Bone Abstracts

Iron overload due to hemochromatosis or chronic blood transfusions has been implicated in the development of osteoporosis. However, the impact of iron overload or iron deficiency on stromal cell functions and the underlying mechanisms are poorly defined. Since the Wnt signaling pathway is a critical regulator of bone remodelling, we aimed to analyse the effects of iron overload and iron deficiency on osteoblast function and further define the role of Wnt signaling in these processes.

Therefore mesenchymal stromal cells were isolated from the bone marrow of wild type mice and differentiated towards osteoblasts. Treatment of the cells with iron (FeCl3) significantly and dose-dependently attenuated osteoblast differentiation in terms of mineralization and osteogenic gene expression whereas iron chelation with deferoxamine (DFO) promoted osteogenic differentiation of the mesenchymal cells in a concentration-dependent manner.

To elucidate the impact of Wnt signaling in iron-chelation osteogenic promoting effects we performed a Wnt signaling array of DFO-treated osteoblasts. Whereas some Wnt inhibitors such as Sfrp1 and Sfrp2 were significant downregulated, the osteoblast key regulator Wnt5a was increased 3- to 5-fold upon DFO treatment in a time- and dose-dependent manner. Further downstream signaling pathway analysis by applying specific pathway inhibitors revealed that DFO promotes Wnt5a-dependent osteogenic differentiation mainly via PI3K signaling pathway.

Finally we could confirm the indispensability of Wnt5a in the DFO-mediated osteoblast promoting effects by analyzing osteoblast differentiation and matrix mineralization of Wnt5a-deficient mesenchymal stromal cells. The DFO-promoting effect on matrix mineralization in wild type cells was completely abolished in Wnt5a^−/− cells.

Thus, these data demonstrate that Wnt5a is a target of DFO and a key mediator of the pro-osteogenic effects of iron chelation via DFO.