Inhibition of DNA-end binding complex formation as a mechanism for celecoxib-induced radioenhancement in human head and neck carcinoma cells

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Selective inhibitors of cyclooxygenase-2 (COX-2) enhance radiosensitivity of tumor cells in vitro and tumor radioresponse in vivo. Our previous report suggested a number of mechanisms likely to underlie the increased cell radiosensitivity including cell cycle redistribution, apoptosis, and inhibition of DNA repair (Raju et al, Int. J. Radiat. Oncol. Biol. Phys., 2002). Because little is known about molecular events underlying the observed inhibition of DNA repair, the present study investigated molecular processes that are activated by selective COX-2 inhibitors that could lead to DNA repair inhibition. The experiments used HN5 human head and neck carcinoma cells and celecoxib, a selective COX-2 inhibitor. Treatment of HN5 cells with celecoxib, 10 or 50 μM for 2 days before the cells were irradiated with 1 to 6 Gy γ-radiation, enhanced radiosensitivity of these cells by a factor of 1.20 and 1.53 respectively, as assessed by in vitro clonogenic cell survival assay. Celecoxib significantly reduced the shoulder region on the radiation cell survival curve, suggesting that it inhibited DNA repair. Celecoxib-treated cells exhibited a reduction in a DNA-end binding complex of proteins consisting of DNA-PKcs, Ku70 and Ku80 involved in DNA repair, which was determined by electrophoretic mobility shift assay. In a separate analysis, it was observed that celecoxib reduced the expression of Ku70, but not Ku80 or DNA-PKcs. In addition, celecoxib down regulated the DNA binding activity of nuclear factor kappa B (NFκB) that is known to regulate Ku70 expression as Ku70 gene promoter region contains kappa B binding sites. In conclusion, our results suggest that celecoxib inhibited DNA repair after ionizing radiation by down regulating NFκB that resulted in the reduction in expression of Ku70 protein. This reduction might have lowered the ability of the DNA-PK complex to bind DNA ends, which is required for successful DNA repair. Thus, celecoxib is a potent sensitizer of HN5 head and neck carcinoma cells, and a major mechanism being inhibition of DNA repair. (Supported in part by NIH Grant CA6294 and Aventis Pharmaceuticals)