher levels during dark phase. In contrast, skeletal muscle mte1 mRNA remained relatively constant over the course of the day. High-fat feeding, fasting, and streptozotocin-induced diabetes, interventions that increase FA availability, muscle PPAR alpha activity, and muscle FAO rates, increased mte1 mRNA in heart and soleus muscle. Conversely, pressure overload and hypoxia, interventions that decrease cardiac PPAR alpha activity and FAO rates, repressed cardiac mte1 expression. Specific activation of PPAR alpha in vivo through WY-14643 administration rapidly induced mte1 mRNA in cardiac and skeletal muscle. WY-14643 also induced mte1 mRNA in isolated adult rat cardiomyocytes dose dependently. Expression of mte1 was markedly lower in hearts and soleus muscles isolated from PPAR alpha-null mice. Alterations in cardiac and skeletal muscle ucp3 expression mirrored that of mte1 in all models investigated. In conclusion, mte1, like ucp3, is a PPAR alpha-regulated gene in cardiac and skeletal muscle.     

禁食及胰岛素水平对大鼠解偶联蛋白-1、2、3基因表达的影响

 为探讨禁食和胰岛素对解偶联蛋白 - 1、2、3基因 (UCP1 ,2 ,3)表达的影响 ,应用 RT- PCR方法观察了在不同禁食时间和应用胰岛素条件下大鼠白色脂肪组织、棕色脂肪组织和骨骼肌中 UCP1 ,2 ,3m RNA水平的变化 .UCP1基因只在大鼠棕色脂肪组织中表达 .UCP2 ,3基因在三种组织中均有表达 ,在白色脂肪组织中以 UCP2表达为主 ;在骨骼肌中以 UCP3表达为主 .过夜禁食使棕色脂肪组织 UCP1 ,3m RNA水平明显下降 (P<0 .0 1 ) ;UCP2 m RNA水平在三种组织中均呈上升反应 ,以白色脂肪组织中表现最为明显 (P<0 .0 5) ;而对白色脂肪组织和骨骼肌中 UCP3基因表达无明显影响 .禁食时间延长至 48h,除棕色脂肪组织中 UCP2 ,3基因有明显下降外 ,各组织中UCPs基因表达基本调节至正常或高于对照组水平 .胰岛素对 UCPs基因表达水平有一定的上调作用 ,这一作用对棕色脂肪组织 UCPs各基因及骨骼肌中 UCP3基因表现得尤为明显 (P<0 .0 5) .大鼠 UCPs基因表达有一定的组织特异性 ;禁食时间对三种组织中 UCPs各成员基因表达的影响有时相上的区别 ;胰岛素可以调 UCPs各成员基因的表达 .结果反映了 UCPs各成员在能量代谢调节上的不同作用 ,这为理解膳食 -产热与体重调节的关系 ,及其能量代谢平衡与疾病关系提供了实验依据     

Expression of glucocorticoid receptors in the regenerating human skeletal muscle

Many stress conditions are accompanied by skeletal muscle dysfunction and regeneration, which is essentially a recapitulation of the embryonic development. However, regeneration usually occurs under conditions of hypothalamus-pituitary-adrenal gland axis activation and therefore increased glucocorticoid (GC) levels. Glucocorticoid receptor (GR), the main determinant of cellular responsiveness to GCs, exists in two isoforms (GRalpha and GRbeta) in humans. While the role of GRalpha is well characterized, GRbeta remains an elusive player in GC signalling. To elucidate basic characteristics of GC signalling in the regenerating human skeletal muscle we assessed GRalpha and GRbeta expression pattern in cultured human myoblasts and myotubes and their response to 24-hour dexamethasone (DEX) treatment. There was no difference in GRalpha mRNA and protein expression or DEX-mediated GRalpha down-regulation in myoblasts and myotubes. GRbeta mRNA level was very low in myoblasts and remained unaffected by differentiation and/or DEX. GRbeta protein could not be detected. These results indicate that response to GCs is established very early during human skeletal muscle regeneration and that it remains practically unchanged before innervation is established. Very low GRbeta mRNA expression and inability to detect GRbeta protein suggests that GRbeta is not a major player in the early stages of human skeletal muscle regeneration.     

