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. 2011;6(6):e21410.
doi: 10.1371/journal.pone.0021410. Epub 2011 Jun 22.

Silymarin protects epidermal keratinocytes from ultraviolet radiation-induced apoptosis and DNA damage by nucleotide excision repair mechanism

Santosh K Katiyar  1 , Sudheer K Mantena, Syed M Meeran
Affiliations

Silymarin protects epidermal keratinocytes from ultraviolet radiation-induced apoptosis and DNA damage by nucleotide excision repair mechanism

Santosh K Katiyar et al. PLoS One. 2011.

Abstract

Solar ultraviolet (UV) radiation is a well recognized epidemiologic risk factor for melanoma and non-melanoma skin cancers. This observation has been linked to the accumulation of UVB radiation-induced DNA lesions in cells, and that finally lead to the development of skin cancers. Earlier, we have shown that topical treatment of skin with silymarin, a plant flavanoid from milk thistle (Silybum marianum), inhibits photocarcinogenesis in mice; however it is less understood whether chemopreventive effect of silymarin is mediated through the repair of DNA lesions in skin cells and that protect the cells from apoptosis. Here, we show that treatment of normal human epidermal keratinocytes (NHEK) with silymarin blocks UVB-induced apoptosis of NHEK in vitro. Silymarin reduces the amount of UVB radiation-induced DNA damage as demonstrated by reduced amounts of cyclobutane pyrimidine dimers (CPDs) and as measured by comet assay, and that ultimately may lead to reduced apoptosis of NHEK. The reduction of UV radiation-induced DNA damage by silymarin appears to be related with induction of nucleotide excision repair (NER) genes, because UV radiation-induced apoptosis was not blocked by silymarin in NER-deficient human fibroblasts. Cytostaining and dot-blot analysis revealed that silymarin repaired UV-induced CPDs in NER-proficient fibroblasts from a healthy individual but did not repair UV-induced CPD-positive cells in NER-deficient fibroblasts from patients suffering from xeroderma pigmentosum complementation-A disease. Similarly, immunohistochemical analysis revealed that silymarin did not reduce the number of UVB-induced sunburn/apoptotic cells in the skin of NER-deficient mice, but reduced the number of sunburn cells in their wild-type counterparts. Together, these results suggest that silymarin exert the capacity to reduce UV radiation-induced DNA damage and, thus, prevent the harmful effects of UV radiation on the genomic stability of epidermal cells.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Treatment of NHEK with silymarin suppresses UVB-induced apoptotic cell death.
(A), Treatment of NHEK with silymarin inhibits UVB-induced cell death as determined by Cell Death Detection ELISA Kit following the manufacturer's protocol. Cells were treated with silymarin for 3 h before UVB irradiation. Cells were harvested 24 h after UVB exposure and subjected to the analysis of cell death. (B), Silymarin inhibits UVB-induced apoptosis in NHEK. NHEK were exposed to UVB with and without the treatment with silymarin. Cells were harvested 24 hours later for the analysis of apoptotic cells by FACS using the Annexin V-Alexa Fluor488 Apoptosis Vybrant Assay Kit following the manufacturer's protocol. (C) Total percent of apoptotic cells (early+ late) in each treatment group was summarized and data are presented as mean±SD of three independent experiments. Sily. = silymarin. Statistically significant difference vs non-silymarin treated UVB exposed control, * P<0.01, P<0.005, † P<0.001.
Figure 2
Figure 2. Silymarin stimulates DNA repair in UV-exposed NHEK.
(A), NHEK were treated with silymarin for 3 h before UVB (150 J/m2) irradiation. Cells were harvested 36 h later, cytospun, and subjected to cytostaining to detect CPD+ cells, as detailed in Materials and methods. CPD-positive cells are dark brown. Magnification, x400. Photomicrographs are representative of three independent experiments. CPD+ cells were not detectable in non-UVB-exposed keratinocytes. (B) The analysis of damaged DNA in the form of CPDs was performed by dot-blot analysis using antibody specific to CPDs or thymine dimers. Genomic DNA from various treatment groups was subjected to dot-blot analysis using an antibody specific to CPDs. Results are shown from a single experiment and is representative of 3 independent experiments.
Figure 3
Figure 3. Silymarin prevents UVB-induced DNA damage as determined by comet assay.
(A), NHEK were exposed to UVB (150 J/m2) radiation with and without the treatment with silymarin (20 µg/mL), as described under Fig. 2. Keratinocytes were harvested 36 h after UVB irradiation, and UVB-induced DNA damage was determined using comet assay, as detailed in Materials and methods. The comet assay was used to determine UVB-induced DNA damage in the form of DNA fragmentation. (B), The tail of the comet was measured in each cell under microscope and expressed in µm as a mean±SD from at least 30 cells in each treatment group. Significant increase in tail length versus non-UVB-exposed control, p<0.001; *Significant decrease in tail length versus UVB alone, p<0.001.
Figure 4
Figure 4. Silymarin stimulates the mRNA levels of NER genes in UVB-exposed NHEK.
NHEK were exposed to UVB with and without the treatment of silymarin (20 µg/mL). Cells were harvested 1 h later and RNA was extracted. The mRNA levels of NER genes were determined using real-time PCR. The data of mRNA expression levels of various NER genes are expressed as mean±SD. Experiments were repeated three times. Statistically significant difference versus UVB alone, *p<0.001, †p<0.05.
Figure 5
Figure 5. Silymarin protects UVB-induced DNA damage and cell death in NER-proficient fibroblasts but not in NER-deficient fibroblasts.
(A), NER-proficient and NER-deficient human fibroblasts were exposed to UVB (150 J/m2) with or without the treatment of silymarin (20 µg/mL) and cells were harvested 36 h later, cytospun, and subjected to cytostaining to detect CPD+ cells. CPD-positive cells are dark brown. CPD+ cells were not detectable in non-UVB-exposed cells. Magnification, x400. (B), The analysis of UVB-induced DNA damage in the form of CPDs was performed by dot-blot analysis. Genomic DNA from various treatment groups was subjected to dot-blot analysis using an antibody specific to CPDs. Results are shown from a single experiment and is representative of 3 independent experiments. (C and D), UVB-induced cell death in NER-proficient and NER-deficient cells was detected by Cell Death Detection ELISA following manufacturer's protocol. Treatment protocol was same as reported in panels A and B. The amount of apoptotic cell death is reflected by increase of absorbance at 405 nm (optical density), as shown on the y-axis. * P<0.001. ND = not detectable.
Figure 6
Figure 6. Effect of silymarin on UVB-induced sunburn cells in the skin of NER-deficient and their wild-types.
(A), Silymarin repairs UVB-induced sunburn cells in NER-proficient mouse skin but not in NER-deficient mouse skin. Mice were exposed to UVB (240 mJ/cm2) with or without the treatment of silymarin, and sacrificed 24 h later. Skin samples were collected and subjected to H&E staining for the analysis of sunburn cells under microscope. Sunburn cells are shown by dark brown, n = 5/group. Some sunburn cells are shown by arrows. (B), The number of sunburn cells were counted per 1 cm length of epidermis from each mouse, and data are summarized in terms of mean±SD, n = 5 mice/group. Significant less number of SCs in NER+/+ mouse skin vs non-silymarin-treated NER+/+ wild-type mice, * P<0.001. ND = not detectable.
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