Kee Anthony J, Combaret Lydie, Tilignac Thomas, Souweine Bertrand, Aurousseau Eveline, Dalle Michel, Taillandier Daniel, Attaix Didier
Muscle Development Unit, Children's Medical Research Institute, Locked Bag 23, Wentworthville NSW 2145, Australia.
J Physiol. 2003 Feb 1;546(Pt 3):765-76. doi: 10.1113/jphysiol.2002.032367.
The central role of the ubiquitin-proteasome system in the loss of skeletal muscle protein in many wasting conditions has been well established. However, it is unclear what factors are responsible for the suppression of this system during periods of protein gain. Thus, the aim of these studies was to examine the short-term effects of insulin release and nutrients on skeletal muscle protein turnover in young rats starved for 48 h, and then infused intravenously with amino acids (AA), or fed an oral diet. Forty-eight hours of starvation (i.e. prolonged starvation in young rats) decreased muscle protein synthesis and increased proteasome-dependent proteolysis. Four-hour AA infusion and 4 h of refeeding increased plasma insulin release and AA concentrations, and stimulated muscle protein synthesis, but had no effect on either total or proteasome-dependent proteolysis, despite decreased plasma corticosterone concentrations. Both muscle proteasome-dependent proteolysis and the rate of ubiquitination of muscle proteins were not suppressed until 10 h of refeeding. The temporal response of these two measurements correlated with the normalised expression of the 14-kDa E2 (a critical enzyme in substrate ubiquitination in muscle) and the expression of the MSS1 subunit of the 19S regulatory complex of the 26S proteasome. In contrast, the starvation-induced increase in mRNA levels for 20S proteasome subunits was normalised by refeeding within 24 h in muscle, and 6 h in jejunum, respectively. In conclusion, unlike protein synthesis, skeletal muscle proteasome-dependent proteolysis is not acutely responsive in vivo to insulin, AA, and/or nutrient intake in refed starved rats. This suggests that distinct and perhaps independent mechanisms are responsible for the nutrient-dependent regulation of protein synthesis and ubiquitin-proteasome-dependent proteolysis following a prolonged period of catabolism. Furthermore, factors other than the expression of ubiquitin-proteasome pathway components appear to be responsible for the suppression of skeletal muscle proteasome-dependent proteolysis by nutrition.
泛素-蛋白酶体系统在许多消耗性疾病中骨骼肌蛋白丢失方面的核心作用已得到充分证实。然而，目前尚不清楚在蛋白质增加期间是什么因素导致该系统受到抑制。因此，这些研究的目的是检查胰岛素释放和营养物质对饥饿48小时的幼鼠骨骼肌蛋白质周转的短期影响，然后给这些幼鼠静脉注射氨基酸（AA）或喂食口服饮食。48小时的饥饿（即幼鼠长期饥饿）会降低肌肉蛋白质合成并增加蛋白酶体依赖性蛋白水解。4小时的AA输注和4小时的重新喂食会增加血浆胰岛素释放和AA浓度，并刺激肌肉蛋白质合成，但对总蛋白水解或蛋白酶体依赖性蛋白水解均无影响，尽管血浆皮质酮浓度降低。直到重新喂食10小时，肌肉蛋白酶体依赖性蛋白水解和肌肉蛋白泛素化速率才受到抑制。这两项测量的时间反应与14 kDa E2（肌肉底物泛素化中的关键酶）的标准化表达以及26S蛋白酶体19S调节复合体的MSS1亚基的表达相关。相比之下，饥饿诱导的20S蛋白酶体亚基mRNA水平的增加在肌肉中分别在重新喂食24小时内和空肠中6小时内恢复正常。总之，与蛋白质合成不同，骨骼肌蛋白酶体依赖性蛋白水解在体内对再喂养饥饿大鼠的胰岛素、AA和/或营养摄入没有急性反应。这表明在长时间分解代谢后，蛋白质合成和泛素-蛋白酶体依赖性蛋白水解的营养依赖性调节是由不同且可能独立的机制负责的。此外，除了泛素-蛋白酶体途径成分的表达外，其他因素似乎也负责营养对骨骼肌蛋白酶体依赖性蛋白水解的抑制作用。
