4周爬梯抗阻训练对低氧诱导大鼠骨骼肌萎缩的影响

目的探究抗阻训练抵抗低氧诱导骨骼肌萎缩的效果,为解决高原训练期间运动员骨骼肌丢失问题提供理论依据。方法 8周龄雄性Sprague-Dawley(SD)大鼠24只,平均体重约230 g,随机分为4组:常氧安静组(C)、常氧抗阻训练组(R)、低氧安静组(H)和低氧抗阻训练组(HR)。H和HR组在模拟海拔4000 m,R和HR组则进行抗阻训练,进行4周的低氧及抗阻训练干预后测试各组大鼠体成分,比目鱼肌、趾长伸肌、腓肠肌、肱二头肌的湿重和肌纤维横截面积。结果观察到HR组瘦体重显著高于H组,H组瘦体重显著低于C组;HR组的肱二头肌湿重和肌纤维横截面积显著高于H组。结论抗阻训练有助于预防低氧诱导骨骼肌萎缩的发生,爬梯形式的抗阻练习可刺激大鼠肱二头肌的肥大。     

低氧暴露所致大鼠骨骼肌萎缩的蛋白转化调节机制

目的 对比低氧暴露和常氧下配对低氧摄食干预(半饥饿状态)下大鼠骨骼肌蛋白质合成和分解相关基因表达的差异,以探讨低氧暴露诱导骨骼肌萎缩发生的可能机制。方法 SD大鼠分为:①常氧正常饮食组(C组);②低氧正常饮食组(H组),氧气浓度为12.4%;③常氧配对饮食组(P组),投食量即为H组前一天摄食量。4周干预后测量大鼠体成分,取比目鱼肌(SOL)和趾长伸肌(EDL),称量湿重;HE染色观察肌纤维形态,计算肌纤维横截面积(FCSA);WB测试骨骼肌中HIF1α、Akt、p-Akt及骨骼肌蛋白合成和分解相关基因蛋白含量。结果 1)H组大鼠体重较C组持续下降,P组与C组间无显著性差异;干预初期H组(P组同)摄食量较C组显著下降,后期两组间无差异;(2)干预后,H组大鼠体质量和肌肉总量较C组和P组显著性降低,P组与C组间无差异;H组两肌肉湿重较C组显著下降;H组EDL的FCSA显著低于C组和P组;(3)H组EDL中HIF1α蛋白含量显著高于C组;H组和P组SOL中p-Akt/Akt比值显著低于C组;H组EDL中mTOR、4EBP1蛋白含量显著低于C组,atrogin1、MuRF1、beclin1蛋白含量及LC3Ⅱ/Ⅰ比值显著高于C组,H组SOL中MuRF1蛋白含量显著高于C组和P组。结论 低氧所致的骨骼肌萎缩由低氧特异性因素诱发,表现为以快肌为主的骨骼肌蛋白合成减少和分解增加,而非低氧下摄食量减少引起。     

运动和制动大鼠腓肠肌内p-Akt/MuRF1/FoxO1的蛋白表达变化

本文旨在研究两种运动方式和制动状态下大鼠腓肠肌内p-Akt/MuRF1/FoxO1的蛋白表达变化,以揭示运动员不同的训练方式和停训状态下肌形态学改变的分子机制。Sprague Dawley(SD)大鼠随机分成对照组、耐力训练组、后肢悬垂组和离心训练组。耐力训练组接受跑台训练,悬吊组接受后肢悬垂,离心训练组接受坡度-16o的跑台训练。各组大鼠取腓肠肌,称取其重量,HE法测定骨骼肌细胞横截面积;免疫组化法测定p-Akt蛋白表达;免疫印迹法测定MuRF1、FoxO1的蛋白表达。结果显示,相对对照组,耐力训练组腓肠肌重量和细胞横截面积无显著变化,而离心训练组和后肢悬垂组显著降低;耐力训练组和离心训练组腓肠肌p-Akt蛋白表达显著增加,后肢悬垂组无明显变化。与对照组相比,耐力训练组MuRF1蛋白表达无明显变化,而离心训练组和后肢悬垂组则显著升高;耐力训练组FoxO1蛋白表达显著降低,而离心训练组与后肢悬垂组则显著升高。以上结果表明,增加活动的运动方式(耐力和离心训练)激活了Akt表达,但没有引起肌肉重量的增加;相反,离心训练和后肢悬垂均显著增加了MuRF1和FoxO1的蛋白表达,导致肌肉萎缩,提示MuRF1和FoxO1是造成肌肉萎缩的主要决定因素。     

