Extracellular phosphate modulates the effect of 1α,25-dihydroxy vitamin D3 (1,25D) on osteocyte like cells

1α,25-dihydroxy vitamin D3 (1,25D) is reported to up-regulate the expression of the osteocyte-derived phosphatonin, fibroblast growth factor 23 (FGF23), an effect increased by high concentrations of extracellular phosphate (Pi). Osteocytes therefore appear to sense Pi directly and this may be an important means, by which FGF23 production is regulated. The intriguing possibility is that the Pi response and 1,25D pathways interact in additional ways. 1,25D also modulates the expression of other genes related to phosphate handling in cells of the osteoblast lineage. These include receptor activator of nuclear factor kappa-B ligand (RANKL) and dentin matrix acidic phosphoprotein 1 (DMP1). These cells are also capable of synthesising 1,25D due to their expression of the 25-hydroxyvitamin D 1α-hydroxylase (CYP27B1). In this study, the mouse cell line, IDG-SW3, which differentiates into an osteocyte-like phenotype expressing Fgf23 mRNA, was utilised to address this question. Cells were differentiated for 35d and the expression level of several Pi handling or vitamin D-related genes was then evaluated in response to short-term culture with varying concentrations of extracellular Pi, in the presence or absence of 1,25D. Pi and 1,25D both increased Fgf23 mRNA expression, as well as that of N-acetylgalactosaminyltransferase 3 (Galnt3), Dmp1, phosphate-regulating gene with homologies to endopeptidases on the X chromosome (Phex), ectonucleotide pyrophosphatase/phosphodiesterase family member 1 (Enpp1) and matrix extracellular phosphoglycoprotein (Mepe). Overall, there was a non-additive, competitive interaction between Pi and 1,25D, which was especially evident with Pi at 10mM. Pi also modulated the 1,25D metabolic pathway, up-regulating Cyp27b1 expression and attenuating 1,25D induction of 25-hydroxyvitamin D 24-hydroxylase (Cyp24a1) mRNA. This study provides evidence that the Pi and 1,25D response in osteocytes is linked in terms of the expression of genes related to phosphate and vitamin D metabolism. This article is part of a Special Issue entitled 'Vitamin D Workshop'.