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Articles in PresS, published online ahead of print October 29, 2001
Am J Physiol Endocrinol Metab, 10.1152/ajpendo.00269.2001
Submitted on June 20, 2001
Accepted on October 25, 2001
1 Department of Cellular and Molecular Physiology, The Pennsylvania State University College of Medicine, Hershey, PA, USA
2 Noll Physiology Research Center, The Pennsylvania State University, University Park, PA, USA
3 ARS, USDA, Houston, TX, USA; Department of Pediatrics, Children's Nutritional Research Center, Baylor College of Medicine, Houston, TX, USA
* To whom correspondence should be addressed. E-mail: skimball{at}psu.edu.
Our previous studies showed that the feeding-induced stimulation of protein synthesis in skeletal muscle of neonatal pigs is accompanied by enhanced phosphorylation of the eIF4E-binding protein, 4E-BP1, and the ribosomal protein S6 kinase, S6K1. These effects of feeding are substantially reduced with development. The goal of the present investigation was to delineate the basis for the reduced responsiveness to feeding observed in the older animals. In these studies, the content and activity of protein kinases located upstream of S6K1 and 4E-BP1 in signal transduction pathways activated by amino acids, insulin, and IGF-I were examined in 7- and 26-day-old pigs that were either fasted overnight or fed porcine milk after an overnight fast. Feeding stimulated PI 3-kinase activity to the same extent in muscle of 7- and 26-day-old pigs, suggesting that PI 3-kinase is not limiting in muscle of older animals. In contrast, protein kinase B (PKB) activity was significantly less in muscle from 26- vs. 7-day-old pigs, regardless of nutritional status, suggesting that its activity is regulated by mechanisms distinct from PI 3-kinase. In part, the reduced PKB responsiveness can be attributed to a developmental decline in PKB content. Likewise, muscle content of the protein kinase termed mammalian target of rapamycin (mTOR) in 26-day-old pigs was less than 25% of that in 7-day-old animals. Finally, in agreement with our earlier work showing that S6K1 phosphorylation is reduced in older animals, S6K1 activity was stimulated to a lesser extent in 26- compared to 7-day-old pigs. Overall, the results suggest that the blunted protein synthetic response observed in 26- vs. 7-day-old neonatal pigs is due in part to decreased content and/or activity of signaling components downstream of PI 3-kinase, e.g. PKB, mTOR, and S6K1.
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