Checkpoint response plays a more protective role in HZE particle-irradiated cells than in X ray-irradiated cells

Abstract

High linear energy transfer (LET) ionizing radiation (IR) kills more cells as compared with low LET IR at the same doses. High LET IR is induced by high-charge (HZE) particles (a component in space radiation), high energy ions or by a special clinical radiotherapy machine. Low LET IR includes X or γ ray (a major component of IR from standard clinical radiotherapy machines), etc. The more cell death induced by high LET IR than by low LET IR at the same dose reflects a relative biological effectiveness (RBE). RBE on all kinds of quantitative effects induced by IR in X-ray exposed cells is 1. The RBE of cell killing induced by high LET IR is ~2-4 in cultured cells (killing ~2-4 times the number of cells at the same dose) depending on cell type. The induction of DNA double strand breaks (DSBs) either by high LET IR or by low-LET IR is a severe threat to genomic integrity that determines radiosensitivity (short term effect) and carcinogenesis (long term effect). It is believed that the higher RBE on cell killing by high LET IR is because of ineffective rejoining of DNA DSBs. [1][2][3][4] Two major DNA DSB repair pathways exist in eukaryotic cells, homologous recombination repair (HRR) and non-homologous end joining (NHEJ). Recently our group and others showed that high LET IR killing more cells than low LET IR is mainly due to the inhibition of Ku-dependent NHEJ. 5,6 After induction of DSBs, proliferating cells actively slow down cell cycle progression via checkpoint activation to provide time for repair. Previously, we and others showed that IR-induced checkpoint response mainly promotes HRR and has little effect on NHEJ. 7-10 These results provide important information that because the NHEJ pathway is inhibited in high LET irradiated cells, HRR might play a more protective role in such cells than that in low LET irradiated cells; because checkpoint facilitates mainly the HRR pathway, checkpoint response might be a more efficient factor for modifying cell sensitivity to high LET IR than to low LET IR. To test this hypothesis, we examined and analyzed the RBE on sensitivity in ATM or ATR, the two most important checkpoint genes, deficient or kinase dead cell lines following either high LET or low LET IR. The results support our hypothesis: both the cell lines either deficient in ATM or with kinase dead ATR, show higher RBE than their wild type counterpart cells (Fig. 1A and B). The RBE difference at 2 Gy is 3 times between AT cells and their wild type counterpart and is 5 times between ATR cells and their wild type counterpart (Fig. 1C). These results indicate that checkpoint response plays a more protective role in HZE particleirradiated cells than in X ray-irradiated cells, suggesting that checkpoint response is a more efficient target for modifying cell sensitivity to high LET IR than that to low LET IR.

DNA repair pathway choice at various conditions immediately post irradiation

“…Previously our group as well as other groups have shown that the enhanced relative biological effectiveness (RBE) on survival in high-LET irradiated cells is via interfering with NHEJ (Lind et al 2003, Okayasu 2006, Wang et al 2008), but not via affecting HRR (Wang et al 2008, Wang et al 2010, Zafar et al 2010). These results indicate that high-LET irradiated cells depend more on HRR to repair DNA DSB and maintain their survival.…”

“…These results indicate that high-LET irradiated cells depend more on HRR to repair DNA DSB and maintain their survival. We have observed that cells deficient in HRR are more sensitive to high-LET IR and showed higher RBE on their survival versus wild type cells (Wang et al 2008). These results provide additional evidence to support that HRR does not affect PLDR.…”

S-Phase Cells Are More Sensitive to High-Linear Energy Transfer Radiation

“…These short DNA DSB fragments could prevent Ku from binding efficiently to the two ends of DNA at the same time and delay Ku-dependent NHEJ (2) because Ku binding to DNA DSBs is the first step in NHEJ, which serves to recruit other essential NHEJ factors to the site of DNA DSBs. Since the checkpoint activation is believed to facilitate mainly the HRR and has little effect on NHEJ (3)(4)(5)(6)(7)(8), this means that checkpoint regulation plays a more protective role in high-LET-irradiated cells than in low-LET-irradiated cells (9).…”

A small peptide mimicking the key domain of MEPE/OF45 interacting with CHK1 protects human cells from radiation-induced killing

“…18 This conclusion is derived from the following facts: (1) high-LET radiation kills more cells than low-LET radiation at the same dose due to the inefficient Ku-dependent nonhomologous end-joining (NHEJ) 19 and thus results in HRR playing a more important role in protecting cells from high- than from low-LET IR-induced killing; (2) the CHK1 pathway protects cells from radiation-induced killing through promoting HRR. 13,14 Based on these results, we initially predicted that the small peptide should show an increased protective role in high-LET irradiated cells than in low-LET irradiated cells.…”