Improved antitumor effect of ionizing radiation in combination with rapamycin for treating nasopharyngeal carcinoma

The aim of the present study is to investigate if rapamycin is a radiosensitizer of nasopharyngeal carcinoma (NPC), and to identify which pathways are involved in radiation sensitization. In vitro, using untreated cells as the control, NPC cells were treated with rapamycin, ionizing radiation (IR) or both. Differences in the phosphorylation of ribosomal protein S6 and glycogen synthase kinase (GSK) 3 beta, expression of cyclin D1, clonogenic survival, number of phosphorylated histone subunit 2AX (gamma H2AX) foci, and cell cycle status between the study groups were compared. The results indicated that rapamycin alone decreased the phosphorylation of S6 and GSK3 beta, as well as the expression of cyclin Dl, in NPC cells. Thus, rapamycin-treated NPC cells had lower cell viability, and higher DNA damage and G1 arrest than control cells. In addition, the combination of rapamycin and IR caused the highest cell death, DNA damage and G1 arrest when compared with the effects caused by either treatment alone. In conclusion, rapamycin improves the anti-tumor effect of IR for treating NPC through inhibiting the Akt/mechanistic target of rapamycin/S6 and Akt/GSK3 beta/cyclin Dl signaling pathways.