Estrogen receptor α in mature osteoblasts regulates the late stage of bone regeneration.

Estrogen deficiency impairs fracture healing and homeostasis of bone tissue. OVX-induced estrogen deficiency in mice attenuates fracture healing and changes the expression ratio of estrogen receptor (ER) α and ERβ in callus during the process of fracture healing. Therefore, ERs may be involved in the regulation of fracture healing. However, the roles of ERs in fracture healing are largely unknown. The purpose of this study was to clarify the significance of ERs during fracture healing using osteoblast-specific ER knockout mice in a mono-cortical drill hole bone regeneration model. The mature osteoblast-specific ER knockout mice were generated using osteocalcin (OCN)-Cre mice, and ERα and ERβ flox mice (OCN-Cre; ERαf/f, ERαΔOb/ΔOb and OCN-Cre; ERβf/f, ERβΔOb/ΔOb). Drill hole surgery was conducted on the tibiae of 8-week-old female mice. The mice were sacrificed 10 or 14 days after surgery and the bones were analyzed by DXA, μCT and bone histomorphometry. DXA analysis revealed that intact femoral BMD was significantly decreased in ERαΔOb/ΔOb mice compared with ERαf/f mice, but there was no difference in bone mass between ERβΔOb/ΔOb and ERβf/f mice. Micro CT analyses showed that the callus volume at the restricted drill hole site in tibiae was significantly less in ERαΔOb/ΔOb compared to ERαf/f mice only at day 14 but not at day 10. In addition to femoral BMD, there was no significant difference in callus volume between ERβΔOb/ΔOb and ERβf/f mice. Bone histomorphometric analyses showed that Ob.S/BS and N.Ob/B.Pm were significantly less in ERαΔOb/ΔOb mice compared with ERαf/f mice only at day 10. In addition, Oc.S/BS and N.Oc/B.Pm were significantly less in ERαΔOb/ΔOb mice compared with ERαf/f mice only at day 14. These results suggest that ERα but not ERβ in osteocalcin-positive osteoblasts may contribute to the late stage of bone regeneration.