Abstract 2385: Involvement of autophagy and senescence in DNA repair in irradiated tumor cells

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA Radiotherapy is used along with other modalities such as surgery, chemotherapy, and immunotherapy to either shrink tumors before surgery or kill surviving tumor cells post surgery. Although radiation may induce different modes of cell death, recurrence of cancer may occur. It has been shown that DNA damage can induce cells to undergo both autophagy and senescence where both responses can function either as a pro-survival mechanism or in promoting loss of self-renewal capacity in the tumor cell. The relationship between autophagy, senescence, and the DNA repair system has not been well-characterized. The primary aims of this work were to understand whether autophagy and senescence might be permissive for DNA repair thereby allowing the tumor cells to overcome the persistent DNA damage and recover. Studies were performed using M059K (DNA-PK proficient) and M059J (DNA-PK deficient) human glioma cells to assess the kinetics of DNA repair after radiation in the absence and presence of autophagy inhibitors. M059J cells were observed to lose their capacity to proliferate as they undergo growth arrest followed by delayed cell death, whereas M059K cells showed proliferative recovery 72 hours post-treatment. Both irradiated cells showed an enlargement in cell size and accumulation of acidic vacuoles, indicating the induction of autophagy. Both cell lines also demonstrated reduced p62 level after radiation, indicative of autophagic flux. Inhibition of autophagy by 3-MA showed a radio-sensitization in M059K cells but not in M059J cells, indicating that radio-induced autophagy in M059K cells might be cytoprotective. Additional studies were performed in HCT116 cells that lack ligase IV to demonstrate a correspondence between the extent of drug or radiation induced senescence and the persistence of DNA damage foci. Ligase IV deficient cells were more sensitive to low radiation doses (2 Gy), whereas the ligase proficient cells showed recovery 96 hrs post-treatment. Both parental HCT116 colon carcinoma cells and Ligase IV deficient cells demonstrated significant increases in senescent populations at low doses of radiation based on β-galactosidase staining and enlargement and extensions of treated cells; senescence was more pronounced in the Ligase IV deficient cells. HCT116 cells were then treated with a dose of radiation (8 Gy) that induces ∼75% of senescence after 96 hrs and then reirradiated with half of that dose (4 Gy) to monitor the ability of senescent cells to repair the DNA damage. Although the majority of the cells were in a state of senescence, we found that these cells were able to repair the DNA damage induced by the second dose.Taken together, it seems that autophagy has an important function in the enhancement of DNA repair during exposure to genotoxic stress, but the role of autophagy may differ according to the status of DNA repair. Also, persistent DNA damage could be a determinant for senescence, and senescent cells might be able to repair these breaks. Citation Format: Moureq R. Alotaibi, Daivd Gewirtz, Lawrence Povirk. Involvement of autophagy and senescence in DNA repair in irradiated tumor cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2385. doi:10.1158/1538-7445.AM2014-2385