Silibinin prevents ultraviolet B radiation-induced epidermal damages in JB6 cells and mouse skin in a p53-GADD45α-dependent manner.

Better preventive strategies are required to reduce ultraviolet (UV)-caused photodamage, the primary etiological factor for non-melanoma skin cancer (NMSC). Accordingly, here we examined the preventive efficacy of silibinin against UVB-induced photodamage using mouse epidermal JB6 cells and SKH1 hairless mouse epidermis. In JB6 cells, silibinin pretreatment protected against apoptosis and accelerated the repair of cyclobutane pyrimidine dimers (CPD) induced by moderate dose of UVB (50 mJ/cm(2)), which we are at risk of daily exposure. Silibinin also reversed UVB-induced S phase arrest, reducing both active DNA synthesizing and inactive S phase populations. In mechanistic studies, UVB-irradiated cells showed a transient upregulation of both phosphorylated (Ser-15 and Ser-392) and total p53, whereas silibinin pretreatment led to a more sustained upregulation and stronger nuclear localization of p53. Silibinin also caused a marked upregulation of GADD45α, a downstream target of p53, implicated in DNA repair and cell cycle regulation. Importantly, under p53 and GADD45α knockdown conditions, cells were more susceptible to UVB-induced apoptosis without any significant S phase arrest, and protective effects of silibinin were compromised. Similar to the in vitro results, topical application of silibinin prior to or immediately after UVB irradiation resulted in sustained increase in p53 and GADD45α levels and accelerated CPD removal in the epidermis of SKH1 hairless mice. Together, our results show for the first time that p53-mediated GADD45α upregulation is the key mechanism by which silibinin protects against UVB-induced photodamage and provides a strong rationale to investigate silibinin in reducing the risk and/or preventing early onset of NMSC.