EXPRESSION OF THE PLATELET-ACTIVATING-FACTOR RECEPTOR RESULTS IN ENHANCED ULTRAVIOLET-B RADIATION-INDUCED APOPTOSIS IN A HUMAN EPIDERMAL-CELL LINE

Recent studies have demonstrated that ultraviolet B radiation (UVB) da mages human keratinocytes in part by inducing oxidative stress and cyt okine production. Severe UVB damage to the keratinocyte can also resul t in apoptosis or programmed cell death, Although the lipid mediator p latelet-activating factor (PAF) is synthesized in response to epiderma l cell damage and epidermal cells express PAF receptors, it is not kno wn whether PAF is involved in UVB-induced epidermal cell apoptosis, Th ese studies examined the role of the PAF system in UVB-induced epiderm al cell apoptosis using a novel model system created by retroviral-med iated transduction of the PAF receptor-negative human epidermal cell l ine KB with the human PAF receptor (PAF-R). Expression of the PAF-R in KB cells did not affect base-line growth or apoptosis, yet resulted i n a decrease in the lag time between treatment of the cells and the in duction of apoptosis following irradiation with 400 J/m(2) UVB, This e ffect was inhibited by pretreatment with the PAF-R antagonists WEB 208 6 and A-85783, confirming involvement of the PAF-R in this process, At lower doses (100-200 J/m(2)) of UVB, only RR cells that expressed the PAF-R became apoptotic. Treatment of PAF-R-expressing EB clones with the metabolically stable PAF-R agonist adexyl-2-N-methylcarbamoyl-3-gI ycerophosphocholine (CPAF) alone did not induce apoptosis but augmente d the degree of apoptosis observed if CPAF was used in combination wit h lower doses (200 J/m(2)) of UVB irradiation. interestingly, UVB irra diation was found to stimulate PAF synthesis only in PAF-R-expressing KB cell clones. The antioxidants N-acetyl cysteine, 1,1,3,3-tetramethy l-2-thiourea, and vitamin E inhibited both WE-induced PAF biosynthesis as well as the augmentation of WE-induced apoptosis in PAF-R-expressi ng HE clones, suggesting the possibility that UVB stimulates the produ ction of oxidized lipid species with PAF-R agonistic activity in this model system. Thus, these studies indicate that a component of UVB-ind uced epidermal cell cytotoxicity can be modulated by PAF-R activation through the production of PAF and PAF-like species.