Prevention of unloading-induced atrophy by vitamin E supplementation: links between oxidative stress and soleus muscle proteolysis?

Abstract

Exposure to reduced activity induces skeletal muscle atrophy. Oxidative stress might contribute to muscle wasting via proteolysis activation. This study aimed to test two hypotheses in rats. First, supplementation of the antioxidant vitamin E, prior and during the phase of unloading, would partly counteract unloading-induced soleus muscle atrophy. Secondly, vitamin E supplementation would decrease the rate of muscle proteolysis by reducing expression of calpains, caspases-3, -9, and -12, and E3 ubiquitin ligases (MuRF1 and MAFbx). Soleus muscle atrophy (-49%) induced by 14 days of hindlimb unloading was reduced to only 32% under vitamin E. Vitamin E partly prevented the decrease in type I and IIa fiber size. Supplementation increased HSP72 content and suppressed the rise in muscle level of thiobarbituric acid-reactive substance caused by unloading but failed to modify the lower ratio of reduced vs oxidized glutathione, the higher uncoupling proteins mRNA, and the antioxidant enzyme activities (superoxide dismutase, catalase, and glutathione peroxidase) observed after unloading. Vitamin E treatment abolished the large upregulation of caspases-9 and -12 and MuRF1 transcripts in unloaded muscle and greatly decreased the upregulation of mu-calpain, caspase-3, and MAFbx mRNA. In conclusion, the protective effect of vitamin E might be due to modulation of muscle proteolysis-related genes rather than to its antioxidant function.

Figures

Fig. 1. Effect of unloading and vitamin E supplementation on reduced to oxidized glutathione ratio (GSH/GSSG) and thiobarbituric acid reactive substances (TBARS) content

C: control rats, C+VE: vitamin E-supplemented control rats, HU: hindlimb unloaded rats, HU+VE: vitamin E-supplemented and unloaded rats. Values are means ± SE for eight animals. * Significantly different from control rats.

Fig. 2. Effect of unloading and vitamin E supplementation on activity of a) cytosolic (SOD Zn2+, Cu2+) and b) mitochondrial (Mn2+) superoxide dismutase, c) glutathione peroxidase (GPx) and d) catalase (CAT)

C: control rats, C+VE: vitamin E-supplemented control rats, HU: hindlimb unloaded rats, HU+VE: vitamin E-supplemented and unloaded rats. Values are means ± SE for eight animals. * Significantly different from control rats. † Significantly different from vitamin E-supplemented control rats.

Fig. 3. Influence of unloading and vitamin E supplementation on UCP 2 (a) and UCP3 (b) mRNA and HSP 72 protein content (c). Note that HSP72 contents are expressed as a percentage of HSP72 levels in control group

C: control rats, C+VE: vitamin E-supplemented control rats, HU: hindlimb unloaded rats, HU+VE: vitamin E-supplemented and unloaded rats. Values are means ± SE for eight animals.* Significantly different from control rats. † Significantly different from vitamin E-supplemented control rats.

Fig. 4. Effect of vitamin E on Caspase 3, 9, 12 mRNA in control and unloaded soleus muscles

C: control rats, C+VE: vitamin E-supplemented control rats, HU: hindlimb unloaded rats, HU+VE: vitamin E-supplemented and unloaded rats. Values are means ± SE. * Significantly different from control rats. † Significantly different from vitamin E- supplemented control rats. # Significantly different from hindlimb unloaded rats.

Fig. 5. Effect of vitamin E on expression of two atrophy-related ubiquitin ligases, MAFbx and MuRF1

C: control rats, C+VE: vitamin E-supplemented control rats, HU: hindlimb unloaded rats, HU+VE: vitamin E-supplemented and unloaded rats. Values are means ± SE. * Significantly different from control rats. † Significantly different from vitamin E-supplemented control rats. # Significantly different from hindlimb unloaded rats.