Chinese hamster ovary cells deficient or proficient in O6-alkylguanine-DNA alkyltransferase activity are equally sensitive to X-rays

Survival of human glioma cells treated with various combination of temozolomide and X-rays

PURPOSE

To investigate the effect of temozolomide, a 3-methyl derivative of mitozolomide in combination with X-rays in human glioma-derived cell lines.

METHODS AND MATERIALS

Glioma cell lines D384 and U251 were treated with temozolomide for various periods of time in combinations with X-rays. Temozolomide administration was repeated every 24 h for exposures up to 96 h. Cytotoxicity was determined with a clonogenic assay.

RESULTS

Incubation of D384 cells with temozolomide during 24 h prior to or following irradiation results in a moderate enhancement of the cytotoxicity. Prolonged treatment with temozolomide, i.e., 48-96 h before X-rays, causes a stronger potentiation. In contrast, no enhancement is observed in irradiated U251 cells in combination with 24-96 h temozolomide treatment. In addition to single-dose irradiation, we investigated the effect of temozolomide in D384 cells with concomitant fractionated irradiation. A 96-h exposure to temozolomide with simultaneous doses of 2 Gy X-rays at 24-h intervals, causes a significant further reduction in cell survival as compared to fractionated irradiation only.

CONCLUSION

Depending on the cell line, treatment of glioma cells with temozolomide and X-rays can have either an additional effect or potentiate cell killing.

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 kinetics, and react differently when facing cytotoxic stimuli. A protein synthesis inhibitor, cycloheximide (CHX), has recently been found to protect neuronal cells from oxidative stress. We investigated whether CHX exerts differential effects on dividing and non-dividing cells in the brain under cytotoxic stimuli. Mitotic C6 rat glioma cells and postmitotic neuronal cells were studied with a cytotoxic regimen combining gamma-irradiation (RT) and 1,3-bis,2-chloroethyl-1-nitrosourea (BCNU). Cells were exposed to BCNU (1 g/ml) for 15 h before gamma-irradiation and incubated with CHX (1 g/ml) from 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 enhancement of cytotoxicity was noticed with the addition of CHX to BCNU and 2-10 Gy of radiation. Our data demonstrated that CHX enhanced cytotoxicity of RT plus BCNU, while no additional toxicity was incurred to the postmitotic neuronal 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. Interestingly, 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 excessive functional deficit is an integral part of a successful brain tumor treatment regimen. Taking advantage of the differential effect of CHX on glioma and neuronal cells may improve tumor control without excessive neural toxicity.

Absence of UV-induced non-homologous recombination in repair-deficient CHO cell lines transfected with ERCC genes

The nucleotide excision repair pathway removes a broad spectrum of DNA lesions, including UV-induced damage. To ascertain whether the repair of the latter has a causative role in the enhancement of non-homologous recombination, Chinese hamster CHO cell lines proficient and deficient in the ability to repair UV-induced damage were transfected with a plasmid containing the bacterial neoR gene. Following UV-treatment an enhancement of non-homologous recombination above the spontaneous level was observed in repair-proficient cells, whereas no increase was observed in repair-deficient cell lines. Hence, the latter were transfected with the corresponding excision repair cross complementing human genes and the resulting repair-proficient transfectants were tested for UV-induced non-homologous recombination. In both untreated and UV-treated transfectants, the frequencies of the event were not significantly different. Cumulatively, the results suggest that non-homologous recombination induced by UV-irradiation is not restored by the correction of the excision repair defect.

Adaptive response induced by mitomycin C measuring the frequency of SCEs in human lymphocyte cultures

The induction of an adaptive response was obtained using mitomycin C (MMC) as both stimulating and challenging agent. Human lymphocyte cultures of two female donors were treated with 5, 10 and 20 ng/ml of MMC as conditioning doses. For the challenging treatments two different protocols were used (200 ng/ml for 4 h, and 400 ng/ml for 1 h). The scoring of sister chromatid exchanges (SCE) in the first challenging combination showed the following inhibition related with the expected SCE damage: 49.2%, 51.4%, and 36.9% for one donor, and 42.0%, 38.6%, and 34.7% for the other (corresponding to the stimulating dosages 5, 10, and 20 ng/ml, respectively). The second challenging combination gave an inhibition of 53.8%, 40.5% and 30.2% in one donor and 43.2%, 45.9% and 30.3% in the other donor.