Clonogenic survival and apoptosis of prostate cancer cells irradiated with X-rays or carbon ions

It is well known that in general, an increased oxidative stress is found in cancer cells (e.g. [1, 2]). Low and high LET radiations induce reactive oxidative species and thus increase the oxidative stress even more. Different cancer cells not only do repair the DNA damage differently, they also cope differently with oxidative stress. Increasing the oxidative stress before irradiation might possibly radiosensitize cancer cells. In order to better understand the different effects of C-ions on two different prostate cancer cell lines, characterized by different genetic profiles, we examined the effects of sodium selenite pre-treatment combined with irradiation on these cell lines. Cell inactivation and apoptosis were measured in radioresistant prostate cancer cells derived from a bone metastasis (PC3) and from a brain metastasis (DU-145). Confluent cells were irradiated in G1-phase with 200 kV X-rays or with spread-out Bragg peak (SOBP) carbon ions (C-ions). Clonogenic survival was measured by standard colony assay. Sodium selenite was applied 6h before irradiation at concentrations non-toxic for humans (7.5 μM for PC-3, 7.5 or 20 μM for DU-145). Apoptosis was measured 24, 48 and 72 h after irradiation by morphological assay (fluorescent microscopy), TUNEL assay and caspase-3 activity assay (ELISA). As can be seen in Fig. 1 and 2, the effects are quite different in the two studied cell lines. SOBP C-ions were more efficient in both cell lines with respect to cell survival (clonogenic assay). The RBE for 10 % survival of SOBP C-ions (without sodium selenite) is about 2 for PC3 cells and 3.5 for DU-145 cells, confirming the superiority of C-ions in tumour therapy. In PC-3 cells, selenite at concentration of 7.5 μM shows the maximal radiosensitization (data not shown). In DU-145 cells, the maximal efficiency was observed at concentration of 20 μM, however for X-rays, only. No radiosensitization effect of selenite pre-treatment was observed for SOBP C-ions in this cell line. In PC-3 cells, where the selenite radiosensitization effect was present for X-rays and SOBP C-ions as well, survival curves indicate the presence of a resistant subpopulation. Apoptotic rates induced in the two cell lines after application of all treatment regimes are in agreement with these cell survival findings (data not shown), even the absence of any radiosensitization due to selenite pre-treatment for SOBP C-ions in DU-145 cells. The role of oxidative stress, repair of oxidative DNA damage and antioxidative capacity in the reaction of the studied cancer cells will be studied in further experiments. In addition, normal prostate epithelial cells PrEC will be examined as well. Fig.1. Clonogenic cell survival of PC-3 cells irradiated with graded doses of 200 kV X-rays (o) or SOBP C-ions (●) in comparison with sodium selenite pre-treated cells, X-rays plus selenite (□) and SOBP C-ions with selenite (■). Vertical bars represent standard deviations.