Transforming growth factor-β1 stimulates the production of insulin-like growth factor-I and insulin-like growth factor-binding protein-3 in human bone marrow stromal osteoblast progenitors

Abstract

While transforming growth factor-β1 (TGF-β1) regulates proliferation and differentiation of human osteoblast precursor cells, the mechanisms underlying these effects are not known. Several hormones and locally acting growth factors regulate osteoblast functions through changes in the insulin-like growth factors (IGFs) and IGF-binding proteins (IGFBPs). Thus, we studied the effects of TGF-β1 on IGFs and IGFBPs in human marrow stromal (hMS) osteoblast precursor cells. TGF-β1 increased the steady-state mRNA level of IGF-I up to 8.5 ± 0.6-fold (P<0.001) in a dose- (0.1-10 ng/ml), and time-dependent (12-72 h) manner. No significant effects on IGF-II gene expression were detectable. Employing RNase protection and nuclear run-on assays, 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 transient transfection of various fragments of the IGF-I promoter 1, we found that TGF-β responsive elements were present in a promoter fragment ranging from - 65 bp to + 55 bp relative to the major transcription start site in exon 1. In addition, TGF-β1 treatment resulted in a dose- and time-dependent increase (2-fold) in the IGFBP-3 steady-state mRNA level as well as in protein production but did not affect IGFBP-2 or IGFBP-4 at mRNA or protein levels. Our results indicate that TGF-β1 exerts significant effects on stimulatory components of the IGF-system and that may represent a mechanism mediating TGF-β effects on the biological functions of osteoblasts.