Appearance of 1-2 Mbp giant DNA fragments as an early common response leading to cell death induced by various substances that cause oxidative stress.
The effects of oxidative stress on double strand DNA breakage were examined in T-24 human bladder tumor cells using various active oxygen producing agents such as hydrogen peroxide (H2O2), bleomycin (BLM), neocarzinostatin (NCS), and x-ray irradiation. Analysis of the DNA by pulsed-field gel electrophoresis (PFGE) revealed that discrete giant DNA fragments of 1-2 Mbp and 200-800 kbp had accumulated in the nuclei of the treated cells. The 1-2 Mbp giant DNA fragments were first observed 2 h after the T-24 cells were exposed to the active oxygen producing agents, or irradiated with x-ray. The appearance and the amounts of 1-2 Mbp and 200-800 kbp giant DNA fragments seemed to depend on the concentration and the type of reagents used or the dose of x-ray. Following the accumulation of giant DNA fragments, another type of DNA fragmentation was detected and DNA fragments smaller than 100 kbp accumulated in the nuclei of the cells irradiated with x-ray or treated with NCS. In addition, DNA ladder formation, which is characteristic of apoptosis, was observed. The giant DNA fragments appeared to arise as a consequence of double-stranded DNA breakage, which occurred earlier than cell lysis, as assessed by 51Cr release. These findings indicate that the formation of giant DNA fragments is a specific characteristic of cells responding to oxidative stress, and it may be an initial event that leads to cell death.