The importance of PGC-1α in contractile activity-induced mitochondrial adaptations

The transcriptional coactivator PPARγ coactivator-1α (PGC-1α) is a critical regulator of mitochondrial content and function in skeletal muscle. PGC-1α may also mediate mitochondrial adaptations in response to chronic contractile activity (CCA). To characterize the essential role of PGC-1α in organelle biogenesis, C₂C₁₂ murine myotubes were transfected with PGC-1α-specific siRNA and subjected to electrical stimulation-evoked CCA. CCA enhanced cytochrome c oxidase (COX) activity along with increases in several nuclear-encoded mitochondrial proteins. Transfection of PGC-1α siRNA decreased protein and mRNA of the coactivator by 60%, resulting in decrements of Tfam and COX-IV proteins. The mRNA expression of the PGC-1 family members PGC-1β and PRC, as well as transcription factors NRF-1/2 and ERRα, did not exhibit compensatory changes in response to PGC-1α depletion. However, phosphorylation of AMPK was enhanced in myotubes with reduced levels of PGC-1α. This suggests the presence of metabolic compensatory stress signals in cells deficient in PGC-1α. Our findings reveal that the CCA-induced increases in COX-IV protein and overall mitochondrial content, using both COX activity and organelle fluorescence, are dependent on PGC-1α. However, this was not the case for all proteins, since decreased levels of the coactivator did not attenuate the increases in Tfam and cytochrome c in response to CCA. These data indicate that PGC-1α is necessary for most of the mitochondrial adaptations that occur with CCA but that there are additional pathways that function in parallel with PGC-1α to mediate the elevated expression of specific nuclear-encoded proteins that are vital for mitochondrial function and cell viability.