S-phase cells are more sensitive to high-linear energy transfer radiation.
OBJECTIVE
S-phase cells are more resistant to low-linear energy transfer (LET) ionizing radiation (IR) than nonsynchronized and G(1)-phase cells, because both nonhomologous end-joining (NHEJ) and homologous recombination repair can repair DNA double-strand breaks (DSBs) in the S phase. Although it was reported 3 decades ago that S-phase cells did not show more resistance to high-LET IR than cells in other phases, the mechanism remains unclear. We therefore attempted to study the phenotypes and elucidate the mechanism involved.


METHODS
Wild-type and NHEJ-deficient cell lines were synchronized using the double-thymidine approach. A clonogenic assay was used to detect the sensitivity of nonsynchronized, synchronized S-phase, and G(2)-phase cells to high- and low-LET IR. The amounts of Ku bound to DSBs in the high- and low-LET-irradiated cells were also examined.


RESULTS
S-phase wild-type cells (but not NHEJ-deficient cells) were more sensitive to high-LET IR than nonsynchronized and G(2)-phase cells. In addition, S-phase wild-type cells showed less efficient Ku protein binding to DSBs than nonsynchronized and G(2)-phase cells in response to high-LET IR, although all cells at all phases showed similarly efficient levels of Ku protein binding to DSBs in response to low-LET IR.


CONCLUSIONS
S-phase cells are more sensitive to high-LET IR than nonsynchronized and G(2)-phase cells, because of the following mechanism: it is more difficult for Ku protein to bind to high-LET IR-induced DNA DSBs in S-phase cells than in cells at other phases, which results in less efficient NHEJ.
Wang
Hongyan
H
Department of Radiation Oncology and Kimmel Cancer Center of Jefferson Medical College, Thomas Jefferson University, Philadelphia, Pennsylvania 19107, USA.
Liu
Shuang
S
Zhang
Piyan
P
Zhang
Shimeng
S
Naidu
Mamta
M
Wang
Huichen
H
Wang
Ya
Y
D000818:Animals; D034961:Antigens, Nuclear / Q000378:metabolism*; D004247:DNA / Q000378:metabolism*; D004260:DNA Repair; D004268:DNA-Binding Proteins / Q000378:metabolism*; D004307:Dose-Response Relationship, Radiation; D016195:G2 Phase / Q000502:physiology / Q000528:radiation effects; D000072200:Ku Autoantigen; D018499:Linear Energy Transfer; D051379:Mice; D010641:Phenotype; D011836:Radiation Tolerance; D016196:S Phase / Q000502:physiology / Q000528:radiation effects*
D034961:Antigens, Nuclear; D004268:DNA-Binding Proteins; D004247:DNA; C000606510:Xrcc6 protein, mouse; D000072200:Ku Autoantigen