Apoptosis assay by annexin V staining under hypoxia versus normoxia on NB4 and THP-1. NB4 and THP-1 were cultured in the same condition as cell proliferation assay for 24 hours in triplicate. Then, apoptotic cells were examined after 12 and 24 hours by annexin V / PI method. Apoptotic cells of NB4 (A) under 24 hours hypoxia were significantly increased (p = 0.0031), although THP-1 (B) did not show increased apoptosis under hypoxia at least 24 hours (p = 0.7876). ▲: under hypoxia, ●: under normoxia. H: under hypoxia, N: under normoxia. *** p < 0.001, ns p > 0.05 (unpaired, two-tailed t-test).