Immediate and delayed apoptotic cell death mechanisms: UVA versus UVB and UVC radiation.

The mechanism of cell death induced by the different waveband regions of ultraviolet radiation (UVR), i.e., UVA1 (340-400 nm), UVB (290-320 nm) and UVC (200-290 nm) was investigated, using equilethal doses (90% reproductive death) on L5178Y-R murine lymphoma cells. To distinguish between necrosis and apoptosis, the following endpoints were monitored over time using flow cytometry and transmission electron microscopy: percentage of remaining cells, membrane permeabilized cells, dead cells, apoptotic cells, and ultrastructural changes. All waveband regions of UVR were found to cause apoptosis as opposed to necrosis. However, UVA1-induced immediate (0-4 h) apoptosis, while UVB- or UVC-induced delayed apoptosis (<34 h). Moreover, the membrane permeability changes that only result from exposure to UVA1 radiation, especially to red blood cells, suggests that the immediate apoptotic mechanism involves membrane damage. Therefore, the results suggest that there are three death mechanisms available to one cell type: necrosis, immediate apoptosis, and delayed apoptosis (or programmed cell death).