Insulin-like growth factor binding protein-3 modulates osteoblast differentiation via interaction with vitamin D receptor.

Insulin-like growth factor (IGF) binding protein-3 (IGFBP-3) is a secreted glycoprotein that reduces the bioavailability of IGFs. This glycoprotein has both IGF-dependent and -independent effects on cell growth. However, the mechanisms responsible for the IGF-independent actions of IGFBP-3 are not fully understood. In the present study, we used multiple methodologies including glutathione S-transferase pull-down assay and co-immunoprecipitation to demonstrate that IGFBP-3 can directly interact with vitamin D receptor (VDR) in vitro and in vivo. Furthermore, immunofluorescence co-localization studies showed that IGFBP-3 and VDR could co-localize in the cell nucleus. Reporter gene experiment showed that IGFBP-3 negatively regulates the growth hormone promoter activity induced by ligand-activated VDR. Moreover, real-time RT-PCR demonstrated that IGFBP-3 can inhibit the osteocalcin and CYP24a1 mRNA transcription induced by 1,25-(OH)2D3 in osteoblastic cells. Finally, alkaline phosphatase activity significantly decreased in osteoblastic cells when the cells were transfected with IGFBP-3 in the presence of 1,25-(OH)2D3. In conclusion, these studies provide evidence that overexpression of IGFBP-3 suppresses osteoblastic differentiation regulated by VDR in the presence of 1,25-(OH)2D3. These findings reveal a novel mechanism by which IGFBP-3 functions.