Insulin-like growth factor (IGF)-binding protein-3 blocks IGF-I-induced receptor down-regulation and cell desensitization in cultured bovine fibroblasts.
Insulin-like growth factor-I (IGF-I) initiates its diverse biological effects by binding to type I IGF receptors on cells. In addition, IGF-I associates with distinct proteins that can modulate its actions. One of these IGF-binding proteins, IGFBP-3, is the major circulating form in adults and is produced by many cells in culture. We investigated the effect of purified bovine IGFBP-3 on IGF-I binding and IGF-I stimulation of amino acid uptake and DNA synthesis in cultured bovine fibroblasts, a cell culture system highly suitable for these types of studies. Incubation of cells with IGF-I resulted in time- and dose-dependent decreases in [125I]IGF-I binding and IGF-I stimulated [3H]aminoisobutyric acid uptake and [3H]thymidine incorporation. Preincubation with 4 nM IGF-I resulted in a 50-60% decrease in IGF-I receptor binding, accompanied by marked decreases in IGF-I-stimulated [3H]aminoisobutyric acid uptake (50-60%) and [3H]thymidine incorporation (80-90%). Preincubation with the IGF-I analog [QAYL]IGF-I (4 nM) or with 100 nM insulin, growth factors that bind and activate type I IGF receptor signalling but have little or no affinity for IGFBP-3, had effects comparable to IGF-I, decreasing both IGF-I binding and action 50-95%. The addition of IGFBP-3 during the preincubation period with IGF-I blocked the decrease in receptor availability and prevented the cells from becoming desensitized. IGFBP-3 did not prevent the [QAYL]IGF-I- or insulin-induced receptor loss and cellular resistance to IGF-I. These data indicate that IGFBP-3 can prevent IGF-I-induced receptor down-regulation, a process that renders cells refractory to further stimulation by IGF-I. Thus, cell-derived IGFBP-3 may function in a buffering capacity to restrict IGF-I and target cell interaction, thereby modulating the biological response to changes in local IGF-I levels.
Conover
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Endocrine Research Unit, Mayo Clinic, Rochester, Minnesota 55905.
Powell
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D000621:Aminoisobutyric Acids / Q000378:metabolism; D000818:Animals; D001667:Binding, Competitive; D002352:Carrier Proteins / Q000494:pharmacology*; D002417:Cattle; D002478:Cells, Cultured; D003432:Cross-Linking Reagents; D015536:Down-Regulation / Q000187:drug effects*; D005347:Fibroblasts / Q000187:drug effects / Q000378:metabolism*; D018969:Insulin-Like Growth Factor Binding Proteins; D007334:Insulin-Like Growth Factor I / Q000378:metabolism / Q000494:pharmacology*; D008970:Molecular Weight; D011956:Receptors, Cell Surface / Q000378:metabolism*; D017451:Receptors, Somatomedin
D000621:Aminoisobutyric Acids; D002352:Carrier Proteins; D003432:Cross-Linking Reagents; D018969:Insulin-Like Growth Factor Binding Proteins; D011956:Receptors, Cell Surface; D017451:Receptors, Somatomedin; C100049:2-aminoisobutyric acid; D007334:Insulin-Like Growth Factor I