Wnt/β-catenin pathway involvement in ionizing radiation-induced invasion of U87 glioblastoma cells

Radiotherapy has been reported to promote the invasion of glioblastoma cells; however, the underlying mechanisms remain unclear. Here, we investigated the role of the Wnt/β-catenin pathway in radiation-induced invasion of glioblastoma cells.

U87 cells were irradiated with 3 Gy or sham irradiated in the presence or absence of the Wnt/β-catenin pathway inhibitor XAV 939. Cell invasion was determined by an xCELLigence real-time cell analyser and matrigel invasion assays. The intracellular distribution of β-catenin in U87 cells with or without irradiation was examined by immunofluorescence and Western blotting of nuclear fractions. We next investigated the effect of irradiation on Wnt/β-catenin pathway activity using TOP/FOP flash luciferase assays and quantitative polymerase chain reaction analysis of β-catenin target genes. The expression levels and activities of two target genes, matrix metalloproteinase (MMP)-2 and MMP-9, were examined further by Western blotting and zymography.

U87 cell invasiveness was increased significantly by ionizing radiation. Interestingly, ionizing radiation induced nuclear translocation and accumulation of β-catenin. Moreover, we found increased β-catenin/TCF transcriptional activities, followed by up-regulation of downstream genes in the Wnt/β-catenin pathway in irradiated U87 cells. Importantly, inhibition of the Wnt/β-catenin pathway by XAV 939, which promotes degradation of β-catenin, significantly abrogated the pro-invasion effects of irradiation. Mechanistically, XAV 939 suppressed ionizing radiation-triggered up-regulation of MMP-2 and MMP-9, and inhibited the activities of these gelatinases.

Our data demonstrate a pivotal role of the Wnt/β-catenin pathway in ionizing radiation-induced invasion of glioblastoma cells, and suggest that targeting β-catenin is a promising therapeutic approach to overcoming glioma radioresistance.