Nonmelanoma skin cancer (NMSC) accounts for more than one million new cases each year in the US alone suggesting that more approaches are needed for its prevention and control. Cancer chemoprevention is a novel and more effective means of cancer control where naturally occurring agents are considered as a less toxic and more effective approach in controlling various malignancies including NMSC. Earlier studies by us have shown that silymarin (a crude form of biologically active silibinin with some other isomers), isolated from milk thistle, affords strong protection against photocarcinogenesis in SKH-1 hairless mice; however, the molecular mechanisms of its efficacy are not known. Here, we assessed the effect of silibinin on ultraviolet B radiation (UVB)-induced cell proliferation, DNA damage and apoptosis in SKH-1 hairless mice employing immuno-histochemistry analyses. Proliferative cell nuclear antigen (PCNA) staining showed that silibinin pre- or post-topical application significantly decreases (20-60%, P<0.001) UVB-induced cell proliferation. Similar silibinin treatments also showed a strong protective effect against UVB-induced thymine dimer formation accounting for 76-85% (P<0.001) inhibition. Analysis of MSH2, a DNA mismatch repair enzyme showed that protective effect of silibinin on UVB-induced DNA damage is not via an activation of DNA repair. In other studies, silibinin treatment resulted in a further up-regulation of p53 by 30-55% (P<0.001) together with an increase (∼2-fold, P< 0.001) in Cip1/p21 protein levels. In addition, silibinin strongly decreased UVB-induced caspase 3 activation and TUNEL positive cells (P<0.001) with a concomitant decrease in sunburn cell formation. These findings suggest that silibinin affords strong protection against UVB-induced damage in epidermis by inhibiting UVB-caused cell proliferation, DNA damage and apoptotic sunburn cell formation possibly via activation of p53-p21 cascade, and inhibition of caspase activation. Comparable effects of silibinin following its pre- or post-UVB application suggest that mechanisms other than sunscreen effect are operational in silibinin efficacy against UVB-caused skin damages.
[Proc Amer Assoc Cancer Res, Volume 45, 2004]