Mild Heat Stress Induces Mitochondrial Biogenesis Associated with Activation of the AMPKSIRT1- PGC-1alpha Pathway in C2C12 Myotubes

During endurance exercise, most (about 75%) of the energy derived from the oxidation of metabolic fuels and ATP hydrolysis of muscle contraction is liberated as heat, the accumulation of which leads to an increase in body temperature. For example, the temperature of exercising muscles can rise to 40 degrees C. Although severe heat injury can be deleterious, several beneficial effects of mild heat stress (HS), such as the improvement of insulin sensitivity in patients with type 2 diabetes have been reported. However, among all cellular events induced by mild HS from physical activities, the direct effects and mechanisms of mild HS on mitochondrial biogenesis in skeletal muscle are least characterized. AMP-activated protein kinase (AMPK) and Sirtuin 1 (SIRT1) are key energy-sensing molecules regulating mitochondrial biogenesis. In C2C12 myotubes, we found that one-hr mild HS at 40 degrees C increased both AMPK activity and SIRT1 expression, as well as increased the expression of several mitochondrial biogenesis regulatory genes including peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) and transcription factors involved in mitochondrial biogenesis. In particular, PGC-1alpha expression was found to be transcriptionally regulated by mild HS. Additionally, after repeated mild HS for 5 days, protein levels of PGC-1alpha and several mitochondrial oxidative phosphorylation subunits also increased. Repeated mild HS also significantly increased mitochondrial DNA copy number. In conclusion, these data suggest that mild HS is sufficient to induce mitochondrial biogenesis associated with activation of the AMPK-SIRT1-PGC-1alpha pathway in C2C12 myotubes. Therefore, it is possible that muscle heat production during exercise plays a role in mitochondrial biogenesis.