A radioresistant variant derived from a human neuroblastoma cell line is less prone to radiation-induced apoptosis

By subjecting radiosensitive human neuroblastoma IMR 32 cells to a regime of fractionated X-irradiation, a radioresistant variant, XRIMR 32, was obtained. Radiation resistance of XRIMR 32 cells was demonstrated by clonogenic and spheroid regrowth delay assays. The XRIMR 32 cultures were phenotypically unstable, with the resistant phenotype being lost after 3 passages in the absence of radiation-selective pressure, but a monoclonal cell line (clone F) was established that maintained its resistance over 35 passages without irradiation. Flow cytometry showed that exponentially growing IMR 32, XRIMR 32, and clone F cells all had very similar cell cycle distributions. Studies of initial DNA damage and repair, using the technique of neutral filter elution, revealed no differences between these lines. Chromosomal damage, as measured by micronucleus frequency following irradiation, was also seen to be very similar. However, studies of apoptosis following irradiation showed significantly higher levels of apoptosis in IMR 32 cells, compared to the resistant lines. This was true at all time points studied between 6 and 42 h after irradiation. p53 status was examined in the IMR 32 and clone F cells. No mutations were detected in exons 5-8 of the cDNA. Both lines showed increased p53 expression after irradiation. These data are consistent with the evolution of cellular resistance as a possible mechanism for the evolution of cellular radioresistance during protracted radiation regimes. However, the molecular mechanism responsible for the increased radioresistance remains to be discovered.