Transforming growth factor-beta 1 stimulates the production of insulin-like growth factor-1 and insulin-like growth factor-binding protein-3 in humanbone marrow stromal osteoblast progenitors

While transforming growth factor-beta1 (TGF-beta1) regulates proliferation and differentiation of human osteoblast precursor cells, the mechanisms und erlying these effects are not known. Several hormones and locally acting gr owth factors regulate osteoblast functions through changes in the: insulin- like growth factors (IGFs) and IGF-binding proteins (IGFBPs), Thus, we stud ied the effects of TGF-beta1 on IGFs and IGFBPs in human marrow stromal (hM S) osteoblast precursor cells. TGF-beta1 increased the steady-state mRNA le vel of IGF-I up to 8.5 +/-0.6-fold (P <0.001) in a dose- (0.1-10 ng/ml), an d time-dependent (12-72 h) manner, No significant effects on IGF-II gene ex pression were detectable. Employing RNase protection and nuclear run-on ass ays, these effects on IGF-I were found to take place at the transcriptional level and were not dependent on de novo protein synthesis. Using the trans ient transfection of various fragments of the IGF-I promoter 1, we found th at TGF-beta responsive elements were present in a promoter fragment ranging from - 65 bp to+55 by relative to the major transcription start site in ex on 1. In addition, TGF-beta1 treatment resulted in a dose- and time-depende nt increase (2-fold) in the IGFBP-3 steady-state mRNA level as well as in p rotein production but did not affect IGFBP-2 or IGFBP-4 at mRNA or protein levels. Our results indicate that TGF-beta1 exerts significant effects on s timulatory components of the IGF-system and that may represent a mechanism mediating TGF-beta effects on the biological functions of osteoblasts.