Estrogen Receptor α Signaling in Osteoblasts is Required for Mechanotransduction in Bone Fracture Healing

Abstract

Biomechanical stimulation by whole-body low-magnitude high-frequency vibration (LMHFV) has demonstrated to provoke anabolic effects on bone metabolism in both non-osteoporotic and osteoporotic animals and humans. However, preclinical studies reported that vibration improved fracture healing and bone formation in osteoporotic, ovariectomized (OVX) mice representing an estrogen-deficient hormonal status, but impaired bone regeneration in skeletally healthy non-OVX mice. These effects were abolished in general estrogen receptor α (ERα)-knockout (KO) mice. However, it remains to be elucidated which cell types in the fracture callus are targeted by LMHFV during bone healing. To answer this question, we generated osteoblast lineage-specific ERα-KO mice that were subjected to ovariectomy, femur osteotomy and subsequent vibration. We found that the ERα specifically on osteoblastic lineage cells facilitated the vibration-induced effects on fracture healing, because in osteoblast lineage-specific ERα-KO (ERαfl/fl; Runx2Cre) mice the negative effects in non-OVX mice were abolished, whereas the positive effects of vibration in OVX mice were reversed. To gain greater mechanistic insights, the influence of vibration on murine and human osteogenic cells was investigated in vitro by whole genome array analysis and qPCR. The results suggested that particularly canonical WNT and Cox2/PGE2 signaling is involved in the mechanotransduction of LMHFV under estrogen-deficient conditions. In conclusion, our study demonstrates a critical role of the osteoblast lineage-specific ERα in LMHFV-induced effects on fracture healing and provides further insights into the molecular mechanism behind these effects.

Estrogen deficiency impedes fracture healing despite eliminating the excessive absorption of the posterior callus in a semi-fixed distal tibial fracture mouse model

“…Whole-body low-magnitude high-frequency vibration (LMHFV) promoted fracture healing in OVX mice, but it had no or opposite effects in healthy mice. This observation suggests that the response of the bony callus to stress stimulation is more sensitive in estrogen-depleted mice than in normal mice [ 10 ] and supports our findings that there were no significant differences in the phenotypes of the anterior and posterior calluses in the OVX group. This indicates that, even though the fixed pin may cause abnormally reduced stress, it can still stimulate bone repair without a site-specific inconsistent manner in estrogen deficiency mice.…”

“…The peak expression of estrogen receptors in the bony callus of OVX-induced osteoporotic fractures was delayed compared to normal fractures. However, stress stimulation in the absence of estrogen promoted the expression of estrogen receptor α and thus increased bone formation [ 9 , 10 ]. These may explain the higher bone volume of the posterior scab in the OVX group compared to the anterior side 28 d post fracture (Fig.…”

“…In addition, a series of clinical and animal studies have shown that post-menopause results in reduced angiogenesis and a disturbed immune response, which further increases the difficulty of fracture healing in this area [ 9 ]. Previous studies investigating the correlation between stress and OVX fracture healing have primarily utilized extracorporeal stress loading techniques, such as whole-body vibration treatment [ 10 , 11 ]. However, there is a dearth of research on a straightforward fracture model that can accurately mimic physiological loading changes.…”

Molecular and Cellular Mechanisms of Osteoporosis

“…It was shown that inhibition of estrogen receptor-α (ER-α) expression suppresses osteoblast differentiation. Several studies in cell culture and animal models of ER-α-knockout mice demonstrate a critical role of the osteoblast ER-α in bone regeneration and fracture healing [6][7][8].…”