不同方式运动对去负荷大鼠比目鱼肌收缩性能的影响*

目的: 探究有氧与抗阻运动对去负荷性肌萎缩大鼠比目鱼肌收缩特性及蛋白MuRF1,PGC-1α和FNDC5表达的影响以及可能的分子生物学机制。方法: 将雄性Wistar大鼠随机分为恢复组(CT)、有氧运动组(A)、抗阻运动组(R)和对照组(C),每组6只。对照组不作任何实验处理,其余3组先进行2周尾部悬吊,而后恢复组安静恢复,有氧组与抗阻组进行2周运动干预。运动方案:有氧组大鼠采用65%最大摄氧量(VO₂max)对应的跑台速度,60 min/d,5 日/周;抗阻组大鼠负重65%最大有意负重(MVCC)爬梯,3次为一组,共5组,每次休息1 min,每组间歇2 min,5 日/周。最后一次运动后禁食24 h,取比目鱼肌观察组织学变化、测试收缩性能并检测MuRF1,PGC-1α和FNDC5表达情况。结果: 与对照组相比,恢复组大鼠体重、比目鱼肌湿重、肌纤维平均横截面积与肌收缩性能都明显降低(P<0.01),PGC-1α/FNDC5表达明显降低(P<0.01)和MuRF1表达明显升高(P<0.01);与恢复组相比,有氧组和抗阻组大鼠体重、比目鱼肌湿重、肌纤维平均横截面积与肌收缩性能都明显升高(P<0.01),PGC-1α/FNDC5表达明显升高(P<0.01)和MuRF1表达明显降低(P<0.01)。与有氧组相比,抗阻组大鼠比目鱼肌PGC-1α表达显著升高(P<0.05)且MuRF1表达显著降低(P<0.05)。结论: 有氧和抗阻运动可明显提高肌收缩性能,上调PGC-1α/FNDC5的表达,抑制MuRF1蛋白表达,表明有氧与抗阻运动改善去负荷性肌萎缩的分子机制可能与PGC-1α和MuRF1蛋白相关。     

八眉猪、长白猪及长×八杂交猪肌肉组织中FoxO1基因的表达

分别取6月龄八眉、长白和 (长×八)杂交猪后腿部比目鱼肌、腓肠肌和趾长伸肌, 提取总RNA, 根据人、黑猩猩及大鼠等物种FoxO1基因同源序列设计并合成引物, 以猪b-actin 基因作为内参, 优化反应条件和体系, RT-PCR 单管扩增猪FoxO1基因, 检测八眉、长白和长×八杂交猪不同类型骨骼肌中FoxO1基因mRNA的表达差异。结果表明: 在不同经济类型猪群和不同类型骨骼肌中FoxO1基因mRNA的表达丰度不同, 即在八眉 猪骨骼肌中的表达普遍高于长白猪 (P<0.01), 在杂交组合 (长×八)骨骼肌中的表达也高于长白猪 (P<0.01); 同时在以Ⅰ型纤维为主的比目鱼肌中表达丰度最低(P<0.01), 在以Ⅱb型纤维为主的趾长伸肌中表达丰度最高(P<0.01)。结果提示, FoxO1基因的表达与Ⅰ型肌纤维的含量成反比; 不同经济类型猪品种骨骼肌的发育与FoxO1基因的调控有关。     

