Abstract
The failure of therapeutic responsiveness to radiation of malignant brain tumors, glioblastomas, remains a significant clinical problem. Tumor suppressor p53, a guardian gene with multiple functions, has been found to contain mutations in more than 50% of high-grade gliomas. Two glioblastoma cell lines, U87 and U138, the latter possessing mutated p53, were used for investigation of the differences in cell survival, cell-cycle checkpoints, apoptotic debris after gamma irradiations. While subtle difference was shown between U87 and U138 in survival curve shape, their major difference was in the cell-cycle checkpoints. After 10 Gy irradiation, U87 exhibited progression perturbation in G1, S and G2, in contrast to those by U138 where a prolonged and stronger G2 arrest but little or no G1 and S checkpoints were indicated. Further, FCM showed more apoptotic debris after the release of the G2 arrest in U138 than in U87. To help elucidate the mechanisms of these cellular effects, we have conducted time-course experiments to measure global gene expression using high-density cDNA and oligonucleotide microarray, with qPCR validations. We will present the outcome of gene expression results with a focus on genes related to cell-cycle control and apoptosis. Based on these results and current knowledge on the molecular pathways of cell-cycle regulation, we propose a model on p53-independent G2 arrest pathways. Our preliminary analysis indicates that in the absence of TP53 function, its family members p63 and TP73 may serve its role to regulate down stream genes such as p21 and GADD45A leading to inactivation of the CyclinB/Cdc2 complex. The alternative pathways are apparently more effective in causing G2 blockade than the p53-dependent pathways and the model is subjected to further test at the protein level. This work is partially supported by Grants NSC91-3112-P-010-003-Y, NSC91-2314-B-010-042, and NHRI-EX92-9115SN. [J Radiat Res 44:371 (2003)]
