Hypoxia is a common environmental stress. Particularly, the center of rapidly-growing solid tumors is easily exposed to hypoxic conditions. Hypoxia is well known to attenuate the therapeutic response to radio and chemotherapies including tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) protein. HIF-1 alpha is a critical mediator of the hypoxic response. However, little is known about the function of hypoxia-inducible factor-1 alpha (HIF-1 alpha) on hypoxic inhibition of TRAIL-mediated apoptosis. In this study, we investigated whether hypoxic inhibition of TRAIL-mediated apoptosis can be regulated by modulating HIF-1 alpha protein. Hypoxia- and DEF-induced HIF-1 alpha activation inhibited the TRAIL-mediated apoptosis in SK-N-SH, HeLa, A549 and SNU-638 cells. And also, HIF-1 alpha inactivating reagents including DOX increased the sensitivity to TRAIL protein in tumor cells exposed to hypoxia. Furthermore, knock-down of HIF-1 alpha using lentiviral RNA interference sensitized tumor cells to TRAIL-mediated cell death under hypoxic condition. Taken together, these results indicate that HIF-1 alpha inactivation increased TRAIL sensitivity in hypoxia-induced TRAIL-resistant tumor cells and also suggest that HIF-1 alpha inhibitors may have benefits in combination therapy with TRAIL against hypoxic tumor cells. (C) 2010 Elsevier Inc. All rights reserved.