Silymarin is a mixture of polyphenolic flavonoids isolated from milk thistle (Silybum marianum) with anticancer activities reported for several organ sites. The present study tested the efficacy of dietary silymarin against mammary carcinogenesis in two rodent models. In the Sprague-Dawley rat model, female rats were fed a purified diet supplemented with none, 0.03, 0.1, 0.3 or 1% (w/w) of silymarin from 21 days of age (DOA) and carcinogenesis was initiated by a single i.p. injection of 1-methyl-1-nitrosourea (MNU) at 51 DOA. Mammary tumor (MT) development was followed till 110 days after carcinogen injection. In the MMTV-neu/HER2 transgenic mouse mammary carcinogenesis model, homozygous transgenic females were fed a purified diet supplemented with none or 0.3% silymarin, either from 28 or 120 DOA and MT development was followed to ∼300 DOA. The results showed that dietary silymarin increased the plasma concentration of free and total silibinin, a major component of silymarin, in a dose-dependent manner in the rat, but did not decrease either MT incidence or number. Instead silymarin modestly increased the number of MNU-induced MTs in rats. Similarly, silymarin increased MT incidence and multiplicity and non-MTs in the neu-transgenic mice. In cell culture, treatment of human MCF-7 breast cancer cells with serum-achievable concentrations of silymarin in the rodent models stimulated their growth, in part through an estrogen-like activity. Because silymarin is being used in the treatment of liver cirrhosis and a variety of other human ailments, and is sold as a dietary supplement, our findings add a cautionary note to its application in breast cancer prevention.
Bibliographical noteFunding Information:
We thank Melissa Bonorden for advices on genotyping of neu-transgenic mice, Todd Schuster for assistance with flow cytometry detection of BrdU index and Andria Hansen for editorial support. This work was supported in part by grant BCTR-2000-297 from the Susan G. Komen Breast Cancer Foundation, the Hormel Foundation and the Eagle’s Telethon (J.L.), and by grant CA101858 from the National Cancer Institute (M.P.C.). The rat study was carried out when Z.W. and J.L. were on the staff of the AMC Cancer Research Center, Denver, CO.