Skeletal Muscle Forkhead Box 3A (FOXO3A) Response to Acute Resistance Exercise in Young and Old Men and Women : Relationship to Muscle Glycogen Content and 5'-AMP Activated Protein Kinase (AMPK) Activity

Abstract

In addition to suppressing protein synthesis, 5'-AMP-activated protein kinase (AMPK) stimulates muscle protein degradation through forkhead box transcription factor 3A (FOXO3A), known to promote transcription of mRNAs encoding degradation pathway proteins. Phosphorylation of FOXO3A at Ser318/321 by Akt (protein kinase B, known to promote muscle growth), normally prevents FOXO3A action by preventing its translocation to the nucleus. However, AMPK inhibits FOXO3A Ser318/321 phosphorylation by Akt, thus allowing its translocation and consequently transcription and upregulation of degradation pathways. AMPK phosphorylation as well as degradation pathways are more greatly elevated in old vs. young rats and humans in response to muscle loading. Moreover, older individuals exhibit lower muscle glycogen content, a condition known to accentuate AMPK activity at rest and during aerobic exercise. However, the possible relationship between muscle glycogen content, AMPK activation, and FOXO3A phosphorylation in response to resistance exercise in young and old individuals has not been previously examined. We hypothesized that FOXO3A (Ser318/321) phosphorylation would be lower in response to acute resistance exercise in the skeletal muscles of older versus younger individuals. It was further hypothesized that this lower FOXO3A phosphorylation response would be related to lower glycogen content and higher AMPK activation in the skeletal muscles of older versus younger individuals. Seven young (21.7 + 2.1 yrs) and 11 old (67.0 + 8.6 yrs) subjects performed an acute bout of leg extension resistance exercise. Muscle biopsies were obtained pre-exercise (PRE), immediately post-exercise (0P), 1-hour post-exercise (1P), and 2-hours post-exercise (2P). Glycogen content was measured in muscle samples, as were the phosphorylations (via western blot) of AMPK, acetyl-CoA carboxylase (ACC; a marker of AMPK activity), and FOXO3A. AMPK phosphorylation was significantly increased in old, but not young, subjects immediately post-exercise; however, no differences in the responses of AMPK activity (assessed by phospho-ACC) or FOXO3A phosphorylation between age groups were observed. Nevertheless, amongst all subjects regardless of age, higher glycogen content was associated with lower phospho/total ACC ratio and higher FOXO3A phosphorylation at the PRE, 0P, and 1P time points. Additionally, the FOXO3A phosphorylation response to resistance exercise was significantly greater in subjects selected for high as compared to low pre-exercise muscle glycogen content. These findings suggest that increasing muscle glycogen content may enhance the FOXO3A Ser318/321 phosphorylation response, and thus potentially lessen the muscle protein degradation response, to resistance exercise regardless of age. However, a strong relationship was not seen between AMPK activity (as measured by the phospho/total ACC ratio) and FOXO3A Ser318/321 phosphorylation, suggesting that another glycogen-mediated factor may play a role in influencing the protein degradation pathway following resistance training in younger and older subjects.