RIKEN Accel. Prog. Rep. 48 (2015)

Ⅲ-4. Radiation Chemistry & Biology

Low-dose high-LET heavy ion-induced bystander signaling
M. Tomita,∗1,∗2 T. Tsukada,∗1 and M. Izumi∗1
The radiation-induced bystander response (RIBR) isa cellular response induced in nonirradiated cells thatreceive bystander signals from directly irradiated cells
within an irradiated cell population.1) The RIBR induced by low doses of high-LET radiations is an important issue concerning the health of astronauts andin heavy-ion radiation cancer therapy. Here, we investigated the molecular mechanisms underlying andbiological implications of the RIBR induced by suchlow doses of high-LET radiations.
Figure 1 shows the clonogenic survival curve of normal human ﬁbroblast WI-38 cells irradiated with Fe
ions (1000 keV/µm). Cells were harvested from thecell culture ﬂask immediately (0 h) or 16–24 h after
irradiation, and the surviving fraction was determinedby using a colony formation assay. At doses below 0.2
Gy, the surviving fractions of the cells harvested 1624 h after exposure to Fe ions were much lower thanthose extrapolated from higher doses above 0.1 Gy using the linear-quadratic model (LQ model).2) On theother hand, the cells harvested immediately after exposure did not show such a high cell killing eﬀect atlower doses. These results suggest that an adequate
incubation period is necessary for the bystander signal
induction and transfer.
Previously, we have reported that reactive oxygen
species (ROS), gap-junction intercellular communication (GJIC), and cyclooxygenase-2 (COX-2) protein as
well as nitric oxide (NO) may be involved in high-LET
radiation-induced bystander signal transfer.3) Here, weshow the progress of results reﬂecting new data [Fig.
2]. Lindane and NS-398 (an inhibitor of GJIC andCOX-2, respectively) were dissolved in DMSO (a scavenger of ROS). DMSO did not signiﬁcantly suppress
the bystander cell killing eﬀect. In contrast, lindane,NS-398, and c-PTIO (a scavenger of NO) signiﬁcantly
suppressed cell death to similar levels. Cells pretreatedwith both c-PTIO and lindane did not show signiﬁcantly higher surviving fraction than those pretreatedwith lindane or c-PITO alone. These results suggestthat bystander signaling through GJIC and the cellculture medium induces the bystander cell killing effect in a coordinated manner.
Currently, we are examining the role of the NFκB/Cox-2/prostaglandin E2 and NF-κB/iNOS/NOpathways,4) which may be activated in bystander cells.

∗1∗2

RIKEN Nishina Center
Radiation Safety Research Center, Central Research Institute of Electric Power Industry

Fig. 1. Survival curves of WI-38 cells. Conﬂuent monolayers of WI-38 cells were irradiated with 90 MeV/u Feions (1000 keV/µm) and the cells were harvested immediately (0 h) or 16–24 h after irradiation. The surviving
fraction was determined by using a colony forming assay. Panel A shows all data obtained in this study.
Panel B shows surviving fractions at doses below 0.5
Gy. The error bars represent the standard deviations.

Fig. 2. Eﬀect of inhibitors or scavengers. DMSO (0.1%),
lindane (Lin, 50 µM), c-PTIO (20 µM), or NS-398 (50µM) was added to the medium 2 h before irradiation.5)
WI-38 cells were irradiated with 0.1 Gy Fe ions. Theerror bars represent the standard errors of the mean
(SEM) (n=3–4).*P < 0.05, for comparison with irradiated control cultures.

References
1) M. Tomita and M. Maeda: J. Radiat. Res. 56, 205(2015).
2) M. Tomita et al.: RIKEN Accel. Prog. Rep. 42, 276(2009).
3) M. Tomita et al.: RIKEN Accel. Prog. Rep. 47, in press.
4) T. K. Hei et al.: Curr. Mol. Pharmacol. 4, 96 (2011).
5) M. Tomita et al.: Radiat. Res. 179, 200 (2013).

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