Although starvation predict ably reduced the phosphorylation of 4E BP1 and increased the binding of 4E BP1 to eIF4E, PDCD4 never depletion had no effects on these parameters. Likewise, in starved myotubes, PDCD4 depletion had no effect on S6K1 or S6 phosphorylation. However, there was a trend towards reduced eIF4G in cells depleted of PDCD4. Furthermore, PDCD4 depletion significantly reduced eIF4G interaction with eIF4E. Discussion In this study, we demonstrated that in myotubes, the regu lation of PDCD4 abundance was reversibly modified by a starvation refeeding cycle. Collectively, the data presented here are the first evidence to demonstrate a requirement for mTORC1 and the proteasome in regulating the abun dance of PDCD4 in muscle cells.

We also presented evi dence that, at least in myotubes, in the absence of growth factors, amino acids had little effect in regulating the abundance of this protein. Finally, in starved myotubes, and contrary to observations in myoblasts and non muscle cells, depletion of PDCD4 had minimal effect on the incorporation of phenylalanine into myotube pro teins. Rather, in starved myotubes, PDCD4 depletion fur ther reduced eIF4G binding to eIF4E. In spite of the fact that PDCD4 has been characterized as a substrate of S6K1 and an inhibitor of cap dependent mRNA translation initiation, there is a paucity of information on the significance of PDCD4 in skeletal muscle. Also, it is unknown if the regulation of PDCD4, like mTORC1 S6K1, is sensitive to nutrients. In the present study, Ser67 and Ser457 phosphorylation of PDCD4 correlates poorly with its abundance.

A requirement for mTORC1 S6K1 in regulating PDCD4 abundance suggests that PDCD4 may be phosphorylated on additional residues. However, PDCD4 degradation appears to depend specifically on Ser67 phosphorylation. It is also possible that phos phorylated PDCD4 does not accumulate because degrad ation by the proteasome is very rapid. However, in refed cells treated with MG132, Ser67 phosphorylated PDCD4 did not accumulate to a greater extent in comparison with cells not treated with the drug. Although amino acids can activate mTORC1, the effects of amino acids require some amount of insulin. Our finding that leucine or a medium that con tained all the 20 amino acids but lacked growth factors had insignificant effects on PDCD4 abundance is consist ent with this view.

AKT too may phosphorylate PDCD4 and target it for degradation. In fact, a require ment for serum rather than amino acids might implicate AKT rather than mTORC1 S6K1 in the phosphorylation and degradation of PDCD4 since AKT does not require amino acid for its activation. Brefeldin_A However, incubation pamycin would not only inhibit mTORC1 S6K1 but should lead to a greater activation of PI3K AKT path way due to the loss of negative inhibition conveyed by ac tivated S6K1.