Silybin, a main component of the milk thistle of Silybum marianum, has been reported to possess anticancer activity. We investigated the effects of
IdB 1016, a complex of
silybin with
phosphatidylcholine, on the development of mammary
tumors appearing spontaneously in HER-2/neu transgenic mice. The mechanisms involved in the antitumor effect of
IdB 1016 were evaluated by studying the apoptosis, senescent-like growth arrest, intratumoral leukocyte infiltrate, and the expression of HER-2/neu and p53 in tumoral mammary glands from transgenic mice and in human breast SKBR3
tumor cells. The administration of
IdB 1016 delayed the development of spontaneous mammary
tumors, reduced the number and size of mammary
tumor masses, and diminished lung metastasization in HER-2/neu transgenic mice. In tumoral mammary glands from IdB 1016-treated mice, a down-regulation of HER-2/neu gene expression was associated with an increased senescent-like growth arrest of
tumor cells, and an increased infiltrate of neutrophils, CD4, and CD8 T cells. Both senescent-like growth arrest and apoptosis were significantly increased and were associated with a reduced p185(HER-2/neu)
protein and an increased p53
mRNA in SKBR3 in vitro treated with
IdB 1016 in comparison with control cells. The results show the antitumor effect of
IdB 1016 in the development of spontaneous mammary
tumors in HER-2/neu transgenic mice. The effect of
IdB 1016 might be related to the down-regulation of HER-2/neu expression and the induction of senescent-like growth arrest and apoptosis through a p53-mediated pathway in
tumor cells.