Vitamin D hormone regulates the expression of hedgehog coreceptor genes Boc and Cdon in osteocytes

Osteocytes are terminally differentiated cells descended from mesenchymal stem cells that are embedded in bone. Osteocytes are known to play an essential role in maintaining bone health, acting as mechanical sensors in bone: translating mechanical load into biochemical signals. Osteocytes have diverse regulatory functions that extend beyond bone, such as lipid regulation and immune system development. These cells are regulated by a number of hormones and cofactors, including the active form of vitamin D, 1,25 dihydroxyvitamin D3 (1,25D3). 1,25D3 binds and activates the vitamin D receptor (VDR) and this complex serves as a transcriptional regulator of various genes. Previously, chromatin immuno‐precipitation coupled to high throughput sequencing (ChIP‐seq) revealed that VDR binds to regions upstream and downstream of the genes coding for Cell adhesion molecule‐related/down‐regulate by oncogenes protein (Cdon) and Brother of Cdon (Boc). These two genes encode cell surface receptor proteins in the Hedgehog signaling pathway, that play an important role in animal tissue development. To better understand 1,25D3’s role in regulating Boc and Cdon, MC3T3‐E1 cells were transfected with luciferase reporter plasmids containing regions of VDRE occupancy identified by ChIP‐seq. These results revealed that these regions of Boc and Cdon mediate up‐regulation by 1,25D3. Specific vitamin D response elements (VDREs) within the enhancer regions were then identified by in silico analyses and selectively mutated. Analysis of these VDRE mutated luciferase reporters revealed an elimination of 1,25D3 activity. In summary, we have shown 1,25D3’s novel role in the up‐regulation of Boc and Cdon and identified the specific VDREs that are responsible for the 1,25D3 response of these genes in osteocytes. Further studies are underway to define the impact of 1,25D3 on Hedgehog signaling in osteocytes. These results contribute to an understanding of 1,25D3’s role in osteocyte and bone health and development. Grant Funding Source : Supported by NIH DK072281