Abstract 1845: Silymarin, a phytochemical from milk thistle, inhibits ultraviolet radiation-induced apoptosis by stimulating DNA repair in skin cells

Abstract Over exposure to solar ultraviolet (UV) radiation is the major etiological factor for over one million new cases of non-melanoma skin cancers in the US each year. One of the hallmark events of exposure to UVB radiation (290-320 nm) is the induction of apoptotic cell death of keratinocytes, the results of which are evident within the epidermis as sunburn cells. The formation of sunburn cells is a protective mechanism for limiting survival of cells with irreparable DNA damage. Because of its protective function, alterations in UVB-induced apoptosis may have a profound impact in the induction of skin cancer. We have shown that topical application of silymarin, a flavonoid from milk thistle (Silybum marianum L.), inhibits UVB-induced immunosuppression in mice, and this protective effect is mediated through the induction of immunoregulatory cytokine IL-12. By using in vitro cell culture and in vivo genetically-modified mouse models, here we report that UVB-induced apoptotic cell death of epidermal keratinocytes was suppressed by silymarin, and in this process UVB-induced DNA damage was significantly reduced or repaired by silymarin-induced IL-12-mediated induction of nucleotide excision repair. Using normal human epidermal keratinocytes (NHEK) as an in vitro model, we found that exposure of NHEK to UVB (20 mJ/cm2) radiation induces apoptosis which was suppressed by pretreatment of NHEK with silymarin (5-20 µg/ml) when analyzed by flow cytometry, and the levels of damaged DNA was determined by dot-blot analysis and immunocytostaining using antibody specific to thymine dimers. The suppression of UVB-induced apoptosis after silymarin treatment was related with the enhancement of IL-12 in silymarin-treated NHEK. Treatment of silymarin-treated NHEK with anti-IL-12 antibody abrogated the anti-apoptotic effect of silymarin in UVB-exposed NHEK which was confirmed by the analysis of DNA damaging cells using cytostaining and comet assay on per cell level. To further confirm the role of silymarin-induced IL-12 in prevention of UVB-induced apoptosis, we used IL-12p40 knock out (IL-12 KO) mice. We observed that UVB-induced sunburn cells were resolved more rapidly in the skin of wild-type mice treated topically with silymarin than untreated control mice. In contrast, the extent of UVB-induced numbers of sunburn cells was not significantly different in the silymarin-treated IL-12 KO mice and untreated control mice. In addition, treatment of xeroderma pigmentosum complementation group A (XPA)-deficient mice, with silymarin did not promote the repair of UVB-induced sunburn cells while promoted the repair of sunburn cells in wild-type of XPA-deficient mice, indicating that silymarin can protect keratinocytes from apoptosis induced by DNA-damaging UV radiation by inducing DNA repair. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1845. doi:10.1158/1538-7445.AM2011-1845