Protective Effects of Silymarin Against Photocarcinogenesis in a Mouse Skin Model

Abstract

Background: Nonmelanoma skin cancer is the most common cancer among humans; solar UV is its major cause. Therefore, it is important to identify agents that can offer protect against this cancer. Purpose: We evaluated the protective effects of silymarin, a flavonoid compound isolated from the milk thistle plant, against UVB radiation-induced non melanoma skin cancer in mice and delineated the mechanism(s) of its action. Methods: For long-term studies, three different protocols of treatment were employed, each evaluating protection by silymarin at a different stage of carcino genesis. Female SKH-1 hairless mice were subjected to 1) UVB-induced tumor initiation followed by phorbol estertumor promotion, 2) 7,12-dimethylbenz[a ]anthra cene-induced tumor initiation followed by UVB-mediated tumor promotion, and 3) UVB-induced complete carcino genesis. Forty mice were used in each protocol and were divided into control and treatment groups. Silymarin was applied topically at a dose of 9 mg per application before UVB exposure, and its effects on tumor incidence (% of mice with tumors), tumor multiplicity (number of tumors per mouse), and average tumor volume per mouse were evalu ated. In short-term studies, the following parameters were measured: formation of sunburn and apoptotic cells, skin edema, epidermal catalase and cyclooxygenase (COX) activi ties, and enzymatic activity and messenger RNA (mRNA) expression for ornithine decarboxylase (ODC), a frequently observed marker at tumor promotion stage. Fisher'ss exact test was used to evaluate differences in tumor incidence, two-sample Wilcoxon rank sum test was used for tumor multiplicity and tumor volume, and Student'ss t test was used for all other measurements. All statistical tests were two-sided. Results: In the protocol with UVB-induced tumor initiation, silymarin treatment reduced tumor incidence from 40% to 20% (P = .30), tumor multiplicity by 67% (P = .10), and tumor volume per mouse by 66% (P = .14). In the protocol with UVB-induced tumor promotion, silymarin treatment reduced tumor incidence from 100% to 60% (P <.003), tumor multiplicity by 78% (P <.0001), and tumor volume per mouse by 90% (P <.003). the effect of silymarin was much more profound in the protocol with uvb-induced complete carcinogenesis, where tumor incidence was reduced from 100% to 25% (P <.0001), tumor multiplicity by 92% (P <.0001), and tumor volume per mouse by 97% (P <.0001). in short-term experiments, silymarin application resulted in statistically significant inhibition in uvb-caused sunburn and apoptotic cell formation, skin edema, depletion of catalase activity, and induction of cox and odc activities and odc mrna expression. Conclusions and Implication: Silymarin can provide substantial protection against different stages of UVB-induced carcinogenesis, possibly via its strong antioxidant properties. Clinical testing of its usefulness is warranted. [J Natl Cancer Inst 1997;89:556–65]