BMP-2 controls alkaline phosphatase expression and osteoblast mineralization by a Wnt autocrine loop

Wnt/beta-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.

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.

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.

In mesenchymal cells, only Wnt's capable of stabilizing beta-catenin induced the expression of alkaline phosphatase (ALP). Wnt3a-mediated ALP induction was inhibited by overexpression of either Xddl, dickkopf 1 (dkk1), or LRP5deltaC, indicating that canonical beta-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 Wntl 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 dkkl reduced extracellular matrix mineralization in a BMP-2-dependent assay.

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.