Effects of dna-dependent protein kinase inhibition by NU7026 on dna repair and cell survival in irradiated gastric cancer cell line N87.
Repair of radiation-induced dna double-strand breaks is a key mechanism in cancer cell radio-resistance. The synthesized compound NU7026 specifically inhibits dna-dependent protein kinase (dna-pk) within the non-homologous end-joining repair mechanism. Earlier studies demonstrated increased radiosensitivity in dna-pk deficient cells compared with wild-type cells. In chronic leukemia cells, NU7026 appears to enhance the cytotoxic effect of chlorambucil. The radio-modifying effects of NU7026 on cell survival, cell cycle, apoptosis, and dna double-strand break repair have yet to be studied in gastric cancer cells.


METHODS
The gastric cancer cell line N87 was treated with 0 Gy or 4 Gy in the presence of NU7026 at a dose range of 0-20 μmol/L. Clonogenic assays were used to assess cell survival after treatment. Cell-cycle distribution was analyzed using propidium iodide with fluorescence-activated cell sorting. Apoptosis was detected using annexin-V and propidium iodide with fluorescence-activated cell sorting. The γH2AX assay was used to measure dna double-strand breaks.


RESULTS
Statistically significant increases in G2/M arrest were observed in N87 cells treated with radiation and NU7026 compared with those treated with radiation alone (p = 0.0004). Combined treatment also led to an increase in apoptosis (p = 0.01). At 24 hours, the γH2AX analysis revealed more dna double-strand breaks in N87 cells treated with radiation and NU7026 than in those treated with radiation alone (p = 0.04). Clonogenic assays demonstrated declining cell survival as both the radiation and the NU7026 dose increased. The dose enhancement factor at 0.1 survival fraction was 1.28 when N87 cells were treated with 4 Gy radiation and 5 μmol/L NU7026.


CONCLUSIONS
In gastric cancer cells, NU7026 appears to enhance the cytotoxic effect of irradiation as assessed by clonogenic assays. This increased cytotoxicity might be the result of an increase in dna double-strand breaks resulting in G2/M cell arrest and possibly higher levels of apoptosis.
Segal Cancer Centre, Department of Oncology, Division of Radiation Oncology, Jewish General Hospital, McGill University, Montreal, QC.
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Niazi
Department of Oncology, Division of Radiation Oncology, Jewish General Hospital, McGill University, Montreal, QC.
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Mok
Department of Experimental Medicine, McGill University, Montreal, QC.
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Heravi
McGill University, Montreal, QC.
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Lee
Segal Cancer Centre, Department of Oncology, Division of Radiation Oncology, Jewish General Hospital, McGill University, Montreal, QC.
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Vuong
Segal Cancer Centre, Lady Davis Institute of Research, Jewish General Hospital, McGill University, Montreal, QC.
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Aloyz
Segal Cancer Centre, Department of Oncology, Division of Radiation Oncology, Jewish General Hospital, McGill University, Montreal, QC.
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Panasci
Segal Cancer Centre, Department of Oncology, Division of Radiation Oncology, Jewish General Hospital, McGill University, Montreal, QC.
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Muanza
Gastric cancer; NU7026; dna pk inhibitor; radiation therapy; radiosensitization