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Department of Cellular and Molecular Physiology, Pennsylvania State College of Medicine, Hershey, Pennsylvania 17033
The goal of these
studies was to investigate the mechanisms by which amino acid supply
regulates global rates of protein synthesis as well as the translation
of ribosomal protein (rp) mRNAs in liver. In the experiments conducted,
male weanling rats were trained over a 2-wk period to consume their
daily food intake within 3 h. On day 14, rats were fed
the control diet or an isocaloric, isonitrogenous diet lacking glycine,
tryptophan, leucine, or the branched-chain amino acids (BCAA) for
1 h. Feeding Trp-, Leu-, or BCAA-deficient diets resulted
in significant reductions in serum insulin, hepatic protein synthesis,
eukaryotic initiation factor 2B (eIF2B) activity, and phosphorylation
of eIF4E-binding protein 1 (4E-BP1) and ribosomal protein S6 kinase
(S6K1). Phosphorylation of eIF2
was inversely related to eIF2B
activity under all conditions. Alterations in the hepatic synthesis of
rp were assessed by changes in the distribution of rp (S4, S8, L26)
mRNAs across sucrose density gradients and compared with non-rp
(
-actin, albumin) mRNAs. In all dietary treatments, non-rp mRNAs
were mostly polysome associated. Conversely, the proportion of rp mRNAs
residing in polysomes was two- to fivefold less in rats fed diets
lacking tryptophan, leucine, or BCAA compared with rats fed the control
diet. Total hepatic abundance of all mRNAs examined did not differ
among treatment groups. For all parameters examined, there were no
differences between rats fed the glycine-deficient diet and rats fed
the control diet. The data suggest that essential amino acid (EAA)
deficiency inhibits global rates of liver protein synthesis via a block
in translation initiation. Additionally, the translation of rp mRNAs is
preferentially repressed in association with decreased S6K1 phosphorylation.
amino acid deficiency; messenger ribonucleic acid translation; ribosomal protein S6 kinase; essential amino acids
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