Reovirus has genomes consisting of 10-segmented double-stranded RNAs, and have received much attention as an oncolytic virus. A previous study reported that reovirus down-regulates hypoxiainducible factor 1 alpha (HIF-1 alpha) protein levels following infection in tumor cells, which contributes to the antitumor effects of reovirus; however, the mechanism remains to be elucidated. In this study, we examined which virus component was involved in reovirus-mediated down-regulation of HIF-1 alpha. Reovirus induced significant down-regulation of HIF-1 alpha protein levels in not only reovirus-permissive tumor cells but also reovirus resistant tumor cells. UV-inactivated reovirus also induced a reduction in HIF-1 alpha protein levels. These data indicate that reovirus induces HIF-1 alpha down-regulation independently of virus replication. Furthermore, transfection with not only reovirus genomes but also polyI:C efficiently induced HIF-1 alpha down-regulation in a manner similar to reovirus, indicating that doublestranded reovirus RNA genomes are a key component for HIF-1 alpha down-regulation. Reovirus-mediated HIF-1 alpha down-regulation was inhibited when tumor cells were pretreated with inhibitors of cathepsins B and L, which play a crucial role in endo-lysosomal escape of virions to the cytoplasm. These data suggest that endo-lysosomal escape of reovirus genome into the cytoplasm is crucial for HIF-1 alpha down-regulation; however, the retinoic acid-inducible gene-I (RIG-I) or interferon-beta promoter stimulator-1 (IPS-1), which are involved in reovirus genome-induced innate immunity in the cytoplasm, did not play a crucial role in reovirus-mediated HIF-1 alpha reduction. (C) 2015 Elsevier Inc. All rights reserved.