Signaling pathways involved in the development of osteoprogenitors induced by Wnts remain poorly understood. In this study, we investigated the role of MAP kinases in the development of mesenchymal cells into osteoprogenitors. In C3H10T1/2 mesenchymal cells, Wnt3a induced a rapid and transient activation of MAP kinases p38 and ERK. DKK1, a selective antagonist of Wnt proteins binding to LRP5/6 did not influence activation of p38 and ERK induced by Wnt3a. A MEK1,2 inhibitor blocked whereas a p38 inhibitor had no effect on Wnt3a-induced cell proliferation. In contrast, both inhibitors significantly reduced ALP stimulation with a more pronounced effect of the p38 inhibitor. The p38 inhibitor also blunted nodules mineralization induced by Wnt3a. Associated with these effects, -catenin transcriptional activity, assessed with the TOPflash system, was dose-dependently decreased by the p38 but not by the ERK inhibitor. Both, the reduced ALP stimulation and blunting of -catenin transcriptional activity were mimicked by expression of dnp38 and dnMkk3/6. Inhibition of -catenin transcriptional activity by the p38 inhibitor as well as by dnp38 and dnMKK3/6 molecules were not associated with changes in cytosolic and nuclear -catenin levels induced by Wnt3a.

In conclusion, Wnt3a activates ERK and p38 in mesenchymal C3H10T1/2 cells by a LRP5/6-independent mechanism. Activation of p38 regulates alkaline phosphatase activity and nodules mineralization induced by Wnt3a probably by interacting with -catenin transcriptional activity. These observations suggest that MAP kinases ERK and p38 are probably essential pathways activated by Wnt proteins for the development of mesenchymal cells into osteoprogenitors.