Membranes as sensitive targets in thymocyte apoptosis.

The role of cellular membranes in thymocyte apoptosis has been examined. Trolox, a water soluble analogue of vitamin E and inhibitor of membrane damage, inhibits DNA fragmentation in thymocytes exposed to gamma-radiation. Trolox is most effective in inhibiting DNA fragmentation when added to cells within 30 min post-irradiation. Exposure to trolox only during irradiation did not prevent DNA fragmentation, suggesting that it does not work by scavenging free radicals generated during radiation exposure. Incubation of the irradiated cell suspension with trolox for 2 h post-irradiation was sufficient to prevent DNA fragmentation measured at 24 h in irradiated cells. This suggests that trolox irreversibly inhibits a cellular lesion required for apoptosis. The induction of DNA fragmentation appears to be related to a concurrent, pronounced flow of Ca2+ into the cell. At 3 h post-irradiation the amount of Ca2+ in irradiated thymocytes was more than twice that of unirradiated thymocytes. Membrane damage has been shown to affect the transport of Ca2+. Trolox treatment completely blocked the radiation-induced influx of Ca2+ into the thymocytes. These results suggest that membrane damage is a critical lesion that is involved in DNA fragmentation in thymocyte apoptosis.