We examined HeLa cell viability and RNA oxidative damage in response to hydrogen peroxide (H2O2) treatment. The level of damaged RNA, measured by the content of 8-hydroxyguanosine (7,8-dihydro-8-oxoguanosine, 8-oxoG), increases depending on H2O2 dosage and is inversely correlated with cell viability. The elevated level of 8-oxoG in RNA decreases after removal of oxidative challenge, suggesting the existence of surveillance mechanism(s) for cleaning up oxidized RNA. Human polynucleotide phosphorylase (hPNPase), an exoribonuclease primarily located in mitochondria, has been previously shown to bind 8-oxoG-RNA with high affinity. The role of hPNPase in HeLa cell under oxidative stress conditions is examined here. Overexpression of hPNPase reduces RNA oxidation and increases cell viability against H2O2 insult. Conversely, hPNPase knockdown decreases viability and increases 8-oxoG level in HeLa cell exposed to H2O2. Our results suggest that hPNPase plays an important role in protecting cells and limiting damaged RNA under oxidative stress. (C) 2008 Elsevier Inc. All rights reserved.