| Research Abstract |
Parathyroid hormone (PTH) via the PTH/PTH related protein (PTHrP) receptor type 1 (PTHR1) 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 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, PMA increased the b-catenin levels. Both H-89,a specific PKA inhibitor, and PKC inhibitors, staurosporine and calphostin C, antagonized the 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 also enhanced by PTH, and this effect was antagonized by cot
… More
ransfection 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 the osteoblastic cells. These findings 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 anti-apoptotic actions of PTH by acting through Smad3 in osteoblasts. Dexamethasone inhibits bone formation, presumably through suppressing smad3-induced transcriptional activity but not by modulating smad3 expression in osteoblasts. Taken together, present findings demonstrated the crucial role of smad3 in bone formation. On the other hand, the present study showed that the abolition of calcium-sensing receptor (CaSR) function results in diminishing alkaline phosphatase acivity, and osteocalcin expression as well as mineralization in osteoblasts, indicating that CaSR may be involved in osteoblastic differentiartion. The present findings provide the rationale for the therapeutic usefulness of PTH, bisphosphonete, statin as well as CaSR agonist as bone-forming agents for senile osteoporosis. Less
|
