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; ERα^f/f, ERα^ΔOb/ΔOb and OCN-Cre; ERβ^f/f, ERβ^ΔOb/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 ERα^ΔOb/ΔOb mice compared with ERα^f/f mice, but there was no difference in bone mass between ERβ^ΔOb/ΔOb and ERβ^f/f mice. Micro CT analyses showed that the callus volume at the restricted drill hole site in tibiae was significantly less in ERα^ΔOb/ΔOb compared to ERα^f/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 ERβ^ΔOb/ΔOb and ERβ^f/f mice. Bone histomorphometric analyses showed that Ob.S/BS and N.Ob/B.Pm were significantly less in ERα^ΔOb/ΔOb mice compared with ERα^f/f mice only at day 10. In addition, Oc.S/BS and N.Oc/B.Pm were significantly less in ERα^ΔOb/ΔOb mice compared with ERα^f/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.