The Ku-dependent non-homologous end-joining pathway contributes to low-dose radiation-stimulated cell survival.

Ionizing radiation (IR)-induced DNA double-strand breaks (DSBs) are a severe threat to cell killing. DNA DSBs are repaired by two major pathways: homologous recombination repair (HRR) and non-homologous end-joining (NHEJ) in mammalian cells. Low doses at ≤0.1 Gy exposure could stimulate DNA repair (Ikushima et al., 1996) and induce cell resistance to high-challenge dose IR-induced killing (Raaphorst et al., 2006). However, it remains unclear which pathway, NHEJ, HRR or both, is involved in the low-dose IR stimulated repair. Low-dose radiation-induced adaptive response was originally used to describe a phenomenon in which pre-exposure of cells to a low dose (adaptive dose) of radiation rendered cells less susceptible to damage induced by a subsequent high dose (challenge dose; Olivieri et al., 1984). Since then, many studies have shown that low-dose radiation-induced adaptive response could prevent genes from high-challenge dose IR-induced mutation (Sanderson and Morley, 1986; Laval, 1988; Schappi-Buchi, 1994; Zhou et al., 1994; Rigaud et al., 1995; Sasaki, 1995; Azzam et al., 1996; Ueno et al., 1996; Broome et al., 2002; Lu et al., 2009), and prevent the mice from subsequent high-challenge dose IR-induced tumors (Redpath and Antoniono, 1998; Mitchel et al., 1999, 2004; Yu et al., 2009), although some studies still challenge the low-dose radiation-induced protective roles in preventing carcinogenesis (Brenner et al., 2003; Mullenders et al., 2009). The mechanism by which adaptive response reduces the high-challenge dose IR-induced damage in vitro and in vivo might be a complicated process that involves protein synthesis (Ikushima, 1989; Wolff et al., 1989), immune response (Liu, 1998), apoptosis (Cregan et al., 1999; Portess et al., 2007) and many other proteins including protein kinase C (Sasaki, 1995), poly-ADP ribose polymerase (Ueno et al., 1996) and nuclear factor (NF)-κB (Fan et al., 2007). The main purpose of this study is to address the question of which DNA DSB repair pathway, HRR, NHEJ or both, is stimulated by the low-dose IR, thus protecting cells from high-challenge dose IR-induced killing. High linear energy transfer (LET) IR (induced by highly charged particles, high-energy ions or a special radiotherapy machine) can kill more cells than low-LET IR (such as X- or γ-rays) at the same doses. The higher relative biological effectiveness (RBE) on cell killing by high-LET IR is because of ineffective DNA repair (Goodhead et al., 1993; Rydberg et al., 1994; Stenerlow and Hoglund, 2002; Hill et al., 2004), which is mainly due to the inefficient Ku-dependent NHEJ pathway (Lind et al., 2003; Okayasu et al., 2006; Wang et al., 2008). Recently, we further demonstrated that high-LET IR when compared with low-LET IR only affects the Ku-dependent NHEJ but not HRR (Wang et al., 2008, 2010). The results related to high-LET IR affecting only NHEJ but not HRR (when compared with low-LET IR), help us identify which repair pathway (NHEJ or HRR) is stimulated by low-dose IR, by comparing the effects of the challenge dose with high- or low-LET IR on cell survival. In this study, we examined the effects of low-dose IR (0.1 Gy of low-LET IR) on high-challenge dose (low- or high-LET) IR-induced killing among different cell lines: wild-type, HRR- or NHEJ-deficient. Our results suggest that the low-dose IR-induced adaptive response might be via promoting the Ku-dependent NHEJ that contributes primarily to protecting cells from high-challenge dose IR-induced killing.