Carboxy-truncated insulin-like growth factor binding protein-5 stimulates mitogenesis in osteoblast-like cells

Recently we demonstrated that a 23 kDa form of IGFBP-5, derived from osteoblast-like cells, stimulates osteoblast mitogenesis and enhances IGF-I action. Because osteoblast-derived IGFBP-5 is smaller than recombinant intact IGFBP-5 (23 vs 30 kDa) and has decreased binding affinity for IGF-I, we proposed that the native 23 kDa form of IGFBP-5 was truncated at a carboxy-terminal position. We now show that a recombinant form of carboxy-truncated IGFBP-5 binds IGF-I with reduced affinity and stimulates mitogenesis in mouse osteoblasts. We also show that 125I-truncated IGFBP-5 specifically binds to osteoblast monolayers with low binding affinity, similar to that seen with native 23 kDa IGFBP-5. These data indicate that carboxy-truncated IGFBP-5 stimulates osteoblast mitogenesis and suggest that reduced IGF-binding and cell-surface attachment are local mediators of this response.