Expression of insulin-like growth factor-I (IGF-I) receptor gene in rat brain and liver during development and in regenerating adult rat liver.

Insulin-like growth factor-I (IGF-I) is involved in the growth and development of liver and brain during fetal life by acting through specific plasma membrane receptors. In an attempt to determine how the changes in IGF-I receptor number are regulated during development, we have compared [125I]IGF-I binding to membrane fractions with the concentration of IGF-I receptor mRNA in rat liver and brain. IGF-I binding to liver membranes was 4.5 times higher in 20-day-old fetuses than in adult rats. After partial hepatectomy (70%) in adult rats a transient 2-fold increase of IGF-I binding to liver membranes was observed. In fetal and regenerating liver increases similar to those observed for IGF-I binding were observed in IGF-I receptor mRNA concentrations. In brain microsomes IGF-I binding was 3.5 times higher in 20-day-old fetuses than in adults. This difference reflects a similar change in the number of IGF-I receptors without modifications in the affinity of the receptor for the ligand. In contrast to the liver, no significant changes in the concentration of IGF-I receptor mRNA were observed in the developing brain when determined by hybridization solution, Northern blot or RNase protection analysis. These findings suggest that in the liver, the IGF-I receptor gene is regulated at pretranslational level during development and regeneration, while in brain it is preferentially regulated at translational or posttranslational level.