Mechanisms of p53-induced mitochondrial biogenesis in skeletal muscle

p53 is a pleiotropic protein that is mutated in many types of cancers. It is involved in regulating various distinct cellular pathways including apoptosis, cell cycle arrest, senescence, autophagy, angiogenesis, and most relevant to this dissertation, cellular metabolism. While extensive research has been conducted on p53 in cancer biology, the role of this protein in modulating mitochondrial function in skeletal muscle has only been recently investigated. Thus the aim of this dissertation was to assess the mechanisms by which p53 induces and controls aerobic metabolism in skeletal muscle, and the subsequent impact it carries on exercise-mediated pathways that elicit an increase in mitochondrial bioenergetic capacity.

First, we examined the effect of p53 on mitochondrial protein import and complex IV assembly. Using subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria isolated from p53 wildtype (WT) and knockout (KO) mice, we discerned no effects of p53 on the rate of protein import. However, the expression of several proteins involved in the import process was reduced in the KO mice. Assembly of complex IV was impaired in the IMF mitochondria, along with the assembly co-factor Surf1, which may facilitate the previously documented attenuation of mitochondrial function in the p53 KO mice.

Next we evaluated whether p53 is recruited in response to endurance exercise. To assess this, we subjected C57B1/6 mice to an acute run at 15m/min for 90 minutes ± 3 hours recovery, and subsequently measured alterations in content, and sub-cellular localization of p53. The nuclear p53 content decreased steadily with acute exercise and post-recovery, in sharp contrast to the increase in p53 abundance in SS and IMF mitochondria. Concomitantly, higher levels of mitochondrial p53 were complexed with Tfam, the mitochondrial DNA (mtDNA) transcription factor, and with mtDNA at the D-loop with exercise and recovery. We identified putative p53 response elements in the D-loop, and hypothesized that p53 could be mediating mtDNA transcription in collaboration with Tfam. Further support for this was derived from the observation that the exercise-induced increase in mtDNA-transcribed protein COX-I was completely abrogated in p53 KO mice.

Lastly, we sought to determine the necessity of p53 to the exercise-induced changes that transpire within the muscle upon an imposed metabolic and physiological challenge such as a bout of endurance exercise. p38 MAPK activation was abolished, whereas AMPK and CaMKII signalling was attenuated with exercise in p53 KO mice. This occurred in tandem with lower levels of PGC-1? translocation into the nucleus, and subsequently attenuated the increase in mRNA content of genes involved in mitochondrial biogenesis. While non-exercised p53 KO mice displayed an impaired ability to undergo autophagy during basal conditions, there was no difference in the activation of autophagic proteins post-exercise in p53 KO vs. WT mice.

Collectively, our data illustrate that p53 is integral to maintaining baseline levels of mitochondrial content and function, and is intimately involved in, and necessary for the signaling process initiated upon acute endurance exercise that mediates mitochondrial biogenesis.