Retinoid-X-receptor-alpha-independent binding of vitamin D receptor to its response element from human osteocalcin...

We have studied the role of retinoid X receptor alpha (RXR alpha) in vitamin D receptor (VDR) responsive interactions using nuclear extracts from human osteoblast-like MG-63 osteosarcoma cells and its specific response element from human osteocalcin gene (OC-VDRE). An RXR alpha-specific antibody was not capable of recognizing the two VDR responsive complexes formed with the OC-VDRE. Addition of in-vitro-produced RXR alpha to the binding reaction resulted in decreased binding of the two VDR-responsive interactions and, simultaneously, formation of a new complex, which was identified with RXR alpha- and VDR-specific antibodies. A similarly migrating RXR alpha- and VDR-responsive complex was also formed when baculovirus-expressed VDR was used with the in-vitro-produced RXR alpha in the absence of a nuclear extract or when VDRE from mouse osteopontin gene (OP-VDRE) was used as a binding site. Characterization of DNA binding properties for this VDR-RXR alpha complex revealed that both half sites of OC-VDRE are required for DNA binding. These results indicate that RXR alpha is probably not the physiological accessory factor in the MG-63 osteosarcoma cells needed for the VDR-VDRE interactions within the human osteocalcin gene promoter, although it is capable of dimerizing with recombinant VDR and the native VDR from these cells and although these dimers are capable of binding in vitro to the OC-VDRE.