Recently, considerable research efforts have been directed toward the identification of naturally occurring dietary/non-dietary agents for both prevention/intervention of colorectal cancer (CRC). One such agent is silibinin, a flavonolignan isolated from the seeds of milk thistle (Silybum marianum), which has been used clinically and as dietary supplement against liver toxicity for over three decades. Several recent studies conducted by us and others have shown that silibinin effectively inhibits the growth of various cancers of epithelial origin. Regarding CRC, silibinin has been shown to inhibit CRC growth and progression in various pre-clinical models; however, the mechanistic basis for its efficacy against CRC remains largely unknown. Here, we demonstrate for the first time that silibinin rapidly induces oxidative stress in human CRC SW480 cells due to ROS generation with a concomitant dissipation of mitochondrial potential, cytochrome c release and apoptotic death. ROS generation preceded the changes in DiOC6 retention indicating that silibinin triggers oxidative stress prior to mitochondrial disruption and apoptosis. Longer-time exposure of cells with silibinin caused an intensified cytoplasmic vacuolization followed by sequestration of the organelles, which inhibited the release of cytochrome c. Interestingly, this decrease in apoptotic response co-related with increased autophagic events as evidenced by tracking the dynamics of LC3-II within the cells. Further mechanistic studies revealed that silibinin strongly inhibits the activation of PI3K-Akt-mTOR pathway. Co-incident with this effect, energy restriction was induced by silibinin where an enhanced AMP/ATP ratio caused an increase in phospho AMPKTh172 resulting in increased phosphorylation and thereby decreased activity of acetyl-CoA carboxylase, an essential enzyme for fatty acid synthesis. Down regulation of mTOR kinase activity by silibinin also resulted in suppression of protein translation initiation complex leading to suppression of Cap-dependent translation. To further confirm that silibinin causes starvation-induced autophagic death, additional studies were performed to assess the metabolic profile of the cells employing investigative metabolomics study utilizing quantitative high-resolution proton nuclear magnetic resonance spectroscopy (1H-NMRS), which showed that silibinin significantly decreases glucose uptake in the cells eventually leading to cell death. Our completed studies show dual efficacy of silibinin where first it initiates an apoptotic response triggered by ROS generation and then imposes energy restriction within the cell activating autophagic machinery to induce autophagic cell death. Together, these might be major molecular mechanisms of silibinin in its chemopreventive efficacy against colon cancer. This work was supported by NCI RO1 grant CA112304.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3790.