Mitochondrial biogenesis elicited by chronic contractile activity with mTORC1 inhibition

Abstract

Chronic contractile activity (CCA) enriches the mitochondrial content of skeletal muscle, and this adaptation requires an increased expression of nuclear genes encoding mitochondrial proteins (NUGEMPs). The mammalian target of rapamycin complex 1 (mTORC1) has been implicated in regulating the expression of NUGEMPs by linking the co-activator PGC-1α to the transcription factor YY1. We wished to ascertain if mTORC1 was required for CCA-induced mitochondrial biogenesis. C2C12 myotubes were subjected to CCA (3 hours/day, 4 days) in the presence of 1nM rapamycin, an mTORC1 inhibitor, or DMSO. CCA increased mitochondrial content, as reflected by cytochrome oxidase (COX) activity. This adaptation was also evident in the presence of rapamycin. Indeed, mTORC1 inhibition led to an increase in COX activity in both the absence (28%) and presence (20%) of CCA. Surprisingly, the mTORC1 component raptor, as well as YY1, were both significantly elevated due to mTORC1 inhibition. However, CCA had no effect on raptor levels, but increased YY1 expression by 1.8-fold. State III respiration declined in rapamycin-treated cells, but this was not observed in the presence of CCA, suggesting that CCA recovered mitochondrial function. These data suggest that mTORC1 may be a negative regulator of NUGEMPs, and that the inhibition of mTORC1 does not blunt CCA-induced mitochondrial biogenesis.