Effects of contractile activity on mitochondrial transcription factor A expression in skeletal muscle

Mitochondrial transcription factor A (Tfam) is a nuclear-encoded gene produ ct that is imported into mitochondria and is required for the transcription of mitochondrial DNA (mtDNA). We hypothesized that conditions known to pro duce mitochondrial biogenesis in skeletal muscle would be preceded by an in crease in Tfam expression. Therefore, rat muscle was stimulated (10 Hz, 3 h /day). Tfam mRNA levels were significantly elevated (by 55%) at 4 days and returned to control levels at 14 days. Tfam import into intermyofibrillar ( IMF) mitochondria was increased by 52 and 61% (P < 0.05) at 5 and 7 days, r espectively. This corresponded to an increase in the level of import machin ery components. Immunoblotting data indicated that IMF Tfam protein content was increased by 63% (P < 0.05) at 7 days of stimulation. This was associa ted with a 49% (P < 0.05) increase in complex formation at the mtDNA promot er and a 65% (P < 0.05) increase in the levels of a mitochondrial transcrip t, cytochrome-c oxidase (COX) subunit III. Similarly, COX enzyme activity w as elevated by 71% (P < 0.05) after 7 days of contractile activity. These r esults indicate that early events in mitochondrial biogenesis include incre ases in Tfam mRNA, followed by accelerations in mitochondrial import and in creased Tfam content, which correspond with increased binding to the mtDNA promoter region. This was accompanied by increased mitochondrial transcript levels and elevated COX activity. These data support the role of Tfam as a regulatory protein involved in contractile activity-induced mitochondrial biogenesis.