Abstract

It is well established that aerobic exercise and training lead to increased mitochondrial density and number (known as mitochondrial biogenesis) which consequently improves the oxidative capacity of mus- cle fibers. However, there is little experimental evidence regarding the molecular events that mediate this adaptation. As a result, and with the valuable contribution of technological development, there is a grow- ing scientific interest in the effect of exercise on gene expression of mitochondrial proteins and the clarifica- tion of the signalling events and transcription factors which determine mitochondrial biogenesis. The available body of knowledge indicates that the mechanical stimulus of exercise is transported to the nu- cleus (where most mitochondrial genes are found) through the increased concentration of Ca2+ and the activation of Ca2+-dependent enzymes, as well as through the activation of AMP-dependent protein kinase as a result of energy imbalance. The increased production of reactive oxygen species is another pu- tative signal path for mitochondrial biogenesis. The most important regulator of mitochondrial biogenesis is considered to be peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1). This, in turn, controls a series of transcription factors, including peroxisome proliferator-activated receptors (PPAR) and   , nuclear respiratory factors 1 and 2, and mitochondrial transcription factor, TFAM. Acute aerobic exercise increases the levels of mRNA and protein of PGC-1 and transcription factors, with diversity re- garding the magnitude and timing of their peaks. Continuing muscle biopsy sampling for 48 h after the end of exercise ensures a more reliable kinetic response. Aerobically trained subjects are characterized by increased levels of mRNA and protein of PGC-1 and PPARs. Finally, a remarkable finding is that high- intensity interval cycling lasting 30 s and interval training of the same characteristics increase gene expres- sion of PGC-1 and promote mitochondrial biogenesis as effectively as the continuous submaximal high- volume training.

Key words: gene expression, transcription factors, mitochondrial DNA