SN-38 acts as a radiosensitizer for colorectal cancer by inhibiting the radiation-induced up-regulation of HIF-1α

Abstract

Background/Aim: Hypoxia offers resistance to therapy in human solid tumors. The aim of the study was to investigate whether SN-38, the active metabolite of irinotecan, acts as a radiosensitizer through inhibition of hypoxia-inducible factor (HIF)-1α in the human colorectal cancer (CRC) cells. Materials and Methods: HT29 and SW480 cells were cultured with SN-38 (0-4 μM) immediately after irradiation (0-8 Gy). HIF-1α expression was assessed using flow-cytometry and western blot analysis. Cell proliferation was evaluated by the calcein assay. Apoptosis and cell cycle were determined by flow-cytometry. Results: Radiation up-regulated HIF-1α, and SN-38 inhibited the radiation-induced HIF-1α. The combination of radiation and SN-38 inhibited cell proliferation more than radiation alone; treatment with SN-38 after radiation exposure did not increase the number of apoptotic cells, whereas, it enhanced the S and G2/M cell-cycle arrest and decreased the population of cells in G1. Conclusion: SN-38 inhibits the radiation-induced up-regulation of HIF-1α and acts as a radiosensitizer by inducing cell-cycle arrest in CRC cells.