The authors have no conflict of interest.
BMP-2 Controls Alkaline Phosphatase Expression and Osteoblast Mineralization by a Wnt Autocrine Loop†
Article first published online: 1 OCT 2003
Copyright © 2003 ASBMR
Journal of Bone and Mineral Research
Volume 18, Issue 10, pages 1842–1853, October 2003
How to Cite
Rawadi, G., Vayssière, B., Dunn, F., Baron, R. and Roman-Roman, S. (2003), BMP-2 Controls Alkaline Phosphatase Expression and Osteoblast Mineralization by a Wnt Autocrine Loop. J Bone Miner Res, 18: 1842–1853. doi: 10.1359/jbmr.2003.18.10.1842
- Issue published online: 2 DEC 2009
- Article first published online: 1 OCT 2003
- Manuscript Accepted: 4 JUN 2003
- Manuscript Revised: 18 APR 2003
- Manuscript Received: 24 JAN 2003
- alkaline phosphatase
Wnt/β-catenin signaling has recently been suggested to be involved in bone biology. The precise role of this cascade in osteoblast differentiation was examined. We show that a Wnt autocrine loop mediates the induction of alkaline phosphatase and mineralization by BMP-2 in pre-osteoblastic cells.
Introduction: Loss of function of LRP5 leads to osteoporosis (OPPG syndrome), and a specific point mutation in this same receptor results in high bone mass (HBM). Because LRP5 acts as a coreceptor for Wnt proteins, these findings suggest a crucial role for Wnt signaling in bone biology.
Materials and Methods: We have investigated the involvement of the Wnt/LRP5 cascade in osteoblast function by using the pluripotent mesenchymal cell lines C3H10T1/2, C2C12, and ST2 and the osteoblast cell line MC3T3-E1. Transfection experiments were carried out with a number of elements of the Wnt/LRP5 pathway. Measuring osteoblast and adipocyte differentiation markers addressed the effect of this cascade on osteoblast differentiation.
Results: In mesenchymal cells, only Wnt's capable of stabilizing β-catenin induced the expression of alkaline phosphatase (ALP). Wnt3a-mediated ALP induction was inhibited by overexpression of either Xdd1, dickkopf 1 (dkk1), or LRP5ΔC, indicating that canonical β-catenin signaling is responsible for this activity. The use of Noggin, a bone morphogenic protein (BMP) inhibitor, or cyclopamine, a Hedgehog inhibitor, revealed that the induction of ALP by Wnt is independent of these morphogenetic proteins and does not require de novo protein synthesis. In contrast, blocking Wnt/LRP5 signaling or protein synthesis inhibited the ability of both BMP-2 and Shh to induce ALP in mesenchymal cells. Moreover, BMP-2 enhanced Wnt1 and Wnt3a expression in our cells. In MC3T3-E1 cells, where endogenous ALP levels are maximal, antagonizing the Wnt/LRP5 pathway led to a decrease of ALP activity. In addition, overexpression of dkk1 reduced extracellular matrix mineralization in a BMP-2-dependent assay.
Conclusions: Our data strongly suggest that the capacity of BMP-2 and Shh to induce ALP relies on Wnt expression and the Wnt/LRP5 signaling cascade. Moreover the effects of BMP-2 on extracellular matrix mineralization by osteoblasts are mediated, at least in part, by the induction of a Wnt autocrine/paracrine loop. These results may help to explain the phenotype of OPPG patients and HBM.