[Radioresistant subline of human glioma cell line MGR2R induced by repeated high dose X-ray irradiation].

Background & Objective: Radiotherapy is one of the most important adjuvant treatments for patients with malignant glioma, but radiosensitivities of gliomas are widely various. This study was to induce human glioma cell line MGR2 to become a stable radioresistant cell subline, and investigate the mechanisms of radioresistance.

Methods: Human glioma cell line MGR2, with survival fraction of 2 Gy (SF(2)) of 0.18+/-0.05, was irradiated by intermittent high dose X-ray (2 Gy for 3 times, 5 Gy for 2 times). After each irradiation, the cells were cultured for 5 to 8 weeks and received irradiation again. The whole process of irradiation and culture lasted for 11 months. The cells derived from MGR2 were obtained and named MGR2R (MGR2 radiation induction). Double times of MGR2 and MGR2R cells were determined by MTT assay. Dose-survival curves, radiobiological parameters and SF2 were determined by colony-forming assay and line-quadratic model. The variation of their cell cycles was investigated by flow cytometry and cell cycle synchronization of serum-starvation.

Results: The double time of MGR2 cells was 3.6 days, while that of MGR2R cells was 4.0 days. The growth rate of MGR2R cells was slower than that of MGR2 cells. Using colony-forming assay and line-quadratic model, the parameters of MGR2 and MGR2R were obtained. The alpha values of MGR2 and MGR2R were 0.447 and 0.089 (t=4.524, P=0.011), the beta values were 0.177 and 0.141 (t=1.562, P=0.193), and the SF(2) were 0.208 and 0.478 (t=-6.062, P=0.040), respectively. The radioresistance of MGR2R cells was stronger than that of MGR2 cells. The distribution of cell cycle in MGR2 cells after synchronization were 54.8% in G(1) phase, 30.9% in S phase, and 14.3% in G(2) phase; 24 h after loss of synchronization, the distribution of cell cycle were 35.9% in G(1) phase, 51.2% in S phase, and 12.8% in G(2) phase. The distribution of cell cycle in MGR2R cells after synchronization were 55.7% in G(1) phase, 27.8% in S phase, and 16.6% in G(2) phase; 24 h after loss of synchronization, the distribution of cell cycle were 56.4% in G1 phase, 26.7% in S phase, and 16.9% in G(2) phase. MGR2R cells appeared G(2) phase arrest before synchronization, and G1 phase arrest after loss of synchronization.

Conclusions: After intermittent high dose X-ray irradiation, radiosensitive cell line MGR2 has been induced to be relatively radioresistant (MGR2R). MGR2R cells grow slower and have G(1) phase and G(2) phase arrest which might relate to its radioresistance.