Effect of hypoxia on the paclitaxel or epirubicin-induced apoptosis. MDA-MB-231 cells were incubated under normoxic (N) or hypoxic (H) conditions with or without paclitaxel (tax, 50 μM) or epirubicin (epi, 10 μM) for 16 hours. (A) The caspase 3 activity was assayed by measuring free AFC released from the cleavage of the caspase 3 specific substrate Ac-DEVD-AFC. Results are expressed in fluorescence intensity, as mean ± 1 SD (n = 3). (B) After the incubation, DNA fragmentation was assayed using an ELISA for soluble nucleosomes (Cell Death Detection Elisa, Roche). Results are expressed as mean ± 1 SD (n = 3). N.S. = non significantly different from control, * = significantly different from control (p < 0.05), ** = significantly different from control (p < 0.01), *** = significantly different from control (p < 0.001); N.S. = no significant difference between N epi and H epi, ### = significant difference between N tax and H tax (p < 0.001).