Translational status of the insulin-like growth factor-I mRNAs in liver of protein-restricted rats

Dietary protein restriction in young rats decreases serum insulin-like growth factor-I (IGF-I) concentrations and the amount of liver IGF-I mRNA, suggesting that regulation for the liver IGF-I gene expression in this model occurs at a pretranslational level. To determine whether there is also translational control, we assessed the association of the liver IGF-I mRNA transcripts with polysomes in livers of normally fed (15% dietary protein) versus protein-restricted (5% dietary protein) rats. One week of dietary protein restriction reduces serum IGF-I concentrations by 54% and the amount of liver IGF-I mRNA by 35%, with the 7.5 kb size-class of IGF-I mRNA being the most affected (-48%). Protein restriction reduces the amount of the polysomal IGF-I mRNAs by 30%, a value in close agreement with the changes in total IGF-I mRNAs. Protein restriction is not associated with changes in the distribution of IGF-I mRNAs between the polysomal and non-polysomal fractions. All major size-classes of IGF-I mRNA transcripts (7.5, 4.7, 1.7, 0.9-1.2 kb) are associated with the polysomes in both dietary groups, suggesting involvement in the initiation phase of the IGF-I translation. We conclude that no untranslatable pool of IGF-I mRNAs is present in the liver of protein-restricted animals. Protein restriction, however, decreases slightly the mean size of polysomes. This decrease in ribosomal number associated with the IGF-I mRNA could decrease translational efficiency. Our data suggest that dietary protein restriction does not impair the initiation of IGF-I mRNA translation.