Nuclear and cell membrane effects contribute independently to the induction of apoptosis in human cells exposed to UVB radiation

UVB-induced DNA damage is a crucial event in UVB-mediated apoptosis. On the other hand, UVB directly activates death receptors on the cell surface inc luding CD95, implying that UVB-induced apoptosis can be initiated at the ce ll membrane through death receptor clustering. This study was performed to measure the relative contribution of nuclear and membrane effects in UVB-in duced apoptosis of the human epithelial cell line HeLa. UVB-mediated DNA da mage can be reduced by treating cells with liposomes containing the repair enzyme photolyase followed by exposure to photoreactivating light. Addition of photolyase followed by photoreactivation after UVB reduced the apoptosi s rate significantly, whereas empty liposomes had no effect. Likewise, phot oreactivating treatment did not affect apoptosis induced by the ligand of C D95, CD95L, UVB exposure at 4 degrees C, which prevents CD95 clustering, al so reduced the apoptosis rate, but to a lesser extent. When cells were expo sed to UVB at 4 degrees C and treated with photolyase plus photoreactivatin g light, UVB-induced apoptosis was almost completely prevented. Inhibition of caspase-3, a downstream protease in the CD95 signaling pathway, blocked both CD95L and UVB-induced apoptosis, whereas blockage of caspase-8, the mo st proximal caspase, inhibited CD95L-mediated apoptosis completely, but UVB -induced apoptosis only partially. Although according to these data nuclear effects seem to be slightly more effective in mediating UVB-induced apopto sis than membrane events; both are necessary for the complete apoptotic res ponse. Thus, this study shows that nuclear and membrane effects are not mut ually exclusive and that both components contribute independently to a comp lete response to UVB.