The
flavonoid antioxidant silymarin is used clinically in Europe and Asia for the treatment of
liver diseases and is sold in the United States and Europe as a dietary supplement. Recently we showed that
silymarin possesses exceptionally high
cancer-preventive effects in different mouse skin
carcinogenesis models and affords strong anticancer effects in human skin, cervical, prostate, and
breast carcinoma cells. More recently, we showed that the anti-
tumor-promoting effect of
silymarin is primarily targeted against stage I
tumor promotion in mouse
skin (Cancer Res 1999;59:622-632). Based on this recent study, in this report, further investigations were made to identify and define the biochemical and molecular mechanisms of
silymarin's effect during stage I
tumor promotion in mouse skin. A single topical application of
silymarin at 3-, 6-, and
9-mg doses onto SENCAR mouse skin followed 30 min later with 12-O-tetradecanoylphorbol 13-acetate (TPA) at a 3-microg dose resulted in a 76-95% inhibition (P < 0.001) of TPA-caused skin
edema. Similarly, these doses of
silymarin also showed 39-90%, 29-85%, and 15-67% protection (P < 0.05 or 0.001), against TPA-caused depletion of epidermal
superoxide dismutase,
catalase, and
glutathione peroxidase activity, respectively. Pretreatment of mice with
silymarin also produced highly significant inhibition of TPA-caused induction of epidermal lipid peroxidation (47-66% inhibition, P < 0.001) and
myeloperoxidase activity (56-100% inhibition, P < 0.001). In additional studies assessing the effect of
silymarin on
arachidonic acid metabolism pathways involving
lipoxygenase and
cyclooxygenase (COX), similar doses of
silymarin showed highly significant inhibition of TPA-caused induction of epidermal
lipoxygenase (49-77% inhibition, P < 0.001) and COX (35-64% inhibition, P < 0.01 or 0.001) activity. Western immunoblot analysis showed that the observed effect of
silymarin on COX activity was due to inhibition of TPA-inducible COX-2 with no change in constitutive COX-1
protein levels. In other studies,
silymarin also showed dose-dependent inhibition of TPA-caused induction of epidermal
interleukin 1alpha (IL-1alpha)
protein (39-72% inhibition, P < 0.005 or 0.001) and
mRNA expression. Taken together, the results from these biochemical and molecular studies further substantiate our recent observation of
silymarin's anti-
tumor-promoting effects primarily at stage I
tumor promotion. Furthermore, the observed inhibitory effects of
silymarin on COX-2 and IL-1alpha should be further explored to develop preventive strategies against those
cancers in which these molecular targets play one of the causative roles, such as non-
melanoma skin, colon, and breast
cancers in humans.