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。

Hypoxia Induces Skeletal Muscle Atrophy by Affecting the Balance of Protein Synthesis and Decomposition

In order to investigate whether skeletal muscle atrophy in the hypoxic environment is caused by hypoxia itself or hypoxia-induced low food intake, the levels related to hypoxia and protein synthesis and decomposition in rat gastrocnemius muscles were examined. 21 male SD rats were randomly divided into three groups: the normoxic group (group N), the hypoxic group (oxygen concentration was 12.4% at a simulated altitude of 4 000 m, group H), the paired group (the food intake of the paired group was the same as that of the hypoxic group, group P), and the body weight and food intake of rats were recorded every day for four weeks. The morphology of muscle fibers was observed by HE staining, and the content of related proteins was tested by Western blotting. Food intake in the group H (group P) decreased significantly compared with the group N at the initial stage of intervention (P<0.05), but no significant difference was detected in the later stage of intervention. The results showed that in the group H the average increase of rats’ body weight (102.10 g), body mass (341.20 ± 16.75 g), muscle mass(226.83 ± 8.33 g) and cross-sectional areas of gastrocnemius muscle fibers (12.67 ± 1.83 mm) decreased significantly compared with the group N (128.00 g; 377.50 ± 20.75 g; 260.50 ± 9.35 g; 15.78 ± 2.38 mm) and the group P (119.40 g; 375.86 ± 11.30 g; 262.29 ± 7.90 g; 15.71 ± 2.82 mm), and the group P was similar to the group N during the intervention period. Compared with the group N, the HIF1α protein (1.42 ± 0.19) expression in the group H was significantly increased (P<0.05), and the relative content of the Akt protein and ratio of p-Akt/Akt (1.44 ± 0.13; 0.47 ± 0.08) was significantly lower than the group N (P<0.05) after four weeks of intervention. There was no significant difference in the relative expression levels of the three proteins above between the P and N groups. In terms of protein synthesis, mTOR (0.63 ± 0.18) in the group H was significantly decreased compared with the group N (P<0.05), while the P and N groups had no obvious distinctions. The protein content of 4EBP1 (1.14 ± 0.14) and p70S6K1 (1.14 ± 0.11) in the gastrocnemius muscle of the group P was apparently higher than the group H (P<0.05). In terms of protein decomposition, the p-Foxo1 protein level and p-Foxo1 /FoxO1 ratio (0.71 ± 0.15; 0.78 ± 0.14, P<0.05) in the group H were significantly lower than that in the group N, while the group P (1.25 ± 0.23) was significantly higher than the group N (P<0.05). In the group H, rat gastrocnemius Atrogin1, MuRF1, Beclin1, LC3 Ⅰ protein contents and LC3 Ⅱ/Ⅰ ratios (1.35 ± 0.12; 1.30 ± 0.22; 1.17 ± 0.11; 1.03 ± 0.11; 1.35 ± 0.13) were higher than that in the group N (P<0.05). We concluded that reduced protein synthesis related gene expression and increased protein decomposition related gene expression in skeletal muscles under the hypoxic environment could lead to skeletal muscle atrophy and weight loss, which was not correlated with reduced food intake.