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

Abstract

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 including CD95, implying that UVB-induced apoptosis can be initiated at the cell membrane through death receptor clustering. This study was performed to measure the relative contribution of nuclear and membrane effects in UVB-induced apoptosis of the human epithelial cell line HeLa. UVB-mediated DNA damage 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 apoptosis rate significantly, whereas empty liposomes had no effect. Likewise, photoreactivating treatment did not affect apoptosis induced by the ligand of CD95, CD95L. UVB exposure at 4°C, which prevents CD95 clustering, also reduced the apoptosis rate, but to a lesser extent. When cells were exposed to UVB at 4°C and treated with photolyase plus photoreactivating light, UVBinduced 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 most proximal caspase, inhibited CD95L-mediated apoptosis completely, but UVBinduced apoptosis only partially. Although according to these data nuclear effects seem to be slightly more effective in mediating UVB-induced apoptosis than membrane events, both are necessary for the complete apoptotic response. Thus, this study shows that nuclear and membrane effects are not mutually exclusive and that both components contribute independently to a complete response to UVB.UV radiation in the middle-wavelength range between 290 and 320 nm (UVB) represents one of the most relevant environmental dangers because of its hazardous effects, including skin aging (1), induction of skin cancer (2), and exacerbation of infections (3). Like other adverse agents (alkylating chemicals, oxidants), UVB induces changes in mammalian cell gene expression (4-6). Elucidation of the underlying molecular mechanisms is of primary importance for the understanding of how UVB can damage cells and thus act as a pathogen. One of the most controversial issues in this context is whether the cellular UVB response is initiated at the cell membrane or in the nucleus (7). To exert its biological effects, UVB must be first absorbed by a cellular chromophore, which transfers the energy into a biochemical signal. Among a number of chromophores (porphyrins, aromatic amino acids, urocanic acid), DNA is regarded as the most important for several reasons. (i)The wavelength dependency of some UVB effects is similar to that for DNA absorption (8).(ii) Acceleration of DNA repair inhibits particular biological UVB effects (8-12). (iii) Lower UVB doses are necessary to achieve the same biological effects in DNA repair-deficient than -proficient cells (13). Thus, these data favor the concept that DNA is the most important mo...

IL-18 Reduces Ultraviolet Radiation-Induced DNA Damage and Thereby Affects Photoimmunosuppression

“…These photolesions have been recognized not only to be the primary basis for malignant transformation but also to be essentially involved in UV-induced signal transduction (7). DNA damage is the crucial molecular trigger for a variety of UV effects, e.g., immunosuppression and apoptotic cell death (8,9). The vast majority of UVB-induced DNA lesions are removed by the nucleotide excision repair (NER), 3 the essential endogenous DNA repair system (10).…”

“…Because the severity of DNA damage is one of the major determinants of cell apoptosis upon UV exposure (8), the impact of IL-18 on the amounts of UV-specific DNA lesions was determined. UV induces two major types of DNA lesions, 6-4 photoproducts and cyclobutane pyrimidine dimers (CPD), with the latter being the predominant lesions (26).…”

The pathophysiology of photosensitivity in lupus erythematosus

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“…4,5 Besides its role in epidermal development, apoptosis is probably the most important mechanism protecting the organism against genotoxic hazards. 4,6 UVB irradiation initiates apoptosis via (i) damage of nuclear DNA, 5 (ii) activation of cell surface death receptors, 7 and (iii) intracellular formation of ROS accompanied by mitochondrial dysfunction and cytochrome c release. 8 The importance of UVB-induced apoptotic cell death as a protective mechanism becomes evident by the fact that the reduced risk of dark-skinned individuals to develop skin cancer is, to a major part, due to facilitation of UVB-induced epidermal apoptosis by melanin.…”