Early events in the apoptotic cascade initiated in cells treated with medium from the progeny of irradiated cells

Genomic instability and the bystander effect have recently been linked experimentally. It has previously been shown that medium from irradiated cells can induce early events in the apoptotic cascade, such as mobilisation of intracellular calcium, loss of mitochondrial membrane potential and an increase in reactive oxygen species, in unhit cells. The aim of this study was to determine if medium from the progeny of irradiated cells could also initiate apoptosis in unhit cells. Human keratinocytes were irradiated (0.5 and 5 Gy) and medium was harvested up to the 7th passage post-irradiation and transferred to unirradiated keratinocytes. Intracellular calcium levels, mitochondrial membrane potential and the level of reactive oxygen species were all monitored for a period of 24 h following medium transfer. Rapid calcium fluxes (within 30 s), loss of mitochondrial membrane potential and increases in reactive oxygen species (from 6 h after medium transfer) were observed. There was no significant difference between medium generated by cells irradiated at the different doses. The data suggest that initiating events in the apoptotic cascade were induced in unhit cells by a signal produced by irradiated cells and that this signal can still be produced in the progeny of irradiated cells.