Crucial roles of canonical Runx2-dependent pathway on Wnt1-induced osteoblastic differentiation of human periodontal ligament fibroblasts

Canonical Wnt signaling is thought to enhance osteogenic differentiation of human periodontal ligament fibroblasts (hPLFs). However, the mechanism of this enhancement has not yet been defined. We investigated the effects of Wnt1 on osteoblast differentiation of hPLFs and explored the mechanisms of the effects. Treating hPLFs with Wnt1 induced cytosolic accumulation and nuclear translocation of β-catenin with concomitant increases in alkaline phosphatase (ALP) activity and calcium content in a time-dependent and dose-dependent manner. Wnt1-stimulated differentiation of hPLFs was accompanied by augmented phosphorylation of glycogen synthase kinase (GSK)-3β and expression of the bone-specific factors runt-related transcription factor 2 (Runx2), osterix2 (Osx2), ALP, type I collagen, osteopontin, and osteocalcin. Pretreatment with Dickkopf-1 inhibited Wnt1-stimulated differentiation of hPLFs by suppressing GSK-3β phosphorylation, nuclear translocation of β-catenin, and expression of the bone-specific factors. Small interfering (si) RNA-mediated knockdown of β-catenin, or pretreatment with FH535, markedly prevented Wnt1-stimulated differentiation of cells by blocking Runx2 and its downstream factors at the mRNA and protein levels. siRNA-mediated silencing of Runx2 also inhibited Wnt1-stimulated mineralization of cells, accompanied by a reduction in the levels of Osx2 and other early and late bone-formation regulatory factors. However, Wnt1-mediated nuclear translocation of β-catenin and GSK-3β phosphorylation were not inhibited by knockdown of Runx2 or FH535. Collectively, our findings suggested that Wnt1 stimulates osteogenic differentiation and mineralization of hPLFs, mainly by activating the canonical Wnt/β-catenin pathway, in which Runx2 is a key downstream regulator.