The Effect Of Rapamycin On The Overall Quality Of Long Bones In Ovariectomized Mice And Its Mechanisms

ObjectiveThe aim of this study was to determine whether rapamycin could improve the overall bone quality of long bones in ovariectomized mice,to evaluate the changes in bone quality in terms of both bone microstructure and biomechanical properties,and to further investigate the mechanism of rapamycin’s effects on cancellous and cortical bone.MethodsThirty female C57BL/6J mice were randomly divided into Sham,OVX and OVX+Rapa group.After 12 weeks,the distal and mid-diaphyses of the femurs were analyzed by micro-CT.The axial 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 in cancellous bone was quantified by tartrate-resistant acid phosphatase（TRAP）staining.Enzyme-linked immunosorbent assay（ELISA）detected the concentration of bone-specific alkaline phosphatase（BALP）in serum to evaluate the activity of osteoblasts.ResultsThe data showed that rapamycin reduced cancellous bone loss,but further aggravated cortical bone loss in the distal femurs,with no significant effect on the cortical bone in the mid-diaphyses.Moreover,rapamycin could not ameliorate the decreased compressive stiffness and bending strength of long bones caused by ovariectomy.Rapamycin reduced the TRAP-positive osteoclasts and the concentration of serum BALP remarkably.Additionally,autophagy was activated and TRAP-positive osteoclasts in cancellous bone were increased in the OVX group compared with the sham group.ConclusionsThis study has demonstrated that rapamycin ameliorates cancellous bone loss and partly exacerbates cortical bone loss,while has no effect on improving the biomechanical performance overall.The excessive long-term autophagy and inhibited mTORCl in osteoclasts and osteoblasts may be the underlying cause.Additionally,the present study also proved that OVX led to both cancellous and cortical bone loss,with attenuated overall biomechanical properties of long bones.