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 receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoporotic model mice. In this study, we investigated whether MDP could alleviate post-menopausal osteoporosis through Wnt signaling regulation in an ovariectomy (OVX)-induced mouse osteoporosis model. MDP-administered OVX mice exhibited higher bone volume and bone mineral density than mice 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 with 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 pretreated with Wnt signaling inhibitors, DKK1 or IWP-2, the induction of pAKT, pGSK3β, and β-catenin was not observed. In addition, nucleotide oligomerization domain-containing protein 2-deficient osteoblasts were not sensitive to MDP. MDP-administered OVX mice exhibited fewer tartrate-resistant acid phosphatase (TRAP)-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 post-menopausal bone loss. © 2023 The Pathological Society of Great Britain and Ireland.