低氧影响蛋白质合成和分解的平衡诱导骨骼肌萎缩

暴露在低氧环境下,可能会引起胃肠功能障碍和摄食量下降,打破骨骼肌蛋白质合成和分解平衡,造成骨骼肌萎缩。为探讨低氧环境下骨骼肌的萎缩是低氧环境引起的还是低氧诱发的摄食量减少所致,本研究检测大鼠腓肠肌中低氧时蛋白质合成与分解相关基因的蛋白质表达。将21只雄性SD大鼠,随机分为3组：常氧对照组、低氧组（氧浓度为12.4%,模拟海拔4 000 m高度）和配对组（大鼠的摄食量与低氧组前1 d的摄食量相同）,每组7只,每天记录大鼠体重和摄食量。4周后,HE染色法观察腓肠肌肌纤维形态,Western印迹测试相关蛋白质水平。低氧组和配对组摄食量在低氧干预初期,较常氧对照组有显著性下降(P<0.05),干预后期差异不明显;干预期间,低氧组大鼠体重平均增加量(102.10 g)、体重(341.20 ± 16.75 g)、肌肉总量（226.83 ± 8.33 g）和腓肠肌肌纤维横截面积(12.67 ± 1.83 mm)较常氧对照组（128.00 g;377.50 ± 20.75 g;260.50 ± 9.35 g;15.78 ± 2.38 mm）和配对组（119.40 g;375.86 ± 11.30 g;262.29 ± 7.90 g;15.71 ± 2.82 mm）均显著下降,配对组较常氧对照组无显著性差异;4周干预后,与常氧对照组相比,低氧组大鼠腓肠肌中与低氧相关的HIF1α显著增加(1.42 ± 0.19, P<0.05),Akt和p-Akt/Akt显著降低 (1.44 ± 0.13; 0.47 ± 0.08, P<0.05),配对组上述3种指标相对表达量均无显著性差异;在蛋白质合成方面,低氧组mTOR较常氧对照组显著下降(0.63 ± 0.18, P<0.05),配对组较常氧对照组差异不明显;低氧组腓肠肌中,4EBP1(1.14 ± 0.14)和p70S6K1(1.14 ± 0.11)较配对组显著下降(P<0.05)。在蛋白质分解方面,低氧组p-FoxO1和p-FoxO1/FoxO1比值较常氧对照组显著下降(0.71 ± 0.15; 0.78 ± 0.14, P<0.05);低氧组大鼠腓肠肌中,Atrogin1、MuRF1、Beclin1、LC3Ⅰ及LC3Ⅱ/Ⅰ比值均高于常氧对照组(1.35 ± 0.12; 1.30 ± 0.22; 1.17 ± 0.11; 1.03 ± 0.11; 1.35 ± 0.13, P<0.05);配对组与常氧对照组间无明显差异。低氧环境下骨骼肌中蛋白质合成相关基因表达减少,蛋白质分解相关基因表达增加,造成骨骼肌萎缩,体重下降,此变化与摄食量减少无关。     

低氧影响蛋白质合成和分解的平衡诱导骨骼肌萎缩

暴露在低氧环境下,可能会引起胃肠功能障碍和摄食量下降,打破骨骼肌蛋白质合成和分解平衡,造成骨骼肌萎缩。为探讨低氧环境下骨骼肌的萎缩是低氧环境引起的还是低氧诱发的摄食量减少所致,本研究检测大鼠腓肠肌中低氧时蛋白质合成与分解相关基因的蛋白质表达。将21只雄性SD大鼠,随机分为3组：常氧对照组、低氧组（氧浓度为12.4%,模拟海拔4 000 m高度）和配对组（大鼠的摄食量与低氧组前1 d的摄食量相同）,每组7只,每天记录大鼠体重和摄食量。4周后,HE染色法观察腓肠肌肌纤维形态,Western印迹测试相关蛋白质水平。低氧组和配对组摄食量在低氧干预初期,较常氧对照组有显著性下降(P<0.05),干预后期差异不明显;干预期间,低氧组大鼠体重平均增加量(102.10 g)、体重(341.20 ± 16.75 g)、肌肉总量（226.83 ± 8.33 g）和腓肠肌肌纤维横截面积(12.67 ± 1.83 mm)较常氧对照组（128.00 g;377.50 ± 20.75 g;260.50 ± 9.35 g;15.78 ± 2.38 mm）和配对组（119.40 g;375.86 ± 11.30 g;262.29 ± 7.90 g;15.71 ± 2.82 mm）均显著下降,配对组较常氧对照组无显著性差异;4周干预后,与常氧对照组相比,低氧组大鼠腓肠肌中与低氧相关的HIF1α显著增加(1.42 ± 0.19, P<0.05),Akt和p-Akt/Akt显著降低 (1.44 ± 0.13; 0.47 ± 0.08, P<0.05),配对组上述3种指标相对表达量均无显著性差异;在蛋白质合成方面,低氧组mTOR较常氧对照组显著下降(0.63 ± 0.18, P<0.05),配对组较常氧对照组差异不明显;低氧组腓肠肌中,4EBP1(1.14 ± 0.14)和p70S6K1(1.14 ± 0.11)较配对组显著下降(P<0.05)。在蛋白质分解方面,低氧组p-FoxO1和p-FoxO1/FoxO1比值较常氧对照组显著下降(0.71 ± 0.15; 0.78 ± 0.14, P<0.05);低氧组大鼠腓肠肌中,Atrogin1、MuRF1、Beclin1、LC3Ⅰ及LC3Ⅱ/Ⅰ比值均高于常氧对照组(1.35 ± 0.12; 1.30 ± 0.22; 1.17 ± 0.11; 1.03 ± 0.11; 1.35 ± 0.13, P<0.05);配对组与常氧对照组间无明显差异。低氧环境下骨骼肌中蛋白质合成相关基因表达减少,蛋白质分解相关基因表达增加,造成骨骼肌萎缩,体重下降,此变化与摄食量减少无关。     

