Abstract
Developmental studies indicate the Wnt/ β-catenin pathway biphasically regulates tissue differentiation. Bone morphogenetic protein (BMP) and Wnt signaling reported function in osteoblast differentiation, but few studies have investigated these complex mechanisms. We hypothesized that Wnt/β-catenin signaling acts, either promoting or inhibiting osteogenesis of human periodontal ligament (HPDL) cells depending on times of treatment of Wnt3A. HPDL cells were treated with single or repeated administration (12- or 24-h intervals) of 10 ng/ml of Wnt3A and cultured for 3 days. Single Wnt3A administration increased alkaline phosphatase (ALP) activity. Conversely, repeated Wnt3A administration significantly decreased the expression of osteoblast marker genes. Single Wnt3A administration resulted in Smad1/5 and Akt phosphorylation; however, repeated Wnt3A administration inhibited it. BMP2/7 or IGF-1 treatment was sufficient to reverse the inhibitory effects of repeated Wnt3A treatment. Single administration of a glycogen synthase kinase 3 inhibitors, CHIR99021, increased ALP expression, but repeated CHIR99021 administration significantly decreased ALP expression and ALP activity compared with single administration. These findings suggest that repeated activation of Wnt signaling inhibits osteoblast differentiation by suppressing the BMP2/7–Smad1/5 and phosphatidylinositide 3-kinase (PI3K) /Akt pathways in HPDL cells. This indicated that crosstalk between the Wnt, Smad1/5, and PI3K/Akt pathways is important for osteoblastic differentiation of HPDL cells.
