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

Abstract

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