Silibinin Prevents UV-Induced HaCaT Cell Apoptosis Partly through Inhibition of Caspase-8 Pathway

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  • Li Lin-Hao
    China–Japan Research Institute of Medical and Pharmaceutical Sciences, Shenyang Pharmaceutical University
  • Wu Li-Jun
    Department of Phytochemistry, Shenyang Pharmaceutical University
  • Tashiro Shin-ichi
    Department of Clinical and Biomedical Sciences, Showa Pharmaceutical University
  • Onodera Satoshi
    Department of Clinical and Biomedical Sciences, Showa Pharmaceutical University
  • Uchiumi Fumiaki
    Department of Gene Regulation, Faculty of Pharmaceutical Sciences, Tokyo University of Science
  • Ikejima Takashi
    China–Japan Research Institute of Medical and Pharmaceutical Sciences, Shenyang Pharmaceutical University

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Silymarin is a polyphenolic flavonoid from milk thistle (Silybum marianum), which has anti-inflammatory, cytoprotective, and anticarcinogenic effects.1) In this study, we assessed the effect of silibinin (Fig. 1), the major active compound in silymarin, on ultraviolet light (UV)-induced cell apoptosis in HaCaT cells, a human keratinocyte cell line. Pretreatment with silibinin 500 μM significantly inhibited UV-induced apoptosis in HaCaT cells after 9 h incubation. The expression of Fas-associating protein with death domain (FADD), a downstream molecule of the death receptor pathway, was completely eliminated by silibinin treatment in UV-irradiated HaCaT cells, followed by inhibition of cleavage of procaspase-8, whose activation induced cell apoptosis2) and decreased the release of cytochrome c from mitochondria. The caspase-8 inhibitor z-IETD-fmk at 10 μM increased the ratio of UV-irradiated HaCaT cell viability, suggesting that UV-induced HaCaT cell apoptosis was partially due to activation of the caspase-8 pathway. Moreover, UV-induced cleavage of procaspase-3 and digestion of its substrates, the inhibitor of caspase-activated DNase (ICAD) and poly-(ADP-ribose) polymerase (PARP), were also reduced by silibinin pretreatment. While unexpectedly, it was found in our study that pretreatment with silibinin increased HaCaT cell death by CD95 agonistic antibody CH11. Consequently, the protective effect of silibinin against UV irradiation in HaCaT cells is exerted by inactivation of caspase-8 after direct down-regulation of FADD expression, resulting in blockage of UV-induced apoptosis.

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