Comparison studies of IGFBP-5 binding to osteoblasts and osteoblast-derived extracellular matrix

Recent studies have identified a specific membrane protein in osteoblast-like cells which binds intact and carboxy-truncated IGFBP-5 with high affinity. The purpose of the present study was to evaluate the IGFBP-5 binding properties of osteoblast-derived extracellular matrix (ECM), with special interest in determining whether ECM proteoglycans were necessary for IGFBP-5 binding. Neonatal mouse osteoblasts and the ECM of these cells both bound intact [¹²⁵I]IGFBP-5 and [¹²⁵I]IGFB_-5^1–169, though the ECM bound both forms with lower affinity when compared to their cellular binding. Treatment of the ECM with heparinase or chondroitinase, to remove glycosaminoglycan (GAG) side-chains of proteoglycans, resulted in 20–34% enhanced binding of intact [¹²⁵I]IGFBP-5 and a 92–100% enhancement of [¹²⁵I]IGFBP-5^1–169 binding. Similar enzymatic treatment of osteoblast monolayers had no effect on the binding of either form of [¹²⁵I]IGFBP-5. These results indicate that GAGs within ECM secreted by neonatal mouse osteoblasts do not mediate the binding of IGFBP-5. This study also shows that intact and carboxy-truncated IGFBP-5 preferentially bind to the osteoblast surface, but that removal of GAGs from osteoblast-derived ECM can increase IGFBP-5 localization to this pericellular space, particularly the carboxy-truncated form of IGFBP-5.