Cells of the osteoblast lineage convert 25-hydroxyvitamin D (25D) to 1,25-dihydroxyvitamin D (1,25D) by virtue of the activity of the CYP27B1 enzyme, which appears to be necessary for the regulation of osteoblastic activity. To further understand the role for local synthesis of 1,25D, we have created two genetically modified mouse models in which Cyp27b1 expression is either ablated or enhanced within mature osteoblasts. Osteoblast-specific Cyp27b1-KO (ObCYP27B1KO) mice were generated with the Osteocalcin-Cre founder line. Osteoblast-specific CYP27B1 transgenic (ObCYP27B1-Tg) mice were generated in which human Cyp27b1 gene over-expression was driven by the 3.6kb human osteocalcin promoter. In both mouse models, no change to serum 1,25D, PTH, calcium, phosphorus or cross-laps were observed. However, six week old ObCYP27B1KO mice demonstrated an 18% reduction on vertebral BV/TV% (P<0.01) when compared to Cyp27b1loxP/loxP littermates due to a reduction in trabecular thickness (TbTh) which was associated with a reduction in mineralising surface (MS/BS). No change to osteoclastic bone resorption measures were observed in ObCYP27B1KO mice. Conversely, 7w old ObCYP27B1-Tg transgenic mice exhibited a 14% increase (P<0.01) in vertebral BV/TV%, associated with both a 9% increase in TbTh (P<0.01) and a trend towards increased MS/BS as well as a 14% increase in serum alkaline phosphatase levels. Although while adult (20w) ObCYP27B1-Tg mice continue to exhibit a 23% increase in vertebral BV/TV%, when raised to be 25D-deplete, the vertebral bone phenotype completely abrogated in ObCYP27B1-Tg mice. Consistent with these findings, MLO-A5 osteoblasts (a mouse cortical bone cell model) exhibited pronounced enhancement of mineralisation in the presence of transient overexpression of the CYP27B1 transgene plus 100nM 25(OH)D. This increase in mineralisation was associated with increased levels of Enpp1 and high levels of Tnap mRNA. Our data strongly suggest that CYP27B1 activity in osteoblasts promotes bone formation and is dependent on adequate supply of 25(OH)D, consistent with our previous studies demonstrating the necessity of adequate serum 25D levels to optimise bone formation.
Disclosure: The authors declared no competing interests. This work was supported by a Project Grant from the National Health and Medical Research Council, Australia (APP1003433) and Career Development Fellowship for P.A. (APP1034698).