Combining poly(ADP-ribose) polymerase 1 (PARP-1) inhibition and radiation in Ewing sarcoma results in lethal DNA damage

摘要

Ewing sarcomas (ES) harbor a chromosomal translocation that fuses the EWS gene to an ETS transcription factor, most commonly FLI1. The EWS-FLI1 fusion acts in a positive feedback loop to maintain expression of poly(ADP-ribose) polymerase 1 (PARP-1), which is involved in repair of DNA damage. Here, we examine the effects of PARP-1 inhibition and radiation therapy (RT) on ES. In proliferation assays, the ES cell lines RD-ES and SK-N-MC were much more sensitive than non-ES cell lines to the PARP-1 inhibitor olaparib (Ola) (IC50 0.5–1 uM vs >5 uM) and to radiation (IC50 2–4 Gy vs >6 Gy). PARP-1 inhibition with shRNA or Ola sensitized ES cells but not non-ES cells to RT in both proliferation and colony formation assays. Using the Comet assay, radiation of ES cells with Ola, compared to without Ola, resulted in more DNA damage at 1 hr (mean tail moment 36–54 vs. 26–28) and sustained DNA damage at 24 hr (24–29 vs. 6–8). This DNA damage led to a 2.9–4.0 fold increase in apoptosis and a 1.6–2.4 fold increase in cell death. The effect of PARP-1 inhibition and RT on ES cells was lost when EWS-FLI1 was silenced by shRNA. A small dose of RT (4 Gy), when combined with PARP-1 inhibition, stopped growth of SK-N-MC flank tumors xenografts. In conclusion, PARP-1 inhibition in ES amplifies the level and duration of DNA damage caused by RT leading to synergistic increases in apoptosis and cell death in a EWS-FLI1 dependent manner.