Biol. Pharm. Bull. 29(6) 1096—1101 (2006)

adverse effects of UV irradiation reinforce the need for novel chemoprevention strategies which involve the use of active compounds from Chinese herbal medicine that might prevent DNA damage or other biological events occurring following UV exposure to the skin. Our previous study demonstrated that silymarin was found to have an anti-apoptotic effect against UV irradiation in human malignant melanoma A375S2 cells. Therefore, in this study, we investigated whether silibinin, the major active compound in silymarin, has a preventive effect against UV-induced skin damage in vitro by employing HaCaT human immortalized keratinocyte cells. Caspases, a family of cysteine proteases, are critical mediators of programmed cell death, and thus far, 14 family members have been identified. Caspases are activated in a sequential cascade of cleavage by other caspase family members. Some of these, such as caspase-8, mediate signal transduction downstream of death receptors located on the cell membrane. Others, such as caspase-9, mediate apoptotic signals after mitochondrial damage. CD95 (Fas/APO-1) is a death-promoting receptor that belongs to the tumor necrosis factor (TNF) receptor family. Triggering of the CD95 molecule either by agonistic antibodies or by the natural ligand CD95L (FasL) induces apoptosis. Ligand binding induces trimerization of CD95, and the trimerized cytoplasmic region then transduces the signal by recruiting FADD which is responsible for downstream signal transduction by recruitment of caspase-8, a key protein in the apoptotic pathway. Since it has been reported that UV irradiation induced apoptosis in HaCaT cells via direct activation of the CD95 pathway by inducing the clustering of CD95, which could be partly prevented by keeping the cells at a low temperature (4 °C), in this study, we investigated the quantitative alteration of proteins involved in the CD95 pathway after silibinin pretreatment in UV-irradiated HaCaT cells.