Ultraviolet B Irradiation Induces Apoptosis of Keratinocytes by Direct Activation of Fas Antigen

Ultraviolet B (UVB) irradiation induces apoptosis of keratinocytes, where p53 has been suggested to play an important role. Recently we have shown that UVB irradiation induces apoptosis of SV40-transformed human keratinocytes (SVHK cells). Because p53 function is impaired in SVHK cells by large T antigen, a UVB-induced p53-independent apoptotic pathway was suggested. We investigated the UVB-induced apoptotic pathway using various keratinocytes. Cultured mouse keratinocytes of homozygous p53 deficient mice (p53(–/–)) were markedly resistant to UVB-induced apoptosis compared with keratinocytes from wild or heterozygous p53 deficient mice (p53(–/+)). Twenty per cent of keratinocytes derived from p53 (–/–) mice, however, induced apoptosis following UVB irradiation. Analysis using caspase inhibitors disclosed activation of caspase 8 and 3 in UVB-irradiated SVHK cells. Keratinocytes derived from MRL/lpr mice, which have mutated Fas antigen, showed diminished UVB-induced apoptosis suggesting that Fas antigen is significantly involved in UVB-induced apoptosis. Immunohistochemical analysis revealed that UVB irradiation induces aggregation of Fas antigen showing a dense dot-like staining, which was also observed in SVHK cells treated with agonistic anti-Fas antibody, CH11. Pre treatment of antagonistic anti-Fas antibody, ZB4, inhibited CH11-induced but not UVB-induced multimerization of Fas antigen. Furthemore, UVB irradiation did not affect the basal expression of Fas ligand mRNA, protein and soluble Fas ligand. These results indicate that UVB irradiation induces multimerization of Fas antigen that results in apoptosis without the Fas ligand