Differential effect of cycloheximide on neuronal and glioma cells treated with chemotherapy and radiation

Dividing cells and non-dividing cells are distinct in their cell cycle kine tics, and react differently when facing cytotoxic stimuli. A protein synthe sis inhibitor, cycloheximide (CHX), has recently been found to protect neur onal cells from oxidative stress. We investigated whether CHX exerts differ ential effects on dividing and non-dividing cells in the brain under cytoto xic stimuli. Mitotic C6 rat glioma cells and postmitotic neuronal cells wer e studied with a cytotoxic regimen combining gamma-irradiation (RT) and 1,3 -bis,2-chloroethyl-1-nitrosurea (BCNU). Cells were exposed to BCNU (1 mu g/ ml) for 15 h before gamma-irradiation and incubated with CHX (1 mu g/ml) fr om 30 min before and until 5 h after irradiation. Clonogenic assay was used to assess cytotoxic effects on C6 glioma cells. LDH assay was used for the viability of H19-7 postmitotic neuronal cells. A 2.27-3.75 fold enhancemen t of cytotoxicity was noticed with the addition of CHX to BCNU and 2-10 Gy of radiation. Our data demonstrated that CHX enhanced cytotoxocity of RT pl us BCNU, while no additional toxicity was incurred to the postmitotic neuro nal cells when CHX was added. We further studied whether the inhibition of DNA repair, assayed by single-cell DNA electrophoresis (comet assay), is a contributing factor for the enhanced cytotoxicity on C6 glioma cells. Inter estingly, the initial DNA damage after RT plus BCNU was equivalent; whereas DNA repair was significantly less at 5 h after radiation in CHX-treated C6 glioma cells. Protecting non-dividing neuronal cells to avoid excesive fun ctional deficit is an integral part of a successful brain tumor treatment r egimen. Taking advantage of the differential effect of CHX on glioma and ne uronal cells may improve tumor control without excessive neural toxicity.