HIF-1α up-regulation sensitizes HUVECs to the proapoptotic and antiangiogenic effects of bortezomib. (A) HUVECs, precultured in either the presence (left) or the absence (right) of DFO, were treated with 10 nM and 100 nM bortezomib and assessed for apoptosis induction by annexin V/PI staining and FACS analysis. Data, expressed as the percentage of annexin V/PI–positive cells, represent mean plus or minus SEM of 3 independent experiments. (B) Caspase 3 activity was evaluated by FACS analysis (top panels); mitochondrial membrane potential was evaluated by FACS analysis using JC-1 (bottom panels). The percentage of events associated with positive green and negative red (lower quadrants) was scored as mitochondrial depolarized cells. Data are representative of 3 independent experiments. (C) The 24-hour proliferation of HUVECs precultured with DFO was evaluated. Data are presented as the mean plus or minus SEM from 3 independent experiments in duplicate. Their ability to form capillaries on Matrigel layers was also assessed (D) (original magnification ×10). **P < .001.