Parathyroid Hormone Increases β-Catenin Levels through Smad3 in Mouse Osteoblastic Cells

PTH, via the PTH/PTH-related protein receptor type 1 that couples to both protein kinase A (PKA) and protein kinase C (PKC) pathways, and the canonical Wnt-β-catenin signaling pathway play important roles in bone formation. In the present study we have examined the interaction between the PTH and Wnt signaling pathways in mouse osteoblastic MC3T3-E1 cells. PTH dose- and time-dependently increased the concentrations of β-catenin. The PKA activator, forskolin, and the PKC activator, phorbol 12-myristate-13-acetate, as well as the PTH analog, [Nle8,18,Tyr34]human PTH-(3–34)amide, all increased β-catenin levels. Both H-89, a specific PKA inhibitor, and PKC inhibitors, staurosporine and calphostin C, antagonized PTH stimulation of β-catenin levels. TGF-β as well as transfection of the TGF-β-signaling molecule, Smad3, enhanced β-catenin levels, and this was antagonized by transfection of a dominant-negative Smad3. The transcriptional activity of transfected dominant-active β-catenin was enhanced by PTH, an effect that was antagonized by cotransfection of a dominant-negative Smad3. PTH as well as LiCl2, which mimics the effects of the Wnt-β-catenin pathway, rescued the dexamethasone- and etoposide-induced apoptosis of osteoblastic cells. In conclusion, the data demonstrate that PTH stimulates osteoblast β-catenin levels via Smad3, and that both PKA and PKC pathways are involved. The canonical Wnt-β-catenin pathway is likely to be involved in the antiapoptotic actions of PTH by acting through Smad3 in osteoblasts.