Diffusing Alpha-Emitters Radio-Therapy (DaRT) treatment is based on insertion of 224Ra – loaded wire(s), which release into the tumor short-lived alpha emitting atoms.
DaRT treatment caused significant damage to squamous cell tumors (SCC, SQ2) but not to pancreatic (Panc02) and breast adenocarcinoma (4T1). These differences led us to investigate the cellular characteristics, which govern the different sensitivity to alpha radiation.
The cell-lines were cultured in well-like devices and were exposed to 0-7 Gy of alpha radiation. Survival curves, obtained from clonogenic assays, revealed that the SQ2 cell line is the most radiosensitive to alpha radiation whereas the 4T1 and Panc02 cells are the least radiosensitive. Since a-radiation causes mainly DNA double strand breaks (DSBs), we investigated whether the differences are controlled by DNA damage or repair mechanisms.
Formation and disappearance of phosphorylated H2AX (g-H2AX) foci (DSBs indicators), and activation of non-homologous DNA breaks repair mechanisms (NHEJ), (recruitment of Ku70 into the nucleus), were quantified using western blot analysis and immunostaining.
The 3 cell lines examined exhibited different damage accumulation and repair kinetics. The radioresistant cell line, 4T1, had the lowest number of DSBs and a peak of Ku70 nuclear recruitment 30 min post radiation, indicating a quick and efficient repair process. The Panc02, a relatively radioresistant cell line, had an intermediate number of DSBs, and quick damage repair shortly after radiation. The sensitive SQ2 cells exhibited high DNA damage and a low and very slow Ku70 nuclear recruitment indicating a slow and not efficient repair process that resulted in rapid cell death.
Identifying the mechanism(s) responsible for the resistance of various tumor cells to alpha radiation might open the possibility to block this mechanism(s) and render the cells more susceptible to alpha particles. This may have practical implications for the treatment of solid tumors by DaRT.
