Activation of topoisomerase II-mediated excision of chromosomal DNA loops during oxidative stress

Hydrogen peroxide (H2O2), a reactive oxygen species (ROS), is known to indu ce oxidative stress and apoptosis. U937 cells treated with H2O2 were shown to produce high molecular weight (HMW) DNA fragments similar to 50 to 100 k b in size in <1 min. The formation of these HMW DNA fragments is reversible and shown to be mediated by DNA topoisomerase II (TOP2). Following this in itial event, formation of irreversible HMW DNA fragments and nucleosomal la dders occurs. Our results thus demonstrate a potential role of TOP2 in oxid ative damage of DNA and apoptotic cell death.