The IGFBP-3 mRNA and protein levels are IGF-I-dependent and GH-independentin MG-63 human osteosarcoma cells

Growth Hormone (GH), Insulin-like Growth Factors (IGFs) and IGF-Binding Pro teins which modulate the IGFs' bioavailability (e.g. IGFBP-3, -4, -5), are essential regulators of bone remodeling. In this study, MG-63 human osteosa rcoma cells were used as a model system to investigate the mechanism(s) whe reby IGF-I and GH control IGFBP-3 gene expression. Physiological concentrat ions of IGF-I (1-20 nM) induced a dose-dependent increase in the steady-sta te amount of IGFBP-3 mRNA (maximal stimulation: similar to 9-10-fold). This increase was delectable 3 h after the onset of IGF-I treatment, was enhanc ed over a 24 h period, then plateaued until at least 30 h. Consistently, a dose-dependent increase in IGFBP-3 secretion (similar to 40-50-fold at IGF- I concentrations greater than or equal to 16 nM) was observed by western li gand- and immune-blot analysis of MG-63 cells conditioned medium, and its t ime course was similar to that observed for IGFBP-3 transcripts. IGFBP-3 mR NA stability (t(1/2) similar to 20 h) was identical in the presence or abse nce of IGF-I treatment. By contrast, human (h) GH treatment (24-72 h) of MG -63 cells did not increase IGFBP-3 secretion in the conditioned medium. Ect opic expression of recombinant rat GH-R resulted in hGH-enhanced expression of GH-responsive reporter gene constructs, but did not increase endogenous IGFBP-3 gene expression, suggesting that the GH unresponsiveness was not o nly due to the very low level of GH binding sites at the plasma membrane le vel. Altogether, these results support the conclusions that in MG-63 cells (i) transcriptional rather post-transcriptional mechanisms are involved in the IGF-I-induced increase of IGFBP-3; (ii) the abundance of GH-R is very l ow at the plasma membrane level; (iii) the dowstream GH-signaling cascade i s fully functional in this human osteosarcoma cell line; and (iv) the endog enous IGFBP-3 gene is not responsive to hGH in human MG-63 osteosarcoma cel ls. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.