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Deprivation of cultured BRL-3A rat liver cells for a single essential amino acid (leucine, methionine, tryptophan, or phenylalanine) under conditions in which the cells remain highly viable causes a decreased secretion of insulin-like growth factor-II (IGF-II) E-domain peptide into the culture medium. This decrease is observed within 8 h after shifting the cells into amino acid-deficient medium. The magnitude of this decrease is greatest in tryptophan-deprived cultures, in which there is a 66% decrease in IGF-II E-domain peptide secretion over a 24-h incubation as compared with E-domain peptide secretion in control cultures incubated in complete medium. Northern blot analysis has indicated that the decrease in IGF-II E-domain peptide secretion observed in amino acid-deprived cells is correlated with a decreased abundance of the major 3.6-kilobase (kb) species of IGF-II mRNA, as well as several minor species. In contrast, the level of a 1-kb species of IGF-II mRNA is not decreased in amino acid-limited cells. In addition, one other specific mRNA, that encoded by the alpha-tubulin gene, also is not significantly decreased in the leucine, phenylalanine, or tryptophan-limited cells. These results indicate that amino acid limitation specifically decreases the level of certain IGF-II mRNA species. The 3.6- and 1-kb IGF-II mRNA species differ only in the length of the 3'-untranslated region, suggesting the existence of a specific regulatory sequence in the long 3'-untranslated region of the 3.6-kb mRNA species that regulates levels of this mRNA species under conditions of amino acid limitation. IGF-II gene transcription is not decreased in the amino acid-limited cells. This indicates that the decrease in IGF-II mRNA and E-domain peptide secretion observed in the amino acid-deprived cells is caused primarily by a post-transcriptional regulatory mechanism. |
