Zhao J, Sharma Y, Agarwal R
Center for Cancer Causation and Prevention, AMC Cancer Research Center, Denver, Colorado 80214, USA.
Mol Carcinog. 1999 Dec;26(4):321-33.
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.
类黄酮抗氧化剂水飞蓟素在欧洲和亚洲临床上用于治疗肝脏疾病，在美国和欧洲作为膳食补充剂出售。最近我们发现，水飞蓟素在不同的小鼠皮肤致癌模型中具有极高的防癌效果，并且对人皮肤癌、宫颈癌、前列腺癌和乳腺癌细胞具有强大的抗癌作用。最近，我们发现水飞蓟素的抗肿瘤促进作用主要针对小鼠皮肤肿瘤促进的I期（《癌症研究》1999年；59：622 - 632）。基于这项最新研究，在本报告中，我们进行了进一步研究，以确定和阐明水飞蓟素在小鼠皮肤肿瘤促进I期作用的生化和分子机制。将3毫克、6毫克和9毫克剂量的水飞蓟素单次局部涂抹于SENCAR小鼠皮肤，30分钟后再涂抹3微克剂量的12 - O - 十四烷酰佛波醇13 - 乙酸酯（TPA），结果导致TPA引起的皮肤水肿受到76 - 95%的抑制（P < 0.001）。同样，这些剂量的水飞蓟素还分别对TPA引起的表皮超氧化物歧化酶、过氧化氢酶和谷胱甘肽过氧化物酶活性的消耗显示出39 - 90%、29 - 85%和15 - 67%的保护作用（P < 0.05或0.001）。用水飞蓟素预处理小鼠也对TPA引起的表皮脂质过氧化诱导（47 - 66%抑制，P < 0.001）和髓过氧化物酶活性（56 - 100%抑制，P < 0.001）产生了高度显著的抑制作用。在评估水飞蓟素对涉及脂氧合酶和环氧化酶（COX）的花生四烯酸代谢途径影响的其他研究中，类似剂量的水飞蓟素对TPA引起的表皮脂氧合酶诱导（49 - 77%抑制，P < 0.001）和COX活性（35 - 64%抑制，P < 0.01或0.001）显示出高度显著的抑制作用。蛋白质免疫印迹分析表明，水飞蓟素对COX活性的观察到的作用是由于对TPA诱导型COX - 2的抑制，而组成型COX - 1蛋白水平没有变化。在其他研究中，水飞蓟素还对TPA引起的表皮白细胞介素1α（IL - 1α）蛋白诱导（39 - 72%抑制，P < 0.005或0.001）和mRNA表达显示出剂量依赖性抑制。综上所述，这些生化和分子研究的结果进一步证实了我们最近观察到的水飞蓟素的抗肿瘤促进作用主要在肿瘤促进I期。此外，水飞蓟素对COX - 2和IL - 1α的观察到的抑制作用应进一步探索，以制定针对这些分子靶点起致病作用之一的癌症的预防策略，如人类的非黑色素瘤皮肤癌、结肠癌和乳腺癌。
