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1 Department of Cellular and Molecular Physiology, Penn State University College of Medicine, Hershey, PA, USA
* To whom correspondence should be addressed. E-mail: tvary{at}psu.edu.
Feeding promotes protein accretion in skeletal muscle through a stimulation of the mRNA translation initiation phase of protein synthesis either secondary to nutrient-induced rises in insulin or owing to direct effects of nutrients themselves. The present set of experiments establish the effects of meal feeding on potential signal transduction pathways that may be important in accelerating mRNA translation initiation. Gastrocnemius from male Sprague Dawley rats trained to consume a meal consisting of rat chow, was sampled prior to, during and following the meal. Meal feeding enhanced the assembly of the active eIF4G.eIF4E complex, which returned to basal levels within 3 hours of removal of food. The increased assembly of the active eIF4G.eIF4E complex was associated with a marked 10-fold rise in phosphorylation of eIF4G(Ser1108) and a decreased assembly of inactive 4E-BP1.eIF4E complex. The reduced assembly of 4E-BP1.eIF4E complex was associated with a 75-fold increase in phosphorylation of 4E-BP1 in the 
-form during feeding. Phosphorylation of S6K1 on Ser789 was increased by meal feeding, although the extent of phosphorylation was greater at one half hour after feeding than after 1 hour. Phosphorylation of mammalian target of rapamycin (mTOR) on Ser2448 or Ser2481, an upstream kinase responsible for phosphorylating both S6K1 and 4E-BP1, was increased at all times during meal feeding although the extent of phosphorylation was greater at one half hour after feeding than after 1 hour. Phosphorylation of PKB, an upstream kinase responsible for phosphorylating mTOR, was only elevated after one half hour of meal feeding for Thr308, whereas phosphorylation Ser473 was significantly elevated at only one half and one hour after initiation of feeding. We conclude from these studies that meal feeding stimulates two signal pathways in skeletal muscle that lead to elevated eIF4G.eIF4E complex assembly through increased phosphorylation of eIF4G and decreased association of 4E-BP1 with eIF4E.
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