The 1α,25-dihydroxy Vitamin D3 receptor preferentially recruits the coactivator SRC-1 during up-regulation of the osteocalcin gene

Binding of 1α,25-dihydroxy Vitamin D3 to the C-terminal domain (LBD) of its receptor (VDR), induces a conformational change that enables interaction of VDR with transcriptional coactivators such as the members of the p160/SRC family or the DRIP (Vitamin D interacting complex)/Mediator complex. These interactions are critical for VDR-mediated transcriptional enhancement of target genes. Recent reports indicate that nuclear receptors, including VDR, interact with p160/SRC members and the DRIP/Mediator complex in a sequential, cyclical, and mutually exclusive manner when bound to a target promoter, exhibiting also a high exchange rate. Here, we present an overview of how these coactivators are recruited to the bone-specific osteocalcin (OC) gene in response to short and long exposures to 1α,25-dihydroxy Vitamin D3. We find that in intact osteoblastic cells VDR and SRC-1 rapidly bind to the OC promoter in response to the ligand. This recruitment correlates with transcriptional enhancement of the OC gene and with increased histone acetylation at the OC promoter. In contrast, binding of the DRIP205 subunit, which anchors the DRIP/Mediator complex to the VDR, is detected at the OC promoter after several hours of incubation with 1α,25-dihydroxy Vitamin D3. Together, our results indicate that VDR preferentially recruits SRC-1 to enhance basal bone-specific OC gene transcription. We propose a model where specific protein-DNA and protein-protein interactions that occur within the context of the OC gene promoter in osteoblastic cells stabilize the preferential association of the VDR-SRC-1 complex.