Silibinin inhibits ultraviolet B radiation-induced mitogenic and survival signaling, and associated biological responses in SKH-1 mouse skin

Ultraviolet B (UVB) radiation is a complete skin carcinogen causing DNA damage as a tumor-initiating event and activating signaling cascades that play a critical role in its tumor-promoting potential. Recently we reported that a naturally occurring flavonoid, silibinin, protects UVB-induced skin damages and prevents photocarcinogenesis. Here we examined silibinin efficacy on acute and chronic UVB-caused mitogen-activated protein kinases (MAPKs) and AKT activation, and associated biological responses in SKH-1 hairless mouse skin. A single UVB exposure at 180 mJ/cm 2 dose resulted in varying degree of ERK1/2, JNK1/2, MAPK/p38 and AKT phosphorylation at various time-points in mouse skin; however, topical application of silibinin prior to or immediately after UVB exposure, or its dietary feeding strongly inhibited the activation of these molecules at all the timepoints examined. Stronger effects of silibinin towards inhibition of UVB-caused phosphorylation of MAPKs and AKT were also observed in a chronic UVB (180 mJ/cm 2 /day for 5 days) exposure protocol. Immunohistochemical analysis of chronically exposed skin sections showed that silibinin treatment in all three protocols increases UVBinduced p53-positive cells; and decreases UVB-caused cell proliferation, apoptotic and sunburn cells. These findings suggest that silibinin inhibits UVB-induced MAPK and AKT signaling and increases p53 in mouse skin, and that these effects of silibinin possibly lead to a decrease in UVB-caused proliferation and apoptosis, which might, in part, be responsible for its overall efficacy against photocarcinogenesis.