E. Odegaard et al., DNA-DEPENDENT PROTEIN-KINASE DOES NOT PLAY A ROLE IN ADAPTIVE SURVIVAL RESPONSES TO IONIZING-RADIATION, Environmental health perspectives, 106, 1998, pp. 301-305
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.