Insulin-induced loss of insulin-like growth factor-I receptors on IM-9 lymphocytes

Preexposure of IM-9 lymphocytes to the somatomedin peptide insulin-growth factor-I (IGF-I) results in a time- and concentration-dependent reduction in specific receptors for IGF-I. Since insulin and proinsulin are structurally homologous to IGF-I, we investigated the ability of insulin analogues to compete for occupancy and to directly modulate IGF-I receptor concentrations. IGF-I binds rapidly and reversibly to IM-9 cells at 15°C, with half-maximal displacement of ¹²⁵I-IGF-I at IGF-I concentrations of 3.6 x 10^-9 M and insulin concentrations of 5 x 10^-7 M. Preexposure of cells at 37°C to either IGF-I or insulin produced a concentration-dependent reduction in binding of ¹²⁵I-IGF-I. A 50% decrease in binding was observed following preincubation of cells with IGF-I at 2.5 x 10^-9 M and insulin at 2 x 10^-7 M. At higher insulin concentrations (10^-6-10^-5 M), up to 70% reduction in ¹²⁵I-IGF-I binding occurred less potently than porcine insulin for the IGF-I receptor, and produced receptor loss in proportion to their ability to occupy the IGF-I receptor. Scatchard analysis indicated that at all insulin concentrations, the decrease in binding was secondary to loss of available IGF-I receptors, with no change in affinity. Receptor loss was evident following 1-2 h preexposure to insulin, with at t( 1/2 ) of 4 h and maximal receptor loss within 10 h. Similarly, IGF-I and IGF-II competed for occupancy of the IM-9 insulin receptor, with 50% displacement of ¹²⁵I-insulin occurring at peptide concentrations of M 3.5 x 10^-9 M (insulin), 3.5 x 10^-8 M (IGF-II), and 3 x 10^-7 M (IGF-I). Preexposure of cells to these peptides at 37°C for 20 h resulted in a concentration-dependent reduction in binding of ¹²⁵-I-insulin, with the order of analogue effectiveness being insulin > IGF-II > IGF-I. These data emphasize the structural and functional homology of insulin and the somatomedin peptides. IGF-I and II, as well as their respective receptors. Additionally, the data support the conclusion that the insulin and somatomedin peptides not only bind to both receptors, but downregulate each receptor in proportion to their ability to occupy that receptor.