Inhibition of mTORC1 by rapamycin results in feedback activation of Akt(S473) and aggravates hallmarks of osteoarthritis in female mice and non-human primates.

PURPOSE: Genetic deletion of mTOR has protected against post-traumatic osteoarthritis (OA) in male mice, however, effects of pharmacological mTOR-inhibition are equivocal and have not been tested in aging models nor in female subjects. Therefore, the goal of this study was to determine if mTOR-inhibition by rapamycin can modify OA pathology in aging non-human primates and female mice. METHODS: Common marmosets were administered oral rapamycin (1mg/kg/day) or vehicle starting near mid-life until death. Five-month-old, female C57BL/6J mice were treated with vehicle or rapamycin (IP, 2mg/kg, 3x/week) for 8-weeks following non-invasive ACL rupture. Knee OA pathology was assessed via microCT and histology. Phosphorylation of mTORC1 (p-RPS6(S235/36)) and mTORC2 (p-Akt(S473), p-NDRG1(T638), p-PKCα(T348)) substrates were evaluated via western blot in articular cartilage, meniscus, and/or infrapatellar fat pad. ATDC5 cells were cultured with rapamycin to determine time and dose effects on mTORC1/2 signaling. RESULTS: In marmosets, rapamycin did not impact age-related radiographic OA severity or cartilage pathology but increased medial meniscus calcification and lowered lateral tibia subchondral thickness, particularly in females. In female mice, rapamycin worsened ACLR-induced meniscus calcification and cartilage pathology. In marmoset and mouse joint tissues, rapamycin inhibited mTORC1 and increased p-Akt(S473) but not p-NDRG1(T638) or p-PKCα(T348). This mTOR signaling pattern was replicated in ATDC5 cells during exposure to low concentrations of rapamycin. CONCLUSIONS: Rapamycin attenuated mTORC1 signaling with feedback activation of Akt(S473) in articular cartilage, meniscus, and/or infrapatellar fat pad and was accompanied by deleterious effects on meniscus calcification and/or cartilage pathology in female mice and common marmosets.