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1,25-(OH)2D3 down-regulates expression of Phex, a marker of the mature osteoblast
1Shriners Hospital, Department of Surgery, McGill University, Montréal, Québec, Canada. becarot@shriners.mcgill.ca
Abstract:
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)2D3. 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)2D3 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)2D3, an inhibitor of matrix mineralization. These findings add support for Phex having an important role in bone mineralization.
Insights
Mutations in the PHEX gene cause X-linked hypophosphatemia. This study shows PHEX (phosphate-regulating endopeptidase homolog, X-linked) expression increases with osteoblast differentiation and is regulated by 1,25-(OH)2D3, suggesting a role in bone mineralization.
Area of Science:
- Molecular biology
- Cell biology
- Biochemistry
Background:
- Mutations in the PHEX gene are linked to X-linked hypophosphatemia.
- The PHEX gene encodes a protein homologous to neutral endopeptidases.
- Understanding PHEX expression is crucial for bone mineralization research.
Purpose of the Study:
- To investigate PHEX messenger RNA (mRNA) and protein expression in osteoblasts.
- To examine the regulation of PHEX expression by 1,25-(OH)2D3.
- To determine the role of PHEX in bone mineralization.
Main Methods:
- Quantification of Phex mRNA using Northern blots and densitometry relative to GAPDH mRNA.
- Evaluation of immunoreactive Phex protein levels via immunoprecipitation.
- Analysis of Phex expression in primary osteoblast cultures and MC3T3-E1 cells under varying conditions (beta-glycerophosphate, 1,25-(OH)2D3).
Main Results:
- Phex mRNA and protein levels increased significantly with beta-glycerophosphate-induced matrix mineralization in primary osteoblasts.
- Phex expression was low in proliferating MC3T3-E1 cells and increased with differentiation, independent of mineralization.
- 1,25-(OH)2D3 inhibited mineral deposition and dose-dependently down-regulated Phex mRNA and protein expression.
Conclusions:
- PHEX is a marker of fully differentiated osteoblasts.
- PHEX expression is stimulated by beta-glycerophosphate-induced mineralization and down-regulated by 1,25-(OH)2D3.
- These findings support a significant role for PHEX in bone mineralization processes.