Inhibitors of mammalian target of rapamycin downregulate MYCN protein expression and inhibit neuroblastoma growth in vitro and in vivo.

Mammalian target of rapamycin (mTOR) has been shown to play an important function in cell proliferation, metabolism and tumorigenesis, and proteins that regulate signaling through mTOR are frequently altered in human cancers. In this study we investigated the phosphorylation status of key proteins in the PI3K/AKT/mTOR pathway and the effects of the mTOR inhibitors rapamycin and CCI-779 on neuroblastoma tumorigenesis. Significant expression of activated AKT and mTOR were detected in all primary neuroblastoma tissue samples investigated, but not in non-malignant adrenal medullas. mTOR inhibitors showed antiproliferative effects on neuroblastoma cells in vitro. Neuroblastoma cell lines expressing high levels of MYCN were significantly more sensitive to mTOR inhibitors compared to cell lines expressing low MYCN levels. Established neuroblastoma tumors treated with mTOR inhibitors in vivo showed increased apoptosis, decreased proliferation and inhibition of angiogenesis. Importantly, mTOR inhibitors induced downregulation of vascular endothelial growth factor A (VEGF-A) secretion, cyclin D1 and MYCN protein expression in vitro and in vivo. Our data suggest that mTOR inhibitors have therapeutic efficacy on aggressive MYCN amplified neuroblastomas.