Sepsis downregulates myostatin mRNA levels without altering myostatin protein levels in skeletal muscle

Myostatin is a negative regulator of muscle mass and has been reported to be upregulated in several conditions characterized by muscle atrophy. The influence of sepsis on myostatin expression and activity is poorly understood. Here, we tested the hypothesis that sepsis upregulates the expression and downstream signaling of myostatin in skeletal muscle. Because sepsis‐induced muscle wasting is at least in part regulated by glucocorticoids, we also determined the influence of glucocorticoids on myostatin expression. Sepsis was induced in rats by cecal ligation and puncture and control rats were sham‐operated. In other experiments, rats were injected intraperitoneally with dexamethasone (10 mg/kg) or corresponding volume of vehicle. Surprisingly, myostatin mRNA levels were reduced and myostatin protein levels were unchanged in muscles from septic rats. Muscle levels of activin A, follistatin, and total and phosphorylated Smad2 (p‐Smad2) were not influenced by sepsis, suggesting that myostatin downstream signaling was not altered during sepsis. Interestingly, total and p‐Smad3 levels were increased in septic muscle, possibly reflecting altered signaling through pathways other than myostatin. Similar to sepsis, treatment of rats with dexamethasone reduced myostatin mRNA levels and did not alter myostatin protein levels. Fasting, an additional condition characterized by muscle wasting, reduced myostatin mRNA and activin A protein levels, increased myostatin protein, and did not influence follistatin and p‐Smad2 levels. Of note, total and p‐Smad3 levels were reduced in muscle during fasting. The results suggest that sepsis and glucocorticoids do not upregulate the expression and activity of myostatin in skeletal muscle. The role of myostatin may vary between different conditions characterized by muscle wasting. Downstream signaling through Smad2 and 3 is probably regulated not only by myostatin but by other mechanisms as well. J. Cell. Biochem. 111: 1059–1073, 2010. © 2010 Wiley‐Liss, Inc.     

Acute effect of fatty acids on metabolism and mitochondrial coupling in skeletal muscle

Acute effects of free fatty acids (FFA) were investigated on: (1) glucose oxidation, and UCP-2 and -3 mRNA and protein levels in 1 h incubated rat soleus and extensor digitorium longus (EDL) muscles, (2) mitochondrial membrane potential in cultured skeletal muscle cells, (3) respiratory activity and transmembrane electrical potential in mitochondria isolated from rat skeletal muscle, and (4) oxygen consumption by anesthetized rats. Long-chain FFA increased both basal and insulin-stimulated glucose oxidation in incubated rat soleus and EDL muscles and reduced mitochondrial membrane potential in C2C12 myotubes and rat skeletal muscle cells. Caprylic, palmitic, oleic, and linoleic acid increased O(2) consumption and decreased electrical membrane potential in isolated mitochondria from rat skeletal muscles. FFA did not alter UCP-2 and -3 mRNA and protein levels in rat soleus and EDL muscles. Palmitic acid increased oxygen consumption by anesthetized rats. These results suggest that long-chain FFA acutely lead to mitochondrial uncoupling in skeletal muscle.     

Acute effect of fatty acids on metabolism and mitochondrial coupling in skeletal muscle

Acute effects of free fatty acids (FFA) were investigated on: (1) glucose oxidation, and UCP-2 and -3 mRNA and protein levels in 1 h incubated rat soleus and extensor digitorium longus (EDL) muscles, (2) mitochondrial membrane potential in cultured skeletal muscle cells, (3) respiratory activity and transmembrane electrical potential in mitochondria isolated from rat skeletal muscle, and (4) oxygen consumption by anesthetized rats. Long-chain FFA increased both basal and insulin-stimulated glucose oxidation in incubated rat soleus and EDL muscles and reduced mitochondrial membrane potential in C2C12 myotubes and rat skeletal muscle cells. Caprylic, palmitic, oleic, and linoleic acid increased O₂ consumption and decreased electrical membrane potential in isolated mitochondria from rat skeletal muscles. FFA did not alter UCP-2 and -3 mRNA and protein levels in rat soleus and EDL muscles. Palmitic acid increased oxygen consumption by anesthetized rats. These results suggest that long-chain FFA acutely lead to mitochondrial uncoupling in skeletal muscle.     

Hyperlipemia: a role in regulating UCP3 gene expression in skeletal muscle during cancer cachexia?

