Abstract: Objective To investigate the role of senescence and autophagy in radiation-induced polyploidy breast cancer cells. Method Breast cancer MDA-MB-231 cells were irradiated with a dose of 7 Gy in 6 MV X-ray mode, and the cell morphology changes were observed on the 3rd (Day 3 group), 5th (Day 5 group), 7th (Day 7 group), 11th (Day 11 group) and 19th (Day 19 group) days, respectively. The cell ploidy was detected by flow cytometry, cell senescence was detected by β-galactosidase, and the expression of senescence and autophagy related proteins was detected by Western blot. The GEPIA tool was used to analyze the difference of PLK1 expression between breast tissues and breast cancer tissues. Results After 7 Gy dose irradiation, the volume of MDA-MB-231 cells in Day 3 group, Day 5 group and Day 7 group became larger, and the proportion of polyploid cell subsets (DNA content>4 N) was significantly higher than that of the control group MDA-MB-231 cells without radiation treatment (all P<0.0001), and cell senescence occurred at the same time. Compared with the control group MDA-MB-231 cells without radiation treatment, the expression of DNA damage repair protein PARP and DNA synthesis related proteins Rb, E2F-1, E2F-2 were down-regulated in Day 3 and Day 5 groups. The ratio of phage-related protein LC3B/LC3A was significantly increased. The expression of nuclear membrane integrity-related protein Lamin B1 and DNA damage repair protein PARP were down-regulated, and the expression of DNA damage response-related protein PLK1 was up-regulated, the differences were statistically significant (all P<0.05). Compared with breast tissues，PLK1 was highly expressed in breast cancer tissues (P<0.05). Conclusions Radiation-induced senescence of breast cancer cells can be a favorable condition for the formation of polyploid cell. Senescence and autophagy may contribute to self-repair and deploid proliferation of polyploid cells.

Key words: Breast cancer, Aging, Polyploidy, Autophagy, MDA-MB-231