title: Role of Smad3, acting independently of transforming growth factor-beta, in the early induction of Wnt-beta-catenin signaling by parathyroid hormone in mouse osteoblastic cells.
abstract: Parathyroid hormone (PTH) exerts an anabolic action on bone but the mechanisms are incompletely understood. We showed previously that PTH interacts with the canonical Wnt-beta-catenin signaling pathway via the transforming growth factor (TGF)-beta signaling molecule, Smad3, to modulate osteoblast differentiation and apoptosis. Here, we examined which actions of Smad3 are TGF-beta-independent in stimulating the osteoblast phenotype and PTH-induced Wnt-beta-catenin signaling. For this, the TGF-beta receptor type 1 [activin receptor-like kinase (ALK5)] inhibitor (SB431542), and a Smad3 mutant in which the site normally phosphorylated by ALK5 is mutated from SSVS to AAVA, was used. PTH induced total beta-catenin and reduced phosphorylated beta-catenin levels at 1, 6, and 24 h in mouse osteoblastic MC3T3-E1 cells. Transient transfection of Smad3AAVA inhibited the PTH induction of total beta-catenin and reduction of phosphorylated beta-catenin levels at 6 and 24 h, but not at 1 h, indicating that the early effects occur independently of TGF-beta receptor signaling. On the other hand, MC3T3-E1 cell clones in which Smad3AAVA was stably expressed demonstrated elevated beta-catenin levels, although alkaline phosphatase (ALP) activity and mineralization were unaltered. In contrast, MC3T3-E1 cell clones in which wild-type Smad3 was stably expressed exhibited increased ALP activity and mineralization that were decreased by the ALK5 inhibitor, SB431542, although the beta-catenin levels induced in these cells were not modulated. In conclusion, the present study indicates that PTH induces osteoblast beta-catenin levels via Smad3 independently of, and dependently on, TGF-beta in the early and later induction phases, respectively.
authors: Division of Diabetes, Metabolism and Endocrinology, Department of Internal Medicine, Kobe University Graduate School of Medicine, Chuo-ku, Kobe 650-0017, Japan.
authors: Yoshifumi
authors: Y
authors: Inoue
authors: Lucie
authors: L
authors: Canaff
authors: Geoffrey N
authors: GN
authors: Hendy
authors: Itoko
authors: I
authors: Hisa
authors: Toshitsugu
authors: T
authors: Sugimoto
authors: Kazuo
authors: K
authors: Chihara
authors: Hiroshi
authors: H
authors: Kaji
mesh_terms: D000818:Animals; D002460:Cell Line; D006801:Humans; D051379:Mice; D010006:Osteoblasts / Q000378:metabolism*; D010281:Parathyroid Hormone / Q000378:metabolism*; D010641:Phenotype; D015398:Signal Transduction; D051900:Smad3 Protein / Q000235:genetics / Q000378:metabolism*; D014162:Transfection; D016212:Transforming Growth Factor beta / Q000378:metabolism*; D051153:Wnt Proteins / Q000378:metabolism*; D051176:beta Catenin / Q000235:genetics / Q000378:metabolism*
chemical_list: D010281:Parathyroid Hormone; D051900:Smad3 Protein; D016212:Transforming Growth Factor beta; D051153:Wnt Proteins; D051176:beta Catenin