1,25-(OH) sub(2)D sub(3) Down-Regulates Expression of Phex, a Marker of the Mature Osteoblast

Mutations in the PHEX/Phex gene, which encodes for a protein with homology to neutral endopeptidases, are responsible for human and murine X-linked hypophosphatemia. The present study examined Phex messenger RNA (mRNA) and protein expression in cultured osteoblasts and its regulation by 1,25-(OH) sub(2)D sub(3). Phex mRNA levels were quantitated on Northern blots by densitometric analysis relatively to GAPDH mRNA levels. Immunoreactive Phex protein levels were evaluated by immunoprecipitation using a polyclonal rabbit antiserum raised against a mouse Phex carboxy-terminal peptide. beta -Glycerophosphate-induced matrix mineralization in primary osteoblast cultures was associated with significant increases in Phex mRNA and protein. Phex mRNA and protein levels were low or undetectable in proliferating preosteoblastic MC3T3-E1 cells and dramatically increased concomitantly with initiation of matrix mineralization. The pattern of Phex expression, however, was similar in nonmineralizing cultures grown in the absence of beta -glycerophosphate, indicating that the induction of Phex expression in MC3T3-E1 cells was related to cell differentiation rather than matrix mineralization. 1,25-(OH) sub(2)D sub(3) inhibited mineral deposition and down-regulated Phex mRNA and protein expression in a time- and dose-dependent manner. These results indicate that Phex is a marker of the fully differentiated osteoblast and that its expression is stimulated during beta -glycerophosphate-induced mineralization in primary osteoblast cultures and down-regulated by 1,25-(OH) sub(2)D sub(3), an inhibitor of matrix mineralization. These findings add support for Phex having an important role in bone mineralization.