自噬途径在低氧暴露诱导骨骼肌萎缩中的调节作用机制

该研究拟通过在体和离体实验观察低氧暴露诱导肌萎缩现象,并探究其作用机制是否与自噬途径有关。SD大鼠进行低氧(氧浓度为12.4%)暴露干预。4周后,测量体质量、瘦体质量、体成分、趾长伸肌(EDL)湿重,观察肌纤维形态,计算肌纤维横截面积(FCSA),PCR芯片分析自噬差异表达基因功能。在低氧环境下使用自噬抑制剂3-MA干预L6肌管细胞,检测自噬关键蛋白选择性自噬接头蛋白(p62)、肌球蛋白样BCL2结合蛋白1(Beclin1)、微管蛋白轻链3(LC3)的表达,统计肌管直径,检测肌萎缩相关蛋白肌球蛋白重链(Myosin)、肌肉特异性环指蛋白1(MuRF1)和肌萎缩F-box蛋白(Atrogin1)的表达。结果显示,大鼠低氧后,体质量、瘦体质量及其百分含量、EDL湿重及其百分含量、FCSA显著降低(P0.05,P0.01);低氧后EDL中自噬差异基因表达以上调为主,功能主要富集于自噬囊泡形成等过程;L6肌管细胞在低氧暴露下,自噬关键蛋白表达增加,肌萎缩相关蛋白表达增加,肌管直径减小,而在低氧条件下使用抑制剂3-MA,可抑制自噬,缓解肌管萎缩(P0.05)。结果表明,低氧暴露可通过提高自噬水平导致肌萎缩的发生,自噬前期的激活在其中发挥重要作用。     

尾部悬吊对达乌尔黄鼠比目鱼肌形态及 mATP 酶活性的影响

采用尾部悬吊法建立后肢骨骼肌废用的动物模型,以肌球蛋白ATP酶(mATPase)法测定比目鱼肌的mATPase活性,依据mATPase染色结果进行肌纤维分型,并测量肌纤维横截面积(Cross-section area,CSA),首次观察了尾部悬吊对达乌尔黄鼠比目鱼肌湿重、CSA和梭外肌、梭内肌纤维mATPase活性的影响,并与尾部悬吊大鼠进行了比较。旨在探讨冬眠动物骨骼肌在废用状态下的变化。结果显示,尾部悬吊14d可使大鼠比目鱼肌湿重体重比下降35.52%(P<0.001),Ⅰ型肌纤维CSA和Ⅱ型肌纤维CSA分别下降18.91%和20.68%(P<0.05);肌纤维平均CSA减少20.45%(P<0.01)。比目鱼肌中Ⅰ型肌纤维的构成比由对照组的80.61%降低为66.83%,Ⅱ型肌纤维的构成比由19.39%增加到33.17%(P<0.001);梭内肌纤维mATPase活性增强,核袋1纤维的mATPase染色由阴性(-)转变为强阳性( ),核袋2纤维和核链纤维由阳性( )转变为强阳性( )。而达乌尔黄鼠在尾部悬吊14d后,比目鱼肌湿重仅比对照组下降0.05%,Ⅰ、Ⅱ型肌纤维CSA与平均CSA分别比对照组减少0.84%、0.63%和0.37%,均无明显差异(P>0.05);与对照组相比,比目鱼肌中Ⅰ型肌纤维的构成比从82.55%减少到77.30%,Ⅱ型肌纤维的构成比由17.45%增加到22.70%(P<0.05);梭内肌纤维mATPase活性亦明显升高,核袋1纤维的mATPase染色由对照组的阴性(-)转化为强阳性( ),核袋2纤维及核链纤维则由对照组的阳性( )转化为核袋2纤维呈阳性( ),核链纤维则呈弱阳性( )。结果表明:尾部悬吊可致大鼠比目鱼肌明显萎缩;达乌尔黄鼠比目鱼肌则无明显萎缩;两者比目鱼肌梭内、外肌mATPase活性均明显升高。     

