Radiation-induced survivin nuclear accumulation is linked to DNA damage repair

Increased expression of survivin has been identified as a negative prognostic marker in a variety of human cancers. We have previously shown that survivin is a radiation-resistance factor and that the therapeutic effect of survivin knock-down might result from an impaired DNA repair capacity. In this study, we aimed to elucidate an interrelationship between survivin's cellular localization and DNA double-strand break repair.

Survivin's cellular distribution and nuclear complex formation were assayed by Western blotting of subcellular fractions, by immunofluorescence staining, and co-immunoprecipitation in SW480 colorectal cancer cells. DNA repair capacity was analyzed by kinetics of gamma-H2AX foci formation, and by DNA-dependent protein kinase (DNA-PKcs) assays in the presence of survivin-specific or nonspecific control siRNA.

Following irradiation, we observed a rapid nuclear accumulation of survivin and subsequent phosphorylation of the protein in the nucleus. Co-immunoprecipitation analyses from nuclear extracts revealed an interaction among survivin, Ku70, gamma-H2AX, MDC1, and DNA-PKcs that was confirmed by immunofluorescence co-localization in nuclear foci. Survivin knock down by siRNA resulted in an impaired DNA double strand break repair, as demonstrated by an increased detection of gamma-H2AX foci/nucleus at 60 min and a higher amount of residual gamma-H2AX foci at 24 hr postirradiation. Furthermore, we detected in survivin-depleted cells a hampered S2056 autophosphorylation of DNA-PKcs and a significantly decreased DNA-PKcs kinase activity.

These data indicate that nuclear survivin is linked to DNA double-strand break repair by interaction with members of the DNA double-strand breaks repair machinery, thus regulating DNA-PKcs activity.