End-joining deficiency and radiosensitization induced by gemcitabine

The mechanism of radiosensitization by gemcitabine (2',2'-dinuoro-2'-deoxyc ytidine, dFdC) is not exactly known. We investigated the possible role of i nhibition of the repair of DNA double-strand breaks by dFdC by measuring th e extent of radiosensitization in different cell lines deficient and profic ient in components of nonhomologous end-joining and in the parental cell li nes. Different cell lines were incubated with 0.5 and 5 muM dFdC for 4 h. C ells deficient in DNA-dependent protein-kinase catalytic subunit (V3) showe d sensitization similar to that of wild-type cells (AA8) and complemented c ells (V3+YAC). Ku80-deficient cells (xrs5 and xrs6) showed even more radios ensitization by dFdC as compared with wild-type CHO-K1. However, Ku80-compl emented cell lines (xrs5+huKu80 and xrs6+haKu80) did not show radiosensitiz ation. The differences in dFdC-mediated radiosensitization were not attribu table to different changes in deoxynucleotide triphosphate levels and cell cycle distribution. We conclude that a functional nonhomologous end-joining pathway is not required for dFdC-mediated radiosensitization.