Muramyl dipeptide alleviates estrogen deficiency-induced osteoporosis through canonical Wnt signaling.

Wnt signaling is a positive regulator of bone formation through the induction of osteoblast differentiation and down-regulation of osteoclast differentiation. We previously reported that muramyl dipeptide (MDP) increases bone volume by increasing osteoblast activity and attenuating osteoclast activity in RANKL-induced osteoporotic model mice. In this study, we investigated whether MDP could alleviate post-menopausal osteoporosis through Wnt signaling regulation through an ovariectomy (OVX)-induced mouse osteoporosis model. MDP-administered OVX mice exhibited higher bone volume and bone mineral density than those of the control group. MDP significantly increased P1NP in the serum of OVX mice, implying increased bone formation. The expression of pGSK3β and β-catenin in the distal femur of OVX mice was lower than that in the distal femur of SHAM-operated mice. Yet, the expression of pGSK3β and β-catenin was increased in MDP-administered OVX mice compared to OVX mice. In addition, MDP increased the expression and transcriptional activity of β-catenin in osteoblasts. MDP inhibited the proteasomal degradation of β-catenin via the down-regulation of its ubiquitination by GSK3β inactivation. When osteoblasts were pre-treated with Wnt signaling inhibitors, DKK1 or IWP-2, the induction of pAKT, pGSK3β, and β-catenin was not observed. In addition, nucleotide-oligomerization domain protein 2-deficient osteoblasts were not sensitive to MDP. MDP-administered OVX mice exhibited fewer TRAP (tartrate resistant scid phosphatase)-positive cells than did OVX mice, attributed to a decrease in the RANKL/OPG ratio. In conclusion, MDP alleviates estrogen deficiency-induced osteoporosis through canonical Wnt signaling and could be an effective therapeutic for the treatment of postmenopausal bone loss. This article is protected by copyright. All rights reserved.