睫状神经营养因子对大鼠去神经骨骼肌的营养作用

目的：了解睫状神经营养因子（CNTF）对去神经引起的肌肉萎缩的治疗作用。方法：离断SD大鼠一侧坐骨神经,连续给予CNTF20d,观察肌肉湿重、蛋白含量、肌纤维横截面积、收缩性能和残肢程度。结果：①给予0.2mg/kg的CNTF,可使损务侧肌纤维横截面积增加35％,肌肉湿重增加38％,胫前肌总蛋白含量增加24％,腓长肌强直收缩强度提高40％,显著改善肢残程度;②0.2mg/kg的CNTF作用明显强于0.05mg/kg的CNTF;③此目鱼肌（慢肌）比伸趾长肌（快肌）对CNTF更敏感。结论：CNTF能显著改善成年大鼠坐骨神经离断后骨骼肌的萎缩和功能丧失,该效应的强弱与用药剂量和肌肉类型有关。     

Deficiency of inducible nitric oxide synthase attenuates immobilization-induced skeletal muscle atrophy in mice

The present study examined the effects of inducible nitric oxide synthase (iNOS) deficiency on skeletal muscle atrophy in single leg-immobilized iNOS knockout (KO) and wild-type (WT) mice. The left leg was immobilized for 1 wk, and the right leg was used as the control. Muscle weight and contraction-stimulated glucose uptake were reduced by immobilization in WT mice, which was accompanied with increased iNOS expression in skeletal muscle. Deficiency of iNOS attenuated muscle weight loss and the reduction in contraction-stimulated glucose uptake by immobilization. Phosphorylation of Akt, mTOR, and p70S6K was reduced to a similar extent by immobilization in both WT and iNOS KO mice. Immobilization decreased FoxO1 phosphorylation and increased mRNA and protein levels of MuRF1 and atrogin-1 in WT mice, which were attenuated in iNOS KO mice. Aconitase and superoxide dismutase activities were reduced by immobilization in WT mice, and deficiency of iNOS normalized these enzyme activities. Increased nitrotyrosine and carbonylated protein levels by immobilization in WT mice were reversed in iNOS KO mice. Phosphorylation of ERK and p38 was increased by immobilization in WT mice, which was reduced in iNOS KO mice. Immobilization-induced muscle atrophy was also attenuated by an iNOS-specific inhibitor N(6)-(1-iminoethyl)-l-lysine, and this finding was accompanied by increased FoxO1 phosphorylation and reduced MuRF1 and atrogin-1 levels. These results suggest that deficiency of iNOS attenuates immobilization-induced skeletal muscle atrophy through reduced oxidative stress, and iNOS-induced oxidative stress may be required for immobilization-induced skeletal muscle atrophy.     

The E3 ubiquitin ligase TRAF6 intercedes in starvation-induced skeletal muscle atrophy through multiple mechanisms

Starvation, like many other catabolic conditions, induces loss of skeletal muscle mass by promoting fiber atrophy. In addition to the canonical processes, the starvation-induced response employs many distinct pathways that make it a unique atrophic program. However, in the multiplex of the underlying mechanisms, several components of starvation-induced atrophy have yet to be fully understood and their roles and interplay remain to be elucidated. Here we unveiled the role of tumor necrosis factor receptor-associated factor 6 (TRAF6), a unique E3 ubiquitin ligase and adaptor protein, in starvation-induced muscle atrophy. Targeted ablation of TRAF6 suppresses the expression of key regulators of atrophy, including MAFBx, MuRF1, p62, LC3B, Beclin1, Atg12, and Fn14. Ablation of TRAF6 also improved the phosphorylation of Akt and FoxO3a and inhibited the activation of 5' AMP-activated protein kinase in skeletal muscle in response to starvation. In addition, our study provides the first evidence of the involvement of endoplasmic reticulum stress and unfolding protein response pathways in starvation-induced muscle atrophy and its regulation through TRAF6. Finally, our results also identify lysine 63-linked autoubiquitination of TRAF6 as a process essential for its regulatory role in starvation-induced muscle atrophy.     

