The radiosensitization effect of mTOR inhibitior in human tongue squamous cell carcinoma.

Abstract

e13585 Background: mTOR plays a critical role in malignant phenotype in many tumors. Everolimus [RAD001 (40-O-(2-hydroxyethyl)-rapamycin)] is a rapamycin rapalog that is being developed as an antitumor agent and is approved for various cancers. Recently, RAD001 has also been reported to increase the radiosensitization following for amplifying the autophagic pathway; however, it could promote an alternative mechanism that leads to apoptosis. In HNSCC, radiation therapy is a common form of cancer treatment, but locoregional recurrence and/or distant metastasis is the most cause of failure after RT and it results in HNSCC significant morbidity. Evidence from previous studies showed that RAD001 enhanced the cytotoxic effects of radiation in HNSCC cell lines. The molecular mechanism of radiosensitization by RAD001 in HNSCC has not cleared.Our aim is to evaluate the role of molecular mechanism of RAD001-mediated radiosensitization in HNSCC. Methods: Radiosensitivity effects of RAD001 were assays on human oral squamous cell carcinoma cell lines (SCC4 and DuFa). RAD001 (30 and 300 nM)and radiation (0~8 Gy) alone and in combination were evaluated for antitumor activity in the clonogenic assay. Next, we also evaluated the RAD001 on radiation-induced signal transduction by western blot analysis. Results: Here, we show that 300nM RAD001 plus 6 Gy cohort with significant HNSCC cell line growth suppression compared to 6 Gy alone (p=0.004) or 30n MRAD001 plus 6 Gy (p=0.008). RAD001 treatment effectively reduces mTOR downstream effector p-S6 protein compared with the vehicle or irradiated alone. RAD001 + 6 Gy significantly increases p53 and PARP cleavage, which facilitates apoptosis. Unexpectedly, autophagy pathway, such as beclin 1 and LC3II were not affected by RAD001. Addiationlly, we discover that Akt phosphorylation (Ser473) is up-regulated after RAD001/and irradiation of HNSCC cells. Conclusions: These findings suggest radiosensitization by the RAD001 involves the induction of apoptosis in HNSCC. Notwithstanding, Akt signaling is increased under RAD001 and it could attenuate anticancer efficacy instead, contribute to the development of resistance.