Specific insulin-like growth factor-binding proteins (IGFBPs) are synthesized and secreted by bovine fibroblasts in vitro. By Western ligand blotting, three molecular forms of IGFBP were identified in conditioned medium from control cultures with mol wt (Mr) of 34, 000, 28, 000, and 24, 000. Concentrations of these three IGFBP forms increased with time in serum-free conditioned medium without benefit of hormonal supplementation. Insulin and IGF-I were potent stimuli for IGFBP production by bovine fibroblasts, whereas bovine GH, epidermal growth factor, or steroid treatment had little or no effect. Insulin and IGF-I enhanced the production of 24, 000, 28, 000, and 34, 000 Mr IGFBPs in a dose-dependent fashion. Moreover, addition of low nanomolar concentrations of insulin or IGF-I to bovine fibroblast cultures specifically induced the secretion of a 42, 000/38, 000 Mr species of IGFBP, which corresponded in size to the IGF-binding subunit of the principal 150, 000 Mr IGFBP complex in serum. After stimulation with insulin or IGF-I, bovine fibroblasts (3 x 105 cells) secreted approximately 30 ng/24 h 42, 000/38, 000 Mr IGFBP. Subunits of 42, 000/38, 000 Mr in bovine fibroblast-conditioned medium did not form macromolecular complexes in either the absence or presence of bovine GH.
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