Alternative signaling pathways from IGF1 or Insulin to Akt activation and Foxo1 nuclear efflux in adult skeletal muscle fibers.

Muscle atrophy is regulated by the balance between protein degradation and synthesis. Foxo1, a transcription factor, helps to determine this balance by activating pro-atrophic gene transcription when present in muscle fiber nuclei. Foxo1 nuclear efflux is promoted by Akt-mediated Foxo1 phosphorylation, eliminating Foxo1's atrophy promoting effect. Akt activation can be promoted by IGF1 or Insulin via a pathway including IGF1 or Insulin, PI3K, and Akt. We used confocal fluorescence time-lapse imaging of Foxo1-GFP in adult isolated living muscle fibers maintained in culture to explore the effects of IGF1 and insulin on Foxo1-GFP nuclear efflux with and without pharmacological inhibitors. We observed that while Akt inhibitor blocks the IGF1 or insulin induced effect on Foxo1 nuclear efflux, PI3K inhibitors, which we show to be effective in these fibers, do not. We also found that inhibition of the protein kinases Ack1 or ATM, contributes to the suppression of Foxo1 nuclear efflux after IGF1. These results indicate a novel pathway that has been unexplored in the IGF1 or insulin induced regulation of Foxo1 and present information useful for both therapeutic interventions for muscle atrophy, as well as further investigative areas into insulin insensitivity and type 2 diabetes.