Comparison of the intracellular itineraries of insulin-like growth factor-I and insulin and their receptors in Rat-1 fibroblasts

In this study we have compared the intracellular itineraries of insulin and insulin-like growth factor-I (IGF-I) and their receptors subsequent to ligand internalization in rat fibroblasts. We found that the endocytic rate constant is approximately 3 times as high for insulin as for IGF-I. The dissociation of internalized ligand from its receptor was monitored by the ability of ligand-receptor complexes to precipitate in the presence of polyethylene glycol (PEG). Insulin loses its ability to precipitate with PEG more rapidly than IGF-I. After 60 min, less than 10% of the intracellular insulin remains PEG precipitable, whereas 44% of intracellular IGF-I stays PEG precipitable. Ligand degradation was determined by precipitation with trichloroacetic acid (TCA). Insulin degradation after internalization is more rapid compared with IGF-I degradation; after 2 h, 80% of intracellular IGF-I, in contrast to only 30% of intracellular insulin, remains intact. We measured retroendocytosis of insulin and IGF-I by asse...