INDUCTION OF INSULIN-LIKE GROWTH-FACTOR BINDING PROTEIN-1 GENE-EXPRESSION IN LIVER OF PROTEIN-RESTRICTED RATS AND IN RAT HEPATOMA-CELLS LIMITED FOR A SINGLE AMINO-ACID

Restriction of the dietary protein intake of young growing animals results in a rapid cessation of growth. In order to gain further insight into the molecular mechanisms for metabolic adaptation to protein restriction, the expression of the insulin-like growth factor binding protein-1 (IGFBP-1) gene was examined in 4-week-old male rats fed isocaloric diets containing 20%, 8%, or 4% protein over a 10-day period. Expression of the IGFBP-1 gene was strongly induced in the protein-restricted animals. Animals on the 8% protein diet exhibited a 14-fold increase, and animals on the 4% protein diet exhibited a 33-fold increase in hepatic IGFBP-1 messenger RNA (mRNA) abundance relative to the abundance of IGFBP-1 mRNA in animals on the 20% protein diet. Expression of the IGFBP-1 gene was also strongly increased by severe energy restriction: IGFBP-1 mRNA abundance was increased 15-fold in animals maintained for 10 days on a diet with energy restricted to 50% of the ad libitum intake rate. In animals fasted for 24 h there was a 6-fold increase in IGFBP-1 mRNA abundance, a lower induction than was observed in either of the two chronic nutritional restriction models. To determine whether limitation for substrate (i.e. amino acids) might have a direct effect on IGFBP-1 gene expression, we examined the effect on IGFBP-1 gene expression of limitation of H4-II-E rat hepatoma cells for a single essential amino acid (phenylalanine, methionine, leucine, or tryptophan) for a period of 24 h. The abundance of IGFBP-1 mRNA was increased by approximately 4- to 5-fold in cultures limited for any of these four amino acids as compared with its abundance in cells incubated in medium containing all essential amino acids. To study further the molecular mechanism for induction of IGFBP-1 gene expression by nutritional restriction, probes specific for intron 3 or intron 1 of the rat IGFBP-1 gene were used to quantify levels of the IGFBP-1 primary nuclear transcript in protein-restricted rats and amino acid-limited cultured cells. The level of the IGFBP-1 primary transcript was increased by 8-fold in animals on the 8% protein diet and 14-fold in animals on the 4% protein diet, suggesting that the induction of IGFBP-1 mRNA was caused largely by an increase in transcription. However, since the induction of the primary transcript was not as great as the induction of IGFBP-1 mRNA, it is possible that there was also a posttranscriptional component for the observed regulation. In the amino acid-limited H4-II-E cells, there was a small increase in IGFBP-1 primary transcript and a substantially greater increase in IGFBP-1 mRNA, suggesting that the induction of IGFBP-1 mRNA was caused by both transcriptional and posttranscriptional mechanisms. The overall results indicate that limitation for essential amino acids may be a factor contributing to the induction of IGFBP-1 gene expression in the liver of protein-restricted animals.