Tumor cells might resist
therapy with ionizing radiation (IR) by non-homologous end-joining (NHEJ) of IR-induced double-strand breaks. One of the key players in NHEJ is
DNA-dependent protein kinase (
DNA-PK). The catalytic subunit of
DNA-PK, i.e.
DNA-
PKcs, can be inhibited with the small-molecule inhibitor
NU7026. In the current study, the in vitro potential of
NU7026 to radiosensitize
neuroblastoma cells was investigated.
DNA-
PKcs is encoded by the PRKDC (
protein kinase, DNA-activated, catalytic
polypeptide) gene. We showed that PRKDC levels were enhanced in
neuroblastoma patients and correlated with a more advanced
tumor stage and poor prognosis, making
DNA-
PKcs an interesting target for radiosensitization of
neuroblastoma tumors. Optimal dose finding for combination treatment with
NU7026 and IR was performed using NGP cells. One hour pre-treatment with 10 μM
NU7026 synergistically sensitized NGP cells to 0.63 Gy IR.
Radiosensitizing effects of
NU7026 increased in time, with maximum effects observed from 96 h after IR-exposure on. Combined treatment of NGP cells with 10 μM
NU7026 and 0.63 Gy IR resulted in apoptosis, while no apoptotic response was observed for either of the
therapies alone. Inhibition of IR-induced
DNA-PK activation by
NU7026 confirmed the capability of NGP cells to, at least partially, resist IR by NHEJ.
NU7026 also synergistically radiosensitized other
neuroblastoma cell lines, while no synergistic effect was observed for low
DNA-
PKcs-expressing non-cancerous fibroblasts. Results obtained for
NU7026 were confirmed by PRKDC knockdown in NGP cells. Taken together, the current study shows that
DNA-
PKcs is a promising target for
neuroblastoma radiosensitization.