In this study we investigated the mechanism(s) by which insulin-like growth factor-binding protein-3 (IGFBP-3) potentiates IGF-I action in cultured bovine fibroblasts. Preincubation of cells with glycosylated or nonglycosylated recombinant human IGFBP-3 enhanced responsiveness to IGF-I in a time-dependent manner. A preincubation period of at least 24 h with IGFBP-3 was required to see a significant effect. Pretreatment with IGFBP-3 for 72 h resulted in a 2-to 4-fold augmentation of IGF-I-stimulated [3H]aminoisobutyric acid uptake; IGFBP-3 had no effect on basal [3H]aminoisobutyric acid uptake. During the preincubation period, exogenous IGFBP-3 associated with the fibroblast surface and exhibited time-dependent processing to lower mol wt forms that retained the ability to bind radiolabeled IGF-I. Initial surface adherence (preincubation time of 24 h or less) was readily reversible. However, IGFBP-3, once processed, appeared to be closely associated with the cell. After 72 h of exposure to bovine fibroblasts, cell-associated IGFBP-3 had a 10-fold lower affinity for IGF-I com pared to intact IGFBP-3 in solution. In addition, incubation of bovine fibroblasts with IGFBP-3 had modifying effects on type I IGF receptor-mediated signalling because 1) the bioeffectiveness of [Gln3, Ala4, Tyr15, Leu16]IGF-I and insulin, IGF-I receptor activators with little or no affinity for IGFBP-3, was potentiated by preincubation with IGFBP-3; and 2) fibroblast responsiveness to IGF-I analogs with different affinities for the type I IGF receptor was enhanced in direct relation to the ability of the peptide to bind to the receptor. There was no evidence for an increase in receptor number or affinity as a result of IGFBP-3 treatment. These data suggest that IGFBP-3 potentiation of IGF-I action in bovine fibroblasts may involve changes in IGFBP-3 and type I IGF receptor responsiveness. Thus, cell-associated IGFBP-3 may provide a mechanism for optimal presentation of IGF-I to its receptor as well as a means to heighten receptor reactivity to IGF-I and related peptides.
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