Comparison of the effects of Vitamin D Metabolites on Osteoblast and Osteocyte Bone Cells

Abstract

While the major source of vitamin D is D3 from ultraviolet exposure, some supplements supply D2. The relative potency of vitamin D2 versus vitamin D3 remains controversial. The aims of the current study were, 1. To optimize the in vitro model, including use of cell lines, vitamin D concentrations, and outcome biomarkers. 2. To compare the potency of vitamin D2 and D3 metabolites on mouse and human bone cellular activity. 3. To explore the expression of VDR in osteoarthritic (OA) bone tissues as well as cellular responses to vitamin D2 and D3 metabolites ex-vivo. In mouse 2T3 osteoblasts, at physiological doses, both vitamin D2 and D3 metabolites increased ALP activity and mineralisation and up-regulated osteoblastic signature genes and proteins. At supra-physiological doses D3 metabolites were more potent inhibitors of 2T3 function than D2 metabolites. Although hBMS cell proliferation was inhibited by both 25(OH)D2 and D3, ALP activity was enhanced by both metabolites. However, 25(OH)D3 was a more potent stimulator of ALP and mineralisation of hBMSCs. D2 and D3 equally stimulated expression of CX43 and PHEX markers in osteocytic cell lines. Immunohistochemistry of femoral heads showed much reduced VDR expression in OA osteocytes and osteoclasts, yet both 25(OH)D2 and D3 increased OA-hBMSCs mineralisation more than non-OA-hBMSCs ex-vivo. While vitamin D2 or D3 increased mouse 2T3 osteoblastic activity at physiological doses, OA and non-OA hBMSCs differentiation was more responsive to 25(OH)D3. Key bone cells such as osteocyte and osteoclasts expressed less VDR in OA. For the first time vitamin D2 metabolites have been thoroughly examined and emerged as a potent stimulator of bone cell differentiation, at least in vitro. Vitamin D3 in contrast is confirmed as highly potent in bone cells, but with toxicity at much lower doses than D2.This thesis is not currently available on ORA