DNA-DEPENDENT PROTEIN-KINASE DOES NOT PLAY A ROLE IN ADAPTIVE SURVIVAL RESPONSES TO IONIZING-RADIATION

We previously demonstrated that exposure of certain human tumor cells to very low chronic doses of ionizing radiation led to their enhanced survival following exposure to subsequent high doses of radiation. Sur vival enhancement due to these adaptive survival responses (ASRs) rang ed from 1.5-fold to 2.2-fold in many human tumor cells. Furthermore, w e showed that ASRs result from altered G(1) checkpoint regulation, pos sibly mediated by overexpression of cyclin D1, proliferating cell nucl ear antigen (PCNA), and the X-ray induction of cyclin A. Because cycli n D1 and PCNA proteins are components of many DNA synthetic and repair processes in the cell, we tested the hypothesis that preexposure of c ells to low doses of ionizing radiation enabled activation of the DNA repair machinery needed for survival recovery after high-dose radiatio n. We examined the role of DNA break repair in ASRs using murine cells deficient (i.e., severe combined immunodeficiency [SCID] cells) or pr oficient (i.e., parental mouse strain [CB-17] cells) in DNA-dependent protein kinase catalytic subunit (DNA-PKcs) expression and DNA double- strand break repair. DNA-PKcs is a nuclear serine/threonine protein ki nase that is activated by DNA breaks and plays a key role in double-st rand break repair. DNA-PKcs also phosphorylates several nuclear DNA-bi nding regulatory transcription factor proteins (e.g., Sp1 and p53), wh ich suggests that DNA-PKcs may play a role in regulating transcription , replication, and recombination as well as DNA repair, after radiatio n. Therefore, we exposed confluent SCID or CB-17 cells to low priming doses of ionizing radiation (i.e., 5 cGy) and compared the survival re sponses of primed cells to those of unprimed cells after an equitoxic high-dose challenge. low-close-primed SCID or CB-17 cells demonstrated 2-fold enhanced survival after a high-dose challenge compared to that of unprimed control cells. These data suggest that expression of the catalytic subunit of DNA-PKcs (expressed in CB-17 not SCID cells) and the presence of active double-strand break repair processes (active in CB-17, deficient in SCID cells) do not play a major role in ASRs in m ammalian cells. Furthermore, we present data that suggest that DNA-PKc s plays a role in the regulation of the G(2)/M cell cycle checkpoint f ollowing extremely high doses of ionizing radiation.