Targeting the PI3K/AKT/mTOR signaling pathway as an effectively radiosensitizing strategy for treating human oral squamous cell carcinoma in vitro and in vivo .

Radiation therapy (RT) is the current standard adjuvant approach for oral squamous cell carcinoma (OSCC) patients. Radioresistance is a major contributor to radiotherapy failure. In this study, we used patient-derived cells and a radiation-resistant cell line in vitro and in vivo for two purposes: evaluate the anti-tumor effects and understand the mechanisms in the dual PI3K/mTOR signaling pathway regulation of radiosensitization. Our findings indicate that in OML1-R cells, the radioresistance phenotype is associated with activation of the PI3K/AKT/mTOR signaling pathway. Compared to a combination of PI3K or mTOR inhibitors and radiation, dual blockade of the PI3K and mTOR kinases significantly improved radiation efficacy in oral cancer and patient-derived OSCC cells. Dual PI3K/mTOR inhibition enhanced the effect of radiation by inhibiting AKT/mTOR signaling pathways and caused G1 phase arrest, which is associated with downregulation of cyclin D1/CDK4 activity, leading to growth inhibition. In nude mice xenografted with radioresistant OML1-R cells, the combined treatment was also more effective than RT alone in reducing tumor growth. This treatment was also demonstrated to be dependent on the inhibition of protein kinase-dependent S6 kinase pathway and eIF4E-mediated cap-dependent translation. These findings indicate that activation of the PI3K/AKT/mTOR signaling pathway has a role in radioresistance of OSCC. We determined that a PI3K/mTOR inhibitor combined with radiation exhibits synergistic inhibition of the AKT/mTOR axis and induces cell cycle arrest. Our results show the therapeutic potential of drugs targeting the PI3K/AKT/mTOR signaling pathway should be new candidate drugs for radiosensitization in radiotherapy.