Insulin-like growth factor binding protein-3 (IGFBP-3) is unique among the IGF binding proteins in its extensive glycosylation in the native state. To determine the functional significance of carbohydrate moieties on IGFBP-3, we examined the effects of nonglycosylated Escherichia coli-derived recombinant human IGFBP-3 (hIGFBP-3E. coli) and glycosylated Chinese hamster ovary cell-derived hIGFBP-3 (hIGFBP-3CHO) on IGF-I action in cultured bovine fibroblasts. Both hIGFBP-3 preparations bound IGF-I with high affinity and were approximately 5-fold more potent than unlabeled IGF-I in inhibiting [125I]IGF-I binding to bovine fibroblasts. Coin-cubation of IGF-I and hIGFBP-3£ or hIGFBP-3CHOproduced a dose-dependent inhibition of IGF-I-but not insulin-stimulated [3H]aminoisobutyric acid (AIB) uptake. In contrast, preincubation of bovine fibroblasts with hIGFBP-3E. colior hIGFBP-3CH0 potentiated subsequent IGF-I-stimulated [3H]AIB uptake. When cells were preincubated with 50 nM hIGFBP-3E. colifor 24 h, [l25I]IGF-I binding to bovine fibroblasts increased 2.4-fold, whereas responsiveness to IGF-I was increased only 25%. After a 72-h preincubation, IGF-I cell binding remained increased 2fold with commensurate enhancement of IGF-I-stimulated [3H] AIB uptake. The increase in [125I]IGF-I binding to bovine fibroblast monolayers was primarily due to association of hIGFBP-3E. coliwith the cell surface; there was no significant change in IGF-I receptor number or affinity under these conditions. Affinity cross-linking experiments indicated that intense binding of [125I]IGF-I to cell-associated 29, 000 MrhIGFBP-3E. coliseen after 24 h of incubation was reduced approximately 70% after 72 h, concomitant with the appearance of smaller bands indicating hIGFBP-3E. coliforms of 12, 000-27, 000 Mr. Cell-associated IGFBP-3E. coli(72 h preincubation conditions) had a 10-fold lower affinity for IGF-I compared to hIGFBP-3E. coliin solution and a 2-fold lower affinity compared to the IGF-I receptor. These data demonstrate that glycosylation is not obligatory for biologically functional IGFBP-3. Furthermore, they suggest that processing of cell-associated IGFBP-3 to forms with altered affinity for IGF-I peptide may underly the potentiating effect of IGFBP-3 on IGF-I action.
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