Radiation-induced apoptosis of Ewing's sarcoma cells: DNA fragmentation and proteolysis of poly(ADP-ribose) polymerase.

Ewing's sarcoma (ES) cells express high levels of poly(ADP-ribose) polymerase (PADPRP) and are responsive to killing by ionizing radiation. We have determined that ionizing radiation induced a pronounced but reversible G2-M phase cell cycle arrest that was maximum by 24 h after exposure. Following the release from this block, floating cells began to appear. These floating cells were shown to be apoptotic by flow cytometric and DNA fragmentation analyses. We found that apoptosis is a significant component of radiation-induced death in ES cells and that this is accomplished in conjunction with proteolytic cleavage of PADPRP. Two fragments of M(r) 25,000 and M(r) 29,000 containing the PADPRP DNA-binding domain were identified in floating (apoptotic) cells, whereas only the full-length M(r) 116,000 native protein was detected in adherent cells that retained DNA intact. These data are consistent with PADPRP cleavage being an early step in the apoptotic cascade of biochemical events in ES cells after ionizing radiation exposure.