Hypoxia-Induced BAX Overexpression and Radiation Killing of Hypoxic Glioblastoma Cells

Abstract

One major challenge in treating glioblastoma multiforme (GBM) has been the presence of radiation-resistant hypoxic cells. The pro-apoptosis protein BAX has been reported to be a possible radiation sensitizer of cancer cells; however, to our knowledge, no studies have reported on the effects of BAX on radiation sensitivity under hypoxic conditions. Therefore, in this study, we specifically overexpressed murine Bax in hypoxic cells in an attempt to enhance radiation-induced cell killing. We have previously stably transfected U-251 MG and U-87 MG human GBM cells with constructs containing murine Bax under the regulation of nine copies of hypoxia-responsive elements (HREs). During hypoxia, the transcriptional complex hypoxia-inducible factor 1 (HIF1) forms and binds to HRE; this binding facilitates the transcription of downstream genes. In the experiments reported here, two protocols were used. In the first protocol, parent and clone cells were exposed to graded doses of X rays under hypoxic conditions, left hypoxic for 0, 4, 16 or 24 h, and then assayed for clonogenic cell survival. In the second protocol, cells were incubated under hypoxic conditions for 20 h, irradiated with graded doses under hypoxia, then left in hypoxic conditions for 4 h before being assayed for clonogenic cell survival. Western blots showed that we had successfully increased Bax expression in both U-251 MG and U-87 MG Bax clone cells after 16 h of hypoxic incubation, yet dose-response curves showed no difference in radiation-induced cell killing between control non-Bax-expressing pNeo clone cells and the U-251 MG Bax clone cells using either protocol. In U-87 MG cells, the first protocol showed no difference in radiation response between control pNeo clone cells and the Bax clone, similar to the results obtained in U-251 cells. However, the second protocol revealed that Bax overexpression did render these cells more sensitive to radiation under hypoxic conditions. Therefore, we conclude that whether Bax is a radiation enhancer under hypoxia not only is cell line-dependent but also depends on when the Bax overexpression occurs.