Abstract #924: Silibinin, an active ingredient of milk thistle extract inhibits the growth of human colorectal carcinoma LoVo cells by causing cell cycle arrest and apoptosis

Colorectal cancer is among the most common cancers worldwide and is one of the leading causes of cancer-related deaths in the USA. Chemoprevention by natural agents plays a potential role in reducing the incidence of several malignancies including colorectal cancer. Currently there has been considerable interest in polyphenolic phytochemicals in the prevention of various malignancies due to their wide range of pharmacologic properties. Herein, we assessed the anticancer efficacy of silibinin, an active ingredient of milk thistle extract, in human colorectal carcinoma LoVo cells by examining changes in cell cycle progression, cell growth and apoptotic death together with the levels of the molecules involved in these biological events. We also investigated the in vivo antitumor potential and associated mechanisms of silibinin in LoVo tumor xenograft in nude mice. We observed that silibinin treatment at concentrations ranging from 50-200 µM for 24 to 72 h strongly inhibits the viability of LoVo cells by 30-83% depending upon the concentration and time duration of treatment. Treatment of cells with silibinin also resulted in cell cycle arrest at G1 phase at all the concentrations and G2/M arrest at the highest concentration. Mechanistic studies showed that silibinin down regulates the levels of cyclin D1, D3, A and B1 along with cyclin dependent kinase 2, 4, 6 and cdc2. The levels of cyclin dependent kinase inhibitors, p21 and p27, were up regulated upon silibinin treatment. Consistent with these changes in cell cycle regulators, a decrease in the phosphorylation of Rb at Ser780 and Ser795/801 was also observed in silibinin-treated cells. In other studies, flow cytometric analysis of annexin V-PI stained cells revealed that silibinin induces apoptotic death at 100 and 200 µM concentrations. Mechanistic studies showed that silibinin causes an activation of caspase-3 and caspase-9 with concomitant increase in the levels of cleaved PARP suggesting that it induces caspase-dependent apoptosis in LoVo cells. In tumor studies, oral administration of silibinin was effective in inhibiting the growth of LoVo tumor xenografts. Immunohistochemical and western blot analyses of tumor xenografts revealed that inhibitory effect of silibinin on tumor xenograft growth was mediated through simultaneous inhibition of proliferation and induction of apoptosis. Together these results suggest that silibinin may be a potential chemopreventive agent against colorectal cancer.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 924.