Superoxide dismutase gene expression in skeletal muscle: fiber-specific effect of age

The influence of ageing on the expression of two superoxide dismutase (SOD) isozymes was examined in three different skeletal muscle fiber types of young (Y, 8 mo) and old (O, 25 mo) rats. Total SOD activity was increased with age in the gastrocnemius (Gas, type II(mix)) and superficial vastus lateralis (SVL, type IIb) but unchanged in the soleus (Sol, type I). The increased SOD activity in SVL was due to increased cytosolic SOD (CuZn SOD), whereas both mitochondrial (Mn SOD) and CuZn SOD activities were increased in Gas. In Sol, Mn SOD activity was significantly increased in aged rats. Mn SOD mRNA level was significantly decreased with age in all three muscles examined, while Mn SOD protein content was not altered. Ageing did not affect CuZn SOD mRNA abundance in any of the muscles, but significantly increased CuZn SOD protein content in aged Gas and Sol. Binding of two redox-sensitive transcription factors, nuclear factor-kappaB (NFkappaB) and activator protein-1 (AP-1) was significantly decreased with age in all three muscle types. These results indicate that increased SOD activity in aged skeletal muscle is not associated with higher levels of gene transcription. Increases in Mn SOD activity seen in aged Gas and Sol are the result of post-translational modification of the enzyme, whereas increases in CuZn SOD activity during ageing may be due to both translational and post-translational control.