MHY1485 activates mTOR and protects osteoblasts from dexamethasone.

Abstract Dexamethasone (Dex) exerts cytotoxic effects to cultured osteoblasts. The potential effect of MHY1485, a small-molecular mammalian target of rapamycin (mTOR) activator, against the process was studied here. In both osteoblastic MC3T3-E1 cells and primary murine osteoblasts, treatment with MHY1485 significantly ameliorated Dex-induced cell death and apoptosis. mTOR inhibition, through mTOR kinase inhibitor OSI-027 or mTOR shRNAs, abolished MHY1485-mediated osteoblast cytoprotection against Dex. Intriguingly, activation of mTOR complex (mTORC1), but not mTORC2, is required for MHY1485's anti-Dex activity. mTORC1 inhibitors (rapamycin and RAD001) or Raptor knockdown almost reversed MHY1485-induced osteoblast cytoprotection. mTORC2 inhibition, via shRNA knockdown of Rictor, failed to affect MHY1485's activity in MC3T3-E1 cells. Further studies showed that MHY1485 treatment in MC3T3-E1 cells and primary murine osteoblasts significantly inhibited Dex-induced mitochondrial death pathway activation, the latter was tested by mitochondrial depolarization, cyclophilin D-ANT-1 association and cytochrome C cytosol release. Together, these results suggest that MHY1485 activates mTORC1 signaling to protect osteoblasts from Dex.