Wnt3a downregulates thyroid hormone-induced osteocalcin expression in osteoblasts

Wnt3a is a crucial modulator of bone metabolism through the canonical Wnt/β-catenin signaling pathway in bone-forming osteoblasts. We previously reported that the expression of osteocalcin is stimulated by triiodothyronine (T₃) at least in part through the activation of p38 mitogen-activated protein (MAP) kinase but not p44/p42 MAP kinase in osteoblast-like MC3T3-E1 cells. In the present study, we investigated the effect of Wnt3a on the T₃-induced osteocalcin expression in these cells. Wnt3a suppressed the release of osteocalcin induced by T₃. The inhibitory effect of Wnt3a was dose-dependent between 0.3 and 30 ng/ml. SB216763, an inhibitor of glycogen synthase kinase-3β, that reduces the phosphorylation of β-catenin, inhibited the T₃-induced osteocalcin release. Wnt3a, as well as SB216763, reduced the expression of osteocalcin mRNA induced by T₃. The transcriptional activity induced by T₃, assessed by a luciferase activity, was also suppressed by both Wnt3a and SB216763. In contrast, Wnt3a did not markedly affect the T₃-stimulated phosphorylation of p38 MAP kinase. These results suggested that Wnt3a downregulates the T₃-stimulated osteocalcin expression in MC3T3-E1 cells, and the suppressive effect of Wnt3a is independent of p38 MAP kinase.