Rats bearing the Yoshida AH-130 ascites hepatoma showed an increased expression of both uncoupling protein-2 (UCP2) (two-fold) and UCP3 (three- to four-fold) in skeletal muscle (both soleus and gastrocnemius). The increase in mRNA content was associated with increased circulating concentrations of fatty acids (two-fold), triglyceride (two-fold) and cholesterol (1.9-fold). Administration of nicotinic acid to tumor-bearing rats abolishes the hyperlipidemic increase associated with tumor burden. The vitamin treatment also resulted in a decreased UCP3 gene expression in soleus muscle but not in gastrocnemius. It is concluded that circulating fatty acids may be involved in the regulation of UCP3 gene expression in aerobic muscles during experimental cancer cachexia. Since the UCP3 protein could have a role in energy expenditure, it may be suggested that hypolipidemic agents may have a beneficial role in the treatment of the cachectic syndrome.     

UCP3 in muscle wasting, a role in modulating lipotoxicity?

UCP3 has been postulated to function in the defense against lipid-induced oxidative muscle damage (lipotoxicity). We explored this hypothesis during cachexia in rats (zymosan-induced sepsis), a condition characterized by increased oxidative stress and supply of fatty acids to the muscle. Muscle UCP3 protein content was increased 2, 6 and 11 days after zymosan injection. Plasma FFA levels were increased at day 2, but dropped below control levels on days 6 and 11. Muscular levels of the lipid peroxidation byproduct 4-hydroxy-2-nonenal (4-HNE) were increased at days 6 and 11 in zymosan-treated rats, supporting a role for UCP3 in modulating lipotoxicity during cachexia.     

Downregulation of uncoupling protein-3 in vivo is linked to changes in muscle mitochondrial energy metabolism as a result of capsiate administration

Although it has been suggested that the skeletal muscle mitochondrial uncoupling protein-3 (UCP3) is involved in regulating energy expenditure, its role is still poorly understood. In the present study, we aimed at investigating noninvasively, using magnetic resonance techniques, metabolic changes occurring in exercising muscle as a result of capsiate treatment, which has been previously linked to UCP3 upregulation. We showed that capsiate ingestion strongly reduced UCP3 gene expression in rat gastrocnemius muscle. This large underexpression was accompanied by a significant increase in the rate of mitochondrial ATP production and phosphocreatine level both at rest and during muscle stimulation. Similarly, the stimulation-induced ATP fall and ADP accumulation were significantly less after capsiate administration than in untreated rats. The larger oxidative ATP production rate could not be explained by a proportional decrease in the anaerobic component, i.e., glycolysis and phosphocreatine breakdown. In addition, the mechanical performance was not affected by capsiate administration. Finally, the plasma free fatty acid (FFA) level increased in capsiate-treated rats, whereas no significant change was observed after muscle stimulation in the control group. Considering the corresponding enhanced UCP3 mRNA expression occurring in the control group after muscle stimulation, one can suggest that changes in FFA level and UCP3 mRNA expression are not mechanistically correlated. Overall, we have shown that capsiate administration induced a UCP3 downregulation coupled with an increased mitochondrial ATP synthesis, whereas the muscle force-generating capacity was unchanged. This suggests that a decrease in muscle efficiency and/or additional noncontractile ATP-consuming mechanisms result from UCP3 downregulation.     

Effect of beta1- and beta2-adrenergic stimulation on energy expenditure,substrate oxidation,and UCP3 expression in humans

In humans, beta-adrenergic stimulation increases energy and fat metabolism. In the case of beta1-adrenergic stimulation, it is fueled by an increased lipolysis. We examined the effect of beta2-adrenergic stimulation, with and without a blocker of lipolysis, on thermogenesis and substrate oxidation. Furthermore, the effect of beta1-and beta2-adrenergic stimulation on uncoupling protein 3 (UCP3) mRNA expression was studied. Nine lean males received a 3-h infusion of dobutamine (DOB, beta1) or salbutamol (SAL, beta2). Also, we combined SAL with acipimox to block lipolysis (SAL+ACI). Energy and substrate metabolism were measured continuously, blood was sampled every 30 min, and muscle biopsies were taken before and after infusion. Energy expenditure significantly increased approximately 13% in all conditions. Fat oxidation increased 47 +/- 7% in the DOB group and 19 +/- 7% in the SAL group but remained unchanged in the SAL+ACI condition. Glucose oxidation decreased 40 +/- 9% upon DOB, remained unchanged during SAL, and increased 27 +/- 11% upon SAL+ACI. Plasma free fatty acid (FFA) levels were increased by SAL (57 +/- 11%) and DOB (47 +/- 16%), whereas SAL+ACI caused about fourfold lower FFA levels compared with basal levels. No change in UCP3 was found after DOB or SAL, whereas SAL+ACI downregulated skeletal muscle UCP3 mRNA levels 38 +/- 13%. In conclusion, beta2-adrenergic stimulation directly increased energy expenditure independently of plasma FFA levels. Furthermore, this is the first study to demonstrate a downregulation of skeletal muscle UCP3 mRNA expression after the lowering of plasma FFA concentrations in humans, despite an increase in energy expenditure upon beta2-adrenergic stimulation.     

