Cell death mechanisms involved in gamma ray-irradiated glioblastoma multiforme (GBM) cells Free

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A better understanding of mechanisms responsible for ionizing radiation induced cell death may help make better therapeutic regimes for the most malignant human brain tumor, glioblastoma multiforme (GBM). In this study, we scrutinized the radiation responses and radiation-induced cell death mechanisms in two human GBM cell lines with different p53 statuses, U87MG (wt-p53) and U138MG (mt-p53). By clonogenic assay, U87GM and U138GM cells showed similar survival curves after irradiated with various doses of gamma ray. Their IC50 values were 1.95 and 2.53 Gy, respectively. By Annexin V staining and flow cytometric analysis, gamma irradiation apparently did not trigger apoptotic process in these 2 cells. To uncover how these 2 GBM cell lines died after gamma ray irradiation, we first analyzed their cell cycle progression by aid of a flow cytometer. U87MG cells irradiated with 10 Gy underwent rapid and transient G2 phase arrest followed by prolonged G1 phase arrest, while only prolonged G2 phase arrest was observed in U138MG cells irradiated with the same dose of gamma ray. By using senescence-associated β-galactosidase (SA-β-Gal) as a senescence marker, 51% of U87MG cells turned into senescence at 5 days after irradiation, whereas only 8% of U138MG cells expressed SA-β-Gal. We also observed that gamma ray irradiation at 10 Gy resulted in significant leaking of lactate dehydrogenase to medium of these 2 cell lines, indicating that these GBM cell lines might die of necrosis. To explore the differential responses of U87MG and U138MG cells to gamma irradiation, we found damage-specific DNA binding protein 2 (DDB2) and Xeroderma pigmentosum complementation group C (XPC), both functioned as damage recognition in nucleotide excision repair, were enhanced in U87MG cells, while only DDB2 was enhanced in U138MG cells. In addition, transforming growth factor alpha, a tumor-promoting paracrine factor, and p21, a negative cell cycle regulator, were also enhanced in U87MG cells. Further in-depth analysis of the changes in gene expression involved in the differences of cell death mechanisms between these 2 GBM cell lines may prompt us to overcome their resistance to ionizing irradiation.