Cycling hypoxia induces higher, long-term HIF-1 activation in glioblastoma cells and xenografts.

<p>(A) Western blot analysis of HIF-1α in GBM8401 and U87 cells after cycling hypoxia. Cells were exposed to hypoxic stress, either non-interrupted or cycling, for 4 h and harvested to determine the amounts of HIF-1α protein in nuclear extracts. (B) Transcriptional activity at hypoxia response elements in GBM8401 cells after cycling hypoxic stress. GBM8401/hif-1-r cells were cultured under hypoxic stress, either non-interrupted or cycling, for 4 h and grown in normoxia for different periods, followed by measurements of reporter gene expression. (C) Kinetics of HIF-1 transcriptional activity in GBM8401/hif-1-r xenografts after cycling hypoxic stress. <i>In vivo</i> fluorescence imaging (FLI) was performed for GBM8401/hif-1-r tumors before hypoxic treatments and at different times after hypoxic treatments. The data represent the mean ± standard deviation of the ratio of average counts within the tumor region of interest (ROI) in GFP and DsRed signals from 6 mice.