Upregulation of uncoupling protein homologues in skeletal muscle but not adipose tissue in posttraumatic insulin resistance

Metabolic alterations after surgical stress include peripheral insulin resistance and increased utilization of fat as a fuel substrate. An up-regulation of skeletal muscle uncoupling proteins (UCPs) has been associated with physiologic states of insulin resistance and enhanced fat metabolism in rodents. We examined whether posttraumatic insulin resistance induced the UCPs in gastrocnemius and soleus muscle and white adipose tissue in an experimental model of surgical trauma. Insulin sensitivity was significantly reduced in isolated soleus muscles but unchanged in adipocytes after trauma. In traumatized rats, mRNA and protein contents of UCP2 and UCP3 and were significantly increased in both muscle types. UCP2 protein content in adipose tissue was unaltered by surgical stress. Circulating NEFAs and glycerol were reduced after surgical trauma. We hypothesize that the changes in UCP2 and UCP3 gene and protein expression are involved in the regulation of substrate utilization in posttraumatic insulin resistance.     

Hyperoxia-mediated oxidative stress increases expression of UCP3 mRNA and protein in skeletal muscle

The uncoupling protein-3 (UCP3) is a mitochondrial protein expressed mainly in skeletal muscle. Among several hypotheses for its physiological function, UCP3 has been proposed to prevent excessive production of reactive oxygen species. In the present study, we evaluated the effect of an oxidative stress induced by hyperoxia on UCP3 expression in mouse skeletal muscle and C2C12 myotubes. We found that the hyperoxia-mediated oxidative stress was associated with a 5-fold and 3-fold increase of UCP3 mRNA and protein levels, respectively, in mouse muscle. Hyperoxia also enhanced reactive oxygen species production and UCP3 mRNA expression in C2C12 myotubes. Our findings support the view that both in vivo and in vitro UCP3 may modulate reactive oxygen species production in response to an oxidative stress.     

A cell-autonomous role for the glucocorticoid receptor in skeletal muscle atrophy induced by systemic glucocorticoid exposure

Glucocorticoids (GCs) are important regulators of skeletal muscle mass, and prolonged exposure will induce significant muscle atrophy. To better understand the mechanism of skeletal muscle atrophy induced by elevated GC levels, we examined three different models: exogenous synthetic GC treatment [dexamethasone (DEX)], nutritional deprivation, and denervation. Specifically, we tested the direct contribution of the glucocorticoid receptor (GR) in skeletal muscle atrophy by creating a muscle-specific GR-knockout mouse line (MGR(e3)KO) using Cre-lox technology. In MGR(e3)KO mice, we found that the GR is essential for muscle atrophy in response to high-dose DEX treatment. In addition, DEX regulation of multiple genes, including two important atrophy markers, MuRF1 and MAFbx, is eliminated completely in the MGR(e3)KO mice. In a condition where endogenous GCs are elevated, such as nutritional deprivation, induction of MuRF1 and MAFbx was inhibited, but not completely blocked, in MGR(e3)KO mice. In response to sciatic nerve lesion and hindlimb muscle denervation, muscle atrophy and upregulation of MuRF1 and MAFbx occurred to the same extent in both wild-type and MGR(e3)KO mice, indicating that a functional GR is not required to induce atrophy under these conditions. Therefore, we demonstrate conclusively that the GR is an important mediator of skeletal muscle atrophy and associated gene expression in response to exogenous synthetic GCs in vivo and that the MGR(e3)KO mouse is a useful model for studying the role of the GR and its target genes in multiple skeletal muscle atrophy models.