Evidence that human bone cells in culture secrete insulin‐like growth factor (IGF)‐II and IGF binding protein‐3 but not acid‐labile subunit both under basal and regulated conditions

Insulin‐like growth factors (IGFs) are found in human circulation predominantly as part of a growth hormone (GH)–dependent complex of 125–150 kD, which is composed of three subunits: IGF‐I or IGF‐II, an acid stable IGF binding protein (IGFBP)‐3, and an acid labile subunit (ALS). Although recent studies demonstrate that a number of cell types in culture secrete IGFs and IGFBP‐3, very little is known with regard to the origin of circulating ALS. To test the hypothesis that human bone cells (HBCs), which produce abundant amounts of IGF‐II and IGFBP‐3, also produce ALS, we measured the IGF‐I, IGF‐II, IGFBP‐3, and ALS levels using specific radioimmunoassays (RIAs) in the conditioned medium (CM) of untransformed normal HBCs and SaOS‐2 osteosarcoma cells treated with various effectors (IGF‐II, osteogenic protein‐1 [OP‐1, bone morphogenetic protein‐7] and human GH) for 48 h. No detectable levels (<3 ng/ml) of ALS were found in the CM of various HBC types under basal conditions. In contrast, CM collected from liver explants in culture contained significant amount of ALS prepared and assayed under identical conditions. The IGF‐I level was also undetectable in the CM of various HBC types. In the IGF‐II (3, 30 ng/ml)‐treated HBC CM, the IGFBP‐3 level was increased in a dose‐dependent manner but neither IGF‐I nor ALS could be detected. In the SaOS‐2 cell culture, OP‐1 (1, 100 ng/ml) increased both IGF‐II and IGFBP‐3 secretion but neither ALS nor IGF‐I secretion. Treatment of HBCs with GH (1, 10, 100 ng/ml) had no significant effect on the secretion of either IGF‐I, IGF‐II, IGFBP‐3, or ALS. The level of IGF‐II in the CM of various HBC types correlated positively with that of IGFBP‐3 (r = 0.84). From these results, we conclude that the production of ALS and IGFBP‐3 are not concomitantly regulated by the same effectors in HBCs. The finding of this study together with the previous findings that GH regulates ALS secretion in liver cells suggest that the primary functions of IGFs produced in the bone and liver may be different (i.e., local versus endocrine effects).