Effects Of Phytosterols On Muscle Protein Metabolism In Mice With Different Energy Levels

Objective: Increased intake of dietary fat would lead to the production of ROS in cells exceeding the antioxidant capacity and induce oxidative stress, the excess ROS would damage biomacromolecules including proteins through oxidation. As the largest protein reservoir, skeletal muscle can also be oxidized by surplus ROS. Phytosterols has been proved to chemically act as an antioxidant as well as a modest radical scavenger, meanwhile, phytosterols could also improve the growth performance and carcass quality of animals. Hence, the purpose of present study was to investigate the impact of dietary fat on mice skeletal muscle redox status and protein metabolism, and whether these changes could be attenuated or prevented by phytosterols supplementation.Methods: A total of 72 male C57BL/6 mice were randomly divided into 9 groups and fed with control diet(5% fat), control diet supplemented with 0.002% and 0.004% phytosterols, medium-fat diet(10% fat), medium-fat diet supplemented with 0.004% and 0.008% phytosterols, high-fat diet(20% fat), high-fat diet supplemented with 0.008% and 0.016% phytosterols for 18 weeks respectively. The Comprehensive Laboratory Animal Monitoring System was used to quantitate individual mice on their food intake, oxygen consumption, carbon dioxide production, heat production, and locomotor activity at the 17 th week. After that, mice were slightly anesthetized and sacri?ced, soleus and gastrocnemius were dissected and weighed. The levels of ROS, oxidative stress indicators, AOPP and carbonyl, as well as the contents of protein and glycogen in gastrocnemius were determined. Meanwhile, the expressions of genes involved in growth and protein metabolism in skeletal muscle were determined by RT-PCR.Results: With the increase in dietary fat content, mice exhibited: 1)significantly increased oxygen consumption, carbon dioxide production and heat production(P<0.05), while no significant difference was found for energy intake when normalized to body weight; 2)significantly increased levels of ROS and MDA, while markedly decreased activities of GSH-Px、CAT、SOD and T-AOC in gastrocnemius(P<0.05); 3)significantly increased levels of AOPP and carbonyl in plasma and gastrocnemius(P<0.05); 4)significantly decreased muscular protein content, and markedly down-regulated expression of protein synthesis-related genes(PI3K、Akt、m TOR、4EBP1、e IF4E、S6K1、rp S6), while up-regulated expression of protein degradation-related genes(Fox O3a、Mu RF1、MAFbx) in gastrocnemius(P<0.05); 5)significantly decreased leg muscle rate, down-regulated expression of myogenin, and up-regulated expression of myostatin in gastrocnemius(P<0.05); 6) significantly decreased locomotor activities and muscular glycogen content(P<0.05). Phytosterols could: 1)significantly increase oxygen consumption, carbon dioxide production, heat production and energy intake of mice(P<0.05); 2)significantly decease the levels of ROS and MDA, while enhance the activities of GSH-Px, T-AOC, SOD and CAT in gastrocnemius(P<0.05); 3)significantly decease the levels of AOPP and carbonyl in plasma and gastrocnemius(P<0.05); 4)significantly increase muscular protein content, and markedly enhance the expression of protein synthesis-related genes, while normalize the expression of protein degradation-related genes in gastrocnemius(P<0.05); 5)significantly increase skeletal muscle mass and leg muscle rate, meanwhile, enhance the expression of myogenin and normalize the expression of myostatin in gastrocnemius(P<0.05); 6)significantly increase locomotor activities and muscular glycogen content(P<0.05). Correlation analysis showed that gastrocnemius muscle oxidative stress biomarkers strongly related to the ratios of skeletal muscle to body weight and muscular protein content.Conclusion: The increase in dietary fat impaired the quality and maintenance of skeletal muscle, and possibly triggered by oxidative stress. Phytosterols could contradict dietary fat-induced skeletal muscle damage by ameliorating the oxidative stress, regulating energy homeostasis, and in part by inducing the PI3K/Akt signaling pathway to stimulate protein synthesis and inhibit protein degradation in skeletal muscle. The stimulatory effect of phytosterols on protein synthesis subsequently translated into whole body anabolic effects, such as the increase in muscle protein, skeletal muscle mass and physical performance.