Regulation of insulin‐like growth factor binding protein 4 by a specific insulin‐like growth factor binding protein 4 proteinase in normal human osteoblast‐like cells: Implications in bone cell physiology

Insulin‐like growth factor binding protein 4 (IGFBP‐4) is secreted by normal human osteoblast‐like cells (hOB) and is a potent inhibitor of insulin‐like growth factor (IGF) action in vitro. In previous studies, IGF treatment of hOB in culture led to markedly reduced medium levels of IGFBP‐4 as detected by western ligand blotting. In the present study, incubation of hOB‐conditioned medium (hOB‐CM) with IGF under cell‐free conditions resulted in a similar loss of IGFBP‐4. Both IGF‐I and IGF‐II were capable of inducing a decrease in IGFBP‐4; however, IGF‐II was more effective. When the six characterized IGFBP were added to hOB‐CM, only IGFBP‐4 disappeared in response to IGF‐II addition. This IGF‐regulated loss of IGFBP‐4 was inhibited by metalloproteinase inhibitors and appeared to be due to a proteinase that cleaved IGFBP‐4 in 18 and 14 kD fragments identified by western immunoblotting. Conditioned media from eight of eight different donor hOB lines tested exhibited IGFBP‐4 proteinase activity. To assess the biologic consequences of IGF‐II‐induced IGFBP‐4 proteolysis, we treated hOB with IGF‐II for 5 h, which decreased medium IGFBP‐4 by 70%, and then measured IGF‐I and insulin stimulation of [³H]thymidine incorporation. IGF‐II itself was not mitogenic and had no effect on insulin‐stimulated [³H]thymidine incorporation. However, pretreatment of cultured hOB with IGF‐II enhanced IGF‐I‐stimulated [³H]thymidine incorporation threefold. In conclusion, these data indicate that (1) regulation of IGFBP‐4 availability can occur via an IGF‐dependent, IGFBP‐4 proteinase secreted by normal hOB, and (2) IGF‐II‐induced IGFBP‐4 proteolysis is associated with enhanced hOB response to IGF‐I. The IGFBP‐4/IGFBP‐4 proteinase system may be involved in local regulation of IGF action in bone.