Rapamycin Alleviates Cancellous Bone Loss and Cortical Bone Porosity in Ovariectomized Mice but Not Overall Biomechanical Performance

Abstract Background: Recent studies have shown that rapamycin (Rapa) rescues cancellous bone loss of osteoporosis models, but its effects on cortical bone and the biomechanical properties remain uncertain. This study was aimed to determine whether rapamycin improve the quality of long bones in ovariectomy (OVX)-induced osteoporosis.Methods: Thirty female C57BL/6J mice were randomly divided into Sham, OVX and OVX+Rapa group. Mice in the OVX+Rapa group were injected intraperitoneally with 1.5 mg/kg rapamycin daily after ovariectomy. After 12 weeks, the microstructures of femurs and the vascular canal porosity tibiae were analyzed by micro-CT. The compressive stiffness of the distal femurs was calculated with the micro-finite element method, and the bending strength of the tibiae was evaluated with a three-point bending test. Western blot of LC3, P62 was used to assess autophagy activity of bone. The number of osteoclasts was quantified by tartrate-resistant acid phosphatase (TRAP) staining. ELISA detected the concentration of bone-specific alkaline phosphatase (BALP) in serum.Results: OVX led to a decrease in cross-sectional areas of the mid-diaphyses and distal femoral cortical bones, increasing the cortical bone vascular canal porosity from 1.42% to 4.79%. The rapamycin reduced cancellous bone loss and decreased cortical bone vascular canal porosity by 3.9%, but further reduced the thickness of the distal femoral cortical bone by 16.7%, with no significant effect on the cortical bone in the mid-diaphyses. Compared to the Sham group, OVX mice showed a decrease in distal femoral stiffness to 6497 N/mm and a decrease in tibial maximum load to 6.08 N, while rapamycin intervention did not significantly improve the decreased biomechanical properties. Moreover, rapamycin remarkably reduced the TRAP-positive osteoclasts and the concentration of serum BALP by 80.6% and 30.8%, respectively. Autophagy was activated in the OVX group compared with the sham group.Conclusions: This study has demonstrated that rapamycin ameliorates cancellous bone loss and cortical bone porosity in ovariectomized mice but partly reduces the size of cortical bone, while has no effect on improving the biomechanical performance. Additionally, the present study also proved that OVX led to both cancellous and cortical bone loss, with attenuated biomechanical properties of long bones.