MTOR and osteosarcoma: Compared to normal bone tissue, metabotropic glutamate receptor (mGluR) has been reported to be highly expressed in osteosarcoma tissues as well as related to poor prognosis (Yang et al. 2014), and it is well known that active mGluR signaling is linked to the PI3/AKT/mTOR pathway (Willard and Koochekpour 2013).