Insulin down-regulates the expression of ubiquitin E3 ligases partially by inhibiting the activity and expression of AMP-activated protein kinase in L6 myotubes

While insulin is an anabolic hormone, AMP-activated protein kinase (AMPK) is not only a key energy regulator, but it can also control substrate metabolism directly by inducing skeletal muscle protein degradation. The hypothesis of the present study was that insulin inhibits AMPK and thus down-regulates the expression of the ubiquitin E3 ligases, muscle atrophy F-box (MAFbx) and muscle RING finger 1 (MuRF1) in skeletal muscle cells. Differentiated L6 myotubes were treated with 5-aminoimidazole-4-carboxamide-1-β-4-ribofuranoside (AICAR) and/or compound C to stimulate and/or block AMPK respectively. These treatments were also conducted in the presence or absence of insulin and the cells were analysed by western blot and quantitative real-time PCR. In addition, nuleotide levels were determined using HPLC. The activation of AMPK with AICAR enhanced the mRNA levels of MAFbx and MuRF1. Insulin reduced the phosphorylation and activity AMPK, which was accompanied by reduced MAFbx and MuRF1 mRNA levels. Using a protein kinase B (PKB/Akt) inhibitor, we found that insulin regulates AMPK through the activation of Akt. Furthermore, insulin down-regulated AMPK α2 mRNA. We conclude that insulin inhibits AMPK through Akt phosphorylation in L6 myotubes, which may serve as a possible signalling pathway for the down-regulation of protein degradation. In addition, decreased expression of AMPK α2 may partially participate in inhibiting the activity of AMPK.     

High CO2 Levels Cause Skeletal Muscle Atrophy via AMP-activated Kinase (AMPK), FoxO3a Protein,and Muscle-specific Ring Finger Protein 1 (MuRF1)

Patients with chronic obstructive pulmonary disease, acute lung injury, and critical care illness may develop hypercapnia. Many of these patients often have muscle dysfunction which increases morbidity and impairs their quality of life. Here, we investigated whether hypercapnia leads to skeletal muscle atrophy. Mice exposed to high CO₂ had decreased skeletal muscle wet weight, fiber diameter, and strength. Cultured myotubes exposed to high CO₂ had reduced fiber diameter, protein/DNA ratios, and anabolic capacity. High CO₂ induced the expression of MuRF1 in vivo and in vitro, whereas MuRF1^−/− mice exposed to high CO₂ did not develop muscle atrophy. AMP-activated kinase (AMPK), a metabolic sensor, was activated in myotubes exposed to high CO₂, and loss-of-function studies showed that the AMPKα2 isoform is necessary for muscle-specific ring finger protein 1 (MuRF1) up-regulation and myofiber size reduction. High CO₂ induced AMPKα2 activation, triggering the phosphorylation and nuclear translocation of FoxO3a, and leading to an increase in MuRF1 expression and myotube atrophy. Accordingly, we provide evidence that high CO₂ activates skeletal muscle atrophy via AMPKα2-FoxO3a-MuRF1, which is of biological and potentially clinical significance in patients with lung diseases and hypercapnia.     

Glycogen synthase kinase-3β is required for the induction of skeletal muscle atrophy

Skeletal muscle atrophy commonly occurs in acute and chronic disease. The expression of the muscle-specific E3 ligases atrogin-1 (MAFbx) and muscle RING finger 1 (MuRF1) is induced by atrophy stimuli such as glucocorticoids or absence of IGF-I/insulin and subsequent Akt signaling. We investigated whether glycogen synthase kinase-3β (GSK-3β), a downstream molecule in IGF-I/Akt signaling, is required for basal and atrophy stimulus-induced expression of atrogin-1 and MuRF1, and myofibrillar protein loss in C(2)C(12) skeletal myotubes. Abrogation of basal IGF-I signaling, using LY294002, resulted in a prominent induction of atrogin-1 and MuRF1 mRNA and was accompanied by a loss of myosin heavy chain fast (MyHC-f) and myosin light chains 1 (MyLC-1) and -3 (MyLC-3). The synthetic glucocorticoid dexamethasone (Dex) also induced the expression of both atrogenes and likewise resulted in the loss of myosin protein abundance. Genetic ablation of GSK-3β using small interfering RNA resulted in specific sparing of MyHC-f, MyLC-1, and MyLC-3 protein levels after Dex treatment or impaired IGF-I/Akt signaling. Interestingly, loss of endogenous GSK-3β suppressed both basal and atrophy stimulus-induced atrogin-1 and MuRF1 expression, whereas pharmacological GSK-3β inhibition, using CHIR99021 or LiCl, only reduced atrogin-1 mRNA levels in response to LY294002 or Dex. In conclusion, our data reveal that myotube atrophy and myofibrillar protein loss are GSK-3β dependent, and demonstrate for the first time that basal and atrophy stimulus-induced atrogin-1 mRNA expression requires GSK-3β enzymatic activity, whereas MuRF1 expression depends solely on the physical presence of GSK-3β.