Short-term effects of radiation in gliolalstoma spheroids

Glioblastoma is the most frequent and malignant primary brain tumor. The standard treatment includes surgery, radiation and chemotherapy. The limited efficacy of the current treatment has been explained by the existence of treatment-resistant stem-like tumor cells. The aim of this study was to investigate the short-term effects of radiation of spheroids containing tumor-initiating stem-like cells. We used a patient-derived glioblastoma stem cell enriched culture (T76) and the standard glioblastoma cell line U87. Primary spheroids were irradiated with doses between 2 and 50 Gy and assessed after two and five days. We found a small reduction in primary spheroid size after radiation and an associated small increase in uptake of the cell death marker propidium iodide. Using immunohistochemistry, P53 expression was found to be significantly increased, whereas the Ki-67 proliferation index was significantly reduced. Both number and size of secondary spheroids formed after radiation were significantly reduced. In a limiting dilution assay, the spheroid formation capacity upon irradiation was higher for T76 compared to U87, but for both T76 and U87 irradiation led to a reduced spheroid formation capacity. Gene expression analysis of nine stem cell- and two hypoxia-related genes did not reveal any upregulation after radiation. In conclusion, this study suggests that a major short-term effect of radiation is pronounced reduction of tumor cell proliferation. We found no upregulation of stem cell-related genes. This may suggest a limited effect of targeting these genes within the first days after radiation therapy.