The Paracrine Feedback Loop Between Vitamin D(3) (1,25(OH)(2)D(3)) and PTHrP in Prehypertrophic Chondrocytes

The endocrine feedback loop between vitamin D(3) (1,25(OH)(2)D(3)) and parathyroid hormone (PTH) plays a central role in skeletal development. PTH-related protein (PTHrP) shares homology and its receptor (PTHR1) with PTH. The aim of this study was to investigate whether there is a functional paracrine feedback loop between 1,25(OH)(2)D(3) and PTHrP in the growth plate, in parallel with the endocrine feedback loop between 1,25(OH)(2)D(3) and PTH. This was investigated in ATDC5 cells treated with 10(−8) M 1,25(OH)(2)D(3) or PTHrP, Col2-pd2EGFP transgenic mice, and primary Col2-pd2EGFP growth plate chondrocytes isolated by FACS, using RT-qPCR, Western blot, PTHrP ELISA, chromatin immunoprecipitation (ChIP) assay, silencing of the 1,25(OH)(2)D(3) receptor (VDR), immunofluorescent staining, immunohistochemistry, and histomorphometric analysis of the growth plate. The ChIP assay confirmed functional binding of the VDR to the PTHrP promoter, but not to the PTHR1 promoter. Treatment with 1,25(OH)(2)D(3) decreased PTHrP protein production, an effect which was prevented by silencing of the VDR. Treatment with PTHrP significantly induced VDR production, but did not affect 1α- and 24-hydroxylase expression. Hypertrophic differentiation was inhibited by PTHrP and 1,25(OH)(2)D(3) treatment. Taken together, these findings indicate that there is a functional paracrine feedback loop between 1,25(OH)(2)D(3) and PTHrP in the growth plate. 1,25(OH)(2)D(3) decreases PTHrP production, while PTHrP increases chondrocyte sensitivity to 1,25(OH)(2)D(3) by increasing VDR production. In light of the role of 1,25(OH)(2)D(3) and PTHrP in modulating chondrocyte differentiation, 1,25(OH)(2)D(3) in addition to PTHrP could potentially be used to prevent undesirable hypertrophic chondrocyte differentiation during cartilage repair or regeneration.