STUDIES ON THE REGULATION OF INSULIN-LIKE GROWTH-FACTOR BINDING-PROTEIN-3 SECRETION IN HUMAN OSTEOSARCOMA CELLS IN-VITRO

Previous studies demonstrated that insulin-like growth factors (IGFs) are important autocrine and paracrine mitogens for human bone cells in vitro and that IGF binding proteins (IGFBPs) are important regulators of the biologic actions of IGFs. Thus, the actions of IGFs may be det ermined not only by their concentrations but also by the type and amou nt of IGFBPs produced by human bone cells at a local site in bone. In this study, we sought to determine the effects of dexamethasone, 1,25- (OH)(2)D-3, and parathyroid hormone (PTH) on the secretion of IGFBP-3 in human osteosarcoma cell lines. Serum-free cultures of low- and high -alkaline phosphatase (ALP) SaOS-2, MG-63, and TE89 human osteosarcoma cells were treated for 24 or 48 h with the effecters and the conditio ned media used for determination of IGFBP-3 using a radioimmunoassay. We report that (1) the basal rate of IGFBP-3 secretion (ng/mg cellular protein) was dependent upon cell type, with TE89 > low-ALP Saos-2 > M G-63 > high-ALP SaOS-2 cells, and did not correlate with either basal cell proliferation or basal cellular ALP activity; (2) dexamethasone ( 10-(12)-10(-7) M) inhibited IGFBP-3 secretion in a dose-dependent mann er in low-ALP SaOS-2, MG-63, and TE89 cells but not in high-ALP SaOS-2 cells; (3) 1,25-(OH)(2)D-3 (10-(11)-10(-8) M) stimulated IGFBP-3 secr etion in a dose-dependent manner in MG-63, low-ALP SaOS-2, and high-AL P SaOS-2 cells, and the coaddition of TGF-beta and 1,25-(OH)(2)D-3 inc reased synergistically IGFBP-3 secretion and cellular ALP activity in MG-63 cells; and (4) human PTH-(1-34) (0.1-100 ng/ml) had no significa nt effect on IGFBP-3 secretion in MG-63, low-ALP SaOS-2, or high-ALP S aOS-2 cells. We conclude that such agents as dexamethasone, 1,25-(OH)( 2)D-3, and PTH differentially regulate 1GFBP-3 secretion in human oste osarcoma cells in vitro.