Quantitative changes in the stem cell population of cervical cancer cell line HeLa under the influence of fractionated G-irradiation in vitro

Radioresistance of cancer stem cells (CSCs) is considered as one of the possible causes of cancer recurrence after radiation therapy. However, the regularities and mechanisms of the effect of fractionated irradiation on this cell population have been insufficiently studied, particularly, there is little evidence of mechanisms of fractionated irradiation effects on cervical cancer stem cells. Therefore, the goal of this work is elucidating quantitative changes in stem cells of cervical cancer cell line HeLa following γ-irradiation in the standard dose fractionation mode (2 Gy daily for 5 days). CSCs were identified by flow cytometry by their ability to exclude the fluorescent dye Hoechst 33342 (SP method) 24 hours after each dose fraction in the cumulative dose range up to 10 Gy. The relative and absolute number of CSCs increased after irradiation at cumulative doses up to 6 Gy. However, further dose increasing resulted in reduction of the number of CSC, and absolute number of the cells remained unchanged. At the same time, increasing radiation exposure caused inhibition of the cells growth as compared to the control, as expected. The obtained results were compared with previously published data on effects of single acute irradiation in the dose range up to 10 Gy on CSCs. Possible mechanisms of the observed effects seem to include the higher radioresistance of CSCs in comparison with the other tumor cells, CSC proliferation during the postradiation repopulation of cancer cells, and dedifferentiation of non-stem cells that survived irradiation and their transition to CSCs pool.